test: add integration tests for git diff --quiet optimization

Author: shaanmajid

crates/prek/tests/skipped_hooks.rs

@@ -206,13 +206,150 @@ fn all_hooks_skipped_multiple_priority_groups() -> Result<()> {
     assert!(stdout.contains("priority-20") && stdout.contains("Skipped"));
     assert!(stdout.contains("priority-30") && stdout.contains("Skipped"));
 
-    // Regression test for #1335: only 1 get_diff call (initial baseline)
-    // Without fix: 4 calls (1 initial + 3 per priority group)
+    // Optimization: when worktree is cleaned (stashed) and all hooks skip, no diff needed.
+    // - #1335 fix: skip diff check when all hooks in a group skip
+    // - --quiet optimization: use has_worktree_changes instead of get_diff
+    // Combined: 0 get_diff calls when all hooks skip in normal workflow
     let stderr = String::from_utf8_lossy(&output.stderr);
     let get_diff_calls = stderr.matches("get_diff").count();
+    assert_eq!(
+        get_diff_calls, 0,
+        "Expected 0 get_diff calls when all hooks skip in stashed workflow.\n\
+         Found {get_diff_calls} get_diff calls.\n\
+         Trace output:\n{stderr}"
+    );
+
+    Ok(())
+}
+
+/// Optimization test: when worktree is stashed (normal workflow), use `--quiet`
+/// check instead of full diff comparison.
+///
+/// In normal pre-commit workflow:
+/// - Unstaged changes are stashed, so worktree matches index
+/// - After hooks run, we only need to check IF changes exist (boolean)
+/// - `git diff --quiet` (exit code) is faster than capturing/comparing full diff
+///
+/// This test verifies:
+/// 1. `has_worktree_changes` is called (not `get_diff`) when hooks run
+/// 2. File modifications by hooks are still detected correctly
+#[test]
+fn uses_has_worktree_changes_when_stashed() -> Result<()> {
+    let context = TestContext::new();
+    context.init_project();
+
+    let cwd = context.work_dir();
+
+    // Hook that modifies files (triggers modification detection)
+    context.write_pre_commit_config(indoc::indoc! {r#"
+        repos:
+          - repo: local
+            hooks:
+              - id: modifier
+                name: modifier
+                language: system
+                entry: python3 -c "open('file.txt', 'a').write('modified')"
+                files: \.txt$
+    "#});
+
+    cwd.child("file.txt").write_str("original")?;
+    context.git_add(".");
+
+    let output = context.run().env("RUST_LOG", "prek::git=trace").output()?;
+
+    // Hook should fail because it modified files
+    assert!(
+        !output.status.success(),
+        "hook should fail due to file modification"
+    );
+
+    let stderr = String::from_utf8_lossy(&output.stderr);
+
+    // Optimization: use has_worktree_changes for first check (avoids initial get_diff).
+    // When changes are detected, get_diff is called to capture state for subsequent groups.
+    let has_worktree_changes_calls = stderr.matches("has_worktree_changes").count();
+    let get_diff_calls = stderr.matches("get_diff").count();
+
+    assert!(
+        has_worktree_changes_calls >= 1,
+        "Expected has_worktree_changes to be called for first modification check.\n\
+         Found {has_worktree_changes_calls} has_worktree_changes calls, {get_diff_calls} get_diff calls.\n\
+         Trace output:\n{stderr}"
+    );
+
+    // Optimization benefit: 1 get_diff call (after change detection) vs 2 (before + after)
+    // The initial get_diff is replaced by the cheaper has_worktree_changes check.
     assert_eq!(
         get_diff_calls, 1,
-        "Expected 1 get_diff call (initial baseline) when all hooks skip, found {get_diff_calls}.\n\
+        "Expected 1 get_diff call (to capture state after change detection).\n\
+         Found {get_diff_calls} get_diff calls.\n\
+         Trace output:\n{stderr}"
+    );
+
+    Ok(())
+}
+
+/// When using `--all-files`, worktree is NOT stashed, so full diff comparison is needed.
+///
+/// With --all-files, the worktree may have existing unstaged changes. To detect if
+/// hooks added MORE changes, we must compare before/after diffs (not just check
+/// existence). This ensures correctness for the edge case where a file is already
+/// modified and a hook modifies it further.
+#[test]
+fn uses_get_diff_when_all_files() -> Result<()> {
+    let context = TestContext::new();
+    context.init_project();
+
+    let cwd = context.work_dir();
+
+    // Hook that modifies files
+    context.write_pre_commit_config(indoc::indoc! {r#"
+        repos:
+          - repo: local
+            hooks:
+              - id: modifier
+                name: modifier
+                language: system
+                entry: python3 -c "open('file.txt', 'a').write('modified')"
+                files: \.txt$
+    "#});
+
+    cwd.child("file.txt").write_str("original")?;
+    // Stage and commit so file is tracked, then use --all-files
+    context.git_add(".");
+    context.configure_git_author();
+    context.git_commit("initial");
+
+    let output = context
+        .run()
+        .arg("--all-files")
+        .env("RUST_LOG", "prek::git=trace")
+        .output()?;
+
+    // Hook should fail because it modified files
+    assert!(
+        !output.status.success(),
+        "hook should fail due to file modification"
+    );
+
+    let stderr = String::from_utf8_lossy(&output.stderr);
+
+    // With --all-files (no stash), should use get_diff for comparison
+    let get_diff_calls = stderr.matches("get_diff").count();
+    let has_worktree_changes_calls = stderr.matches("has_worktree_changes").count();
+
+    assert!(
+        get_diff_calls >= 1,
+        "Expected get_diff to be called for --all-files workflow.\n\
+         Found {get_diff_calls} get_diff calls, {has_worktree_changes_calls} has_worktree_changes calls.\n\
+         Trace output:\n{stderr}"
+    );
+
+    // has_worktree_changes should NOT be used in --all-files workflow
+    assert_eq!(
+        has_worktree_changes_calls, 0,
+        "Expected 0 has_worktree_changes calls in --all-files workflow.\n\
+         Found {has_worktree_changes_calls} has_worktree_changes calls.\n\
          Trace output:\n{stderr}"
     );