Add quickjs-2021-03-27 to third_party

third_party/quickjs/examples/pi_bigfloat.js

@@ -0,0 +1,66 @@
+/*
+ * PI computation in Javascript using the QuickJS bigfloat type
+ * (binary floating point)
+ */
+"use strict";
+
+/* compute PI with a precision of 'prec' bits */
+function calc_pi() {
+    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 = BigFloat(G * (CHUD_B * b + CHUD_A));
+            if (b & 1n)
+                P = -P;
+            G = BigFloat(G);
+            Q = BigFloat(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 = 0l;
+        }
+        return [P, Q, G];
+    }
+
+    var n, P, Q, G;
+    /* number of serie terms */
+    n = BigInt(Math.ceil(BigFloatEnv.prec / CHUD_BITS_PER_TERM)) + 10n;
+    [P, Q, G] = chud_bs(0n, n, false);
+    Q = Q / (P + Q * BigFloat(CHUD_A));
+    G = BigFloat((CHUD_C / 12n)) * BigFloat.sqrt(BigFloat(CHUD_C));
+    return Q * G;
+}
+
+(function() {
+    var r, n_digits, n_bits;
+    if (typeof scriptArgs != "undefined") {
+        if (scriptArgs.length < 2) {
+            print("usage: pi n_digits");
+            return;
+        }
+        n_digits = scriptArgs[1];
+    } else {
+        n_digits = 1000;
+    }
+    n_bits = Math.ceil(n_digits * Math.log2(10));
+    /* we add more bits to reduce the probability of bad rounding for
+       the last digits */
+    BigFloatEnv.setPrec( () => {
+        r = calc_pi();
+        print(r.toFixed(n_digits, BigFloatEnv.RNDZ));
+    }, n_bits + 32);
+})();