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refactor(git-sync): extract shared buildLcsTable for the two block diffs (PR #119 review)

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The two-way block diff (yjs-body-merge.diffBlocks) and the three-way merge
planner (three-way-merge.lcsPairs) built the identical backward-filled LCS DP
table inline. Extract it to lcs.ts (buildLcsTable); each caller keeps its own
traceback. No behavior change — merge specs unchanged and green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
This commit is contained in:
claude code agent 227
2026-06-24 01:03:33 +03:00
parent 182c580cf7
commit d117a3f821
3 changed files with 31 additions and 22 deletions
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26
apps/server/src/integrations/git-sync/services/lcs.ts Normal file
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@@ -0,0 +1,26 @@
/**
* Backward-filled LCS length table for sequences `a` and `b`: `dp[i][j]` is the
* length of the longest common subsequence of the suffixes `a[i:]` and `b[j:]`.
* O(n*m) time/space — fine for page block counts.
*
* Shared by the two-way block diff (`yjs-body-merge.diffBlocks`) and the
* three-way merge planner (`three-way-merge.lcsPairs`) so the (identical) table
* construction lives in ONE place; each caller does its own traceback over the
* returned table.
*/
export function buildLcsTable(a: string[], b: string[]): number[][] {
const n = a.length;
const m = b.length;
const dp: number[][] = Array.from({ length: n + 1 }, () =>
new Array(m + 1).fill(0),
);
for (let i = n - 1; i >= 0; i--) {
for (let j = m - 1; j >= 0; j--) {
dp[i][j] =
a[i] === b[j]
? dp[i + 1][j + 1] + 1
: Math.max(dp[i + 1][j], dp[i][j + 1]);
}
}
return dp;
}

14
apps/server/src/integrations/git-sync/services/three-way-merge.ts
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@@ -19,21 +19,13 @@
* instances (and the human's in-flight edits) in place.
*/
import { buildLcsTable } from './lcs';
/** Matched index pairs of the longest common subsequence of `a` and `b`. */
function lcsPairs(a: string[], b: string[]): Array<[number, number]> {
const n = a.length;
const m = b.length;
const dp: number[][] = Array.from({ length: n + 1 }, () =>
new Array(m + 1).fill(0),
);
for (let i = n - 1; i >= 0; i--) {
for (let j = m - 1; j >= 0; j--) {
dp[i][j] =
a[i] === b[j]
? dp[i + 1][j + 1] + 1
: Math.max(dp[i + 1][j], dp[i][j + 1]);
}
}
const dp = buildLcsTable(a, b);
const pairs: Array<[number, number]> = [];
let i = 0;
let j = 0;

13
apps/server/src/integrations/git-sync/services/yjs-body-merge.ts
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@@ -1,6 +1,7 @@
import * as Y from 'yjs';
import { diff3Plan } from './three-way-merge';
import { buildLcsTable } from './lcs';
/**
* Block-level merge of an incoming (git) page body into a LIVE Yjs document,
@@ -90,17 +91,7 @@ type Op =
export function diffBlocks(a: string[], b: string[]): Op[] {
const n = a.length;
const m = b.length;
const dp: number[][] = Array.from({ length: n + 1 }, () =>
new Array(m + 1).fill(0),
);
for (let i = n - 1; i >= 0; i--) {
for (let j = m - 1; j >= 0; j--) {
dp[i][j] =
a[i] === b[j]
? dp[i + 1][j + 1] + 1
: Math.max(dp[i + 1][j], dp[i][j + 1]);
}
}
const dp = buildLcsTable(a, b);
const ops: Op[] = [];
let i = 0;
let j = 0;