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.go

@@ -2,11 +2,14 @@
 package pkg
 
 import (
+	"bytes"
 	"crypto/sha512"
 	"encoding/base64"
+	"encoding/binary"
 	"errors"
 	"io"
 	"os"
+	"slices"
 	"sync"
 
 	"hakurei.app/container/check"
@@ -39,6 +42,11 @@ func MustDecode(s string) (checksum Checksum) {
 // deterministically but might not currently be available in memory or on the
 // filesystem.
 type Artifact interface {
+	// Kind returns the [Kind] of artifact. This is usually unique to the
+	// concrete type but two functionally identical implementations of
+	// [Artifact] is allowed to return the same [Kind] value.
+	Kind() Kind
+
 	// ID returns a globally unique identifier referring to the current
 	// [Artifact]. This value must be known ahead of time and guaranteed to be
 	// unique without having obtained the full contents of the [Artifact].
@@ -71,6 +79,39 @@ type File interface {
 	Artifact
 }
 
+// Kind corresponds to the concrete type of [Artifact] and is used to create
+// identifier for an [Artifact] with dependencies.
+type Kind uint64
+
+const (
+	// KindHTTP is the kind of [Artifact] returned by [Cache.NewHTTP].
+	KindHTTP Kind = iota
+	KindTar
+)
+
+// Ident returns a deterministic identifier for the supplied params and
+// dependencies. The caller is responsible for ensuring params uniquely and
+// deterministically describes the current [Artifact].
+func (k Kind) Ident(params []byte, deps ...Artifact) ID {
+	type extIdent [len(ID{}) + wordSize]byte
+	identifiers := make([]extIdent, len(deps))
+	for i, a := range deps {
+		id := a.ID()
+		copy(identifiers[i][wordSize:], id[:])
+		binary.LittleEndian.PutUint64(identifiers[i][:], uint64(a.Kind()))
+	}
+	slices.SortFunc(identifiers, func(a, b extIdent) int { return bytes.Compare(a[:], b[:]) })
+	slices.Compact(identifiers)
+
+	h := sha512.New384()
+	h.Write(binary.LittleEndian.AppendUint64(nil, uint64(k)))
+	h.Write(params)
+	for _, e := range identifiers {
+		h.Write(e[:])
+	}
+	return ID(h.Sum(nil))
+}
+
 const (
 	// dirIdentifier is the directory name appended to Cache.base for storing
 	// artifacts named after their [ID].