Initial data commit

Task/Statistics-Normal-distribution/Java/statistics-normal-distribution.java

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+import static java.lang.Math.*;
+import static java.util.Arrays.stream;
+import java.util.Locale;
+import java.util.function.DoubleSupplier;
+import static java.util.stream.Collectors.joining;
+import java.util.stream.DoubleStream;
+import static java.util.stream.IntStream.range;
+
+public class Test implements DoubleSupplier {
+
+    private double mu, sigma;
+    private double[] state = new double[2];
+    private int index = state.length;
+
+    Test(double m, double s) {
+        mu = m;
+        sigma = s;
+    }
+
+    static double[] meanStdDev(double[] numbers) {
+        if (numbers.length == 0)
+            return new double[]{0.0, 0.0};
+
+        double sx = 0.0, sxx = 0.0;
+        long n = 0;
+        for (double x : numbers) {
+            sx += x;
+            sxx += pow(x, 2);
+            n++;
+        }
+
+        return new double[]{sx / n, pow((n * sxx - pow(sx, 2)), 0.5) / n};
+    }
+
+    static String replicate(int n, String s) {
+        return range(0, n + 1).mapToObj(i -> s).collect(joining());
+    }
+
+    static void showHistogram01(double[] numbers) {
+        final int maxWidth = 50;
+        long[] bins = new long[10];
+
+        for (double x : numbers)
+            bins[(int) (x * bins.length)]++;
+
+        double maxFreq = stream(bins).max().getAsLong();
+
+        for (int i = 0; i < bins.length; i++)
+            System.out.printf(" %3.1f: %s%n", i / (double) bins.length,
+                    replicate((int) (bins[i] / maxFreq * maxWidth), "*"));
+        System.out.println();
+    }
+
+    @Override
+    public double getAsDouble() {
+        index++;
+        if (index >= state.length) {
+            double r = sqrt(-2 * log(random())) * sigma;
+            double x = 2 * PI * random();
+            state = new double[]{mu + r * sin(x), mu + r * cos(x)};
+            index = 0;
+        }
+        return state[index];
+
+    }
+
+    public static void main(String[] args) {
+        Locale.setDefault(Locale.US);
+        double[] data = DoubleStream.generate(new Test(0.0, 0.5)).limit(100_000)
+                .toArray();
+
+        double[] res = meanStdDev(data);
+        System.out.printf("Mean: %8.6f, SD: %8.6f%n", res[0], res[1]);
+
+        showHistogram01(stream(data).map(a -> max(0.0, min(0.9999, a / 3 + 0.5)))
+                .toArray());
+    }
+}