setup-python/node_modules/w3c-hr-time/lib/calculate-clock-offset.js
Danny McCormick 39c08a0eaa Initial pass
2019-06-26 21:12:00 -04:00

38 lines
1.3 KiB
JavaScript

"use strict";
// This files implements the calculation of the offset between the global monotonic clock and UNIX time. This value is
// known as |t1| in the calculation of "time origin timestamp" in the spec. This value needs to be calculated once and
// can be used in all subsequent Performance instances.
//
// However, if the clock is not fast enough, the export is undefined to signify that we should use Date.now() to get the
// time origin timestamp with millisecond accuracy, per spec.
const { getGlobalMonotonicClockMS } = require("./global-monotonic-clock");
const clockIsAccurate = require("./clock-is-accurate");
// This function assumes the clock is accurate.
function calculateClockOffset() {
const start = Date.now();
let cur = start;
while (cur === start) {
cur = Date.now();
}
// At this point |cur| "just" became equal to the next millisecond -- the unseen digits after |cur| are approximately
// all 0, and |cur| is the closest to the actual value of the UNIX time. Now, get the current global monotonic clock
// value and do the remaining calculations.
return cur - getGlobalMonotonicClockMS();
}
if (clockIsAccurate) {
// Warm up the function.
calculateClockOffset();
calculateClockOffset();
calculateClockOffset();
module.exports = calculateClockOffset;
} else {
module.exports = undefined;
}