Sync

Task/Knuths-algorithm-S/Clojure/knuths-algorithm-s-1.clj

@@ -0,0 +1,19 @@
+(defn s-of-n-fn-creator [n]
+  (fn [[sample iprev] item]
+    (let [i (inc iprev)]
+      (if (<= i n)
+        [(conj sample item) i]
+        (let [r (rand-int i)]
+          (if (< r n)
+            [(assoc sample r item) i]
+            [sample i]))))))
+
+(def s-of-3-fn (s-of-n-fn-creator 3))
+
+(->> #(reduce s-of-3-fn [[] 0] (range 10))
+    (repeatedly 100000)
+    (map first)
+    flatten
+    frequencies
+    sort
+    println)

Task/Knuths-algorithm-S/Clojure/knuths-algorithm-s-2.clj

@@ -0,0 +1 @@
+([0 29924] [1 30053] [2 30018] [3 29765] [4 29974] [5 30225] [6 30082] [7 29996] [8 30128] [9 29835])

Task/Knuths-algorithm-S/Clojure/knuths-algorithm-s-3.clj

@@ -0,0 +1,5 @@
+(defn s-of-n-creator [n]
+  (let [state (atom [[] 0])
+        s-of-n-fn (s-of-n-fn-creator n)]
+    (fn [item]
+      (first (swap! state s-of-n-fn item)))))

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

@@ -1,15 +1,20 @@
 import std.stdio, std.random, std.algorithm;
 
-struct SOfN(int n) {
+double random01(ref Xorshift rng) {
+    immutable r = rng.front / cast(double)rng.max;
+    rng.popFront;
+    return r;
+}
+
+struct SOfN(size_t n) {
     size_t i;
     int[n] sample = void;
-    static rng = Xorshift(0);
 
-    int[] next(in int item) {
+    int[] next(in size_t item, ref Xorshift rng) {
         i++;
         if (i <= n)
             sample[i - 1] = item;
-        else if (uniform(0.0, 1.0, rng) < (cast(double)n / i))
+        else if (rng.random01 < (cast(double)n / i))
             sample[uniform(0, n, rng)] = item;
         return sample[0 .. min(i, $)];
     }
@@ -18,12 +23,13 @@ struct SOfN(int n) {
 void main() {
     enum nRuns = 100_000;
     size_t[10] bin;
+    auto rng = Xorshift(0);
 
-    foreach (trial; 0 .. nRuns) {
-        SOfN!(3) sofn;
-        foreach (item; 0 .. bin.length - 1)
-            sofn.next(item);
-        foreach (s; sofn.next(bin.length - 1))
+    foreach (immutable trial; 0 .. nRuns) {
+        SOfN!3 sofn;
+        foreach (immutable item; 0 .. bin.length - 1)
+            sofn.next(item, rng);
+        foreach (immutable s; sofn.next(bin.length - 1, rng))
             bin[s]++;
     }
     writefln("Item counts for %d runs:\n%s", nRuns, bin);