affected

Detect which packages in a monorepo are affected by recent changes.

Steps

1. Detect the monorepo tool in use (Turborepo, Nx, Lerna, pnpm workspaces, Cargo workspace).
2. Get the list of changed files since the comparison point:
   • Default: git diff --name-only origin/main...HEAD.
   • Or since a specific commit/tag if provided.
3. Map changed files to packages:
   • Read workspace configuration to map directories to packages.
   • For each changed file, determine which package it belongs to.
4. Build the dependency graph:
   • Parse package.json (or equivalent) for each package.
   • Map inter-package dependencies.
5. Calculate the full affected set:
   • Direct: packages with changed files.
   • Transitive: packages that depend on directly affected packages.
6. Determine which tasks need to run:
   • Build: affected packages and their dependents.
   • Test: affected packages.
   • Lint: only directly changed packages.
7. Output the affected package list with recommended actions.

Format

Affected Packages (since <comparison>)

Changed files: 

Directly affected:
  - @scope/pkg-a (3 files changed)
  - @scope/pkg-b (1 file changed)

Transitively affected:
  - @scope/pkg-c (depends on pkg-a)

Recommended:
  build: @scope/pkg-a @scope/pkg-b @scope/pkg-c
  test:  @scope/pkg-a @scope/pkg-b
  lint:  @scope/pkg-a @scope/pkg-b

Rules

• Always include transitive dependents in the build affected set.
• Changes to shared config files (tsconfig, eslint) affect all packages.
• Changes to root package.json or lock files affect all packages.
• Use the native monorepo tool's affected detection if available.
• Support filtering by task type (build, test, lint, deploy).