40 lines
1.5 KiB
JavaScript
40 lines
1.5 KiB
JavaScript
"use strict";
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// This files implements the calculation of the offset between the global monotonic clock and UNIX time. This value is
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// known as |t1| in the calculation of "time origin timestamp" in the spec. This value needs to be calculated once and
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// can be used in all subsequent Performance instances.
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//
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// However, if the clock is not fast enough, the export is undefined to signify that we should use Date.now() to get the
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// time origin timestamp with millisecond accuracy, per spec.
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const { getGlobalMonotonicClockMS } = require("./global-monotonic-clock");
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const clockIsAccurate = require("./clock-is-accurate");
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// This function assumes the clock is accurate.
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function calculateClockOffset() {
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const start = Date.now();
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let cur = start;
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// Limit the iterations, just in case we're running in an environment where Date.now() has been mocked and is
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// constant.
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for (let i = 0; i < 1e6 && cur === start; i++) {
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cur = Date.now();
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}
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// At this point |cur| "just" became equal to the next millisecond -- the unseen digits after |cur| are approximately
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// all 0, and |cur| is the closest to the actual value of the UNIX time. Now, get the current global monotonic clock
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// value and do the remaining calculations.
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return cur - getGlobalMonotonicClockMS();
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}
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if (clockIsAccurate) {
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// Warm up the function.
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calculateClockOffset();
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calculateClockOffset();
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calculateClockOffset();
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module.exports = calculateClockOffset;
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} else {
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module.exports = undefined;
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}
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