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Merged
ExplodingCabbage merged 1 commit into from
Feb 17, 2025
Merged

Purge more quadratic-time regexes #581

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30 changes: 16 additions & 14 deletions src/diff/word.js
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Original file line number Diff line number Diff line change
@@ -1,5 +1,5 @@
import Diff from './base';
import { longestCommonPrefix, longestCommonSuffix, replacePrefix, replaceSuffix, removePrefix, removeSuffix, maximumOverlap } from '../util/string';
import { longestCommonPrefix, longestCommonSuffix, replacePrefix, replaceSuffix, removePrefix, removeSuffix, maximumOverlap, leadingWs, trailingWs } from '../util/string';

// Based on https://en.wikipedia.org/wiki/Latin_script_in_Unicode
//
Expand Down Expand Up @@ -193,10 +193,10 @@ function dedupeWhitespaceInChangeObjects(startKeep, deletion, insertion, endKeep
// * Just a "delete"
// We handle the three cases separately.
if (deletion && insertion) {
const oldWsPrefix = deletion.value.match(/^\s*/)[0];
const oldWsSuffix = deletion.value.match(/\s*$/)[0];
const newWsPrefix = insertion.value.match(/^\s*/)[0];
const newWsSuffix = insertion.value.match(/\s*$/)[0];
const oldWsPrefix = leadingWs(deletion.value);
const oldWsSuffix = trailingWs(deletion.value);
const newWsPrefix = leadingWs(insertion.value);
const newWsSuffix = trailingWs(insertion.value);

if (startKeep) {
const commonWsPrefix = longestCommonPrefix(oldWsPrefix, newWsPrefix);
Expand All @@ -218,16 +218,18 @@ function dedupeWhitespaceInChangeObjects(startKeep, deletion, insertion, endKeep
// whitespace and deleting duplicate leading whitespace where
// present.
if (startKeep) {
insertion.value = insertion.value.replace(/^\s*/, '');
const ws = leadingWs(insertion.value);
insertion.value = insertion.value.substring(ws.length);
}
if (endKeep) {
endKeep.value = endKeep.value.replace(/^\s*/, '');
const ws = leadingWs(endKeep.value);
endKeep.value = endKeep.value.substring(ws.length);
}
// otherwise we've got a deletion and no insertion
} else if (startKeep && endKeep) {
const newWsFull = endKeep.value.match(/^\s*/)[0],
delWsStart = deletion.value.match(/^\s*/)[0],
delWsEnd = deletion.value.match(/\s*$/)[0];
const newWsFull = leadingWs(endKeep.value),
delWsStart = leadingWs(deletion.value),
delWsEnd = trailingWs(deletion.value);

// Any whitespace that comes straight after startKeep in both the old and
// new texts, assign to startKeep and remove from the deletion.
Expand Down Expand Up @@ -255,16 +257,16 @@ function dedupeWhitespaceInChangeObjects(startKeep, deletion, insertion, endKeep
// We are at the start of the text. Preserve all the whitespace on
// endKeep, and just remove whitespace from the end of deletion to the
// extent that it overlaps with the start of endKeep.
const endKeepWsPrefix = endKeep.value.match(/^\s*/)[0];
const deletionWsSuffix = deletion.value.match(/\s*$/)[0];
const endKeepWsPrefix = leadingWs(endKeep.value);
const deletionWsSuffix = trailingWs(deletion.value);
const overlap = maximumOverlap(deletionWsSuffix, endKeepWsPrefix);
deletion.value = removeSuffix(deletion.value, overlap);
} else if (startKeep) {
// We are at the END of the text. Preserve all the whitespace on
// startKeep, and just remove whitespace from the start of deletion to
// the extent that it overlaps with the end of startKeep.
const startKeepWsSuffix = startKeep.value.match(/\s*$/)[0];
const deletionWsPrefix = deletion.value.match(/^\s*/)[0];
const startKeepWsSuffix = trailingWs(startKeep.value);
const deletionWsPrefix = leadingWs(deletion.value);
const overlap = maximumOverlap(startKeepWsSuffix, deletionWsPrefix);
deletion.value = removePrefix(deletion.value, overlap);
}
Expand Down
26 changes: 26 additions & 0 deletions src/util/string.js
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Original file line number Diff line number Diff line change
Expand Up @@ -101,3 +101,29 @@ export function hasOnlyWinLineEndings(string) {
export function hasOnlyUnixLineEndings(string) {
return !string.includes('\r\n') && string.includes('\n');
}

export function trailingWs(string) {
// Yes, this looks overcomplicated and dumb - why not replace the whole function with
// return string match(/\s*$/)[0]
// you ask? Because:
// 1. the trap described at https://markamery.com/blog/quadratic-time-regexes/ would mean doing
// this would cause this function to take O(n²) time in the worst case (specifically when
// there is a massive run of NON-TRAILING whitespace in `string`), and
// 2. the fix proposed in the same blog post, of using a negative lookbehind, is incompatible
// with old Safari versions that we'd like to not break if possible (see
// https://github.com/kpdecker/jsdiff/pull/550)
// It feels absurd to do this with an explicit loop instead of a regex, but I really can't see a
// better way that doesn't result in broken behaviour.
let i;
for (i = string.length - 1; i >= 0; i--) {
if (!string[i].match(/\s/)) {
break;
}
}
return string.substring(i + 1);
}

export function leadingWs(string) {
// Thankfully the annoying considerations described in trailingWs don't apply here:
return string.match(/^\s*/)[0];
}