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Polyfill: Replace unreachable overshoot block with assertion in calendarToIsoDate #3292

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Merged
ptomato merged 3 commits into from
Mar 5, 2026
Merged

Polyfill: Replace unreachable overshoot block with assertion in calendarToIsoDate #3292

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69da812
Polyfill: Replace unreachable overshoot with assertion in calendarToI…
jessealama Mar 5, 2026
501d4ae
(fixup) Convert assertNotReached into assert
ptomato Mar 5, 2026
faf750a
(fixup) Simplify and update comments
ptomato Mar 5, 2026
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43 changes: 11 additions & 32 deletions polyfill/lib/calendar.mjs
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Original file line number Diff line number Diff line change
Expand Up @@ -982,43 +982,22 @@ const nonIsoHelperBase = {
sign = this.compareCalendarDates(date, roundtripEstimate);
}
}
// If the initial guess is not in the same month, then bisect the
// distance to the target, starting with 8 days per step.
let increment = 8;
// If the initial guess is not in the same month, search in 8-day increments
// towards the target, until we get into the same month. From there we can
// calculate the number of days directly.
const increment = 8;
const oldSign = sign;
while (sign) {
isoEstimate = ES.AddDaysToISODate(isoEstimate, sign * increment);
const oldRoundtripEstimate = roundtripEstimate;
roundtripEstimate = this.isoToCalendarDate(isoEstimate, cache);
const oldSign = sign;
sign = this.compareCalendarDates(date, roundtripEstimate);
if (sign) {
diff = simpleDateDiff(date, roundtripEstimate);
if (diff.years === 0 && diff.months === 0) {
isoEstimate = calculateSameMonthResult(diff.days);
// Signal the loop condition that there's a match.
sign = 0;
} else if (oldSign && sign !== oldSign) {
if (increment > 1) {
// If the estimate overshot the target, try again with a smaller increment
// in the reverse direction.
increment /= 2;
} else {
// Increment is 1, and neither the previous estimate nor the new
// estimate is correct. The only way that can happen is if the
// original date was an invalid value that will be constrained or
// rejected here.
if (overflow === 'reject') {
throw new RangeErrorCtor(`Can't find ISO date from calendar date: ${JSONStringify({ ...originalDate })}`);
} else {
// To constrain, pick the earliest value
const order = this.compareCalendarDates(roundtripEstimate, oldRoundtripEstimate);
// If current value is larger, then back up to the previous value.
if (order > 0) isoEstimate = ES.AddDaysToISODate(isoEstimate, -1);
sign = 0;
}
}
}
if (sign === 0) break;
diff = simpleDateDiff(date, roundtripEstimate);
if (diff.years === 0 && diff.months === 0) {
isoEstimate = calculateSameMonthResult(diff.days);
break;
}
assert(sign === oldSign, 'calendarToIsoDate search should not overshoot a month entirely');
}
cache.set(key, isoEstimate);
if (keyOriginal) cache.set(keyOriginal, isoEstimate);
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