1 files changed, 220 insertions, 0 deletions
diff --git a/internal/gitdiff/io.go b/internal/gitdiff/io.go
new file mode 100644
index 0000000..8143238
--- /dev/null
+++ b/internal/gitdiff/io.go
@@ -0,0 +1,220 @@
+package gitdiff
+
+import (
+	"errors"
+	"io"
+)
+
+const (
+	byteBufferSize  = 32 * 1024 // from io.Copy
+	lineBufferSize  = 32
+	indexBufferSize = 1024
+)
+
+// LineReaderAt is the interface that wraps the ReadLinesAt method.
+//
+// ReadLinesAt reads len(lines) into lines starting at line offset. It returns
+// the number of lines read (0 <= n <= len(lines)) and any error encountered.
+// Line numbers are zero-indexed.
+//
+// If n < len(lines), ReadLinesAt returns a non-nil error explaining why more
+// lines were not returned.
+//
+// Lines read by ReadLinesAt include the newline character. The last line does
+// not have a final newline character if the input ends without one.
+type LineReaderAt interface {
+	ReadLinesAt(lines [][]byte, offset int64) (n int, err error)
+}
+
+type lineReaderAt struct {
+	r     io.ReaderAt
+	index []int64
+	eof   bool
+}
+
+func (r *lineReaderAt) ReadLinesAt(lines [][]byte, offset int64) (n int, err error) {
+	if offset < 0 {
+		return 0, errors.New("ReadLinesAt: negative offset")
+	}
+	if len(lines) == 0 {
+		return 0, nil
+	}
+
+	count := len(lines)
+	startLine := offset
+	endLine := startLine + int64(count)
+
+	if endLine > int64(len(r.index)) && !r.eof {
+		if err := r.indexTo(endLine); err != nil {
+			return 0, err
+		}
+	}
+	if startLine >= int64(len(r.index)) {
+		return 0, io.EOF
+	}
+
+	buf, byteOffset, err := r.readBytes(startLine, int64(count))
+	if err != nil {
+		return 0, err
+	}
+
+	for n = 0; n < count && startLine+int64(n) < int64(len(r.index)); n++ {
+		lineno := startLine + int64(n)
+		start, end := int64(0), r.index[lineno]-byteOffset
+		if lineno > 0 {
+			start = r.index[lineno-1] - byteOffset
+		}
+		lines[n] = buf[start:end]
+	}
+
+	if n < count {
+		return n, io.EOF
+	}
+	return n, nil
+}
+
+// indexTo reads data and computes the line index until there is information
+// for line or a read returns io.EOF. It returns an error if and only if there
+// is an error reading data.
+func (r *lineReaderAt) indexTo(line int64) error {
+	var buf [indexBufferSize]byte
+
+	offset := r.lastOffset()
+	for int64(len(r.index)) < line {
+		n, err := r.r.ReadAt(buf[:], offset)
+		if err != nil && err != io.EOF {
+			return err
+		}
+		for _, b := range buf[:n] {
+			offset++
+			if b == '\n' {
+				r.index = append(r.index, offset)
+			}
+		}
+		if err == io.EOF {
+			if offset > r.lastOffset() {
+				r.index = append(r.index, offset)
+			}
+			r.eof = true
+			break
+		}
+	}
+	return nil
+}
+
+func (r *lineReaderAt) lastOffset() int64 {
+	if n := len(r.index); n > 0 {
+		return r.index[n-1]
+	}
+	return 0
+}
+
+// readBytes reads the bytes of the n lines starting at line and returns the
+// bytes and the offset of the first byte in the underlying source.
+func (r *lineReaderAt) readBytes(line, n int64) (b []byte, offset int64, err error) {
+	indexLen := int64(len(r.index))
+
+	var size int64
+	if line > indexLen {
+		offset = r.index[indexLen-1]
+	} else if line > 0 {
+		offset = r.index[line-1]
+	}
+	if n > 0 {
+		if line+n > indexLen {
+			size = r.index[indexLen-1] - offset
+		} else {
+			size = r.index[line+n-1] - offset
+		}
+	}
+
+	b = make([]byte, size)
+	if _, err := r.r.ReadAt(b, offset); err != nil {
+		if err == io.EOF {
+			err = errors.New("ReadLinesAt: corrupt line index or changed source data")
+		}
+		return nil, 0, err
+	}
+	return b, offset, nil
+}
+
+func isLen(r io.ReaderAt, n int64) (bool, error) {
+	off := n - 1
+	if off < 0 {
+		off = 0
+	}
+
+	var b [2]byte
+	nr, err := r.ReadAt(b[:], off)
+	if err == io.EOF {
+		return (n == 0 && nr == 0) || (n > 0 && nr == 1), nil
+	}
+	return false, err
+}
+
+// copyFrom writes bytes starting from offset off in src to dst stopping at the
+// end of src or at the first error. copyFrom returns the number of bytes
+// written and any error.
+func copyFrom(dst io.Writer, src io.ReaderAt, off int64) (written int64, err error) {
+	buf := make([]byte, byteBufferSize)
+	for {
+		nr, rerr := src.ReadAt(buf, off)
+		if nr > 0 {
+			nw, werr := dst.Write(buf[0:nr])
+			if nw > 0 {
+				written += int64(nw)
+			}
+			if werr != nil {
+				err = werr
+				break
+			}
+			if nr != nw {
+				err = io.ErrShortWrite
+				break
+			}
+			off += int64(nr)
+		}
+		if rerr != nil {
+			if rerr != io.EOF {
+				err = rerr
+			}
+			break
+		}
+	}
+	return written, err
+}
+
+// copyLinesFrom writes lines starting from line off in src to dst stopping at
+// the end of src or at the first error. copyLinesFrom returns the number of
+// lines written and any error.
+func copyLinesFrom(dst io.Writer, src LineReaderAt, off int64) (written int64, err error) {
+	buf := make([][]byte, lineBufferSize)
+ReadLoop:
+	for {
+		nr, rerr := src.ReadLinesAt(buf, off)
+		if nr > 0 {
+			for _, line := range buf[0:nr] {
+				nw, werr := dst.Write(line)
+				if nw > 0 {
+					written++
+				}
+				if werr != nil {
+					err = werr
+					break ReadLoop
+				}
+				if len(line) != nw {
+					err = io.ErrShortWrite
+					break ReadLoop
+				}
+			}
+			off += int64(nr)
+		}
+		if rerr != nil {
+			if rerr != io.EOF {
+				err = rerr
+			}
+			break
+		}
+	}
+	return written, err
+}