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Fix bisector non-termination on arrays larger than 2³¹ elements #284

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Fix bisector non-termination on arrays larger than 2³¹ elements #284

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4 changes: 2 additions & 2 deletions src/bisector.js
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Original file line number Diff line number Diff line change
Expand Up @@ -23,7 +23,7 @@ export default function bisector(f) {
if (lo < hi) {
if (compare1(x, x) !== 0) return hi;
do {
const mid = (lo + hi) >>> 1;
const mid = Math.trunc((lo + hi) / 2);
if (compare2(a[mid], x) < 0) lo = mid + 1;
else hi = mid;
} while (lo < hi);
Expand All @@ -35,7 +35,7 @@ export default function bisector(f) {
if (lo < hi) {
if (compare1(x, x) !== 0) return hi;
do {
const mid = (lo + hi) >>> 1;
const mid = Math.trunc((lo + hi) / 2);
if (compare2(a[mid], x) <= 0) lo = mid + 1;
else hi = mid;
} while (lo < hi);
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18 changes: 18 additions & 0 deletions test/bisect-test.js
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Expand Up @@ -165,3 +165,21 @@ it("bisectRight(array, value, lo, hi) keeps non-comparable values to the right",
assert.strictEqual(bisectRight(values, undefined), 4);
assert.strictEqual(bisectRight(values, NaN), 4);
});

it("bisectRight(array, value) terminates for arrays larger than 2³¹ elements (#261)", () => {
// A virtual sorted array of length > 2³¹, without allocating it. Reading past
// ~log2(length) elements means the midpoint overflowed and the search stalls.
const length = 2 ** 32 - 1;
const threshold = 3e9; // a[i] = i < threshold ? 0 : 1; boundary is above 2³¹
let reads = 0;
const a = new Proxy({}, {get(_, k) {
if (k === "length") return length;
const i = Number(k);
if (Number.isInteger(i)) {
if (++reads > 64) throw new Error("bisect did not converge");
return i < threshold ? 0 : 1;
}
}});
assert.strictEqual(bisectRight(a, 0), threshold);
assert.strictEqual(bisectLeft(a, 1), threshold);
});