Combin.java

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*/

package com.google.refine.expr.functions.math;

import java.util.Properties;

import org.json.JSONException;
import org.json.JSONWriter;

import com.google.refine.expr.EvalError;
import com.google.refine.grel.ControlFunctionRegistry;
import com.google.refine.grel.Function;

public class Combin implements Function {

    @Override
    public Object call(Properties bindings, Object[] args) {
        if(args.length != 2) {
            return new EvalError(ControlFunctionRegistry.getFunctionName(this) + " expects two numbers");
        }
        if(args[0] == null || !(args[0] instanceof Number)) {
            return new EvalError(ControlFunctionRegistry.getFunctionName(this) + " expects the first argument to be a number");
        }
        if(args[1] == null || !(args[1] instanceof Number)) {
            return new EvalError(ControlFunctionRegistry.getFunctionName(this) + " expects the second argument to be a number");
        }

        return Combin.combination(((Number) args[0]).intValue(), ((Number) args[1]).intValue());
    }

    /*
     * Compute binomial coefficient using dynamic programming which takes 
     * advantage of Pascal's identity as described in:
     * http://introcs.cs.princeton.edu/java/96optimization/
     */
    public static long combination(int n, int k) {
        long[][] binomial = new long[n+1][k+1];

        for (int j = 1; j <= k; j++) {
            binomial[0][j] = 0;
        }
        for (int i = 0; i <= n; i++) {
            binomial[i][0] = 1;
        }

        for (int i = 1; i <= n; i++) {
            for (int j = 1; j <= k; j++) {
                binomial[i][j] = binomial[i-1][j-1] + binomial[i-1][j];
                if (binomial[i][j] > Long.MAX_VALUE || binomial[i][j] < 0) {
                    throw new RuntimeException("Range limit exceeded");
                }
            }
        }

        return binomial[n][k];
    }

    @Override
    public void write(JSONWriter writer, Properties options)
        throws JSONException {

        writer.object();
        writer.key("description"); writer.value("Returns the number of combinations for n elements as divided into k");
        writer.key("params"); writer.value("number d");
        writer.key("returns"); writer.value("number");
        writer.endObject();
    }
}