Improve fragment validation, track context lines

Track leading and trailing context lines because it's easy and Git
includes this information in the fragment type. Add validation for when
the fragment does not agree with the header or with the new/deleted
state of the file.

gitdiff/gitdiff.go

@@ -39,6 +39,9 @@ type Fragment struct {
 	LinesAdded   int64
 	LinesDeleted int64
 
+	LeadingContext  int64
+	TrailingContext int64
+
 	Lines []FragmentLine
 }
 

gitdiff/parser.go

@@ -107,7 +107,29 @@ func (p *parser) ParseNextFileHeader() (*File, error) {
 // ParseFragments parses fragments until the next file header or the end of the
 // stream and attaches them to the given file.
 func (p *parser) ParseFragments(f *File) error {
-	panic("TODO(bkeyes): unimplemented")
+	for {
+		frag, err := p.ParseFragment()
+		if err != nil {
+			return err
+		}
+		if frag == nil {
+			// TODO(bkeyes): this could mean several things:
+			//  - binary patch
+			//  - reached the next file header
+			//  - reached the end of the patch
+			return nil
+		}
+
+		lines := int64(len(frag.Lines) + 1) // +1 for the header
+		if f.IsNew && frag.OldLines > 0 {
+			return p.Errorf(-lines, "new file depends on old contents")
+		}
+		if f.IsDelete && frag.NewLines > 0 {
+			return p.Errorf(-lines, "deleted file still has contents")
+		}
+
+		f.Fragments = append(f.Fragments, frag)
+	}
 }
 
 // Next advances the parser by one line. It returns any error encountered while
@@ -211,7 +233,7 @@ func (p *parser) ParseFragment() (*Fragment, error) {
 	}
 
 	oldLines, newLines := frag.OldLines, frag.NewLines
-	for oldLines > 0 && newLines > 0 {
+	for oldLines > 0 || newLines > 0 {
 		line := p.Line(0)
 		switch line[0] {
 		case '\n':
@@ -219,14 +241,21 @@ func (p *parser) ParseFragment() (*Fragment, error) {
 		case ' ':
 			oldLines--
 			newLines--
+			if frag.LinesAdded == 0 && frag.LinesDeleted == 0 {
+				frag.LeadingContext++
+			} else {
+				frag.TrailingContext++
+			}
 			frag.Lines = append(frag.Lines, FragmentLine{OpContext, line[1:]})
 		case '-':
 			frag.LinesDeleted++
 			oldLines--
+			frag.TrailingContext = 0
 			frag.Lines = append(frag.Lines, FragmentLine{OpDelete, line[1:]})
 		case '+':
 			frag.LinesAdded++
 			newLines--
+			frag.TrailingContext = 0
 			frag.Lines = append(frag.Lines, FragmentLine{OpAdd, line[1:]})
 		default:
 			// this could be "\ No newline at end of file", which we allow
@@ -246,7 +275,9 @@ func (p *parser) ParseFragment() (*Fragment, error) {
 		}
 	}
 
-	// TODO(bkeyes): verify stuff about the fragment
+	if oldLines != 0 || newLines != 0 {
+		return nil, p.Errorf(0, "invalid fragment: remaining lines: %d old, %d new", oldLines, newLines)
+	}
 
 	return frag, nil
 }