1 files changed, 230 insertions, 0 deletions
diff --git a/pkg/gitdiff/gitdiff.go b/pkg/gitdiff/gitdiff.go
new file mode 100644
index 0000000..5365645
--- /dev/null
+++ b/pkg/gitdiff/gitdiff.go
@@ -0,0 +1,230 @@
+package gitdiff
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"strings"
+)
+
+// File describes changes to a single file. It can be either a text file or a
+// binary file.
+type File struct {
+	OldName string
+	NewName string
+
+	IsNew    bool
+	IsDelete bool
+	IsCopy   bool
+	IsRename bool
+
+	OldMode os.FileMode
+	NewMode os.FileMode
+
+	OldOIDPrefix string
+	NewOIDPrefix string
+	Score        int
+
+	// TextFragments contains the fragments describing changes to a text file. It
+	// may be empty if the file is empty or if only the mode changes.
+	TextFragments []*TextFragment
+
+	// IsBinary is true if the file is a binary file. If the patch includes
+	// binary data, BinaryFragment will be non-nil and describe the changes to
+	// the data. If the patch is reversible, ReverseBinaryFragment will also be
+	// non-nil and describe the changes needed to restore the original file
+	// after applying the changes in BinaryFragment.
+	IsBinary              bool
+	BinaryFragment        *BinaryFragment
+	ReverseBinaryFragment *BinaryFragment
+}
+
+// String returns a git diff representation of this file. The value can be
+// parsed by this library to obtain the same File, but may not be the same as
+// the original input.
+func (f *File) String() string {
+	var diff strings.Builder
+	newFormatter(&diff).FormatFile(f)
+	return diff.String()
+}
+
+// TextFragment describes changed lines starting at a specific line in a text file.
+type TextFragment struct {
+	Comment string
+
+	OldPosition int64
+	OldLines    int64
+
+	NewPosition int64
+	NewLines    int64
+
+	LinesAdded   int64
+	LinesDeleted int64
+
+	LeadingContext  int64
+	TrailingContext int64
+
+	Lines []Line
+}
+
+// String returns a git diff format of this fragment. See [File.String] for
+// more details on this format.
+func (f *TextFragment) String() string {
+	var diff strings.Builder
+	newFormatter(&diff).FormatTextFragment(f)
+	return diff.String()
+}
+
+// Header returns a git diff header of this fragment. See [File.String] for
+// more details on this format.
+func (f *TextFragment) Header() string {
+	var hdr strings.Builder
+	newFormatter(&hdr).FormatTextFragmentHeader(f)
+	return hdr.String()
+}
+
+// Validate checks that the fragment is self-consistent and appliable. Validate
+// returns an error if and only if the fragment is invalid.
+func (f *TextFragment) Validate() error {
+	if f == nil {
+		return errors.New("nil fragment")
+	}
+
+	var (
+		oldLines, newLines                     int64
+		leadingContext, trailingContext        int64
+		contextLines, addedLines, deletedLines int64
+	)
+
+	// count the types of lines in the fragment content
+	for i, line := range f.Lines {
+		switch line.Op {
+		case OpContext:
+			oldLines++
+			newLines++
+			contextLines++
+			if addedLines == 0 && deletedLines == 0 {
+				leadingContext++
+			} else {
+				trailingContext++
+			}
+		case OpAdd:
+			newLines++
+			addedLines++
+			trailingContext = 0
+		case OpDelete:
+			oldLines++
+			deletedLines++
+			trailingContext = 0
+		default:
+			return fmt.Errorf("unknown operator %q on line %d", line.Op, i+1)
+		}
+	}
+
+	// check the actual counts against the reported counts
+	if oldLines != f.OldLines {
+		return lineCountErr("old", oldLines, f.OldLines)
+	}
+	if newLines != f.NewLines {
+		return lineCountErr("new", newLines, f.NewLines)
+	}
+	if leadingContext != f.LeadingContext {
+		return lineCountErr("leading context", leadingContext, f.LeadingContext)
+	}
+	if trailingContext != f.TrailingContext {
+		return lineCountErr("trailing context", trailingContext, f.TrailingContext)
+	}
+	if addedLines != f.LinesAdded {
+		return lineCountErr("added", addedLines, f.LinesAdded)
+	}
+	if deletedLines != f.LinesDeleted {
+		return lineCountErr("deleted", deletedLines, f.LinesDeleted)
+	}
+
+	// if a file is being created, it can only contain additions
+	if f.OldPosition == 0 && f.OldLines != 0 {
+		return errors.New("file creation fragment contains context or deletion lines")
+	}
+
+	return nil
+}
+
+func lineCountErr(kind string, actual, reported int64) error {
+	return fmt.Errorf("fragment contains %d %s lines but reports %d", actual, kind, reported)
+}
+
+// Line is a line in a text fragment.
+type Line struct {
+	Op   LineOp
+	Line string
+}
+
+func (fl Line) String() string {
+	return fl.Op.String() + fl.Line
+}
+
+// Old returns true if the line appears in the old content of the fragment.
+func (fl Line) Old() bool {
+	return fl.Op == OpContext || fl.Op == OpDelete
+}
+
+// New returns true if the line appears in the new content of the fragment.
+func (fl Line) New() bool {
+	return fl.Op == OpContext || fl.Op == OpAdd
+}
+
+// NoEOL returns true if the line is missing a trailing newline character.
+func (fl Line) NoEOL() bool {
+	return len(fl.Line) == 0 || fl.Line[len(fl.Line)-1] != '\n'
+}
+
+// LineOp describes the type of a text fragment line: context, added, or removed.
+type LineOp int
+
+const (
+	// OpContext indicates a context line
+	OpContext LineOp = iota
+	// OpDelete indicates a deleted line
+	OpDelete
+	// OpAdd indicates an added line
+	OpAdd
+)
+
+func (op LineOp) String() string {
+	switch op {
+	case OpContext:
+		return " "
+	case OpDelete:
+		return "-"
+	case OpAdd:
+		return "+"
+	}
+	return "?"
+}
+
+// BinaryFragment describes changes to a binary file.
+type BinaryFragment struct {
+	Method BinaryPatchMethod
+	Size   int64
+	Data   []byte
+}
+
+// BinaryPatchMethod is the method used to create and apply the binary patch.
+type BinaryPatchMethod int
+
+const (
+	// BinaryPatchDelta indicates the data uses Git's packfile encoding
+	BinaryPatchDelta BinaryPatchMethod = iota
+	// BinaryPatchLiteral indicates the data is the exact file content
+	BinaryPatchLiteral
+)
+
+// String returns a git diff format of this fragment. Due to differences in
+// zlib implementation between Go and Git, encoded binary data in the result
+// will likely differ from what Git produces for the same input. See
+// [File.String] for more details on this format.
+func (f *BinaryFragment) String() string {
+	var diff strings.Builder
+	newFormatter(&diff).FormatBinaryFragment(f)
+	return diff.String()
+}