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docs(firestore): add an example to the detailed comment in compareUtf8Strings() #9154

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Jul 9, 2025
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docs(firestore): add an example to the detailed comment in compareUtf8Strings() #9154

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docs(firestore): add an example to the detailed comment in compareUtf…
dconeybe Jul 8, 2025
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dconeybe Jul 8, 2025
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16 changes: 16 additions & 0 deletions packages/firestore/src/util/misc.ts
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Expand Up @@ -95,6 +95,22 @@ export function compareUtf8Strings(left: string, right: string): number {
// used to represent code points greater than 0xFFFF which have 4-byte UTF-8 representations
// and are lexicographically greater than the 1, 2, or 3-byte representations of code points
// less than or equal to 0xFFFF.
//
// An example of why Case 2 is required is comparing the following two Unicode code points:
//
// |-----------------------|------------|---------------------|-----------------|
// | Name | Code Point | UTF-8 Encoding | UTF-16 Encoding |
// |-----------------------|------------|---------------------|-----------------|
// | Replacement Character | U+FFFD | 0xEF 0xBF 0xBD | 0xFFFD |
// | Grinning Face | U+1F600 | 0xF0 0x9F 0x98 0x80 | 0xD83D 0xDE00 |
// |-----------------------|------------|---------------------|-----------------|
//
// A lexicographical comparison of the UTF-8 encodings of these code points would order
// "Replacement Character" _before_ "Grinning Face" because 0xEF is less than 0xF0. However, a
// direct comparison of the UTF-16 code units, as would be done in case 1, would erroneously
// produce the _opposite_ ordering, because 0xFFFD is _greater than_ 0xD83D. As it turns out,
// this relative ordering holds for all comparisons of UTF-16 code points requiring a surrogate
// pair with those that do not.
const length = Math.min(left.length, right.length);
for (let i = 0; i < length; i++) {
const leftChar = left.charAt(i);
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