From c928ba282785297df6ac642969799680acf3b648 Mon Sep 17 00:00:00 2001
From: Billy Keyes <bluekeyes@gmail.com>
Date: Sat, 16 Mar 2019 22:42:14 -0700
Subject: [PATCH] Implement index header line parsing

---
 gitdiff/gitdiff.go |  4 +++-
 gitdiff/parser.go  | 28 ++++++++++++++++++++++++----
 2 files changed, 27 insertions(+), 5 deletions(-)

diff --git a/gitdiff/gitdiff.go b/gitdiff/gitdiff.go
index a63331b..c41e6f4 100644
--- a/gitdiff/gitdiff.go
+++ b/gitdiff/gitdiff.go
@@ -18,7 +18,9 @@ type File struct {
 	OldMode os.FileMode
 	NewMode os.FileMode
 
-	Score int
+	OldOID string
+	NewOID string
+	Score  int
 
 	Fragments []*Fragment
 }
diff --git a/gitdiff/parser.go b/gitdiff/parser.go
index dfad4ef..237edca 100644
--- a/gitdiff/parser.go
+++ b/gitdiff/parser.go
@@ -126,7 +126,7 @@ func (p *parser) ParseNextFileHeader() (file *File, err error) {
 // ParseFileChanges parses file changes until the next file header or the end
 // of the stream and attaches them to the given file.
 func (p *parser) ParseFileChanges(f *File) error {
-	panic("unimplemented")
+	panic("TODO(bkeyes): unimplemented")
 }
 
 func (p *parser) ParseGitFileHeader(f *File, header string) error {
@@ -153,7 +153,7 @@ func (p *parser) ParseGitFileHeader(f *File, header string) error {
 }
 
 func (p *parser) ParseTraditionalFileHeader(f *File, oldFile, newFile string) error {
-	panic("unimplemented")
+	panic("TODO(bkeyes): unimplemented")
 }
 
 // Line reads and returns the next line. The first call to Line after a call to
@@ -186,7 +186,7 @@ func (p *parser) Errorf(msg string, args ...interface{}) error {
 }
 
 func isMaybeFragmentHeader(line string) bool {
-	shortestValidHeader := "@@ -0,0 +1 @@\n"
+	const shortestValidHeader = "@@ -0,0 +1 @@\n"
 	return len(line) >= len(shortestValidHeader) && strings.HasPrefix(line, fragmentHeaderPrefix)
 }
 
@@ -351,7 +351,27 @@ func parseGitScore(f *File, line string) error {
 }
 
 func parseGitIndex(f *File, line string) error {
-	panic("TODO(bkeyes): unimplemented")
+	const minOIDSize = 40
+	const sep = ".."
+
+	parts := strings.SplitN(line, " ", 2)
+	oids := strings.SplitN(parts[0], sep, 2)
+
+	if len(oids) < 2 {
+		return fmt.Errorf("invalid index line: missing %q", sep)
+	}
+	if len(oids[0]) < minOIDSize {
+		return fmt.Errorf("invalid index line: invalid old OID")
+	}
+	if len(oids[1]) < minOIDSize {
+		return fmt.Errorf("invalid index line: invalid new OID")
+	}
+	f.OldOID, f.NewOID = oids[0], oids[1]
+
+	if len(parts) > 1 {
+		return parseGitOldMode(f, parts[1])
+	}
+	return nil
 }
 
 func parseMode(s string) (os.FileMode, error) {