internal/pkg: standardise artifact IR

This should hopefully provide good separation between the artifact curing backend implementation and the (still work in progress) language. Making the IR parseable also guarantees uniqueness of the representation.

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

internal/pkg/tar.go

@@ -4,7 +4,6 @@ import (
 	"archive/tar"
 	"compress/bzip2"
 	"compress/gzip"
-	"encoding/binary"
 	"errors"
 	"fmt"
 	"io"
@@ -29,7 +28,7 @@ type tarArtifact struct {
 	// Caller-supplied backing tarball.
 	f Artifact
 	// Compression on top of the tarball.
-	compression uint64
+	compression uint32
 }
 
 // tarArtifactNamed embeds tarArtifact for a [fmt.Stringer] tarball.
@@ -47,7 +46,7 @@ func (a *tarArtifactNamed) String() string { return a.name + "-unpack" }
 // NewTar returns a new [Artifact] backed by the supplied [Artifact] and
 // compression method. The source [Artifact] must be compatible with
 // [TContext.Open].
-func NewTar(a Artifact, compression uint64) Artifact {
+func NewTar(a Artifact, compression uint32) Artifact {
 	ta := tarArtifact{a, compression}
 	if s, ok := a.(fmt.Stringer); ok {
 		if name := s.String(); name != "" {
@@ -62,7 +61,7 @@ func NewHTTPGetTar(
 	hc *http.Client,
 	url string,
 	checksum Checksum,
-	compression uint64,
+	compression uint32,
 ) Artifact {
 	return NewTar(NewHTTPGet(hc, url, checksum), compression)
 }
@@ -71,8 +70,16 @@ func NewHTTPGetTar(
 func (a *tarArtifact) Kind() Kind { return KindTar }
 
 // Params writes compression encoded in little endian.
-func (a *tarArtifact) Params(ctx *IContext) {
-	ctx.GetHash().Write(binary.LittleEndian.AppendUint64(nil, a.compression))
+func (a *tarArtifact) Params(ctx *IContext) { ctx.WriteUint32(a.compression) }
+
+func init() {
+	register(KindTar, func(r *IRReader) Artifact {
+		a := NewTar(r.Next(), r.ReadUint32())
+		if _, ok := r.Finalise(); ok {
+			panic(ErrUnexpectedChecksum)
+		}
+		return a
+	})
 }
 
 // Dependencies returns a slice containing the backing file.