Code Size & Complexity Limits

Complexity is the strongest single predictor of defect density, and code written by agents balloons fastest — an agent under pressure adds one more branch to an existing function far more readily than it extracts a new one. These five SwiftLint threshold rules are the deterministic cap: they keep functions small enough that tests can actually cover their branches, and they keep agents from wrangling with their own tangles.

When This Skill Activates

Use this skill when:

• Setting up quality gates for a project where agents write most of the code
• A review keeps flagging "this function is too long/branchy" by hand
• Functions have grown past what their tests plausibly cover
• Adopting the deterministic gauntlet (see testing/fitness-functions/)

The Rules

The fragment at rules/swiftlint.yml configures five built-in SwiftLint rules (all enabled by default in SwiftLint — this fragment sets the thresholds):

Rule	Warning	Error	What it caps
cyclomatic_complexity	10	20	Independent paths through one function
function_body_length	50	100	Lines per function body
type_body_length	300	500	Lines per type body
file_length	500	1000	Lines per file
function_parameter_count	5	8	Parameters per function

Merge the fragment into the project's .swiftlint.yml (alongside any opt_in_rules/custom_rules fragments already adopted).

Brownfield Adoption: Baseline, Never Mass-Refactor

Turning these rules on in an existing codebase must not trigger a refactoring spree — bulk rewrites to satisfy a linter are how working code gets broken. Freeze today's violations and gate only new ones:

# One-time: record existing violations (SwiftLint 0.55+)
swiftlint lint --write-baseline .swiftlint-baseline.json

# The gate from then on — fails only on NEW violations
swiftlint lint --strict --baseline .swiftlint-baseline.json

Commit .swiftlint-baseline.json. Shrink it opportunistically: when touching a baselined function for other reasons, fix its violation and regenerate the baseline in the same commit.

Fallback for older SwiftLint (before 0.55, no baseline support): record the violation count instead —

swiftlint lint --quiet | wc -l > .swiftlint-count
# Gate: current count must not exceed the committed count

When a Rule Fires

The fix is structural, not suppressive:

• cyclomatic_complexity / function_body_length → extract the branches into named functions, or replace the branch ladder with a table/enum dispatch
• type_body_length / file_length → split by responsibility (extensions in separate files count separately)
• function_parameter_count → introduce a parameter struct

// swiftlint:disable is not a fix. The rare legitimate exemption (a generated file, a big-but-flat data table) belongs in .swiftlint.yml's excluded: list with a comment — visible in one place, not scattered through source.

Common Pitfalls

Pitfall	Problem	Solution
Enabling without a baseline	Day-one wall of violations, gate ignored	--write-baseline first, gate on new only
Mass-refactoring to satisfy thresholds	Working code broken for a number	Freeze old violations; fix opportunistically
Inline swiftlint:disable comments	Exemptions scattered and invisible	excluded: list in config, with reasons
Thresholds loosened per-project	Cap quietly stops capping	Change thresholds only with a written reason

References

• testing/fitness-functions/ — set/count invariants a per-line rule can't express
• testing/coverage-ratchet/ — the coverage half of the metrics gauntlet
• ios/coding-best-practices/ — iOS-specific lint fragment (style/API rules)
• generators/ci-cd-setup/ — base .swiftlint.yml template these fragments merge into