Lesson 14 · Classifying the path

Classifying the path

The path is the one component always present, and its first character decides how it is interpreted during resolution. Today you classify a path as absolute, rootless, or empty, the distinction the reference-resolution algorithm hinges on later.

The goal

Classify a path as absolute, rootless, or empty.

Start here - the target

TO DO

Scenario: Which kind of path

Giventhe paths "/a/b", "a/b", and the empty string

Wheneach path is classified

Then"/a/b" is absolute, "a/b" is rootless, and "" is empty

Andat the boundary, a lone "/" is absolute and a lone "." is rootless, since a path is absolute exactly when it begins with a slash

Background

The path holds the hierarchical identifier - the /over/there of a URL. Unlike the other components it is never absent; the most it can be is empty. RFC 3986 distinguishes a handful of path forms, but the one that matters for what follows is simple: a path is absolute if it starts with a /, rootless (relative) if it is non-empty and does not, and empty if it has no characters at all.

This three-way split feels trivial today but it is exactly what the resolution algorithm branches on. When you resolve a relative reference against a base, an empty reference path means “keep the base’s path”, an absolute reference path replaces it wholesale, and a rootless path gets merged onto the base’s directory. Each of those is a different code path selected by this classification, so pinning it down now - with the rule stated as plainly as “absolute iff it begins with a slash” - gives the resolver a clean predicate to lean on. The dot-segment cleanup you build next also cares about the leading slash.

Make it work

type PathKind int
const ( PathEmpty PathKind = iota; PathAbsolute; PathRootless )
func classifyPath(p string) PathKind {
  // empty? starts with '/'? otherwise rootless
}

CheckpointDONE

A path now reports whether it is absolute, rootless, or empty. Commit and stop here.