add file source digest support (#1914)

Signed-off-by: Alex Goodman <wagoodman@users.noreply.github.com>

internal/file/digest.go

@@ -0,0 +1,76 @@
+package file
+
+import (
+	"crypto"
+	"fmt"
+	"hash"
+	"io"
+	"strings"
+
+	"github.com/anchore/syft/syft/file"
+)
+
+func supportedHashAlgorithms() []crypto.Hash {
+	return []crypto.Hash{
+		crypto.MD5,
+		crypto.SHA1,
+		crypto.SHA224,
+		crypto.SHA256,
+		crypto.SHA384,
+		crypto.SHA512,
+	}
+}
+
+func NewDigestsFromFile(closer io.ReadCloser, hashes []crypto.Hash) ([]file.Digest, error) {
+	// create a set of hasher objects tied together with a single writer to feed content into
+	hashers := make([]hash.Hash, len(hashes))
+	writers := make([]io.Writer, len(hashes))
+	for idx, hashObj := range hashes {
+		hashers[idx] = hashObj.New()
+		writers[idx] = hashers[idx]
+	}
+
+	size, err := io.Copy(io.MultiWriter(writers...), closer)
+	if err != nil {
+		return nil, err
+	}
+
+	if size == 0 {
+		return make([]file.Digest, 0), nil
+	}
+
+	result := make([]file.Digest, len(hashes))
+	// only capture digests when there is content. It is important to do this based on SIZE and not
+	// FILE TYPE. The reasoning is that it is possible for a tar to be crafted with a header-only
+	// file type but a body is still allowed.
+	for idx, hasher := range hashers {
+		result[idx] = file.Digest{
+			Algorithm: CleanDigestAlgorithmName(hashes[idx].String()),
+			Value:     fmt.Sprintf("%+x", hasher.Sum(nil)),
+		}
+	}
+
+	return result, nil
+}
+
+func Hashers(names ...string) ([]crypto.Hash, error) {
+	hashByName := make(map[string]crypto.Hash)
+	for _, h := range supportedHashAlgorithms() {
+		hashByName[CleanDigestAlgorithmName(h.String())] = h
+	}
+
+	var hashers []crypto.Hash
+	for _, hashStr := range names {
+		hashObj, ok := hashByName[CleanDigestAlgorithmName(hashStr)]
+		if !ok {
+			return nil, fmt.Errorf("unsupported hash algorithm: %s", hashStr)
+		}
+		hashers = append(hashers, hashObj)
+	}
+	return hashers, nil
+}
+
+func CleanDigestAlgorithmName(name string) string {
+	lower := strings.ToLower(name)
+	return strings.ReplaceAll(lower, "-", "")
+}