tildefriends/deps/quickjs/examples/pi.js
2019-10-06 22:19:08 +00:00

67 lines
1.9 KiB
JavaScript

/*
* PI computation in Javascript using the QuickJS bignum extensions
*/
"use strict";
"use bigint";
/* compute PI with a precision of 'prec' bits */
function calc_pi(prec) {
const CHUD_A = 13591409;
const CHUD_B = 545140134;
const CHUD_C = 640320;
const CHUD_C3 = 10939058860032000; /* 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 - 1)) {
G = (2 * b - 1) * (6 * b - 1) * (6 * b - 5);
P = BigFloat(G * (CHUD_B * b + CHUD_A));
if (b & 1)
P = -P;
G = BigFloat(G);
Q = BigFloat(b * b * b * CHUD_C3);
} else {
c = (a + b) >> 1;
[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 = 0;
}
return [P, Q, G];
}
var n, P, Q, G;
/* number of serie terms */
n = Math.ceil(BigFloatEnv.prec / CHUD_BITS_PER_TERM) + 10;
[P, Q, G] = chud_bs(0, n, false);
Q = Q / (P + Q * CHUD_A);
G = (CHUD_C / 12) * BigFloat.sqrt(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);
})();