1 files changed, 94 insertions, 0 deletions
diff --git a/object/commit/parse.go b/object/commit/parse.go
new file mode 100644
index 00000000..d0257003
--- /dev/null
+++ b/object/commit/parse.go
@@ -0,0 +1,94 @@
+package commit
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+
+	"codeberg.org/lindenii/furgit/object/id"
+	"codeberg.org/lindenii/furgit/object/signature"
+)
+
+// Parse decodes a commit object body.
+func Parse(body []byte, algo id.Algorithm) (*Commit, error) {
+	c := new(Commit)
+
+	i := 0
+	for i < len(body) {
+		rel := bytes.IndexByte(body[i:], '\n')
+		if rel < 0 {
+			return nil, errors.New("object: commit: missing newline")
+		}
+
+		line := body[i : i+rel]
+		i += rel + 1
+
+		if len(line) == 0 {
+			break
+		}
+
+		key, value, found := bytes.Cut(line, []byte{' '})
+		if !found {
+			return nil, errors.New("object: commit: malformed header")
+		}
+
+		switch string(key) {
+		case "tree":
+			id, err := id.FromHex(algo, string(value))
+			if err != nil {
+				return nil, fmt.Errorf("object: commit: tree: %w", err)
+			}
+
+			c.Tree = id
+		case "parent":
+			id, err := id.FromHex(algo, string(value))
+			if err != nil {
+				return nil, fmt.Errorf("object: commit: parent: %w", err)
+			}
+
+			c.Parents = append(c.Parents, id)
+		case "author":
+			idt, err := signature.Parse(value)
+			if err != nil {
+				return nil, fmt.Errorf("object: commit: author: %w", err)
+			}
+
+			c.Author = *idt
+		case "committer":
+			idt, err := signature.Parse(value)
+			if err != nil {
+				return nil, fmt.Errorf("object: commit: committer: %w", err)
+			}
+
+			c.Committer = *idt
+		case "change-id":
+			c.ChangeID = string(value)
+		case "gpgsig", "gpgsig-sha256":
+			for i < len(body) {
+				nextRel := bytes.IndexByte(body[i:], '\n')
+				if nextRel < 0 {
+					return nil, errors.New("object: commit: unterminated gpgsig")
+				}
+
+				if body[i] != ' ' {
+					break
+				}
+
+				i += nextRel + 1
+			}
+		default:
+			c.ExtraHeaders = append(c.ExtraHeaders, ExtraHeader{
+				Key:   string(key),
+				Value: append([]byte(nil), value...),
+			})
+		}
+	}
+
+	if i > len(body) {
+		return nil, errors.New("object: commit: parser position out of bounds")
+	}
+
+	c.Message = append([]byte(nil), body[i:]...)
+
+	return c, nil
+}