Add LineReaderAt interface and implementation

This is a line-oriented parallel to io.ReaderAt, meant for text applies.
While the mapping isn't quite as clean as in the binary case, a text
apply still reads a fixed chunk of lines starting at a specific line
number and modifies them. This also allows a consistent interface for
strict and fuzzy applies.

The implementation wraps an io.ReaderAt and reads data in chunks,
indexing line boundaries as it goes. This is probably not the most
efficient way to implement this interface, but it works and allows file
application to take a consistent interface.

gitdiff/io.go

@@ -2,6 +2,7 @@ package gitdiff
 
 import (
 	"bufio"
+	"errors"
 	"fmt"
 	"io"
 )
@@ -77,3 +78,134 @@ func unwrapLineReader(lr LineReader) io.Reader {
 		panic(fmt.Sprintf("%T does not implement io.Reader and is not a gitdiff wrapper", lr))
 	}
 }
+
+// LineReaderAt is the interface that wraps the ReadLinesAt method.
+//
+// ReadLinesAt reads len(lines) into lines starting at line offset in the
+// input source. It returns number of full 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.
+//
+// Each full line includes the line ending character(s). If the last line of
+// the input does not have a line ending character, ReadLinesAt returns the
+// content of the line and io.EOF.
+//
+// If the content of the input source changes after the first call to
+// ReadLinesAt, the behavior of future calls is undefined.
+type LineReaderAt interface {
+	ReadLinesAt(lines [][]byte, offset int64) (n int, err error)
+}
+
+// NewLineReaderAt creates a LineReaderAt from an io.ReaderAt.
+func NewLineReaderAt(r io.ReaderAt) LineReaderAt {
+	return &lineReaderAt{r: r}
+}
+
+type lineReaderAt struct {
+	r     io.ReaderAt
+	index []int64
+	eof   bool
+}
+
+func (r *lineReaderAt) ReadLinesAt(lines [][]byte, offset int64) (n int, err error) {
+	// TODO(bkeyes): revisit variable names
+	//  - it's generally not clear when something is bytes vs lines
+	//  - offset is a good example of this
+
+	if len(lines) == 0 {
+		return 0, nil
+	}
+
+	endLine := offset + int64(len(lines))
+	if endLine > int64(len(r.index)) && !r.eof {
+		if err := r.indexTo(endLine); err != nil {
+			return 0, err
+		}
+	}
+	if offset > int64(len(r.index)) {
+		return 0, io.EOF
+	}
+
+	// TODO(bkeyes): check usage of int / int64
+	// - interface uses int64 for arbitrarily large files
+	// - implementation is limited to int lines by index array
+
+	// offset <= len(r.index) means that it must fit in int without loss
+	size, readOffset := lookupLines(r.index, int(offset), len(lines))
+
+	b := make([]byte, size)
+	if _, err := r.r.ReadAt(b, readOffset); err != nil {
+		if err == io.EOF {
+			err = errors.New("ReadLinesAt: corrupt line index or changed source data")
+		}
+		return 0, err
+	}
+
+	for n = 0; n < len(lines) && offset+int64(n) < int64(len(r.index)); n++ {
+		i := int(offset) + n
+		start, end := readOffset, r.index[i]
+		if i > 0 {
+			start = r.index[i-1]
+		}
+		lines[n] = b[start-readOffset : end-readOffset]
+	}
+
+	if n < len(lines) || b[size-1] != '\n' {
+		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 [1024]byte
+
+	var offset int64
+	if len(r.index) > 0 {
+		offset = r.index[len(r.index)-1]
+	}
+
+	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 n > 0 && buf[n-1] != '\n' {
+				r.index = append(r.index, offset)
+			}
+			r.eof = true
+			break
+		}
+	}
+	return nil
+}
+
+// lookupLines gets the byte offset and size of a range of lines from an index
+// where the value at n is the offset of the first byte after line number n.
+func lookupLines(index []int64, start, n int) (size int64, offset int64) {
+	if start > len(index) {
+		offset = index[len(index)-1]
+	} else if start > 0 {
+		offset = index[start-1]
+	}
+	if n > 0 {
+		// TODO(bkeyes): check types for overflow
+		if start+n > len(index) {
+			size = index[len(index)-1] - offset
+		} else {
+			size = index[start+n-1] - offset
+		}
+	}
+	return
+}