pre-commit-hook: trigger tailwind rebuild conditionally

pre-commit

@@ -1,8 +1,16 @@
 #!/bin/sh
 
-files=`git diff --cached --name-status`
+# this script must be placed in .git/hooks/pre-commit for it to run automatically
+# it is put here in the repo so that it can be used by anyone who clones the repo
 
 . bin/activate
-python manage.py tailwind build
 
+should_run=$(git diff --cached --name-only -z | python tools/is_tracked_by_tailwind.py)
+
+if [ "$should_run" = "yes" ]; then
+    echo "Building Tailwind CSS..."
+    python manage.py tailwind build
     git add theme/static/css/dist/styles.css
+else
+    echo "No relevant changes; skipping Tailwind build."
+fi

tools/is_tracked_by_tailwind.py

@@ -0,0 +1,138 @@
+#!/usr/bin/env python3
+
+# Determine whether the Tailwind build must run based on staged files.
+# Reads null-separated paths from stdin, compares against globs listed in theme/static_src/tailwind.config.js
+
+import pathlib
+import re
+import sys
+
+
+def expand_optional_dirs(pattern):
+    """
+    Python's Path.match() treats '**/' strictly — it requires at least one directory to match.
+    But in globbing systems like Tailwind or bash, '**/' can also mean 'zero directories'.
+    This function expands all combinations where each '**/' is either kept or removed,
+    so that Path.match() can simulate this more flexible behavior.
+
+    For example:
+    'theme/**/templates/**/*.html' →
+    [
+        'theme/**/templates/**/*.html',
+        'theme/templates/**/*.html',
+        'theme/**/templates/*.html',
+        'theme/templates/*.html',
+    ]
+
+    Use this to match paths more like Tailwind does, where '**' can mean "nothing here".
+    """
+    parts = pattern.split("**/")
+    if len(parts) == 1:
+        return [pattern]  # no '**/' present
+
+    results = []
+
+    # There are N-1 '**/' positions
+    num_stars = len(parts) - 1
+
+    # For each combination of keeping or removing the '**/' parts
+    for bits in range(2 ** num_stars):
+        new_pattern = parts[0]
+        for i in range(num_stars):
+            if (bits >> i) & 1:
+                new_pattern += "**/"
+            new_pattern += parts[i + 1]
+        results.append(new_pattern)
+
+    return results
+
+
+def path_matches_glob(path, glob_pattern):
+    """ Check if the given path matches any variant of the glob pattern (including '**'-as-nothing)."""
+    for variant in expand_optional_dirs(glob_pattern):
+        if pathlib.Path(path).match(variant):
+            return True
+    return False
+
+
+def files_from_stdin():
+    # Read from stdin as a single binary stream
+    raw = sys.stdin.buffer.read()
+    files = raw.decode("utf-8").split("\0")
+    files = [f for f in files if f]  # Remove empty final item
+    return files
+
+
+def parse_tailwind_config():
+    # tailwind.config.js is not JSON, so we can't use json.load().
+    # We extract the 'content: [...]' array with a regex,
+    # then pull all string literals using a regex.
+    # Any staged file under theme/static_src/ triggers a rebuild.
+
+    config_path = pathlib.Path("theme/static_src/tailwind.config.js")
+    text = config_path.read_text()
+
+    # Extract the 'content: [ ... ]' block.
+    match = re.search(r'content\s*:\s*\[(.*?)\]', text, re.DOTALL)
+    if not match:
+        raise RuntimeError("Could not find content array in Tailwind config")
+    raw_content_block = match.group(1)
+
+    string_literals = []
+
+    for line in raw_content_block.splitlines():
+        line = line.strip()
+        if not line or not line.startswith(("'", '"')):
+            continue
+
+        # Match a string at the start of the line
+        match = re.match(r'''(['"])(.*?)\1''', line)
+        if not match:
+            raise ValueError(f"Invalid quoted string: {line}")
+
+        string_literals.append(match.group(2))
+
+    return string_literals
+
+
+def globs_from_string_literals(string_literals):
+    """Globs: resolve paths relative to the repo root instead of the config file."""
+
+    result = []
+    for string_literal in string_literals:
+        if string_literal.startswith("../../"):
+            result.append(string_literal[6:])  # Remove '../../' prefix
+        elif string_literal.startswith("../"):
+            result.append("theme/" + string_literal[3:])
+        else:
+            result.append(string_literal)
+
+    return result
+
+
+def any_file_matches_globs(files, globs):
+    for f in files:
+        for g in globs:
+            if path_matches_glob(f, g):
+                return True
+    return False
+
+
+if __name__ == "__main__":
+    files = files_from_stdin()
+    if not files:
+        print("no")
+        sys.exit(0)
+
+    if any(f.startswith("theme/static_src/") for f in files):
+        print("yes")
+        sys.exit(0)
+
+    string_literals = parse_tailwind_config()
+    globs = globs_from_string_literals(string_literals)
+
+    if any_file_matches_globs(files, globs):
+        print("yes")
+        sys.exit(0)
+
+    print("no")