/*************************************************************************
 *  Compilation:  javac StdRandom.java
 *  Execution:    java StdRandom
 *  
 *************************************************************************/

public class StdRandom {

    // return a real number uniformly between a and b
    public static double uniform(double a, double b) {
        return a + Math.random() * (b-a);
    } 

    // return an integer uniformly between 0 and N-1
    public static int uniform(int N) {
        return (int) (Math.random() * N);
    } 

    // return a boolean, which is true with prob p and false otherwise
    public static boolean bernoulli(double p) {
        return Math.random() < p;
    } 

    // return a real number with a standard Gausian distribution
    public static double gaussian() {
        double r, x, y;
        do {
            x = uniform(-1.0, 1.0);
            y = uniform(-1.0, 1.0);
            r = x*x + y*y;
        } while (r >= 1 || r == 0);
        return x * Math.sqrt(-2 * Math.log(r) / r);

        // Remark:  y * Math.sqrt(-2 * Math.log(r) / r)
        // is an independent random gaussian
    } 

    // return a real number from a gaussian distribution with given mean and stddev
    public static double gaussian(double mean, double stddev) {
        return mean + stddev * gaussian();
    } 

    // return i with probability a[i]
    // precondition: sum of array entries equals 1
    public static int discrete(double[] a)  { 
        double r = Math.random();
        double sum = 0.0;
        for (int i = 0; i < a.length; i++) {
            sum = sum + a[i];
            if (sum >= r) return i;
        }
        assert(false);
        return -1;
    } 

    // exponential random variable with rate lambda
    public static double exp(double lambda) {
        return -Math.log(1 - Math.random()) / lambda;
    }


    // swaps array elements i and j
    public static void exch(String[] a, int i, int j) {
        String swap = a[i];
        a[i] = a[j];
        a[j] = swap;
    }

    // take as input an array of strings and rearrange them in random order
    public static void shuffle(String[] a) {
        int N = a.length;
        for (int i = 0; i < N; i++) {
            int r = i + uniform(N-i);     // between i and N-1
            String temp = a[i];
            a[i] = a[r];
            a[r] = temp;
        }
    }



}