hooks: clang-format pre-commit (changed lines only) + .clang-format

git-clang-format scopes formatting to the lines each commit touches -
a full-file reformat would destroy the deliberately dense hand
formatting (opcode tables, multi-statement helpers). The hook re-adds
formatted files (git-clang-format --staged fixes the worktree, not the
index) and refuses partially-staged C files rather than mixing staged
and unstaged changes. Enable once: git config core.hooksPath hooks

3 files changed, 69 insertions, 0 deletions

diff --git a/.clang-format b/.clang-format
new file mode 100644
index 0000000..e338db1
--- /dev/null
+++ b/.clang-format
@@ -0,0 +1,23 @@
+# House style for sl0pboy/gbos C: 4-space, attached braces, dense-but-readable.
+# Tuned to minimize churn against the existing hand-formatted code.
+BasedOnStyle: LLVM
+IndentWidth: 4
+TabWidth: 4
+UseTab: Never
+ColumnLimit: 100
+PointerAlignment: Right
+BreakBeforeBraces: Attach
+AllowShortCaseLabelsOnASingleLine: true
+AllowShortFunctionsOnASingleLine: All
+AllowShortIfStatementsOnASingleLine: AllIfsAndElse
+AllowShortLoopsOnASingleLine: true
+AllowShortBlocksOnASingleLine: Always
+AlignTrailingComments: true
+SpacesBeforeTrailingComments: 2
+AlignConsecutiveMacros: Consecutive
+SortIncludes: Never
+IndentCaseLabels: true
+MaxEmptyLinesToKeep: 2
+KeepEmptyLinesAtTheStartOfBlocks: true
+AlignArrayOfStructures: None
+ReflowComments: false
diff --git a/README.md b/README.md
index 150e842..d53f728 100644
--- a/README.md
+++ b/README.md
@@ -14,6 +14,7 @@ Resize the terminal and `SIGWINCH` triggers a full repaint.
 ## Build & run
 
 ```sh
+git config core.hooksPath hooks   # once: clang-format pre-commit hook
 make
 ./build/sl0pboy path/to/rom.gb             # just play it (ROM goes last)
 ./build/sl0pboy --test 30 rom.gb           # run 30s, serial -> stderr (test roms)
diff --git a/hooks/pre-commit b/hooks/pre-commit
new file mode 100755
index 0000000..7b1778c
--- /dev/null
+++ b/hooks/pre-commit
@@ -0,0 +1,45 @@
+#!/bin/sh
+# pre-commit: clang-format the C lines this commit touches.
+#
+# Scoped to *changed lines* via git-clang-format, NOT whole files: the
+# codebase deliberately packs multiple statements per line (opcode tables,
+# ALU helpers) and a full-file reformat would destroy that. New/edited
+# lines converge on .clang-format style; untouched history stays intact.
+#
+# Enable once per clone:   git config core.hooksPath hooks
+# Bypass for one commit:   git commit --no-verify
+
+# staged C files (added/copied/modified/renamed)
+staged=$(git diff --cached --name-only --diff-filter=ACMR -- '*.c' '*.h')
+[ -z "$staged" ] && exit 0
+
+if ! command -v git-clang-format >/dev/null 2>&1; then
+    echo "pre-commit: git-clang-format not found (install clang-format)." >&2
+    echo "pre-commit: bypass with: git commit --no-verify" >&2
+    exit 1
+fi
+
+# git-clang-format --staged applies fixes to the WORKTREE, so we must re-add
+# the files afterwards. That is only safe when index == worktree for every
+# staged C file; refuse partially-staged files instead of mixing changes.
+partial=$(git diff --name-only -- $staged)
+if [ -n "$partial" ]; then
+    echo "pre-commit: partially staged C files (index != worktree):" >&2
+    printf '    %s\n' $partial >&2
+    echo "pre-commit: stage them fully (git add) or bypass with --no-verify." >&2
+    exit 1
+fi
+
+out=$(git clang-format --staged --quiet --extensions c,h 2>&1)
+rc=$?
+if [ $rc -ge 2 ]; then                      # real error (1 = "changes made")
+    echo "pre-commit: git-clang-format failed:" >&2
+    echo "$out" >&2
+    exit 1
+fi
+if [ $rc -eq 1 ]; then
+    git add -- $staged
+    echo "pre-commit: clang-formatted changed lines in:" >&2
+    printf '    %s\n' $staged >&2
+fi
+exit 0