Add quickjs-2021-03-27 to third_party

third_party/quickjs/examples/pi_bigint.js

@@ -0,0 +1,118 @@
+/*
+ * PI computation in Javascript using the BigInt type
+ */
+"use strict";
+
+/* return floor(log2(a)) for a > 0 and 0 for a = 0 */
+function floor_log2(a)
+{
+    var k_max, a1, k, i;
+    k_max = 0n;
+    while ((a >> (2n ** k_max)) != 0n) {
+        k_max++;
+    }
+    k = 0n;
+    a1 = a;
+    for(i = k_max - 1n; i >= 0n; i--) {
+        a1 = a >> (2n ** i);
+        if (a1 != 0n) {
+            a = a1;
+            k |= (1n << i);
+        }
+    }
+    return k;
+}
+
+/* return ceil(log2(a)) for a > 0 */
+function ceil_log2(a)
+{
+    return floor_log2(a - 1n) + 1n;
+}
+
+/* return floor(sqrt(a)) (not efficient but simple) */
+function int_sqrt(a)
+{
+    var l, u, s;
+    if (a == 0n)
+        return a;
+    l = ceil_log2(a);
+    u = 1n << ((l + 1n) / 2n);
+    /* u >= floor(sqrt(a)) */
+    for(;;) {
+        s = u;
+        u = ((a / s) + s) / 2n;
+        if (u >= s)
+            break;
+    }
+    return s;
+}
+
+/* return pi * 2**prec */
+function calc_pi(prec) {
+    const CHUD_A = 13591409n;
+    const CHUD_B = 545140134n;
+    const CHUD_C = 640320n;
+    const CHUD_C3 = 10939058860032000n; /* C^3/24 */
+    const CHUD_BITS_PER_TERM = 47.11041313821584202247; /* log2(C/12)*3 */
+    
+    /* return [P, Q, G] */
+    function chud_bs(a, b, need_G) {
+        var c, P, Q, G, P1, Q1, G1, P2, Q2, G2;
+        if (a == (b - 1n)) {
+            G = (2n * b - 1n) * (6n * b - 1n) * (6n * b - 5n);
+            P = G * (CHUD_B * b + CHUD_A);
+            if (b & 1n)
+                P = -P;
+            Q = b * b * b * CHUD_C3;
+        } else {
+            c = (a + b) >> 1n;
+            [P1, Q1, G1] = chud_bs(a, c, true);
+            [P2, Q2, G2] = chud_bs(c, b, need_G);
+            P = P1 * Q2 + P2 * G1;
+            Q = Q1 * Q2;
+            if (need_G)
+                G = G1 * G2;
+            else
+                G = 0n;
+        }
+        return [P, Q, G];
+    }
+
+    var n, P, Q, G;
+    /* number of serie terms */
+    n = BigInt(Math.ceil(Number(prec) / CHUD_BITS_PER_TERM)) + 10n;
+    [P, Q, G] = chud_bs(0n, n, false);
+    Q = (CHUD_C / 12n) * (Q << prec) / (P + Q * CHUD_A);
+    G = int_sqrt(CHUD_C << (2n * prec));
+    return (Q * G) >> prec;
+}
+
+function main(args) {
+    var r, n_digits, n_bits, out;
+    if (args.length < 1) {
+        print("usage: pi n_digits");
+        return;
+    }
+    n_digits = args[0] | 0;
+
+    /* we add more bits to reduce the probability of bad rounding for
+      the last digits */
+    n_bits = BigInt(Math.ceil(n_digits * Math.log2(10))) + 32n;
+    r = calc_pi(n_bits);
+    r = ((10n ** BigInt(n_digits)) * r) >> n_bits;
+    out = r.toString();
+    print(out[0] + "." + out.slice(1));
+}
+
+var args;
+if (typeof scriptArgs != "undefined") {
+    args = scriptArgs;
+    args.shift();
+} else if (typeof arguments != "undefined") {
+    args = arguments;
+} else {
+    /* default: 1000 digits */
+    args=[1000];
+}
+
+main(args);