Lesson 08 · Separating the port

Separating the port

The last piece of the authority is the optional port after a colon. Today you split it off the end of the host, again tracking present-but-empty, which sets up the one tricky case the next lesson has to handle.

The goal

Split an optional port off the end of the host on the last colon.

Start here - the target

TO DO

Scenario: Host and port

Giventhe input "//example.com:8080/path"

WhenParse is called

ThenHost is "example.com", HasPort is true, and Port is "8080"

AndParse("//example.com:") has HasPort true with Port "", and Parse("//example.com") has HasPort false

Background

The port is a decimal number after the host, introduced by a colon: example.com:8080. Once the userinfo is stripped, whatever remains is host optionally followed by :port, so split on the colon - the host is before it, the port after. As with every optional piece, example.com: (a trailing colon with no digits) is an empty-but-present port, distinct from example.com with no port, so set HasPort the instant you see the colon.

Splitting on the colon works cleanly here because a plain registered name or IPv4 host contains no colons of its own. That is about to change. An IPv6 address is written with colons inside it - ::1 - so a naive “split on the colon” would mistake part of the address for a port. RFC 3986 solves this by wrapping IPv6 hosts in square brackets, and the next lesson teaches the parser to look for the closing bracket before it goes hunting for a port colon. For now, splitting on the last colon is the right rule for the hosts you have.

Make it work

// after userinfo is gone, a ':' introduces the port
if i := strings.LastIndexByte(a, ':'); i >= 0 {
  u.Host = a[:i]
  u.Port = a[i+1:]
  u.HasPort = true
} else {
  u.Host = a
}

CheckpointDONE

parseAuthority now separates host and port. Commit and stop here.