commit-hygiene

Commit Hygiene Skill

Purpose: Keep commits atomic, PRs reviewable, and git history clean. Advise when it's time to commit before changes become too large.

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Core Philosophy

┌─────────────────────────────────────────────────────────────────┐
│  ATOMIC COMMITS                                                  │
│  ─────────────────────────────────────────────────────────────  │
│  One logical change per commit.                                  │
│  Each commit should be self-contained and deployable.            │
│  If you need "and" to describe it, split it.                     │
├─────────────────────────────────────────────────────────────────┤
│  SMALL PRS WIN                                                   │
│  ─────────────────────────────────────────────────────────────  │
│  < 400 lines changed = reviewed in < 1 hour                      │
│  > 1000 lines = likely rubber-stamped or abandoned               │
│  Smaller PRs = faster reviews, fewer bugs, easier reverts        │
├─────────────────────────────────────────────────────────────────┤
│  COMMIT EARLY, COMMIT OFTEN                                      │
│  ─────────────────────────────────────────────────────────────  │
│  Working code? Commit it.                                        │
│  Test passing? Commit it.                                        │
│  Don't wait for "done" - commit at every stable point.           │
└─────────────────────────────────────────────────────────────────┘

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Commit Size Thresholds

Warning Thresholds (Time to Commit!)

Metric	Yellow Zone	Red Zone	Action
Files changed	5-10 files	> 10 files	Commit NOW
Lines added	150-300 lines	> 300 lines	Commit NOW
Lines deleted	100-200 lines	> 200 lines	Commit NOW
Total changes	250-400 lines	> 400 lines	Commit NOW
Time since last commit	30-60 min	> 60 min	Consider committing

Ideal Commit Size

┌─────────────────────────────────────────────────────────────────┐
│  IDEAL COMMIT                                                    │
│  ─────────────────────────────────────────────────────────────  │
│  Files: 1-5                                                      │
│  Lines: 50-200 total changes                                     │
│  Scope: Single logical unit of work                              │
│  Message: Describes ONE thing                                    │
└─────────────────────────────────────────────────────────────────┘

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Check Current State (Run Frequently)

Quick Status Check

# See what's changed (staged + unstaged)
git status --short

# Count files and lines changed
git diff --stat
git diff --cached --stat  # Staged only

# Get totals
git diff --shortstat
# Example output: 8 files changed, 245 insertions(+), 32 deletions(-)

Detailed Change Analysis

# Full diff summary with file names
git diff --stat HEAD

# Just the numbers
git diff --numstat HEAD | awk '{add+=$1; del+=$2} END {print "+"add" -"del" total:"add+del}'

# Files changed count
git status --porcelain | wc -l

Pre-Commit Check Script

#!/bin/bash
# scripts/check-commit-size.sh

# Thresholds
MAX_FILES=10
MAX_LINES=400
WARN_FILES=5
WARN_LINES=200

# Get stats
FILES=$(git status --porcelain | wc -l | tr -d ' ')
STATS=$(git diff --shortstat HEAD 2>/dev/null)
INSERTIONS=$(echo "$STATS" | grep -oE '[0-9]+ insertion' | grep -oE '[0-9]+' || echo 0)
DELETIONS=$(echo "$STATS" | grep -oE '[0-9]+ deletion' | grep -oE '[0-9]+' || echo 0)
TOTAL=$((INSERTIONS + DELETIONS))

echo "📊 Current changes: $FILES files, +$INSERTIONS -$DELETIONS ($TOTAL total lines)"

# Check thresholds
if [ "$FILES" -gt "$MAX_FILES" ] || [ "$TOTAL" -gt "$MAX_LINES" ]; then
    echo "🔴 RED ZONE: Commit immediately! Changes are too large."
    echo "   Consider splitting into multiple commits."
    exit 1
elif [ "$FILES" -gt "$WARN_FILES" ] || [ "$TOTAL" -gt "$WARN_LINES" ]; then
    echo "🟡 WARNING: Changes getting large. Commit soon."
    exit 0
else
    echo "🟢 OK: Changes are within healthy limits."
    exit 0
fi

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When to Commit

Commit Triggers (Any One = Commit)

Trigger	Example
Test passes	Just got a test green → commit
Feature complete	Finished a function → commit
Refactor done	Renamed variable across files → commit
Bug fixed	Fixed the issue → commit
Before switching context	About to work on something else → commit
Clean compile	Code compiles/lints clean → commit
Threshold hit	> 5 files or > 200 lines → commit

Commit Immediately If

• ✅ Tests are passing after being red
• ✅ You're about to make a "big change"
• ✅ You've been coding for 30+ minutes
• ✅ You're about to try something risky
• ✅ The current state is "working"

Don't Wait For

• ❌ "Perfect" code
• ❌ All features done
• ❌ Full test coverage
• ❌ Code review from yourself
• ❌ Documentation complete

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Atomic Commit Patterns

Good Atomic Commits

✅ "Add email validation to signup form"
   - 3 files: validator.ts, signup.tsx, signup.test.ts
   - 120 lines changed
   - Single purpose: email validation

✅ "Fix null pointer in user lookup"
   - 2 files: userService.ts, userService.test.ts
   - 25 lines changed
   - Single purpose: fix one bug

✅ "Refactor: Extract PaymentProcessor class"
   - 4 files: payment.ts → paymentProcessor.ts + types
   - 180 lines changed
   - Single purpose: refactoring

Bad Commits (Too Large)

❌ "Add authentication, fix bugs, update styles"
   - 25 files changed
   - 800 lines changed
   - Multiple purposes mixed

❌ "WIP"
   - Unknown scope
   - No clear purpose
   - Hard to review/revert

❌ "Updates"
   - 15 files changed
   - Mix of features, fixes, refactors
   - Impossible to review properly

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Splitting Large Changes

Strategy 1: By Layer

Instead of one commit with:
  - API endpoint + database migration + frontend + tests

Split into:
  1. "Add users table migration"
  2. "Add User model and repository"
  3. "Add GET /users endpoint"
  4. "Add UserList component"
  5. "Add integration tests for user flow"

Strategy 2: By Feature Slice

Instead of one commit with:
  - All CRUD operations for users

Split into:
  1. "Add create user functionality"
  2. "Add read user functionality"
  3. "Add update user functionality"
  4. "Add delete user functionality"

Strategy 3: Refactor First

Instead of:
  - Feature + refactoring mixed

Split into:
  1. "Refactor: Extract validation helpers" (no behavior change)
  2. "Add email validation using new helpers" (new feature)

Strategy 4: By Risk Level

Instead of:
  - Safe changes + risky changes together

Split into:
  1. "Update dependencies" (safe, isolated)
  2. "Migrate to new API version" (risky, separate)

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PR Size Guidelines

Optimal PR Size

Metric	Optimal	Acceptable	Too Large
Files	1-10	10-20	> 20
Lines changed	50-200	200-400	> 400
Commits	1-5	5-10	> 10
Review time	< 30 min	30-60 min	> 60 min

PR Size vs Defect Rate

┌─────────────────────────────────────────────────────────────────┐
│  RESEARCH FINDINGS (Google, Microsoft studies)                  │
│  ─────────────────────────────────────────────────────────────  │
│  PRs < 200 lines: 15% defect rate                               │
│  PRs 200-400 lines: 23% defect rate                             │
│  PRs > 400 lines: 40%+ defect rate                              │
│                                                                 │
│  Review quality drops sharply after 200-400 lines.              │
│  Large PRs get "LGTM" rubber stamps, not real reviews.          │
└─────────────────────────────────────────────────────────────────┘

When PR is Too Large

# Check PR size before creating
git diff main --stat
git diff main --shortstat

# If too large, consider:
# 1. Split into multiple PRs (stacked PRs)
# 2. Create feature flag and merge incrementally
# 3. Use draft PR for early feedback

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Commit Message Format

Structure

<type>: <description> (50 chars max)

[optional body - wrap at 72 chars]

[optional footer]

Types

Type	Use For
feat	New feature
fix	Bug fix
refactor	Code change that neither fixes nor adds
test	Adding/updating tests
docs	Documentation only
style	Formatting, no code change
chore	Build, config, dependencies

Examples

feat: Add email validation to signup form

fix: Prevent null pointer in user lookup

refactor: Extract PaymentProcessor class

test: Add integration tests for checkout flow

chore: Update dependencies to latest versions

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Git Workflow Integration

Pre-Commit Hook for Size Check

#!/bin/bash
# .git/hooks/pre-commit

MAX_LINES=400
MAX_FILES=15

FILES=$(git diff --cached --name-only | wc -l | tr -d ' ')
STATS=$(git diff --cached --shortstat)
INSERTIONS=$(echo "$STATS" | grep -oE '[0-9]+ insertion' | grep -oE '[0-9]+' || echo 0)
DELETIONS=$(echo "$STATS" | grep -oE '[0-9]+ deletion' | grep -oE '[0-9]+' || echo 0)
TOTAL=$((INSERTIONS + DELETIONS))

if [ "$TOTAL" -gt "$MAX_LINES" ]; then
    echo "❌ Commit too large: $TOTAL lines (max: $MAX_LINES)"
    echo "   Consider splitting into smaller commits."
    echo "   Use 'git add -p' for partial staging."
    exit 1
fi

if [ "$FILES" -gt "$MAX_FILES" ]; then
    echo "❌ Too many files: $FILES (max: $MAX_FILES)"
    echo "   Consider splitting into smaller commits."
    exit 1
fi

echo "✅ Commit size OK: $FILES files, $TOTAL lines"

Partial Staging (Split Large Changes)

# Stage specific hunks interactively
git add -p

# Stage specific files
git add path/to/sp

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