Busy month

Between a busy month at work and juggling the duties associated with buying a home, I’ve haven’t had the time to write anything up for the month of July. I do have some interesting stuff in the works for August and following months. Until then, I’m sure you’ll find something worth reading in the archive.

Sudoku Solver in Haskell

This month will be a bit of short article since I haven’t had a whole lot of time on my hands lately. Haskell is a wonderful little language that has begun to pick up a bit of moment in the past year that I’ve been playing with on-and-off now for several years. Since I don’t post enough on Haskell, I figured I’d post my bare-bones Haskell Sudoku Solver.

import Data.List
import Data.Maybe

toRowColumn :: Int -> (Int, Int)
toRowColumn index = (r, c)
	where	r = div index 9
		c = mod index 9

toIndex :: (Int, Int) -> Int
toIndex (r, c) = r * 9 + c

toRegion :: (Int, Int) -> Int
toRegion (r, c) = (div r 3) * 3 + (div c 3)

columnIndicies :: Int -> [Int]
columnIndicies c = [c, c + 9..80]

regionIndicies :: Int -> [Int]
regionIndicies g = [toIndex(r + x, c + y) |
		x <- [0..2],
		y  [Int]
rowIndicies r = [9 * r..9 * (r + 1) - 1]

values :: [Int] -> [Int] -> [Int]
values board indicies = filter (>0) (map (board!!) indicies)

possibleValues :: [Int] -> (Int, Int) -> [Int]
possibleValues board rowColumn =
	foldl () [1..9] (
	  map (values board) (
	    map (f -> (fst f . snd f) rowColumn) (
	      zip [rowIndicies, columnIndicies, regionIndicies] [fst, snd, toRegion]
	    )
	  )
	)

validBoard :: [Int] -> Bool
validBoard board = (length board == 81) && (and $ map (==0) l)
	where	l = map length s
		s = map ([1..9]) v
		v = [values board (xIndicies x) |
			x <- [0..8],
			xIndicies  Bool
solvedBoard board = and $ map (>0) board

hasUnassigned :: [Int] -> Bool
hasUnassigned board = isJust $ elemIndex 0 board

assignFirstUnassigned :: [Int] -> Int -> [Int]
assignFirstUnassigned (b:bs) value
	| b == 0 = value : bs
	| otherwise = b : (assignFirstUnassigned bs value)

possibleBoards :: [Int] -> [Int] -> [[Int]]
possibleBoards board possibleAssignments = map (assignFirstUnassigned board) possibleAssignments

solve :: [Int] -> [[Int]]
solve board
	| not (validBoard board) = [[]]
	| solvedBoard board = [board]
	| not (hasUnassigned board) = [[]]
	| otherwise = concated
	where	concated = concat mapped
		mapped = map solve filtered
		filtered = filter (not . null) possibleSolved
		possibleSolved = possibleBoards board possibleAssignments
		possibleAssignments = possibleValues board unassignedRowColumn
		unassignedRowColumn = toRowColumn unassignedIndex
		unassignedIndex = fromJust $ elemIndex 0 board

demo :: [Int]
demo = [2,0,0,0,8,0,3,0,0,
       0,6,0,0,7,0,0,8,4,
       0,3,0,5,0,0,2,0,9,
       0,0,0,1,0,5,4,0,8,
       0,0,0,0,0,0,0,0,0,
       4,0,2,7,0,6,0,0,0,
       3,0,1,0,0,7,0,4,0,
       7,2,0,0,4,0,0,6,0,
       0,0,4,0,1,0,0,0,3]

Sudoku Solver in C# using Lambda Expressions

Seems that everywhere you look someone has a Sudoku Solver that they want to showcase, well, I’m no different so I figured I’d post my take on the subject. Microsoft has introduced/included/borrowed a number of functional programming features into the latest version of C# (3.0) that have made it easier for developers to write better, cleaner code. One of those features continues the trend of improving anonymous methods, which extend delegates which extend interfaces now known as Lambda expressions. E.g., the following are all the same for the expression :

Func<int, int> square = (x) => x * x;

Func<int, int> square = new Func<int, int>(delegate(int x) { return x * x; });

public interface Func<T, R> {
    R Evaluate(T x);
}

public class Square : Func<int, int> {
    public int Evaluate(int x) {
        return x*x;
    }
}

...

Func<int,int> square = new Square();

Given that level of expressive power, I thought I would approach this implementation using as many lambda expressions as possible to see how concise and easy to follow an implementation I could create.

To start off, the solver will assume that the board will be passed in as a 81 character array containing digits 0-9. Zero shall represent an unassigned cell.

The core loop is pretty simple: Start with the initial board on a stack, in a loop take the first board off the stack and check if it is valid. A board is said to be valid if for every row, column and region each structure contains one and only one instance of the digits 1-9. If the board is not valid, no further work should be done.

Next we need to check if the board is solved. If it is not, then we need to explore the possible boards that can be derived from that board. To do so, we will need to find the first possible cell that is unassigned and push on to the the stack a derived board using the possible values that can be assigned to the identified cell.

Once the loop finally exits, print out the solved board.

using System;
using System.Collections.Generic;

namespace Sudoku {
    public class Program {
        static public void Main(string[] args) {
            string input = "200080300060070084030500209000105408000000000402706000301007040720040060004010003";
            Board inputBoard = null;
            try {
                inputBoard = new Board(input);
            } catch (InvalidInputException iie) {
                Console.WriteLine(iie.InvalidInput);
                return;
            }

            Stack boards = new Stack();
            boards.Push(inputBoard);
            Board board = null;
            do {
                board = boards.Pop();
                if (!board.Valid)
                    continue;
                if (!board.Solved)
                    board.FirstAvailable((r, c) =>
                        board.PossibleValuesAt(r, c, (v) =>
                            boards.Push(board.DeriveUsing(r, c, v))
                        )
                    );
            } while (boards.Count > 0 && !board.Solved);
            Console.WriteLine(board);
        }
    }
}

I’m going to start with the private member methods since they form the basic grammar that I will use to implement the public member methods and properties.

The first thing you’ll notice is the private class Structure; it is a simple pair class that contains two member properties for accessing a function that iterates over all possible structures and a function that iterates over all the cells in a specific structure.

The private constructor instantiates a private member array containing the function pointers that enumerate over Rows, Column and Regions.

assertStructure method which iterates over every instance of a structure in the table, and verifies that one and only one instance of the digits 1-9 exist in that structure instance.

indexInStructure iterates over all of the indices of a structure- in this case, 0-8.

using System;
using System.Text;

namespace Sudoku {
    public partial class Board {
        private class Struct {
            public Action<Action<int>> In { get; set; }
            public Action<int, Action<int>> ForValues { get; set; }
            public Struct(Action<Action<int>> _in, Action<int, Action<int>> forValues) {
                In = _in; ForValues = forValues;
            }
        }

        private Struct[] structures;

        private Board(int[] board) {
            this.board = board;
            structures = new Struct[] {
                new Struct(Rows, ValuesInRow), new Struct(Columns, ValuesInColumn),
                new Struct(Regions, ValuesInRegion)
            };
        }

        private bool assertStructure(Action<Action<int>> structure, Action<int, Action<int>>
            valuesInStructure) {
            bool asserted = true;
            structure((x) => {
                int[] used = new int[10];
                valuesInStructure(x, (y) => used[y]++);
                used[0] = 0;
                asserted &= forAllValues((v) => v < 2, used);
            });
            return asserted;
        }

        private void indexInStructure(Action<int> actOnIndex) {
            indexInStructure((n) => true, actOnIndex);
        }

        private void indexInStructure(Predicate<int> p, Action<int> actOnIndex) {
            indexFromToWhere(0, 9, p, actOnIndex, false);
        }

The forAllValues method is simply a way of writing the predicate applied to members of the universe of discourse (the array that was passed in).

indexFromToWhere is a simple wrapper around a common for loop with filtering and the option to break after the first object to pass through the filter is found.

        private bool forAllValues(Predicate f, T[] A) {
            bool held = true;
            for (int n = 0; n < A.Length && held; n++)
                held &= f(A[n]);
            return held;
        }

        private void indexFromToWhere(int min, int max, Predicate<int> p, Action<int> actOnIndex,
            bool breakAfterFirst) {
            for (int n = min; n < max; n++)
                if (p(n)) {
                    actOnIndex(n);
                    if (breakAfterFirst)
                        break;
                }
        }

The following methods are all related to working with the board representation. I choose to implement the board as an integer array. The at method maps a logical row and column to a row order array value within board. Each of the different indexInBoard methods allows for iterating over the indices of the board.

        private int[] board;

        private int at(int row, int col) {
            return row * 9 + col;
        }

        private void indexInBoard(Action<int> actOnIndex) {
            indexInBoard((n) => true, actOnIndex);
        }

        private void indexInBoard(Predicate<int> p, Action<int> actOnIndex) {
            indexInBoard(p, actOnIndex, false);
        }

        private void indexInBoard(Predicate<int> p, Action<int> actOnIndex, bool breakAfterFirst) {
            indexFromToWhere(0, board.Length, p, actOnIndex, breakAfterFirst);
        }
    }
}

The first set of public member methods and properties we can look at are for managing the state of the board. The board is only Solved if each index is assigned. The board is only Valid if every structure in the board is asserted to be true. The constructor checks the input to make sure it is valid, delegates the some work to the private constructor and loads the input string in to the integer array.

using System;
using System.Text;

namespace Sudoku {
    public partial class Board {
        public bool Solved {
            get {
                return forAllValues((x) => x > 0, board);
            }
        }
        public bool Valid {
            get {
                return forAllValues((s) => assertStructure(s.In, s.ForValues), structures);
            }
        }

        public Board(string input) : this (new int[81]) {
            if (string.IsNullOrEmpty(input))
                throw new InvalidInputException(InvalidInput.Empty);
            if (input.Length != 81)
                throw new InvalidInputException(InvalidInput.Length);
            for (int n = 0; n  board[n] = input[n] - '0');
        }

        override public string ToString() {
            StringBuilder S = new StringBuilder(board.Length);
            indexInBoard((n) => S.Append((char)('0' + board[n])));
            return S.ToString();
        }

Next, we need a way to operate on each of the structures in the board. Each row from top to bottom, each column from left to right and each region from top left to bottom right (zig-zag) will be assigned an index from 0-8 respectively.

        public void Columns(Action<int> actOnColumn) {
            indexInStructure(actOnColumn);
        }

        public void Regions(Action<int> actOnRegion) {
            indexInStructure(actOnRegion);
        }

        public void Rows(Action<int> actOnRow) {
            indexInStructure(actOnRow);
        }

It is easy to then iterate the cells in a given structure. The values in a column are simply a trip down the Rows and we only want to act when the value is defined. The values in a row are just as easy by traveling across the Columns and acting when the value is defined. Iterating over the values in the region is a little more involved, but nonetheless just as easy to follow- map the region to the appropriate reference row and column and then iterate over the 3×3 grid and act only when the value is defined.

        public void ValuesInColumn(int column, Action<int> actOnValue) {
            Rows((r) => {
                int value = board[at(r, column)];
                if (value > 0)
                    actOnValue(value);
            });
        }

        public void ValuesInRegion(int region, Action<int> actOnValue) {
            int row = (region / 3) * 3;
            int column = (region % 3) * 3;
            int value = 0;
            for (int r = 0; r < 3; r++)
                for (int c = 0; c  0)
                        actOnValue(value);
                }
        }

        public void ValuesInRow(int row, Action<int> actOnValue) {
            Columns((c) => {
                int value = board[at(row, c)];
                if (value > 0)
                    actOnValue(value);
            });
        }

Finally, we have the interesting methods used in the main loop. DeriveUsing will copy the board into a cloned integer array and then assign the derived at row and column with the value supplied. The newly derived board is then returned.

FirstAvailable iterates over all the indices of the board until it finds an unassigned value and then it acts upon the reference row and column.

PossibleValuesAt goes and collects a list of possible values by first collecting the values used in the row, column and region that the reference row and column reside within, then each value not found is acted upon.

        public Board DeriveUsing(int row, int colum, int withValue) {
            int[] derived = new int[81];
            indexInBoard((n) => derived[n] = board[n]);
            derived[at(row, colum)] = withValue;
            return new Board(derived);
        }

        public void FirstAvailable(Action<int, int> actOnRowAndColumn) {
            indexInBoard((n) =>
                board[n] == 0, (n) => actOnRowAndColumn(n / 9, n % 9), true);
        }

        public void PossibleValuesAt(int row, int column, Action<int> actOnPossibleValue) {
            bool[] used = new bool[10];
            ValuesInRow(row, (x) => used[x] = true);
            ValuesInColumn(column, (x) => used[x] = true);
            ValuesInRegion((row / 3) * 3 + (column / 3), (x) => used[x] = true);
            indexFromToWhere(1, used.Length, (n) => !used[n], actOnPossibleValue, false);
        }
    }
}

Having approached this implementation as I did, I found some interesting bugs that I hadn’t come across before and I figure I’ll close with one that caught me off guard. I had spent an hour writing all my code and figured I’d run it to see what kind of output I got. To my surprise I got an immediate StackOverflowExeception. So I spent and an additional 10 minutes debugging and found the following offending code. Take a look at it and see if you can see what’s wrong with it before reading on.

public void act(Func<int,int> assign){
    act((n) => board[n] = assign(n));
}

public void act(Action<int> actOn) {
    for(int n = 0; n < board.Length; n++)
      actOn(n);
}

After a minute I had my ah-ha moment (as I’m sure have as well) and remembered that the assignment operator returns the value of the assignment so the compiler was turning (n) => board[n] = assign(n), into Func<int, int> instead of Action<int> as I had hoped. To fix the bug, I had to do the following to get the compiler to pick the statement up as Action<int>.

public void act(Func<int,int> assign) {
    for(int n = 0; n < board.Length; n++)
        act((n) => { board[n] = assign(n); });
}

Having made the change, I tested my input string and out printed the solution immediately. I decided against writing similar function overloading in the final implementation to prevent any unintended bugs.

Integer Factorization by Dynamic Programming with Number Theoretic Applications

Having been a participant of a number of mathematical programming competitions over the years, I’ve had to find a number of efficient ways of implementing many common Number Theoretic Functions. In this write up, I’m going to go over a method I’ve found useful for easily factoring numbers using a sieving method, go over some the implementation of a few number theory functions along with time complexity analysis of each. The cornerstone to many of these implementations relies on the ability to quickly factor integers and find primes.

One of the most popular methods of for finding primes is the Sieve of Eratosthenes. The algorithm starts by populating a table with every positive integer from 2 to a ceiling value. Then find the first integer not yet crossed off, in the case 2, and eliminate every multiple of 2 from the table then return to 2 and find the next positive integer not yet crossed off and repeat the procedure until the end of the table is reached. The method is fine and all, but a lot of really great information is lost in that computation. Here is a sample implementation:

bool[] isPrime = new bool[400];
for (uint n = 2; n < isPrime.Length; n++)
    isPrime[n] = true;
for (uint n = 2; n < isPrime.Length; n++)
    if (isPrime[n])
        for (uint m = 2, c = 0; (c = m * n) < isPrime.Length; m++)
            isPrime[c] = false;

On the other hand, say we approach sieve a little differently. Create an empty table as large as the ceiling value. Start at 2 and for every multiple of 2, create a record that has two parts: 2 and half of the multiple. Return to 2 and find the next integer in the table that has yet to be recorded, in this case 3. For every multiple of 3, create a record that has two parts: 3 and third of the multiple (only if the multiple was not previously recorded. E.g., 6 because 2 previously recorded the record). Return to 3 and find the next integer in the table that has yet to be recorded so on and so forth until every integer in the table has been recorded.

The following graphic demonstrates this process for a ceiling values of 25. If we wish to factor 16, we go to 16’s record (2, 8), follow to 8’s record (2,4), again follow 4’s record (2, 2) and finally 2’s record (2, λ). Thus the prime factorization of 16 is 2, 2, 2, 2.

It should be apparent that this algorithm is a simple dynamic programming solution that yields two major results:

1. We have factored every positive integer up to a ceiling value.
2. We have found every positive prime integer up to a ceiling value.

And one major draw back

1. Uses a lot of memory as a trade off for speed.

Let’s get into the C# implementation. To start off, we need a record class that’ll store the information about the first prime that divides an entry and the composite to jump to if the record corresponds to a composite.

public class Record {
    public uint Prime {get; set;}
    public uint? JumpTo {get; set;}
}

We’ll have a class called NumberTheory and assume that it has the following structure. If you want, you could make this a Singleton class but I felt it was unnecessary for the scope of this write up.

public class NumberTheory {
    private Record[] table;

    ...
}

It makes sense to put the core algorithm in the constructor and then have member methods for each of the functions we’d like to have. It should be assumed that for the lifetime of the class that the largest value ever called on the methods will be N otherwise an exception should be thrown by the methods (omitted here for brevity).

public NumberTheory(uint N){
    uint c = 0;
    table = new Record[N+1];
    for(uint n = 2; n < table.Length; n++) {
        if(table[n] != null)
            continue;
        table[n] = new Record() { Prime = n };
        for(uint m = 2; (c = n * m) < table.Length; m++)
            if(table[c] == null)
                table[c] = new Record() { JumpTo = m, Prime = n };
    }
}

The time complexity of the implementation can be derived using some analysis and by having some knowledge of certain identities. Starting from 2 there are N – 2 numbers to check of which 1/2 will be visited by the interior loop, starting from 3 there are N – 3 numbers to check of which 1/3 will be visited visited by the interior loop, so on and so forth leading to the following summation:

If we separate the summation into the harmonic series of primes (HSP) and prime counting function (aggregate 1 for every prime less than n) we get:

Asymptotically, the HSP tends towards and towards .

Giving us our final asymptotic time complexity of .

The first member method we’ll implement is a trivial check to see if a given number is prime by checking the table’s Record’s JumpTo property for null.

public bool IsPrime(uint N){
    return !table[N].JumpTo.HasValue;
}

The time complexity here is a simple .

From IsPrime, we can easily implement a function that will get every single prime up to a given value by iterating over the table.

public void PrimesLessThan(uint value, Action<uint> actOnPrime) {
    for(int n = 2; n < value; n++)
	if(IsPrime(n))
		actOnPrime(n);
}

We can get the prime counting function but utilizing the PrimeLessThan function.

public uint CountPrimes(uint n) {
	uint count = 0;
	PrimesLessThan(n, (p) => {count++;});
	return count;
}

Here the time complexity is the same as PrimesLessThan: .

We can get the prime factorization of a composite easily. To do so, we simply iterate over the records until we reach a Record with no JumpTo value.

public void PrimeFactorsOf(uint composite, Action actOnFactor) {
    Record temp = table[composite];
    while(temp != null) {
        actOnFactor(temp.Prime);
        if(temp.JumpTo.HasValue)
            temp = table[temp.JumpTo.Value];
        else
            temp = null;
    }
}

The time complexity of this implementation relies on the the prime omega function which is the number of prime factors (not necessarily distinct) of n. The function tends to .

From PrimeFactorsOf, we can also easily implement Euler’s Totient Function - the function tells us how many positive integers less than n are coprime to n. It is defined as:

Which essentially states that if you multiply all of the repeated prime factors of n together by all of the non-repeat prime factors – 1 of n together, you will have the result of .

public uint EulerTotient(uint n){
    uint phi = 1, last = 0;
    PrimeFactorsOf(n, (p) => {
        if(p != last) {
            phi *= p - 1;
            last = p;
        } else {
            phi *= p;
        }
    });
    return phi;
}

A similar function known as the Dedekind Function defined as

can be implemented in a similar way as :

public uint DedekindPsi(uint n){
    uint phi = 1, last = 0;
    PrimeFactorsOf(n, (p) => {
        if(p != last) {
            phi *= p + 1;
            last = p;
        } else {
            phi *= p;
        }
    });
    return phi;
}

The Von Mangoldt Function is another interesting function, unfortunately I haven’t had a chance to use it, but it is trivial to implement so I will include it here for completeness. It is defined as

public double VonMangoldt(uint n){
    uint P = 0;
    PrimeFactorsOf(n, (p) => {
        if(P == 0) {
            P = p;
        } else if (P != p) {
            P = 1;
        }
    });
    return Math.Log(P);
}

The Möbius Function is a handy little function for determining if a number is square free or not (among many more interesting things). It is defined as:

public int MoebiusFunction(uint N){
    if(N == 1)
        return 1;
    bool distinct = true;
    uint last = 0, k = 0;
    PrimeFactorsOf(N, (p) => {
        if(p == last) {
            distinct = false;
        } else {
            k++;
            last = p;
        }
    });

    if(distinct)
        return ((k & 1) == 0) ? 1 : -1;
    return 0;
}

Since EulerTotient, DedekindPsi, VonMangoldt and Moebius each use PrimeFactorsOf without any additional lifting, their time complexities are the same as PrimeFactorsOf – .

The last function I’ll implement is the Mertens Function which is simply defined as :

public int Mertens(uint n){
    int m = 0;
    for(uint k = 1; k <= n; k++)
        m += MoebiusFunction(k);
    return m;
}

The implementation’s time complexity is simply .

Generalized Interpreter Framework in C#

Introduction

Like most Computer Science graduates, I had taken a Programming Languages course during college in which we built a compiler and studied the consequences of our design decisions. I had also taken a course in symbolic logic- particularly, looking at the sentential and predicate logic. Lately, I had been thinking about these two courses and thought it would be interesting to write an interpreter that would take in a sentence valid under sentential logic and output the resulting truth table.

After looking at what I had built, it became apparent that I could generalize my solution into a flexible framework where I could easily create a new interpreter for a desired (albeit simple) formal grammar and produce the desired outputs that other applications could then use for whatever purpose. The following is an overview of this framework and an example implementation for generating truth tables for sentences valid under sentential logic.

Framework

Specification

An Interpreter takes a string and produces an object hierarchy that a client can perform functions upon. The Interpreter does so by following the rules of a grammar that the string is assumed (although not garaunteed) to be valid under. The Interpreter will only be valid for context-free-grammars (CFG). A CFG consists of Terminals and Nonterminals. A Terminal is a character or sequence of characters that terminate a definition. A Nonterminal is a rule that defines the relationships between other Nonterminals and Terminals.

Design

Interpreter designs differs from traditional compiler design, in that only the so-called front-end is implemented. There are three principal components involved in this process: Tokenizer, Parser and Semantic Analyzer.

A Tokenizer takes a string of characters and returns a collection of Tokens, where each Token is a tuple containing the accepted substring and meta data describing what the substring represents. Tokens are identified by looking for Terminals in the input string.

The Parser then builds (formally) a concrete syntax tree (informally a syntax tree) following the rules defined by the grammar. This is done by looking for Nonterminals in the collection of Tokens.

Finally, the Semantic Analyzer builds (formally) an abstract syntax tree (informally a semantic tree) by ignoring the syntax in the parse tree and constructing the appropriate tree for performing functions upon.

In the event of an exception, the Interpreter will write the details of the exception to a text writer (e.g., Console.Error) provided by the client of the Interpreter.

Exceptions are to be thrown under the following conditions:

• The input string is null or empty, or none of the Terminals are able to accept a character in the input string.
• None of the Nonterminals are able to accept a set of tokens, or the number of tokens is deficient for the Nonterminals to identify a match.

Implementation

abstract public class Tokenizer<T> {
    protected TerminalList<T> Terminals { get; private set; }

    public Tokenizer() {
        Terminals = new TerminalList<T>();
        loadTerminals();
    }

    public TokenList<T> Tokenize(string input) {
        if (string.IsNullOrEmpty(input))
            throw new UnexpectedEndOfInputException();

        TokenList<T> tokens = new TokenList<T>();
        Token<T> candiate;

        for (int n = 0; n < input.Length; n++) {
            if (char.IsWhiteSpace(input[n]))
                continue;

            if (!terminalExists(input, n, out candiate))
                throw new UnexpectedInputException(n);

            tokens.Add(candiate);
            n += candiate.Lexeme.Length - 1;
        }

        return tokens;
    }

    private bool terminalExists(string input, int n, out Token<T> candiate) {
        candiate = null;
        foreach (Terminal<T> terminal in Terminals)
            if (terminal.Exists(input, n, out candiate))
                return true;
        return false;
    }

    abstract protected void loadTerminals();
}

abstract public class Parser<T, P> {
    protected NonterminalList<T, P> Nonterminals { get; private set; }
    protected T[] FirstExpectedTokens { get; private set; }

    public Parser() {
        Nonterminals = new NonterminalList<T, P>();
        loadNonterminals();

        FirstExpectedTokens = new T[Nonterminals.Count];
        for (int n = 0; n < Nonterminals.Count; n++)
            FirstExpectedTokens[n] = Nonterminals[n].FirstExpectedToken;
    }

    public P Parse(TokenList<T> tokens) {
        return Parse(tokens, 0, tokens.Count - 1);
    }

    public P Parse(TokenList<T> tokens, int n, int m) {
        P parsedSentence;
        foreach (Nonterminal<T, P> nonterminal in Nonterminals)
            if (nonterminal.Exists(tokens, n, m, out parsedSentence))
                return parsedSentence;

        throw new UnexpectedTokenException<T>(tokens[n], FirstExpectedTokens);
    }

    abstract protected void loadNonterminals();
}

abstract public class SemanticAnalyzer<P, S> {
    abstract public S Analyze(P sentence);
}

abstract public class Interpreter<T, P, S> {
    private TextWriter externalLogger;

    abstract protected Tokenizer<T> Tokenizer { get; }
    abstract protected Parser<T, P> Parser { get; }
    abstract protected SemanticAnalyzer<P, S> SemanticAnalyzer { get; }

    public Interpreter(TextWriter externalLogger) {
        this.externalLogger = externalLogger;
    }

    public bool Interpret(string candidateSentence, out S sentence, out TokenList<T> tokens) {
        sentence = default(S);
        tokens = null;

        try {
            tokens = Tokenizer.Tokenize(candidateSentence);
            sentence = SemanticAnalyzer.Analyze(Parser.Parse(tokens));

            return true;
        } catch (UnexpectedInputException uie) {
            logger(uie.Message);
            logger("'{0}'", candidateSentence);
            logger(" {0}^", string.Empty.PadLeft(uie.AtIndex, '.'));

        } catch (UnexpectedEndOfInputException ueie) {
            logger(ueie.Message);
            logger("'{0}'", candidateSentence);
            logger(" {0}^", string.Empty.PadLeft(candidateSentence.Length, '.'));

        } catch (UnexpectedTokenException<T> pe) {
            logger(pe.Message);
            logger("'{0}'", candidateSentence);
            logger(" {0}^", string.Empty.PadLeft(pe.Unexpected.StartsAtIndex, '.'));
        }

        return false;
    }

    private void logger(string format, params object[] values) {
        externalLogger.WriteLine(format, values);
    }
}

Example

Specification

SL Backus-Naur Form (BNF)
sentence	::=	sentence_letter
	|	~ sentence
	|	( sentence connective sentence )
sentence_letter	::=	letter
	|	letter number
connective	::=	v
	|	^
	|	->
	|	<->
letter	::=	a
	|	…
	|	z
	|	A
	|	…
	|	Z
number	::=	number digit
	|	digit
digit	::=	0
	|	…
	|	9

Sentential Logic (SL) is a simple formal system that consists of two semantic elements: primitive types called Sentence Letters that may either be true or false, and expressions built upon Sentence Letters called Sentences. A Sentence Letter is any character a-z (lower or upper) followed by an optional number. Sentences are defined in terms of other sentences, the simplest is a plain Sentence Letter. Any negated ¬ sentence is also a sentence. Any two sentences connected by a conjunction ∧, disjunction ∨, material conditional → or material biconditional ↔ and enclosed by parentheses () is also a sentence under SL. No other arrangement of symbols constitutes a sentence under SL.

Connectives and negation Truth Tables
	¬		∧	T	F		∨	T	F		→	T	F		↔	T	F
T	F			T	F			T	T			T	F			T	F
F	T			F	F			T	F			T	T			F	T

A truth table has a column for each of the sentence letter that show up in a sentence and a column for the sentence. Each row is one Cartesian coordinate in the space where n is the number of the number sentence letters. By convention, one lists the rows in order all sentence letters being true, to the last row where all sentence letters are false.

Implementation

TerminalSyntax and TerminalSentenceLetter are subclasses of Terminal and implement the logic for determining if a certain syntax or sentence letter exists in an input string. TerminalSyntax takes in the TokenType and string to find in the input as configuration options. If one felt inclined, one could have implemented a TerminalRegularExpression class instead of two separate classes but I felt it was unnecessary.

public class SLTokenizer : Tokenizer<TokenType> {
    protected override void loadTerminals() {
        Terminals.AddRange(new Terminal<TokenType>[] {
            new TerminalSyntax(TokenType.Conjunction, "^"),
            new TerminalSyntax(TokenType.Disjunction, "v"),
            new TerminalSyntax(TokenType.LeftParen, "("),
            new TerminalSyntax(TokenType.MaterialBiconditional, "<->"),
            new TerminalSyntax(TokenType.MaterialConditional, "->"),
            new TerminalSyntax(TokenType.Negation, "~"),
            new TerminalSyntax(TokenType.RightParen, ")"),
            new TerminalSentenceLetter()
        });
    }
}

The three Nonterminal types for Connectives, Negation and Letters are subclasses of Nonterminal and implement the logic for determining if a collection of tokens matches the rules associated with each Nonterminal type.

public class SLParser : Parser<TokenType, ParsedSentence> {
    protected override void loadNonterminals() {
        Nonterminals.AddRange(new Nonterminal<TokenType, ParsedSentence>[] {
            new NonterminalSentenceConnective(this),
            new NonterminalSentenceNegation(this),
            new NonterminalSentenceLetter()
        });
    }
}

Nothing particularly exciting here, simply mapping the parsed sentences to actual sentences that will be used by the client code. Sentence implementations have the logic for evaluating themselves.

public class SLSemanticAnalyzer : SemanticAnalyzer<ParsedSentence, Sentence> {
    public override Sentence Analyze(ParsedSentence sentence) {
        ParsedSentenceLetter letter = sentence as ParsedSentenceLetter;
        if (letter != null)
            return new SentenceLetter(letter);

        ParsedSentenceUnary negation = sentence as ParsedSentenceUnary;
        if (negation != null)
            return new SentenceUnaryNegation(negation, Analyze(negation.Sentence));

        ParsedSentenceBinary connective = sentence as ParsedSentenceBinary;
        if (connective != null) {
            switch (connective.Operation.TokenType) {
                case TokenType.Conjunction:
                    return new SentenceBinaryConjunction(connective, Analyze(connective.Left), Analyze(connective.Right));
                case TokenType.Disjunction:
                    return new SentenceBinaryDisjunction(connective, Analyze(connective.Left), Analyze(connective.Right));
                case TokenType.MaterialBiconditional:
                    return new SentenceBinaryMaterialBiconditional(connective, Analyze(connective.Left), Analyze(connective.Right));
                case TokenType.MaterialConditional:
                    return new SentenceBinaryMaterialConditional(connective, Analyze(connective.Left), Analyze(connective.Right));
            }
        }

        throw new NotImplementedException();
    }
}

Nothing extra to implement in the SLInterpreter to add other than the constructor and properties.

public class SLInterpreter : Interpreter<TokenType, ParsedSentence, Sentence> {
    public SLInterpreter(TextWriter externalLogger) : base(externalLogger) { 

    }

    override protected Tokenizer<TokenType> Tokenizer {
        get { return new SLTokenizer(); }
    }

    override protected Parser<TokenType, ParsedSentence> Parser {
        get { return new SLParser(); }
    }

    override protected SemanticAnalyzer<ParsedSentence, Sentence> SemanticAnalyzer {
        get { return new SLSemanticAnalyzer(); }
    }
}

Once all of the Interpreter code has been completed, it becomes trivial to go and implement the truth table.

private void table(string input) {
    SLInterpreter interpreter = new SLInterpreter(Console.Out);
    TokenList<TokenType> tokens;
    Sentence sentence;

    if (!interpreter.Interpret(input, out sentence, out tokens))
        return;

    List<Variable> variables = tokens
       .Where((x) => x.TokenType == TokenType.Letter)
       .Select((x) => x.Lexeme)
       .Distinct()
       .Select((x) => new Variable(x))
       .ToList();

    foreach (Variable variable in variables)
        Console.Write("{0} ", variable.Identifier);
    Console.WriteLine("| {0}", input);

    for (int n = (1 << variables.Count) - 1; n >= 0; n--) {
        int N = n;
        for (int m = variables.Count - 1; m >= 0; m--) {
            variables[m].Value = (N & 1) == 1;
            N >>= 1;
        }

        IDictionary<Token<TokenType>, bool> tableRow = new Dictionary<Token<TokenType>, bool>();
        sentence.Evaluate(variables, tableRow);

        int offset = 2;
        foreach (Variable variable in variables) {
            Console.Write("{0} ", variable.Value ? "T" : "F");
            offset += 1 + variable.Identifier.Length;
        }

        Console.Write("| ");
        foreach (Token<TokenType> token in tableRow.Keys) {
            Console.CursorLeft = token.StartsAtIndex + offset;
            Console.Write(tableRow[token] ? "T" : "F");
        } Console.WriteLine();
    }
}

Ouroboros: reinventing Nibbles

Introduction

I talked about some classic arcade games in a previous post that I had worked on over the years and mentioned that I’d get around to posting some implementation details of one of them. A few months later here we are and the following is an overview of the implementation details of Ouroboros- my revisioning of the classic arcade game Nibbles that I enjoyed playing and learning about back in my QBasic days.

This write-up will go over the activities associated with the software development process from specification to implementation. Before I get into the details, here’s a game play of what is that I’ll be explaining how to make:

Specification

The goal of the game is to collect rewards. Each time a reward is collected, the user’s score is increased. The snake is constantly moving in the direction last requested by the user. The user can direct the snake to move either left, up, right or down. To make the game more challenging, the snake will grow whenever the snake consumes a reward. The game then ends once the snake spans the entire board or the snake collides with itself. When the snake “hits” a wall, its position wraps around the board. When either of the terminating conditions is meet the user is asked if he or she wishes to play again.

Requirements

The user may control the direction the snake may move by using the keyboard. The following keys are valid: {←, ↑, →, ↓} and {a, w, d, s} to the directions {left, up, right and down}.

The game is to be displayed to the user as as command line interface (CLI), 2D graphical user interface (2DGUI) using WinForms and a 3D GUI (3DGUI) using Windows Presentation Foundation. The CLI and 2DGUI shall appear as boards that the snake and reward appear on. The 3DGUI shall appear as a torus that the snake and reward appear on.

The game may not appear to be slower as the length of the snake increases.

Design

The Board

The board is a simple coordinate system with a fixed side length . Each coordinate may be occupied by at most one snake segment. For each view, a mapping from the model space, , to the view space, , is necessary to achieve the required behavior.

For the CLI view, let since every coordinate maps one-to-one with a cursor position on the console.

The 2DGUI view requires a scaling factor, , otherwise the board would appear to be too small to play- unless for example, upon a cellphone LCD. Let .

The 3DGUI view requires an initial mapping from a coordinate to a system. This is accomplished by where is the length of the edge of the board. A torus is defined in terms of an interior radius, , and the swept radius, . Thus a torus is defined as the following:

The Snake

The snake is conceptually a sequence of segments that I choose to represent as a singly-linked list where each node contains a pointer to the next segment and the segment’s position. The following illustrates a snake of length five:

To achieve movement, the position of the head segment is passed to the next segment, and the next segment on to its next segment so on and so forth until the tail is reached.

Each time the snake moves, its coordinate will be calculated as .

Scoring should be done in such a way that rewards become more valuable as time continues. Since the initial length of the snake is 1, a snake of length will have collected rewards. Thus, let represent the scoring function. Where 100 is maximum score for a reward, -1/10 is the decay factor.

Once a snake has consumed a reward, a new node is added to the tail with a location identical to the tail location.

To determine if a the snake is on top of a reward, each segment’s position will be compared to the reward’s position. If a segment and reward are identical then the snake is on top of the reward. If no match is found, then the snake is not on top of the reward. This process can be done in linear time. Constant time, if you choose to generate rewards that are not on top of snake.

When drawing the game it is useful to observe that the only thing that ever changes between time step is the the head and tail of the snake. Thus, it is prudent to only draw the current head position and erase the previous tail position. This will produce a length independent drawing method so that the game does not appear to be slower as the snake gets larger.

Implementations may be written using recursion, but beware that with larger board sizes that you run the risk of a stack overflow on systems that don’t give you much memory to work with. Using a cursor to search the singly-linked list may be more appropriate when using larger board sizes.

Implementation

I decided to go with a Model-View-Controller (MVC) pattern since I’d like to be able to view the CLI, 2DGUI and 3DGUI all at once. Below is a complete UML class diagram of all the MVC components that I choose to implement.

The following is the core engine of the game; it perform each of the core tasks of performing logic, drawing the board, getting user input and maintaining time.

public class Program {
    [STAThread]
    static public void Main(string[] args) {
        List views = new List(new IGameView[] {
            new CLIGameView(),
            new GUI2DGameView(),
            new GUI3DGameView()
        });
        IGameController controller = new CLIGameController();

        int boardSize = 32;
        double maxScore = double.MinValue;

        views.ForEach((view) => view.initialize(boardSize));

        do {
            SnakeDirection desiredDirection = SnakeDirection.Up;
            SnakePoint reward = SnakePoint.Random(boardSize);
            SnakeSegment snake = new SnakeSegment(SnakePoint.Random(boardSize));

            views.ForEach((view) => view.drawBoard());

            do {
                if (controller.InputAvailable) {
                    SnakeDirection possible = controller.getDirection();
                    if (possible != SnakeDirection.Nop)
                        desiredDirection = possible;
                }

                if (snake.isOnTopOf(reward)) {
                    snake.grow();

                    if (snake.Length != boardSize * boardSize) {
                        do {
                            reward = SnakePoint.Random(boardSize);
                        } while (snake.isOnTopOf(reward));
                    }

                    maxScore = Math.Max(maxScore, snake.Score);

                    views.ForEach((view) => view.drawScore(snake.Score, maxScore));
                }

                SnakePoint oldTail = snake.Tail.Location;

                snake.move(desiredDirection, boardSize);

                views.ForEach((view) => view.drawSnake(snake, oldTail));
                views.ForEach((view) => view.drawReward(reward));

                System.Threading.Thread.Sleep(1000 / 15);
            } while (!snake.selfCollision());

            views.ForEach((view) => view.drawGameOver());
            views.ForEach((view) => view.drawPlayAgain());

        } while (controller.playAgain());

        views.ForEach((view) => view.deinitialize());
        views.Clear();
    }
}

public class GUI3DGameView : IGameView {
    private int boardSize;
    private Form canvas;
    private ScoreLabel score;
    private TorusScene scene;

    public int BoardSize {
        get { return boardSize; }
    }

    public GUI3DGameView() {
        canvas = new Form();
        canvas.BackColor = System.Drawing.Color.FromArgb(0x33,0x33,0x33);
        canvas.FormBorderStyle = FormBorderStyle.FixedToolWindow;
        canvas.MaximizeBox = false;
        canvas.MinimizeBox = false;
        canvas.SizeGripStyle = SizeGripStyle.Hide;
        canvas.Text = "GUI3DGameView";
        canvas.ClientSize = new Size(384, 384);

        ElementHost host = new ElementHost();
        host.Child = scene = new TorusScene();
        host.Dock = DockStyle.Fill;
        canvas.Controls.Add(host);

        score = new ScoreLabel();
        score.Dock = DockStyle.Bottom;

        canvas.Controls.Add(score);
    }

    public void initialize(int boardSize) {
        this.boardSize = boardSize;

        if (!canvas.Visible)
            canvas.Show();
    }

    public void deinitialize() {
        canvas.Dispose();
    }

    public void drawBoard() {
        score.reset();
        scene.removeSnake();
    }

    public void drawGameOver() {

    }

    public void drawPlayAgain() {

    }

    public void drawReward(SnakePoint reward) {
        scene.moveReward(reward.x, reward.y);
    }

    public void drawScore(double current, double max) {
        score.setScore(current, max);
    }

    public void drawSnake(SnakeSegment head, SnakePoint oldTail) {
        scene.addSegment(head.Location.x, head.Location.y, head.Length);
    }
}

using System;
using System.Collections.Generic;
using System.Windows.Controls;
using System.Windows.Media;
using System.Windows.Media.Media3D;

namespace Snake.View.GUI3D {
    public class TorusScene : Viewport3D {
        private Queue<ModelVisual3D> patches;
        private ModelVisual3D reward;

        public TorusScene() {
            Camera = new PerspectiveCamera(new Point3D(10, 10, 10), new Vector3D(-10, -10, -10), new Vector3D(0, 1, 0), 60);

            AmbientLight aLight = new AmbientLight(Color.FromRgb(0x33,0x33,0x33));
            ModelVisual3D aLightHost = new ModelVisual3D();
            aLightHost.Content = aLight;
            Children.Add(aLightHost);

            DirectionalLight light = new DirectionalLight(Colors.Orange, new Vector3D(0, -10, 0));
            ModelVisual3D lightHost = new ModelVisual3D();
            lightHost.Content = light;
            Children.Add(lightHost);

            DirectionalLight rearLight = new DirectionalLight(Colors.LightBlue, new Vector3D(0, 10, 0));
            ModelVisual3D rearLightHost = new ModelVisual3D();
            rearLightHost.Content = rearLight;
            Children.Add(rearLightHost);

            Model3DGroup torus = new Model3DGroup();
            double N = 16.0;
            double dTheta = Math.PI / N, dPhi = Math.PI / N;
            double R = 5.0, r = 2.0;

            Color surface = SnakeColors.MGround;

            for (double theta = 0.0; theta <= 2.0 * Math.PI; theta += dTheta) {
                for (double phi = 0.0; phi <= 2.0 * Math.PI; phi += dPhi) {
                    Point3D[] S = square(dTheta, dPhi, R, r, theta, phi);
                    torus.Children.Add(triangle(S[0], S[1], S[3], surface));
                    torus.Children.Add(triangle(S[3], S[2], S[0], surface));
                }
            }

            ModelVisual3D model = new ModelVisual3D();
            model.Content = torus;
            Children.Add(model);

            patches = new Queue<ModelVisual3D>();
        }

        public void addSegment(double u, double v, int max) {
            ModelVisual3D snake = addSphere(u, v, 0.5, SnakeColors.MHead);

            if (patches.Count != 0 && patches.Count == max)
                Children.Remove(patches.Dequeue());
            patches.Enqueue(snake);

            Point3D[] S = square(Math.PI / 16.0, Math.PI / 16.0, 5.0, 2.5, u / 16.0 * Math.PI, v / 16.0 * Math.PI);
            double r = 30.0 / Math.Sqrt(3.0) / Math.Sqrt(S[0].X * S[0].X + S[0].Y * S[0].Y + S[0].Z * S[0].Z);
            Camera.SetValue(PerspectiveCamera.PositionProperty, new Point3D(r * S[0].X, r * S[0].Y, r * S[0].Z));
            Camera.SetValue(PerspectiveCamera.LookDirectionProperty, new Vector3D(-r * S[0].X, -r * S[0].Y, -r * S[0].Z));
        }

        public void moveReward(double u, double v) {
            if (reward != null) {
                Children.Remove(reward);
                reward = null;
            }

            reward = addSphere(u, v, 0.25, SnakeColors.MReward);
        }

        public void removeSnake() {
            while (patches.Count != 0)
                Children.Remove(patches.Dequeue());
        }

        private ModelVisual3D addSphere(double u, double v, double r, Color color) {
            Model3DGroup sphere = new Model3DGroup();
            Point3D center = parameterized(5.0, 2.0 + r, u / 16.0 * Math.PI, v / 16.0 * Math.PI);
            Vector3D vec = new Vector3D(center.X, center.Y, center.Z);

            double dTheta, dPhi;
            dTheta = dPhi = Math.PI / 3.0;

            for (double theta = 0.0; theta <= 2.0 * Math.PI; theta += dTheta) {
                for (double phi = 0.0; phi <= 2.0 * Math.PI; phi += dPhi) {
                    Point3D[] S = square(dTheta, dPhi, 0, r, theta, phi);
                    for (int n = 0; n < S.Length; n++)
                        S[n] = Point3D.Add(S[n], vec);

                    sphere.Children.Add(triangle(S[0], S[1], S[3], color));
                    sphere.Children.Add(triangle(S[3], S[2], S[0], color));
                }
            }

            ModelVisual3D model = new ModelVisual3D();
            model.Content = sphere;
            Children.Add(model);

            return model;
        }

        private Point3D parameterized(double R, double r, double theta, double phi) {
            return new Point3D(
                (R + r * Math.Cos(phi)) * Math.Cos(theta),
                r * Math.Sin(phi),
                (R + r * Math.Cos(phi)) * Math.Sin(theta)
            );
        }

        private Point3D[] square(double dTheta, double dPhi, double R, double r, double theta, double phi) {
            return new Point3D[] {
                parameterized(R, r, theta, phi),
                parameterized(R, r, theta, phi + dPhi),
                parameterized(R, r, theta + dTheta, phi),
                parameterized(R, r, theta + dTheta, phi + dPhi)
            };
        }

        private Model3DGroup triangle(Point3D a, Point3D b, Point3D c, Color color) {
            MeshGeometry3D mesh = new MeshGeometry3D();
            mesh.Positions.Add(a);
            mesh.Positions.Add(b);
            mesh.Positions.Add(c);
            mesh.TriangleIndices.Add(0);
            mesh.TriangleIndices.Add(1);
            mesh.TriangleIndices.Add(2);

            Material material = new DiffuseMaterial(new SolidColorBrush(color));
            GeometryModel3D model = new GeometryModel3D(mesh, material);
            Model3DGroup group = new Model3DGroup();
            group.Children.Add(model);
            return group;
        }
    }
}

Embedded Streaming Radio Project

It’s been a while since I posted anything here, partly a function of a lack of interest, time and viewership. Part of my new years resolution was to try and post something around here maybe once a month. So, I figure I’d post on a hardware project that I’ve been tinkering around with the past couple months. It took me a while to gather this info so hopefully it will save others time looking to build a similar project.

I enjoy listening to the SHOUTcast streaming radio service using Winamp but after awhile I really want to be able to just listen to the stations on my shelf stereo system rather than on my laptop. Thus a pretty simple little idea:

Build a device that has a SHOUTcast complaint client that I can configure to point to a local shoutcast sever running on my laptop so that I can stream my mp3 collection or listen to any streaming station through my stereo shelf system.

Given that simple problem statement, I started thinking about how I would go about getting everything set up:

With all of that in mind, I have enough for an initial laundry list of requirements for the hardware component:

• RCA out to connect to my stereo system
• Built-in 1000/100/10 Gigabit Ethernet or 802.11b/g wireless to access the radio streams
• http server running on port 80/8080 so that I could remotely configure it all
• Maybe include a LCD for kicks or a touch screen for album art/misc content

Now there are couple paths to go down from here: the first was to run with the project thinking that I could go out and purchase all the hardware components described above- assemble them, write a simple http server and do some low level assembly. Second option is to go out and see what available open source options are out there and try to purchase a pre-built embedded system. The last option was to search the market for a similar product.

The first option sounds like a lot of fun, but doesn’t seem very feasible from a time stand point. Third option does offer a perfect solution through Logitech’s Squeezebox Classic which is great from a time stand point but not so great from a satisfaction standpoint so that leads one to option number two.

So the hunt was on to find the perfect combo of pre-existing components and figuring out how to get them all to play nicely. First thing I went looking for was to see if there were any open source SHOUTcast clients out there. Turns out there is a really great project out there called the SnackAmp Music Player that does everything that I’d like to do. In fact, it comes along with a great remote control feature targeted exactly at the kind of scenario I’m trying to build.

Next step was to look at what platforms SnackAmp would run on. Being an open source project they have builds available for standard i386 Linux distributions and of course Microsoft Windows. Not being a person particular about operating systems I decided that I would go with Linux since it’s free and since there are a number of minimalistic distributions out there.

Finally, I needed to decide on a piece of hardware that would meet my needs. I want something compact that I can place next to my stereo that is low power and quiet- something a little smaller than a DSL modem or smaller than standard CD wallet. Looking around it seems that VIA is the key player in the mini-itx market which the form factor of most of the boards I was looking at. mini-itx.com and mini-box.com have a number of solutions but few of their offerings had the 802.11b/g that I was looking for. I’m not 100% decided on what I would like to purchase from either site, but the average cost appears to be around 250-300 USD for a board, memory and enclosure.

Surmising all the above, I think I’d like to go with the following technology stack:

Stereo System	SnackAmp	Firefox	SHOUTcast Server	NAS
	ttylinux	Windows XP SP2
	mini itx	Laptop
	802.11b/g LAN

Total cost would be in the 250-300 USD range -add on an additional 100-300 USD for a cheap NAS- since the hardware is the only thing that I need to get. Now, I mentioned previously that Logitech offers an almost identical device for 300 USD (at the time of this post). So the question arises, which one is the better buy?

The commercial solution has been tested and it works with multiple services and seems to have a pretty large community built around it- but then again there’s nothing to learn other than how to use it. On the other hand, a custom built solution can be repurposed to fit future use cases that I have yet to think up so it may offer in the long run a great utility compared to its competition.

Short answer is that I have yet to decide on which option is right since I’m still looking around to see what’s out there. (If you know of additional products post them in the comments). If I move forward with any of this I’ll be sure to post details as they come along.

Radial tree drawing

Although not as interesting as a sunburst diagram, the radial tree view can hold its color against a number of more primitive information visualizations. A radial tree view places the root at the center of the screen then fans out each child node. Each child node then fans out its children within a restricted span and continues on until each leaf is reached. The strengths of the technique allow for any easy to digest depiction of the structure behind the data in a compact space. A common application is visualizing computer networks. It is worthwhile to examine the algorithm behind the technique because it is an exercise in identifying simplicity.

While in college, I would have approached this problem by trying to identify the location of nodes in terms of after all, I want a radial tree view- makes sense to sprint out the gate with a polar system right? While possible, this is a bad path to head down, as you end up drowning in a sea of extraneous details. Rather, it is better to think in terms of and then map to . To clarify that position, let’s think about how we’d go about drawing the run of the mill tree view as in the figure below:

First some observations:

• Every node at a given depth lies on the same line.
• Every child at a given depth is given an equal share of horizontal space independent of necessity relative to the space owned by its parent.

We can construct a simple recursive definition for drawing the tree if we think about these two facts. Given a node, we want to center the node at the top of a region then carve up a region into the number of child nodes where each sub region is equally wide and the same height as the parent minus a layering distance, then draw a line from the parent node to the child node. Continue doing so until all of the nodes have been drawn. All that remains is mapping this tree to the radial tree view below:

To achieve this last step, we want to map each node at to a point Where is the center of the display area. The radius is simply the node’s present coordinate. can be determined as the ratio between the node’s present coordinate and the display width times . And thus, the mapping is complete.

Solutions to some Microsoft interview questions

A couple of weeks ago, someone on reddit posted a link to a collection of Microsoft Interview Questions. As someone who interviewed with them while in college, I was curious to see what kind question they were asking folks. After reviewing the list, I thought I’d work out a few that looked interesting:

Imagine an analog clock set to 12 o’clock. Note that the hour and minute hands overlap. How many times each day do both the hour and minute hands overlap? How would you determine the exact times of the day that this occurs?

Thus^*: 12:00:00 AM, 01:05:27 AM, 02:10:54 AM, 03:16:21 AM, 04:21:49 AM, 05:27:16 AM, 06:32:43 AM, 07:38:10 AM, 08:43:38 AM, 09:49:05 AM, 10:54:32 AM, 12:00:00 PM, 01:05:27 PM, 02:10:54 PM, 03:16:21 PM, 04:21:49 PM, 05:27:16 PM, 06:32:43 PM, 07:38:10 PM, 08:43:38 PM, 09:49:05 PM and 10:54:32 PM.

^* Floor the conversion from to hour, minute and second of day.

Pairs of primes separated by a single number are called prime pairs. Examples are 17 and 19. Prove that the number between a prime pair is always divisible by 6 (assuming both numbers in the pair are greater than 6). Now prove that there are no ‘prime triples.’

Assuming:

Twin Primes:

Prime Triples:

There are 4 women who want to cross a bridge. They all begin on the same side. You have 17 minutes to get all of them across to the other side. It is night. There is one flashlight. A maximum of two people can cross at one time. Any party who crosses, either 1 or 2 people, must have the flashlight with them. The flashlight must be walked back and forth, it cannot be thrown, etc. Each woman walks at a different speed. A pair must walk together at the rate of the slower woman’s pace.

For example if Woman 1 and Woman 4 walk across first, 10 minutes have elapsed when they get to the other side of the bridge. If Woman 4 then returns with the flashlight, a total of 20 minutes have passed and you have failed the mission. What is the order required to get all women across in 17 minutes? Now, what’s the other way?

One way:

1. Woman 1 and 2 Cross, woman 1 returns. 3 minutes total.
3. Woman 3 and 4 Cross, woman 2 returns. 15 minutes total.
4. Woman 1 and 2 Cross. 17 Minutes total.

Other way:

1. Woman 1 and 2 Cross, woman 2 returns. 4 minutes total.
3. Woman 3 and 4 Cross, woman 1 returns. 11 minutes total.
4. Woman 1 and 2 Cross. 17 minutes total.

If you had an infinite supply of water and a 5 quart and 3 quart pail, how would you measure exactly 4 quarts?

1. Fill 5 quart pail with 5 quarts of water from source.
2. Fill 3 quart pail with 3 quarts of water from 5 quart pail.
3. Empty 3 quart pail.
4. Empty 5 quart pail containing 2 quarts of water into 3 quart pail.
5. Fill 5 quart pail with 5 quarts of water from source.
6. Fill 3 quart pail with water from 5 quart pail.
7. 5 quart pail now contains 4 quarts of water.

Suppose you have an array of 1001 integers. The integers are in random order, but you know each of the integers is between 1 and 1000 (inclusive). In addition, each number appears only once in the array, except for one number, which occurs twice. Assume that you can access each element of the array only once. Describe an algorithm to find the repeated number. If you used auxiliary storage in your algorithm, can you find an algorithm that does not require it?

public int duplicateNumber(int[] A) {
	int count = 0;
	for(int k = 0; k < A.Length; k++)
		count += A[k];
	return count - (A.Length * (A.Length - 1) >> 1);
}

Count the number of set bits in a number. Now optimize for speed. Now optimize for size.

public int bitsUsed(int n) {
	int count = 0;
	while(n != 0) {
		count += n & 1;
		n >>= 1;
	}

	return count;
}

Reimplementing arcade classics

Arcade games are a fun exercise in trying out different techniques that ultimately yield the same result: a responsive graphical interface where an agent is controlled by the user’s keyboard. I’ve had a chance to design a few applications in a couple of languages and thought I’d go over the design decisions of each. Plus a little variety never hurts in tuning your skill set.

Tetris was a simple game pushed by Nintendo to fuel sales of the Game Boy in the early 90s. I wrote a variant awhile ago that uses blocks rather than tetrominoes to cut out unnecessary complexity. This C# implementation revolved around the .Net WinForms and falls under the umbrella of Object Oriented and Event Driven designs. A one dimensional array keeps tabs of how deep a block can fall along with a list of fallen blocks. Any time a block lands on top of another block or the user hits a key, an event handler processes the event and causes the screen to be repainted. This design felt forced but otherwise worked as needed. I hope to refine this approach in future applications.

Pacman was the iconic flagship of 80s arcade armada. During college I wrote a simple version of Pacman in C that relied on the prototypical input, logic and draw loop. In this implementation, an array is kept that represents the inanimate actors: nothing, reward and walls. In addition separate cursors are kept to keep track of pacman and each of the ghosts. Design-wise, this worked out really well. Input was parsed and applied to pacman, each of the ghosts move towards pacman, termination conditions are checked and finally the array and animated actors are painted on the screen. Out of these designs, this one felt the most versatile.

Snake, also known as Nibbler or Nibbles was a classic game that used to get put onto cell phones (when phones used to come with games for free…) where you have a snake that grows as it consumes rewards. The snake moves around a torus (represented as a 2d surface) and the game is over when the snake covers the surface or when part of the snake crosses over itself. This implementation went with C# relying purely on the Console. The snake is represented as a linked list where every node holds a direction and location. As each loop passes, the direction and location of the head is passed onto the next segment and so on until the tail is reached. If the snake is on top of a reward a new segment is appended to the tail. The design is the same as my Pacman implementation, however there is no underlying array to maintain.

If the comments call for it, I’ll post more implementation details on any of the above.