feat(imageref): parse image references into repo, tag and digest

internal/imageref/imageref.go

@@ -0,0 +1,43 @@
+// Package imageref splits container image references into repository, tag
+// and digest.
+package imageref
+
+import "strings"
+
+// Ref is an image reference split into its parts. Tag and Digest are empty
+// when the reference has none.
+type Ref struct {
+	Repo   string
+	Tag    string
+	Digest string
+}
+
+// Parse splits ref into repository, tag and digest. A colon before the last
+// "/" belongs to a registry host and port ("localhost:5000/foo"), not a tag.
+func Parse(ref string) Ref {
+	var r Ref
+	if at := strings.Index(ref, "@"); at >= 0 {
+		r.Digest = ref[at+1:]
+		ref = ref[:at]
+	}
+	slash := strings.LastIndex(ref, "/")
+	last := ref[slash+1:]
+	if colon := strings.Index(last, ":"); colon >= 0 {
+		r.Tag = last[colon+1:]
+		ref = ref[:slash+1+colon]
+	}
+	r.Repo = ref
+	return r
+}
+
+// String reassembles the reference as repo[:tag][@digest].
+func (r Ref) String() string {
+	s := r.Repo
+	if r.Tag != "" {
+		s += ":" + r.Tag
+	}
+	if r.Digest != "" {
+		s += "@" + r.Digest
+	}
+	return s
+}

internal/imageref/imageref_test.go

@@ -0,0 +1,29 @@
+package imageref
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestParse(t *testing.T) {
+	cases := []struct {
+		in   string
+		want Ref
+	}{
+		{"postgres", Ref{Repo: "postgres"}},
+		{"postgres:17.10", Ref{Repo: "postgres", Tag: "17.10"}},
+		{"gitea/gitea:1.27.3@sha256:87a6", Ref{Repo: "gitea/gitea", Tag: "1.27.3", Digest: "sha256:87a6"}},
+		{"lscr.io/linuxserver/jellyfin@sha256:0f42", Ref{Repo: "lscr.io/linuxserver/jellyfin", Digest: "sha256:0f42"}},
+		{"localhost:5000/foo", Ref{Repo: "localhost:5000/foo"}},
+		{"localhost:5000/foo:v1", Ref{Repo: "localhost:5000/foo", Tag: "v1"}},
+		{"localhost:5000/foo:v1@sha256:ab", Ref{Repo: "localhost:5000/foo", Tag: "v1", Digest: "sha256:ab"}},
+	}
+	for _, c := range cases {
+		t.Run(c.in, func(t *testing.T) {
+			got := Parse(c.in)
+			assert.Equal(t, c.want, got)
+			assert.Equal(t, c.in, got.String(), "String must round-trip")
+		})
+	}
+}