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