From 269c09267427746423c6bbe6d715ffd824befc64 Mon Sep 17 00:00:00 2001
From: claude code agent 227 <claude_code@vvzvlad.xyz>
Date: Wed, 24 Jun 2026 01:03:33 +0300
Subject: [PATCH] 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>
---
 .../src/integrations/git-sync/services/lcs.ts | 26 +++++++++++++++++++
 .../git-sync/services/three-way-merge.ts      | 14 +++-------
 .../git-sync/services/yjs-body-merge.ts       | 13 ++--------
 3 files changed, 31 insertions(+), 22 deletions(-)
 create mode 100644 apps/server/src/integrations/git-sync/services/lcs.ts

diff --git a/apps/server/src/integrations/git-sync/services/lcs.ts b/apps/server/src/integrations/git-sync/services/lcs.ts
new file mode 100644
index 00000000..250de948
--- /dev/null
+++ b/apps/server/src/integrations/git-sync/services/lcs.ts
@@ -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;
+}
diff --git a/apps/server/src/integrations/git-sync/services/three-way-merge.ts b/apps/server/src/integrations/git-sync/services/three-way-merge.ts
index fd87e9ac..eb1d2126 100644
--- a/apps/server/src/integrations/git-sync/services/three-way-merge.ts
+++ b/apps/server/src/integrations/git-sync/services/three-way-merge.ts
@@ -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;
diff --git a/apps/server/src/integrations/git-sync/services/yjs-body-merge.ts b/apps/server/src/integrations/git-sync/services/yjs-body-merge.ts
index c1dab666..2c459505 100644
--- a/apps/server/src/integrations/git-sync/services/yjs-body-merge.ts
+++ b/apps/server/src/integrations/git-sync/services/yjs-body-merge.ts
@@ -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;