internal/pkg: compute identifier from deps

This provides infrastructure for computing a deterministic identifier based on current artifact kind, opaque parameters data, and optional dependency kind and identifiers.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pkg/pkg_test.go

@@ -18,6 +18,46 @@ import (
 	"hakurei.app/internal/pkg"
 )
 
+// A stubArtifact implements [Artifact] with hardcoded kind and identifier.
+type stubArtifact struct {
+	kind pkg.Kind
+	id   pkg.ID
+}
+
+func (a stubArtifact) Kind() pkg.Kind                     { return a.kind }
+func (a stubArtifact) ID() pkg.ID                         { return a.id }
+func (a stubArtifact) Hash() (pkg.Checksum, error)        { panic("unreachable") }
+func (a stubArtifact) Pathname() (*check.Absolute, error) { panic("unreachable") }
+
+func TestIdent(t *testing.T) {
+	t.Parallel()
+
+	testCases := []struct {
+		name   string
+		kind   pkg.Kind
+		params []byte
+		deps   []pkg.Artifact
+		want   pkg.ID
+	}{
+		{"tar", pkg.KindTar, []byte{
+			1, 0, 0, 0, 0, 0, 0, 0,
+		}, []pkg.Artifact{
+			stubArtifact{pkg.KindHTTP, pkg.ID{}},
+		}, pkg.MustDecode("HnySzeLQvSBZuTUcvfmLEX_OmH4yJWWH788NxuLuv7kVn8_uPM6Ks4rqFWM2NZJY")},
+	}
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			if got := tc.kind.Ident(tc.params, tc.deps...); got != tc.want {
+				t.Errorf("Ident: %s, want %s",
+					base64.URLEncoding.EncodeToString(got[:]),
+					base64.URLEncoding.EncodeToString(tc.want[:]))
+			}
+		})
+	}
+}
+
 // cacheTestCase is a test case passed to checkWithCache where a new instance
 // of [pkg.Cache] is prepared for the test case, and is validated and removed
 // on test completion.