linter: Add custom gitlint rule to check for imperative mood.

.gitlint

@@ -1,6 +1,8 @@
 [general]
 ignore=title-trailing-punctuation, body-min-length, body-is-missing
 
+extra-path=tools/lib/gitlint-rules
+
 [title-match-regex]
 regex=^.+\.$
 

tools/lib/gitlint-rules/rules.py

@@ -0,0 +1,137 @@
+from typing import Text, List
+
+import gitlint
+from gitlint.rules import LineRule, RuleViolation, CommitMessageTitle
+
+# Word list from https://github.com/m1foley/fit-commit
+# Copyright (c) 2015 Mike Foley
+# License: MIT
+# Ref: fit_commit/validators/tense.rb
+WORD_SET = {
+    'adds', 'adding', 'added',
+    'allows', 'allowing', 'allowed',
+    'amends', 'amending', 'amended',
+    'bumps', 'bumping', 'bumped',
+    'calculates', 'calculating', 'calculated',
+    'changes', 'changing', 'changed',
+    'cleans', 'cleaning', 'cleaned',
+    'commits', 'committing', 'committed',
+    'corrects', 'correcting', 'corrected',
+    'creates', 'creating', 'created',
+    'darkens', 'darkening', 'darkened',
+    'disables', 'disabling', 'disabled',
+    'displays', 'displaying', 'displayed',
+    'drys', 'drying', 'dryed',
+    'ends', 'ending', 'ended',
+    'enforces', 'enforcing', 'enforced',
+    'enqueues', 'enqueuing', 'enqueued',
+    'extracts', 'extracting', 'extracted',
+    'finishes', 'finishing', 'finished',
+    'fixes', 'fixing', 'fixed',
+    'formats', 'formatting', 'formatted',
+    'guards', 'guarding', 'guarded',
+    'handles', 'handling', 'handled',
+    'hides', 'hiding', 'hid',
+    'increases', 'increasing', 'increased',
+    'ignores', 'ignoring', 'ignored',
+    'implements', 'implementing', 'implemented',
+    'improves', 'improving', 'improved',
+    'keeps', 'keeping', 'kept',
+    'kills', 'killing', 'killed',
+    'makes', 'making', 'made',
+    'merges', 'merging', 'merged',
+    'moves', 'moving', 'moved',
+    'permits', 'permitting', 'permitted',
+    'prevents', 'preventing', 'prevented',
+    'pushes', 'pushing', 'pushed',
+    'rebases', 'rebasing', 'rebased',
+    'refactors', 'refactoring', 'refactored',
+    'removes', 'removing', 'removed',
+    'renames', 'renaming', 'renamed',
+    'reorders', 'reordering', 'reordered',
+    'requires', 'requiring', 'required',
+    'restores', 'restoring', 'restored',
+    'sends', 'sending', 'sent',
+    'sets', 'setting',
+    'separates', 'separating', 'separated',
+    'shows', 'showing', 'showed',
+    'skips', 'skipping', 'skipped',
+    'sorts', 'sorting',
+    'speeds', 'speeding', 'sped',
+    'starts', 'starting', 'started',
+    'supports', 'supporting', 'supported',
+    'takes', 'taking', 'took',
+    'testing', 'tested',  # 'tests' excluded to reduce false negative
+    'truncates', 'truncating', 'truncated',
+    'updates', 'updating', 'updated',
+    'uses', 'using', 'used'
+}
+
+imperative_forms = [
+    'add', 'allow', 'amend', 'bump', 'calculate', 'change', 'clean', 'commit',
+    'correct', 'create', 'darken', 'disable', 'dry', 'end', 'enforce',
+    'enqueue', 'extract', 'finish', 'fix', 'format', 'guard', 'handle',
+    'hide', 'increase', 'ignore', 'implement', 'improve', 'keep', 'kill',
+    'make', 'merge', 'move', 'permit', 'prevent', 'push', 'rebase',
+    'refactor', 'remove', 'rename', 'reorder', 'require', 'restore', 'send',
+    'set', 'separate', 'show', 'skip', 'sort', 'speed', 'start', 'support',
+    'take', 'test', 'truncate', 'update', 'use'
+]
+
+
+def head_binary_search(key, words):
+    # type: (Text, List[str]) -> str
+    """ Find the imperative mood version of `word` by looking at the first
+    3 characters. """
+
+    # Edge case: disable and display have the same 4 starting letters.
+    if key in ['displays', 'displaying', 'displayed']:
+        return 'display'
+
+    lower = 0
+    upper = len(words) - 1
+
+    while True:
+        if lower > upper:
+            # Should not happen
+            raise Exception("Cannot find imperative mood of {}".format(key))
+
+        mid = (lower + upper) // 2
+        imperative_form = words[mid]
+
+        if key[:3] == imperative_form[:3]:
+            return imperative_form
+        elif key < imperative_form:
+            upper = mid - 1
+        elif key > imperative_form:
+            lower = mid + 1
+
+
+class ImperativeMood(LineRule):
+    """ This rule will enforce that the commit message title uses imperative
+    mood. """
+
+    name = "title-imperative-mood"
+    id = "Z1"
+    target = CommitMessageTitle
+
+    error_msg = ('Title contains non imperative mood '
+                 '("{word}" -> "{imperative}"): "{title}"')
+
+    def validate(self, line, commit):
+        # type: (Text, gitlint.commit) -> List[RuleViolation]
+        violations = []
+        words = line.lower().split(" ")
+
+        for word in words:
+            if word in WORD_SET:
+                imperative = head_binary_search(word, imperative_forms)
+                violation = RuleViolation(self.id, self.error_msg.format(
+                    word=word,
+                    imperative=imperative,
+                    title=commit.message.title
+                ))
+
+                violations.append(violation)
+
+        return violations