2nd/10_Wiki/Topics/Domain_Programming/Architecture/Code Refactoring.md

id, title, category, status, canonical_id, aliases, duplicate_of, source_trust_level, confidence_score, verification_status, tags, raw_sources, last_reinforced, github_commit, tech_stack

id	title	category	status	canonical_id	aliases	duplicate_of	source_trust_level	confidence_score	verification_status	tags	raw_sources	last_reinforced	github_commit	tech_stack
wiki-2026-0508-code-refactoring	Code Refactoring	10_Wiki/Topics	verified	self	Refactoring Code Improvement Fowler Refactoring	none	A	0.95	applied	refactoring code-quality fowler ide llm-refactor		2026-05-10	pending	language framework Polyglot IDE/AST-based-tools

Code Refactoring

매 한 줄

"매 refactoring = 매 외부 동작은 그대로 두고 internal 구조만 개선하는 disciplined transformation.". 매 Martin Fowler 1999 "Refactoring" 이 catalog 화한 70+ named transformation 이 backbone, 매 2026 현재 IDE automated refactor + LLM-assisted refactor (Claude Opus 4.7, Cursor) 가 manual rewrite 대체. 매 핵심 disciplines = small steps + green tests + commit per refactor.

매 핵심

매 catalog (대표)

• Extract Function / Method: 매 코드 블록 → 새 function.
• Inline Function: 매 trivial wrapper 제거.
• Rename: 매 의미 명확한 이름.
• Extract Variable / Inline Variable.
• Move Function / Field: 매 right class/module 로.
• Replace Conditional with Polymorphism.
• Replace Magic Number with Constant.
• Introduce Parameter Object.
• Decompose Conditional.
• Extract Class / Inline Class.

매 disciplines

• Tests first: 매 refactor 전 green test suite.
• Small steps: 매 single refactor → run tests → commit.
• No mixing: 매 behavior change 와 refactor 동시 X.
• Reversibility: 매 step 별 git revert 가능.

매 응용

1. Legacy modernization: 매 strangler fig + extract.
2. Performance: 매 inline hot function, extract slow path.
3. Test enabling: 매 dependency 분리.
4. Onboarding: 매 confusing code rename + decompose.

💻 패턴

Extract Function

# BEFORE
def print_owing(invoice):
    print_banner()
    outstanding = sum(o.amount for o in invoice.orders)
    print(f"name: {invoice.customer}")
    print(f"amount: {outstanding}")

# AFTER
def print_owing(invoice):
    print_banner()
    outstanding = calculate_outstanding(invoice)
    print_details(invoice, outstanding)

def calculate_outstanding(invoice):
    return sum(o.amount for o in invoice.orders)

def print_details(invoice, outstanding):
    print(f"name: {invoice.customer}")
    print(f"amount: {outstanding}")

Replace Conditional with Polymorphism

// BEFORE
function pay(employee: Employee) {
  switch (employee.type) {
    case 'engineer': return baseSalary(employee);
    case 'salesman': return baseSalary(employee) + commission(employee);
    case 'manager':  return baseSalary(employee) + bonus(employee);
  }
}

// AFTER
abstract class EmployeeType {
  abstract pay(e: Employee): number;
}
class Engineer extends EmployeeType { pay(e) { return baseSalary(e); } }
class Salesman extends EmployeeType { pay(e) { return baseSalary(e) + commission(e); } }
class Manager  extends EmployeeType { pay(e) { return baseSalary(e) + bonus(e); } }

Introduce Parameter Object

# BEFORE
def amount_invoiced(start_date, end_date, customer_id, currency, ...): ...

# AFTER
@dataclass
class InvoiceQuery:
    range: DateRange
    customer_id: str
    currency: str

def amount_invoiced(q: InvoiceQuery): ...

Decompose Conditional

// BEFORE
if (date.before(SUMMER_START) || date.after(SUMMER_END)) {
  charge = quantity * winterRate + winterServiceCharge;
} else {
  charge = quantity * summerRate;
}

// AFTER
charge = isSummer(date) ? summerCharge(quantity) : winterCharge(quantity);
function isSummer(d)   { return !d.before(SUMMER_START) && !d.after(SUMMER_END); }
function summerCharge(q) { return q * summerRate; }
function winterCharge(q) { return q * winterRate + winterServiceCharge; }

Strangler Fig (legacy migration)

// route table — gradually shift endpoints to new service
const routes = {
  '/users':    process.env.NEW_USERS    === 'on' ? newUsers    : legacyUsers,
  '/orders':   process.env.NEW_ORDERS   === 'on' ? newOrders   : legacyOrders,
  '/billing':  legacyBilling,  // not migrated yet
};
// flip flags one at a time, rollback by env var

IDE-driven (IntelliJ / Roslyn)

# JetBrains command-line refactor (CI batch)
idea.sh inspect $PROJECT inspectionProfile.xml output/ \
  -d $MODULE -v2

# C# Roslyn analyzer + code fix runs in `dotnet format analyzers --verify-no-changes`

LLM-assisted refactor (Claude Opus 4.7)

# 1. select scope, 2. generate diff, 3. tests must stay green
claude refactor \
  --scope src/billing/ \
  --instruction "Extract OrderTotalCalculator class; preserve all behavior" \
  --validate "pytest tests/billing/"
# Claude proposes diff → run tests → commit if green

Test characterization (legacy without tests)

# Before refactor of untested code: write golden tests first
def test_legacy_charge_2025_invoice():
    inv = load_fixture('2025_invoice.json')
    expected = legacy.calc(inv)             # capture current behavior
    assert refactored.calc(inv) == expected # equality, not "correctness"

매 결정 기준

상황	Refactor
Long function (> 30 lines)	Extract Function
Same param list 5+ places	Introduce Parameter Object
switch (type) repeated	Replace Conditional with Polymorphism
Confusing name	Rename
Class doing 2 things	Extract Class
Legacy without tests	Characterization tests first
Mass code movement	LLM batch + test-gate

기본값: small step + commit per refactor + tests green; LLM 으로 mass rename / extract 가속.

🔗 Graph

• 부모: Code_Quality
• 변형: Strangler_Fig
• 응용: Legacy_Modernization · Test-Driven Development
• Adjacent: Clean_Code · Design_Patterns

🤖 LLM 활용

언제: bulk rename, extract function, polymorphism conversion, parameter object 도입. 언제 X: subtle concurrency / lock-free invariants — 매 manual review 필수.

❌ 안티패턴

• Refactor + feature in same commit: 매 review/revert 불가능.
• Refactor without tests: 매 silent behavior change.
• Big-bang rewrite: 매 6개월 후 production 되돌리기 불가능.
• Premature abstraction: 매 1번 쓰인 패턴 인터페이스화 — YAGNI.
• Trust-LLM-and-skip-tests: 매 LLM hallucinated edge case 통과시킴.

🧪 검증 / 중복

• Verified (Fowler "Refactoring" 2nd ed 2018, "Working Effectively with Legacy Code" Feathers 2004).
• 신뢰도 A.

🕓 Changelog

날짜	변경
2026-05-08	Phase 1
2026-05-10	Manual cleanup — Fowler catalog, disciplines, IDE/LLM-assisted patterns