move artifact package over

Signed-off-by: Meredith Lancaster <malancas@github.com>

pkg/cmd/attestation/artifact/artifact.go

@@ -0,0 +1,74 @@
+package artifact
+
+import (
+	"fmt"
+	"net/url"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"github.com/github/gh-attestation/pkg/artifact/oci"
+)
+
+type artifactType int
+
+const (
+	ociArtifactType artifactType = iota
+	fileArtifactType
+)
+
+// DigestedArtifact abstracts the software artifact being verified
+type DigestedArtifact struct {
+	URL       string
+	digest    string
+	digestAlg string
+}
+
+func normalizeReference(reference string, pathSeparator rune) (normalized string, artifactType artifactType, err error) {
+	switch {
+	case strings.HasPrefix(reference, "oci://"):
+		return reference[6:], ociArtifactType, nil
+	case strings.HasPrefix(reference, "file://"):
+		uri, err := url.ParseRequestURI(reference)
+		if err != nil {
+			return "", 0, fmt.Errorf("failed to parse reference URI: %w", err)
+		}
+		var path string
+		if pathSeparator == '/' {
+			// Unix paths use forward slashes like URIs, so no need to modify
+			path = uri.Path
+		} else {
+			// Windows paths should be normalized to use backslashes
+			path = strings.ReplaceAll(uri.Path, "/", string(pathSeparator))
+			// Remove leading slash from Windows paths if present
+			if strings.HasPrefix(path, string(pathSeparator)) {
+				path = path[1:]
+			}
+		}
+		return filepath.Clean(path), fileArtifactType, nil
+	}
+	// Treat any other reference as a local file path
+	return filepath.Clean(reference), fileArtifactType, nil
+}
+
+func NewDigestedArtifact(client oci.Client, reference, digestAlg string) (artifact *DigestedArtifact, err error) {
+	normalized, artifactType, err := normalizeReference(reference, os.PathSeparator)
+	if err != nil {
+		return nil, err
+	}
+	if artifactType == ociArtifactType {
+		// TODO: should we allow custom digestAlg for OCI artifacts?
+		return digestContainerImageArtifact(normalized, client)
+	}
+	return digestLocalFileArtifact(normalized, digestAlg)
+}
+
+// Digest returns the digest of the artifact
+func (a *DigestedArtifact) Digest() string {
+	return a.digest
+}
+
+// DigestWithAlg returns the digest:algorithm of the artifact
+func (a *DigestedArtifact) DigestWithAlg() string {
+	return fmt.Sprintf("%s:%s", a.digestAlg, a.digest)
+}