1 files changed, 281 insertions, 0 deletions
diff --git a/internal/gitdiff/format.go b/internal/gitdiff/format.go
new file mode 100644
index 0000000..d97aba9
--- /dev/null
+++ b/internal/gitdiff/format.go
@@ -0,0 +1,281 @@
+package gitdiff
+
+import (
+	"bytes"
+	"compress/zlib"
+	"fmt"
+	"io"
+	"strconv"
+)
+
+type formatter struct {
+	w   io.Writer
+	err error
+}
+
+func newFormatter(w io.Writer) *formatter {
+	return &formatter{w: w}
+}
+
+func (fm *formatter) Write(p []byte) (int, error) {
+	if fm.err != nil {
+		return len(p), nil
+	}
+	if _, err := fm.w.Write(p); err != nil {
+		fm.err = err
+	}
+	return len(p), nil
+}
+
+func (fm *formatter) WriteString(s string) (int, error) {
+	fm.Write([]byte(s))
+	return len(s), nil
+}
+
+func (fm *formatter) WriteByte(c byte) error {
+	fm.Write([]byte{c})
+	return nil
+}
+
+func (fm *formatter) WriteQuotedName(s string) {
+	qpos := 0
+	for i := 0; i < len(s); i++ {
+		ch := s[i]
+		if q, quoted := quoteByte(ch); quoted {
+			if qpos == 0 {
+				fm.WriteByte('"')
+			}
+			fm.WriteString(s[qpos:i])
+			fm.Write(q)
+			qpos = i + 1
+		}
+	}
+	fm.WriteString(s[qpos:])
+	if qpos > 0 {
+		fm.WriteByte('"')
+	}
+}
+
+var quoteEscapeTable = map[byte]byte{
+	'\a': 'a',
+	'\b': 'b',
+	'\t': 't',
+	'\n': 'n',
+	'\v': 'v',
+	'\f': 'f',
+	'\r': 'r',
+	'"':  '"',
+	'\\': '\\',
+}
+
+func quoteByte(b byte) ([]byte, bool) {
+	if q, ok := quoteEscapeTable[b]; ok {
+		return []byte{'\\', q}, true
+	}
+	if b < 0x20 || b >= 0x7F {
+		return []byte{
+			'\\',
+			'0' + (b>>6)&0o3,
+			'0' + (b>>3)&0o7,
+			'0' + (b>>0)&0o7,
+		}, true
+	}
+	return nil, false
+}
+
+func (fm *formatter) FormatFile(f *File) {
+	fm.WriteString("diff --git ")
+
+	var aName, bName string
+	switch {
+	case f.OldName == "":
+		aName = f.NewName
+		bName = f.NewName
+
+	case f.NewName == "":
+		aName = f.OldName
+		bName = f.OldName
+
+	default:
+		aName = f.OldName
+		bName = f.NewName
+	}
+
+	fm.WriteQuotedName("a/" + aName)
+	fm.WriteByte(' ')
+	fm.WriteQuotedName("b/" + bName)
+	fm.WriteByte('\n')
+
+	if f.OldMode != 0 {
+		if f.IsDelete {
+			fmt.Fprintf(fm, "deleted file mode %o\n", f.OldMode)
+		} else if f.NewMode != 0 {
+			fmt.Fprintf(fm, "old mode %o\n", f.OldMode)
+		}
+	}
+
+	if f.NewMode != 0 {
+		if f.IsNew {
+			fmt.Fprintf(fm, "new file mode %o\n", f.NewMode)
+		} else if f.OldMode != 0 {
+			fmt.Fprintf(fm, "new mode %o\n", f.NewMode)
+		}
+	}
+
+	if f.Score > 0 {
+		if f.IsCopy || f.IsRename {
+			fmt.Fprintf(fm, "similarity index %d%%\n", f.Score)
+		} else {
+			fmt.Fprintf(fm, "dissimilarity index %d%%\n", f.Score)
+		}
+	}
+
+	if f.IsCopy {
+		if f.OldName != "" {
+			fm.WriteString("copy from ")
+			fm.WriteQuotedName(f.OldName)
+			fm.WriteByte('\n')
+		}
+		if f.NewName != "" {
+			fm.WriteString("copy to ")
+			fm.WriteQuotedName(f.NewName)
+			fm.WriteByte('\n')
+		}
+	}
+
+	if f.IsRename {
+		if f.OldName != "" {
+			fm.WriteString("rename from ")
+			fm.WriteQuotedName(f.OldName)
+			fm.WriteByte('\n')
+		}
+		if f.NewName != "" {
+			fm.WriteString("rename to ")
+			fm.WriteQuotedName(f.NewName)
+			fm.WriteByte('\n')
+		}
+	}
+
+	if f.OldOIDPrefix != "" && f.NewOIDPrefix != "" {
+		fmt.Fprintf(fm, "index %s..%s", f.OldOIDPrefix, f.NewOIDPrefix)
+
+		// Mode is only included on the index line when it is not changing
+		if f.OldMode != 0 && ((f.NewMode == 0 && !f.IsDelete) || f.OldMode == f.NewMode) {
+			fmt.Fprintf(fm, " %o", f.OldMode)
+		}
+
+		fm.WriteByte('\n')
+	}
+
+	if f.IsBinary {
+		if f.BinaryFragment == nil {
+			fm.WriteString("Binary files ")
+			fm.WriteQuotedName("a/" + aName)
+			fm.WriteString(" and ")
+			fm.WriteQuotedName("b/" + bName)
+			fm.WriteString(" differ\n")
+		} else {
+			fm.WriteString("GIT binary patch\n")
+			fm.FormatBinaryFragment(f.BinaryFragment)
+			if f.ReverseBinaryFragment != nil {
+				fm.FormatBinaryFragment(f.ReverseBinaryFragment)
+			}
+		}
+	}
+
+	// The "---" and "+++" lines only appear for text patches with fragments
+	if len(f.TextFragments) > 0 {
+		fm.WriteString("--- ")
+		if f.OldName == "" {
+			fm.WriteString("/dev/null")
+		} else {
+			fm.WriteQuotedName("a/" + f.OldName)
+		}
+		fm.WriteByte('\n')
+
+		fm.WriteString("+++ ")
+		if f.NewName == "" {
+			fm.WriteString("/dev/null")
+		} else {
+			fm.WriteQuotedName("b/" + f.NewName)
+		}
+		fm.WriteByte('\n')
+
+		for _, frag := range f.TextFragments {
+			fm.FormatTextFragment(frag)
+		}
+	}
+}
+
+func (fm *formatter) FormatTextFragment(f *TextFragment) {
+	fm.FormatTextFragmentHeader(f)
+	fm.WriteByte('\n')
+
+	for _, line := range f.Lines {
+		fm.WriteString(line.Op.String())
+		fm.WriteString(line.Line)
+		if line.NoEOL() {
+			fm.WriteString("\n\\ No newline at end of file\n")
+		}
+	}
+}
+
+func (fm *formatter) FormatTextFragmentHeader(f *TextFragment) {
+	fmt.Fprintf(fm, "@@ -%d,%d +%d,%d @@", f.OldPosition, f.OldLines, f.NewPosition, f.NewLines)
+	if f.Comment != "" {
+		fm.WriteByte(' ')
+		fm.WriteString(f.Comment)
+	}
+}
+
+func (fm *formatter) FormatBinaryFragment(f *BinaryFragment) {
+	const (
+		maxBytesPerLine = 52
+	)
+
+	switch f.Method {
+	case BinaryPatchDelta:
+		fm.WriteString("delta ")
+	case BinaryPatchLiteral:
+		fm.WriteString("literal ")
+	}
+	fm.Write(strconv.AppendInt(nil, f.Size, 10))
+	fm.WriteByte('\n')
+
+	data := deflateBinaryChunk(f.Data)
+	n := (len(data) / maxBytesPerLine) * maxBytesPerLine
+
+	buf := make([]byte, base85Len(maxBytesPerLine))
+	for i := 0; i < n; i += maxBytesPerLine {
+		base85Encode(buf, data[i:i+maxBytesPerLine])
+		fm.WriteByte('z')
+		fm.Write(buf)
+		fm.WriteByte('\n')
+	}
+	if remainder := len(data) - n; remainder > 0 {
+		buf = buf[0:base85Len(remainder)]
+
+		sizeChar := byte(remainder)
+		if remainder <= 26 {
+			sizeChar = 'A' + sizeChar - 1
+		} else {
+			sizeChar = 'a' + sizeChar - 27
+		}
+
+		base85Encode(buf, data[n:])
+		fm.WriteByte(sizeChar)
+		fm.Write(buf)
+		fm.WriteByte('\n')
+	}
+	fm.WriteByte('\n')
+}
+
+func deflateBinaryChunk(data []byte) []byte {
+	var b bytes.Buffer
+
+	zw := zlib.NewWriter(&b)
+	_, _ = zw.Write(data)
+	_ = zw.Close()
+
+	return b.Bytes()
+}