Lesson 20 · Ignoring a directory ignores its contents - Build a Glob and Gitignore Matcher

Ignoring a directory ignores its contents

When a directory is ignored, everything inside it is ignored too - and git will not let you re-include a file whose parent directory is excluded. Today Ignored checks a path's ancestors before deciding.

The goal

Make a path ignored when any ancestor directory is ignored.

Start here - the target

TO DO

Scenario: An ignored ancestor directory ignores everything under it

Givena rule that ignores a directory

WhenIgnored is asked about a path inside it

Thencontents are ignored: Compile("build/").Ignored("build/app.o", false) is true, and the directory itself Compile("build/").Ignored("build", true) is true

Andan excluded parent cannot be re-included: Compile("build/\n!build/app.o").Ignored("build/app.o", false) is true, while an unrelated path Compile("build/").Ignored("src/main.go", false) is false

Background

Ignoring a directory means ignoring its whole subtree - build/ does not just hide the build directory, it hides build/app.o and everything below. Git enforces this strictly: once a parent directory is excluded, you cannot re-include a file inside it with a ! rule, because git never even descends into an ignored directory to see the exception. This is the one place ordering does not save you.

The implementation is a short ancestor walk. Extract the last-match logic from the previous lesson into a helper - call it decide - that judges a single path. Then Ignored first asks decide about each ancestor directory of the path (each prefix, treated as a directory); if any is ignored, the answer is ignored, full stop. Only if no ancestor is excluded does it fall through to decide on the path itself. So build/app.o is ignored because build is, even alongside !build/app.o, while a path with no ignored ancestor is judged normally. This is the last piece of real gitignore behaviour - the capstones put it all together.

Make it work

// rename the last-match walk to decide(path, isDir); wrap it:
func (g *Gitignore) Ignored(path string, isDir bool) bool {
  segs := strings.Split(path, "/")
  for i := 1; i < len(segs); i++ {          // each ancestor directory
    if g.decide(strings.Join(segs[:i], "/"), true) {
      return true                            // parent excluded -> excluded
    }
  }
  return g.decide(path, isDir)
}

CheckpointDONE

A path under an ignored directory is itself ignored. Commit and stop here.