internal/pkg: implement file artifact

This is an Artifact implementing File, backed by a constant, caller-supplied byte slice.

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

internal/pkg/file.go

@@ -0,0 +1,40 @@
+package pkg
+
+import (
+	"crypto/sha512"
+	"syscall"
+)
+
+// A fileArtifact is an [Artifact] that cures into data known ahead of time.
+type fileArtifact []byte
+
+var _ KnownChecksum = fileArtifact{}
+
+// NewFile returns a [File] that cures into a caller-supplied byte slice.
+//
+// Caller must not modify data after NewBin returns.
+func NewFile(data []byte) File { return fileArtifact(data) }
+
+// Kind returns the hardcoded [Kind] constant.
+func (a fileArtifact) Kind() Kind { return KindFile }
+
+// Params returns the result of Data.
+func (a fileArtifact) Params() []byte { return a }
+
+// Dependencies returns a nil slice.
+func (a fileArtifact) Dependencies() []Artifact { return nil }
+
+// Checksum computes and returns the checksum of caller-supplied data.
+func (a fileArtifact) Checksum() Checksum {
+	h := sha512.New384()
+	h.Write(a)
+	return Checksum(h.Sum(nil))
+}
+
+// Data returns the caller-supplied data.
+func (a fileArtifact) Data() ([]byte, error) { return a, nil }
+
+// Cure returns syscall.ENOTSUP. Callers should use Data instead.
+func (a fileArtifact) Cure(*CureContext) (err error) {
+	return syscall.ENOTSUP
+}