UltraghostRandom.java

package com.github.donkirkby.vograbulary.ultraghost;

import java.util.Random;

/**
 * Source of random choices for Ultraghost. Pulled into a separate class to
 * make it easier to mock out in tests.
 * @author don
 *
 */
public class UltraghostRandom {
    private Random random;
    
    // Each entry holds the odds of that letter appearing in that position.
    private double[] startingOdds;
    private double[] interiorOdds;
    private double[] endingOdds;

    public UltraghostRandom() {
        this(new Random());
    }

    public UltraghostRandom(Random random) {
        this.random = random;
    }

    /**
     * Generate a new puzzle.
     * @return a string with three upper-case letters
     */
    public String generatePuzzle() {
        StringBuilder builder = new StringBuilder(3);
        builder.append(generateLetter(startingOdds));
        builder.append(generateLetter(interiorOdds));
        builder.append(generateLetter(endingOdds));
        
        return builder.toString();
    }
    
    /**
     * Choose the index of the starting student.
     * @param studentCount the total number of students.
     */
    public int chooseStartingStudent(int studentCount) {
        return random.nextInt(studentCount);
    }

    private char generateLetter(double[] letterOdds) {
        double remaining = random.nextDouble();
        int i = 0;
        boolean isDone = false;
        while ( ! isDone) {
            remaining -= letterOdds[i];
            if (i < letterOdds.length-1 && remaining >= 0) {
                i++;
            }
            else {
                isDone = true;
            }
        }
        return (char) ('A' + i);
    }

    /**
     * Load a list of words that can be used to choose letter frequencies
     * for generating puzzles.
     */
    public void loadWordList(Iterable<String> wordList) {
        int alphabetSize = 26;
        startingOdds = new double[alphabetSize];
        interiorOdds = new double[alphabetSize];
        endingOdds = new double[alphabetSize];
        int wordCount = 0;
        int interiorCount = 0;
        for (String word : wordList) {
            wordCount++;
            startingOdds[word.charAt(0) - 'A'] += 1;
            endingOdds[word.charAt(word.length()-1) - 'A'] += 1;
            for (int i = 1; i < word.length()-1; i++) {
                interiorCount++;
                interiorOdds[word.charAt(i) - 'A'] += 1;
            }
        }
        
        for (int i = 0; i < alphabetSize; i++) {
            startingOdds[i] /= wordCount;
            interiorOdds[i] /= interiorCount;
            endingOdds[i] /= wordCount;
        }
    }
}