skip complex conflicts in merge auto-resolution

src/commands/merge.ts

@@ -6,6 +6,8 @@ import { spinner } from "../spinner.ts"
 import { die } from "../fmt.ts"
 import { action as closeAction } from "./close.ts"
 
+const MAX_DIFF_LINES = 300
+
 function truncate(text: string, maxLines = 200): string {
   const lines = text.split("\n")
   if (lines.length <= maxLines) return text
@@ -42,13 +44,41 @@ export async function action(branch: string) {
     const branchLog = await git.commitLog(`${base}..${branch}`, root)
     const branchDiffStat = await git.diffStat(`${base}..${branch}`, root)
 
-    for (const file of conflicts) {
+    // Fetch diffs for all conflicted files and partition by complexity
+    spin.text = "Checking conflict complexity"
+    const fileDiffs = await Promise.all(
+      conflicts.map(async (file) => {
+        const [oursDiff, theirsDiff] = await Promise.all([
+          git.fileDiff(base, "HEAD", file, root),
+          git.fileDiff(base, branch, file, root),
+        ])
+        return { file, oursDiff, theirsDiff }
+      }),
+    )
+
+    const resolvable: typeof fileDiffs = []
+    const skipped: string[] = []
+    for (const entry of fileDiffs) {
+      const oursLines = entry.oursDiff ? entry.oursDiff.split("\n").length : 0
+      const theirsLines = entry.theirsDiff ? entry.theirsDiff.split("\n").length : 0
+      if (oursLines > MAX_DIFF_LINES || theirsLines > MAX_DIFF_LINES) {
+        skipped.push(entry.file)
+      } else {
+        resolvable.push(entry)
+      }
+    }
+
+    if (resolvable.length === 0) {
+      await git.abortMerge(root)
+      spin.fail("All conflicts are too complex for auto-resolution")
+      console.log("\nThe following files need manual resolution:")
+      for (const file of skipped) console.log(`  - ${file}`)
+      process.exit(1)
+    }
+
+    for (const { file, oursDiff, theirsDiff } of resolvable) {
       spin.text = `Resolving ${file}`
 
-      const [oursDiff, theirsDiff] = await Promise.all([
-        git.fileDiff(base, "HEAD", file, root),
-        git.fileDiff(base, branch, file, root),
-      ])
       const content = await Bun.file(join(root, file)).text()
 
       const context = [
@@ -87,6 +117,14 @@ export async function action(branch: string) {
       await git.stageFile(file, root)
     }
 
+    if (skipped.length > 0) {
+      spin.succeed(`Resolved ${resolvable.length} of ${conflicts.length} conflict(s)`)
+      console.log("\nThe following files are too complex for auto-resolution:")
+      for (const file of skipped) console.log(`  - ${file}`)
+      console.log("\nResolve them manually, then run: git commit --no-edit")
+      return
+    }
+
     await git.commitMerge(root)
     spin.succeed(`Resolved ${conflicts.length} conflict(s) and merged ${branch}`)
   } catch (err) {