crap4go

CRAP (Change Risk Anti-Pattern) metric for Go projects.

Combines cyclomatic complexity with test coverage to identify functions and methods that are both complex and under-tested - the riskiest code to change.

Quick Start

Install or run with go run:

go run github.com/unclebob/crap4go/cmd/crap4go@latest

From a Go module:

crap4go    # deletes old coverage, runs go test ./..., analyzes

crap4go automatically deletes stale coverage reports, runs:

go test ./... -coverprofile=target/coverage/coverage.out

and then analyzes the results.

Override the coverage test command when a project needs a narrower or tagged test invocation:

crap4go --test-command "go test -tags appunit ./internal/app ./internal/sim"

crap4go appends -coverprofile=target/coverage/coverage.out to the command. If you need to control where the coverage flag appears, include {coverprofile} in the command:

crap4go --test-command "go test ./internal/app {coverprofile} -run TestEditor"

Source-file analysis runs in parallel and defaults to half the available logical CPUs. Override it when needed:

crap4go --max-workers 4

Output

CRAP Report
===========
Function                       Package                               CC   Cov%     CRAP
-------------------------------------------------------------------------------------
Widget.Run                     widget                                12   45.0%    130.2
simple                         widget                                 1  100.0%      1.0

Filtering

Pass module name fragments as arguments to filter:

crap4go internal/combat movement    # only files matching those path fragments

Coverage Mapping Notes

crap4go reads Go coverage profiles from target/coverage/coverage.out. Coverage is matched to functions by source file and line range. If go test emits module-qualified source paths, crap4go also matches by path suffix so normal local paths still resolve.

Files skipped during analysis:

• _test.go files
• target/
• vendor/
• .git/

CRAP Formula

CRAP(fn) = CC² x (1 - coverage)³ + CC

• CC = cyclomatic complexity (decision points + 1)
• coverage = fraction of covered Go coverage statements in the function range

Score	Risk
1-5	Low - clean code
5-30	Moderate - refactor or add tests
30+	High - complex and under-tested

What It Counts

Decision points that increase cyclomatic complexity:

• if
• for
• range
• switch and type switch case clauses
• select communication clauses
• &&
• ||

Claude Code Skill

crap4go includes a SKILL.md for use as a Claude Code skill. Add it to your project's .claude/settings.json:

{
  "skills": [
    "https://github.com/unclebob/crap4go/blob/master/SKILL.md"
  ]
}

Then ask Claude Code for a "CRAP report" and it will know how to set up and run the tool.

Development

go test ./...                          # run tests
go run ./cmd/crap4go                   # run on own source
go run ./cmd/crap4go internal/coverage # filter to one module

License

Copyright (c) Robert C. Martin. All rights reserved.