67 lines
1.9 KiB
JavaScript
67 lines
1.9 KiB
JavaScript
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/*
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* PI computation in Javascript using the QuickJS bigfloat type
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* (binary floating point)
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*/
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"use strict";
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/* compute PI with a precision of 'prec' bits */
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function calc_pi() {
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const CHUD_A = 13591409n;
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const CHUD_B = 545140134n;
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const CHUD_C = 640320n;
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const CHUD_C3 = 10939058860032000n; /* C^3/24 */
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const CHUD_BITS_PER_TERM = 47.11041313821584202247; /* log2(C/12)*3 */
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/* return [P, Q, G] */
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function chud_bs(a, b, need_G) {
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var c, P, Q, G, P1, Q1, G1, P2, Q2, G2;
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if (a == (b - 1n)) {
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G = (2n * b - 1n) * (6n * b - 1n) * (6n * b - 5n);
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P = BigFloat(G * (CHUD_B * b + CHUD_A));
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if (b & 1n)
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P = -P;
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G = BigFloat(G);
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Q = BigFloat(b * b * b * CHUD_C3);
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} else {
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c = (a + b) >> 1n;
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[P1, Q1, G1] = chud_bs(a, c, true);
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[P2, Q2, G2] = chud_bs(c, b, need_G);
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P = P1 * Q2 + P2 * G1;
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Q = Q1 * Q2;
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if (need_G)
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G = G1 * G2;
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else
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G = 0l;
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}
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return [P, Q, G];
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}
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var n, P, Q, G;
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/* number of serie terms */
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n = BigInt(Math.ceil(BigFloatEnv.prec / CHUD_BITS_PER_TERM)) + 10n;
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[P, Q, G] = chud_bs(0n, n, false);
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Q = Q / (P + Q * BigFloat(CHUD_A));
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G = BigFloat((CHUD_C / 12n)) * BigFloat.sqrt(BigFloat(CHUD_C));
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return Q * G;
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}
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(function() {
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var r, n_digits, n_bits;
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if (typeof scriptArgs != "undefined") {
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if (scriptArgs.length < 2) {
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print("usage: pi n_digits");
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return;
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}
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n_digits = scriptArgs[1];
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} else {
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n_digits = 1000;
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}
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n_bits = Math.ceil(n_digits * Math.log2(10));
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/* we add more bits to reduce the probability of bad rounding for
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the last digits */
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BigFloatEnv.setPrec( () => {
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r = calc_pi();
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print(r.toFixed(n_digits, BigFloatEnv.RNDZ));
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}, n_bits + 32);
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})();
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