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authorMinteck <contact@minteck.org>2023-01-10 14:54:04 +0100
committerMinteck <contact@minteck.org>2023-01-10 14:54:04 +0100
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+"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;
+ // Limit the iterations, just in case we're running in an environment where Date.now() has been mocked and is
+ // constant.
+ for (let i = 0; i < 1e6 && cur === start; i++) {
+ 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;
+}