A-M baby

Task/Knuths-algorithm-S/D/knuths-algorithm-s-2.d

@@ -0,0 +1,30 @@
+import std.stdio, std.random, std.algorithm;
+
+struct SOfN(int n) {
+    size_t i;
+    int[n] sample = void;
+    static rng = Xorshift(0);
+
+    int[] next(in int item) {
+        i++;
+        if (i <= n)
+            sample[i - 1] = item;
+        else if (uniform(0.0, 1.0, rng) < (cast(double)n / i))
+            sample[uniform(0, n, rng)] = item;
+        return sample[0 .. min(i, $)];
+    }
+}
+
+void main() {
+    enum nRuns = 100_000;
+    size_t[10] bin;
+
+    foreach (trial; 0 .. nRuns) {
+        SOfN!(3) sofn;
+        foreach (item; 0 .. bin.length - 1)
+            sofn.next(item);
+        foreach (s; sofn.next(bin.length - 1))
+            bin[s]++;
+    }
+    writefln("Item counts for %d runs:\n%s", nRuns, bin);
+}