1 files changed, 93 insertions, 0 deletions
diff --git a/src/cli/diff.ts b/src/cli/diff.ts
new file mode 100644
index 0000000..e00c8c0
--- /dev/null
+++ b/src/cli/diff.ts
@@ -0,0 +1,93 @@
+// Diff logic for comparing local files against the last-pulled snapshot
+
+import { join, relative, sep } from 'path'
+import { readFile, readdir, stat } from 'fs/promises'
+import { createHash } from 'crypto'
+
+export type ChangeType = 'added' | 'modified' | 'deleted'
+
+export interface FileChange {
+  path: string        // relative path
+  type: ChangeType
+  isText: boolean
+}
+
+// Text extensions matching Overleaf's set (from fileSyncBridge.ts)
+const TEXT_EXTENSIONS = new Set([
+  'tex', 'latex', 'sty', 'cls', 'bst', 'bib', 'bibtex', 'txt', 'tikz',
+  'mtx', 'rtex', 'md', 'asy', 'lbx', 'bbx', 'cbx', 'm', 'lco', 'dtx',
+  'ins', 'ist', 'def', 'clo', 'ldf', 'rmd', 'qmd', 'lua', 'py', 'gv',
+  'mf', 'yml', 'yaml', 'lhs', 'lean', 'lean4', 'hs', 'mk', 'xmpdata',
+  'cfg', 'rnw', 'ltx', 'inc',
+  'fd', 'r', 'sh', 'json', 'xml', 'csv', 'tsv', 'html', 'css', 'js',
+  'ts', 'c', 'cpp', 'h', 'hpp', 'java', 'rb', 'pl'
+])
+
+const EDITABLE_FILENAMES = new Set(['latexmkrc', '.latexmkrc', 'makefile', 'gnumakefile'])
+
+export function isTextFile(relPath: string): boolean {
+  const name = relPath.split('/').pop()?.toLowerCase() || ''
+  if (EDITABLE_FILENAMES.has(name)) return true
+  const ext = name.split('.').pop() || ''
+  return TEXT_EXTENSIONS.has(ext)
+}
+
+/** Hash file contents with sha256 */
+export async function hashFile(absPath: string): Promise<string> {
+  const data = await readFile(absPath)
+  return createHash('sha256').update(data).digest('hex')
+}
+
+/** Walk a directory, returning relative paths (forward slashes, no leading /) */
+export async function walkDir(dir: string, base?: string): Promise<string[]> {
+  const root = base || dir
+  const results: string[] = []
+  try {
+    const entries = await readdir(dir, { withFileTypes: true })
+    for (const entry of entries) {
+      // Skip the state file and hidden files
+      if (entry.name === '.lattex-cli.json') continue
+      if (entry.name.startsWith('.')) continue
+      const full = join(dir, entry.name)
+      if (entry.isDirectory()) {
+        results.push(...await walkDir(full, root))
+      } else {
+        results.push(relative(root, full).split(sep).join('/'))
+      }
+    }
+  } catch { /* directory may not exist */ }
+  return results
+}
+
+/** Compute diff between local files and stored hashes from last pull */
+export async function computeDiff(
+  dir: string,
+  storedHashes: Record<string, string>,
+  knownPaths: Set<string>
+): Promise<FileChange[]> {
+  const changes: FileChange[] = []
+  const localFiles = await walkDir(dir)
+  const localSet = new Set(localFiles)
+
+  // Check for modified and added files
+  for (const relPath of localFiles) {
+    const absPath = join(dir, relPath)
+    const hash = await hashFile(absPath)
+    const storedHash = storedHashes[relPath]
+
+    if (!storedHash && !knownPaths.has(relPath)) {
+      changes.push({ path: relPath, type: 'added', isText: isTextFile(relPath) })
+    } else if (storedHash && hash !== storedHash) {
+      changes.push({ path: relPath, type: 'modified', isText: isTextFile(relPath) })
+    }
+  }
+
+  // Check for deleted files
+  for (const relPath of Object.keys(storedHashes)) {
+    if (!localSet.has(relPath)) {
+      changes.push({ path: relPath, type: 'deleted', isText: isTextFile(relPath) })
+    }
+  }
+
+  return changes
+}