Viterbi.java

package socket;

public class Viterbi {
	private static String[] states = { "#", "NN", "VB" };
	private static String[] observations = { "I", "write", "a letter" };
	private static double[] start_probability = { 0.3, 0.4, 0.3 };
	private static double[][] transition_probability = { { 0.2, 0.2, 0.6 },
			{ 0.4, 0.1, 0.5 }, { 0.1, 0.8, 0.1 } };
	private static double[][] emission_probability = { { 0.01, 0.02, 0.02 },
			{ 0.8, 0.01, 0.5 }, { 0.19, 0.97, 0.48 } };
	private static class TNode {
		public int[] v_path;
		public double v_prob;

		public TNode( int[] v_path, double v_prob) {
			this.v_path = copyIntArray(v_path);
			this.v_prob = v_prob;
		}
	}

	private static int[] copyIntArray(int[] ia) {
		int[] newIa = new int[ia.length];
		for (int i = 0; i < ia.length; i++) {
			newIa[i] = ia[i];
		}
		return newIa;
	}

	private static int[] copyIntArray(int[] ia, int newInt) {
		int[] newIa = new int[ia.length + 1];
		for (int i = 0; i < ia.length; i++) {
			newIa[i] = ia[i];
		}
		newIa[ia.length] = newInt;
		return newIa;
	}

	// forwardViterbi(observations, states, start_probability,
	// transition_probability, emission_probability)
	public int[] forwardViterbi(String[] y, String[] X, double[] sp,
			double[][] tp, double[][] ep) {
		TNode[] T = new TNode[X.length];
		for (int state = 0; state < X.length; state++) {
			int[] intArray = new int[1];
			intArray[0] = state;
			T[state] = new TNode( intArray, sp[state] * ep[state][0]);
		}

		for (int output = 1; output < y.length; output++) {
			TNode[] U = new TNode[X.length];
			for (int next_state = 0; next_state < X.length; next_state++) {
				int[] argmax = new int[0];
				double valmax = 0;
				for (int state = 0; state < X.length; state++) {
					int[] v_path = copyIntArray(T[state].v_path);
					double v_prob = T[state].v_prob;
					double p = ep[next_state][output] * tp[state][next_state];
					v_prob *= p;
					if (v_prob > valmax) {
						if (v_path.length == y.length) {
							argmax = copyIntArray(v_path);
						} else {
							argmax = copyIntArray(v_path, next_state);
						}
						valmax = v_prob;

					}
				}
				U[next_state] = new TNode( argmax, valmax);
			}
			T = U;
		}
		// apply sum/max to the final states:
		int[] argmax = new int[0];
		double valmax = 0;
		for (int state = 0; state < X.length; state++) {
			int[] v_path = copyIntArray(T[state].v_path);
			double v_prob = T[state].v_prob;
			if (v_prob > valmax) {
				argmax = copyIntArray(v_path);
				valmax = v_prob;
			}
		}
		System.out.print("Viterbi path: [");
		for (int i = 0; i < argmax.length; i++) {
			System.out.print(states[argmax[i]] + ", ");
		}
		System.out.println("].\n Probability of the whole system: " + valmax);
		return argmax;
	}

	public static void main(String[] args) throws Exception {
		System.out.print("\nStates: ");
		for (int i = 0; i < states.length; i++) {
			System.out.print(states[i] + ", ");
		}
		System.out.print("\n\nObservations: ");
		for (int i = 0; i < observations.length; i++) {
			System.out.print(observations[i] + ", ");
		}
		System.out.print("\n\nStart probability: ");
		for (int i = 0; i < states.length; i++) {
			System.out.print(states[i] + ": " + start_probability[i] + ", ");
		}
		System.out.println("\n\nTransition probability:");
		for (int i = 0; i < states.length; i++) {
			System.out.print(" " + states[i] + ": {");
			for (int j = 0; j < states.length; j++) {
				System.out.print("  " + states[j] + ": "
						+ transition_probability[i][j] + ", ");
			}
			System.out.println("}");
		}
		System.out.println("\n\nEmission probability:");
		for (int i = 0; i < states.length; i++) {
			System.out.print(" " + states[i] + ": {");
			for (int j = 0; j < observations.length; j++) {
				System.out.print("  " + observations[j] + ": "
						+ emission_probability[i][j] + ", ");
			}
			System.out.println("}");
		}
		Viterbi v=new Viterbi();
		v.forwardViterbi(observations, states, start_probability,
				transition_probability, emission_probability);
		
	}
}