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Flip core algorithm so everything is no longer the mirror image of Myers's paper #440

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Merged
merged 2 commits into from
Dec 27, 2023
Merged

Flip core algorithm so everything is no longer the mirror image of Myers's paper #440

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de93e91
Fix core algorithm implementation being mirrored relative to the Myer…
ExplodingCabbage Dec 27, 2023
9d8384d
Fix comment
ExplodingCabbage Dec 27, 2023
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51 changes: 29 additions & 22 deletions src/diff/base.js
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Original file line number Diff line number Diff line change
Expand Up @@ -34,11 +34,11 @@ Diff.prototype = {
maxEditLength = Math.min(maxEditLength, options.maxEditLength);
}

let bestPath = [{ newPos: -1, lastComponent: undefined }];
let bestPath = [{ oldPos: -1, lastComponent: undefined }];

// Seed editLength = 0, i.e. the content starts with the same values
let oldPos = this.extractCommon(bestPath[0], newString, oldString, 0);
if (bestPath[0].newPos + 1 >= newLen && oldPos + 1 >= oldLen) {
let newPos = this.extractCommon(bestPath[0], newString, oldString, 0);
if (bestPath[0].oldPos + 1 >= oldLen && newPos + 1 >= newLen) {
// Identity per the equality and tokenizer
return done([{value: this.join(newString), count: newString.length}]);
}
Expand All @@ -47,35 +47,42 @@ Diff.prototype = {
function execEditLength() {
for (let diagonalPath = -1 * editLength; diagonalPath <= editLength; diagonalPath += 2) {
let basePath;
let addPath = bestPath[diagonalPath - 1],
removePath = bestPath[diagonalPath + 1],
oldPos = (removePath ? removePath.newPos : 0) - diagonalPath;
if (addPath) {
let removePath = bestPath[diagonalPath - 1],
addPath = bestPath[diagonalPath + 1];
if (removePath) {
// No one else is going to attempt to use this value, clear it
bestPath[diagonalPath - 1] = undefined;
}

let canAdd = addPath && addPath.newPos + 1 < newLen,
canRemove = removePath && 0 <= oldPos && oldPos < oldLen;
let canAdd = false;
if (addPath) {
// what newPos will be after we do an insertion:
const addPathNewPos = addPath.oldPos - diagonalPath;
canAdd = addPath && 0 <= addPathNewPos && addPathNewPos < newLen;
}

let canRemove = removePath && removePath.oldPos + 1 < oldLen;
if (!canAdd && !canRemove) {
// If this path is a terminal then prune
bestPath[diagonalPath] = undefined;
continue;
}

// Select the diagonal that we want to branch from. We select the prior
// path whose position in the new string is the farthest from the origin
// path whose position in the old string is the farthest from the origin
// and does not pass the bounds of the diff graph
if (!canAdd || (canRemove && addPath.newPos < removePath.newPos)) {
basePath = self.addToPath(removePath, undefined, true, 0);
// TODO: Remove the `+ 1` here to make behavior match Myers algorithm
// and prefer to order removals before insertions.
if (!canRemove || (canAdd && removePath.oldPos + 1 < addPath.oldPos)) {
basePath = self.addToPath(addPath, true, undefined, 0);
} else {
basePath = self.addToPath(addPath, true, undefined, 1);
basePath = self.addToPath(removePath, undefined, true, 1);
}

oldPos = self.extractCommon(basePath, newString, oldString, diagonalPath);
newPos = self.extractCommon(basePath, newString, oldString, diagonalPath);

// If we have hit the end of both strings, then we are done
if (basePath.newPos + 1 >= newLen && oldPos + 1 >= oldLen) {
if (basePath.oldPos + 1 >= oldLen && newPos + 1 >= newLen) {
return done(buildValues(self, basePath.lastComponent, newString, oldString, self.useLongestToken));
} else {
// Otherwise track this path as a potential candidate and continue.
Expand Down Expand Up @@ -112,25 +119,25 @@ Diff.prototype = {
}
},

addToPath(path, added, removed, newPosInc) {
addToPath(path, added, removed, oldPosInc) {
let last = path.lastComponent;
if (last && last.added === added && last.removed === removed) {
return {
newPos: path.newPos + newPosInc,
oldPos: path.oldPos + oldPosInc,
lastComponent: {count: last.count + 1, added: added, removed: removed, previousComponent: last.previousComponent }
};
} else {
return {
newPos: path.newPos + newPosInc,
oldPos: path.oldPos + oldPosInc,
lastComponent: {count: 1, added: added, removed: removed, previousComponent: last }
};
}
},
extractCommon(basePath, newString, oldString, diagonalPath) {
let newLen = newString.length,
oldLen = oldString.length,
newPos = basePath.newPos,
oldPos = newPos - diagonalPath,
oldPos = basePath.oldPos,
newPos = oldPos - diagonalPath,

commonCount = 0;
while (newPos + 1 < newLen && oldPos + 1 < oldLen && this.equals(newString[newPos + 1], oldString[oldPos + 1])) {
Expand All @@ -143,8 +150,8 @@ Diff.prototype = {
basePath.lastComponent = {count: commonCount, previousComponent: basePath.lastComponent};
}

basePath.newPos = newPos;
return oldPos;
basePath.oldPos = oldPos;
return newPos;
},

equals(left, right) {
Expand Down