1 files changed, 100 insertions, 0 deletions
diff --git a/pkg/file.go b/pkg/file.go
new file mode 100644
index 00000000..14467153
--- /dev/null
+++ b/pkg/file.go
@@ -0,0 +1,100 @@
+package pkg
+
+import (
+	"bytes"
+	"crypto/sha512"
+	"fmt"
+	"io"
+)
+
+// A fileArtifact is an [Artifact] that cures into data known ahead of time.
+type fileArtifact []byte
+
+var _ KnownChecksum = new(fileArtifact)
+var _ CuresExempt = new(fileArtifact)
+var _ RevisionArtifact = new(fileArtifact)
+
+// fileArtifactNamed embeds fileArtifact alongside a caller-supplied name.
+type fileArtifactNamed struct {
+	fileArtifact
+	// Caller-supplied user-facing reporting name.
+	name string
+}
+
+var _ fmt.Stringer = new(fileArtifactNamed)
+var _ KnownChecksum = new(fileArtifactNamed)
+
+// String returns the caller-supplied reporting name.
+func (a *fileArtifactNamed) String() string { return a.name }
+
+// Params writes the caller-supplied reporting name and the file body.
+func (a *fileArtifactNamed) Params(ctx *IContext) {
+	ctx.WriteString(a.name)
+	ctx.Write(a.fileArtifact)
+}
+
+// NewFile returns a [FileArtifact] that cures into a caller-supplied byte slice.
+//
+// Caller must not modify data after NewFile returns.
+func NewFile(name string, data []byte) FileArtifact {
+	f := fileArtifact(data)
+	if name != "" {
+		return &fileArtifactNamed{f, name}
+	}
+	return &f
+}
+
+// Kind returns the hardcoded [Kind] constant.
+func (*fileArtifact) Kind() Kind { return KindFile }
+
+// Params writes an empty string and the file body.
+func (a *fileArtifact) Params(ctx *IContext) {
+	ctx.WriteString("")
+	ctx.Write(*a)
+}
+
+func init() {
+	register(KindFile, func(r *IRReader) Artifact {
+		name := r.ReadString()
+		data := r.ReadStringBytes()
+		if _, ok := r.Finalise(); !ok {
+			panic(ErrExpectedChecksum)
+		}
+		return NewFile(name, data)
+	})
+}
+
+// Inputs returns a nil slice.
+func (*fileArtifact) Inputs() []Artifact { return nil }
+
+// IsExclusive returns false: Cure returns a prepopulated buffer.
+func (*fileArtifact) IsExclusive() bool { return false }
+
+// 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))
+}
+
+// Revision satisfies [RevisionArtifact] for incompatible IsExecutable behaviour.
+func (*fileArtifact) Revision() uint64 { return 0 }
+
+// IsExecutable returns whether the contents begin with shebang or the ELF
+// magic number.
+func (a *fileArtifact) IsExecutable() bool {
+	return a != nil &&
+		(bytes.HasPrefix(
+			*a, []byte{'#', '!'},
+		) || bytes.HasPrefix(
+			*a, []byte{0x7f, 'E', 'L', 'F'},
+		))
+}
+
+// Cure returns the caller-supplied data.
+func (a *fileArtifact) Cure(*RContext) (io.ReadCloser, error) {
+	return io.NopCloser(bytes.NewReader(*a)), nil
+}
+
+// CuresExempt exempts the cheap [KindFile] implementation.
+func (*fileArtifact) CuresExempt() {}