CDE

Task/Ackermann-function/D/ackermann-function-2.d

@@ -0,0 +1,50 @@
+import std.stdio, std.bigint, std.conv;
+
+/*pure nothrow*/ BigInt ipow(/*in*/ BigInt base, /*in*/ BigInt exp){
+    auto result = BigInt(1);
+    //while (exp) {
+    while (exp != 0) {
+        //if (exp & 1)
+        if (exp % 2)
+            result *= base;
+        exp >>= 1;
+        base *= base;
+    }
+
+    return result;
+}
+
+/*pure nothrow*/ BigInt ackermann(in int m, in int n)
+in {
+    assert(m >= 0 && n >= 0);
+} out(result) {
+    //assert(result >= 0);
+    assert(cast()result >= 0);
+} body {
+    /*pure nothrow*/ static BigInt ack(in int m, /*in*/ BigInt n) {
+        switch (m) {
+            case 0: return n + 1;
+            case 1: return n + 2;
+            case 2: return 3 + 2 * n;
+            //case 3: return 5 + 8 * (2 ^^ n - 1);
+            case 3: return 5 + 8 * (ipow(BigInt(2), n) - 1);
+            default: if (n == 0)
+                        return ack(m - 1, BigInt(1));
+                     else
+                        return ack(m - 1, ack(m, n - 1));
+        }
+    }
+
+    return ack(m, BigInt(n));
+}
+
+void main() {
+    foreach (m; 1 .. 4)
+        foreach (n; 1 .. 9)
+            writefln("ackermann(%d, %d): %s", m, n, ackermann(m, n));
+    writefln("ackermann(4, 1): %s", ackermann(4, 1));
+
+    auto a = text(ackermann(4, 2));
+    writefln("ackermann(4, 2)) (%d digits):\n%s...\n%s",
+             a.length, a[0 .. 94], a[$-96 .. $]);
+}