Making parsekit.reader both simpler and more complex (more complex by adopting some buffer allocation logic from the built-in bytes package, to not be copying memory all the time during the read operations.

reader/reader.go

@@ -41,6 +41,7 @@ package reader
 
 import (
 	"bufio"
+	"errors"
 	"fmt"
 	"io"
 	"strings"
@@ -58,12 +59,11 @@ import (
 //
 // The parserkit.reader.Reader is used internally by parsekit.TokenAPI.
 type Reader struct {
-	bufio        *bufio.Reader // Used for ReadRune()
-	buffer       []rune        // Input buffer, holding runes that were read from input
-	bufferOffset int           // The offset of the buffer, relative to the start of the input
-	bufferLen    int           // Input size, the number of runes in the buffer
-	err          error         // A read error, if one occurred
-	errOffset    int           // The offset in the buffer at which the read error was encountered
+	bufio         *bufio.Reader // Used for ReadRune()
+	buffer        []rune        // Input buffer, holding runes that were read from input
+	err           error         // A read error, if one occurred
+	errOffset     int           // The offset in the buffer at which the read error was encountered
+	firstReadDone bool          // Whether or not the first read was done
 }
 
 // New initializes a new reader struct, wrapped around the provided input.
@@ -74,8 +74,7 @@ type Reader struct {
 // - bufio.Reader
 func New(input interface{}) *Reader {
 	return &Reader{
-		bufio:  makeBufioReader(input),
-		buffer: []rune{},
+		bufio: makeBufioReader(input),
 	}
 }
 
@@ -108,6 +107,8 @@ func makeBufioReader(input interface{}) *bufio.Reader {
 // When reading failed, the rune will be utf8.RuneError and the error will
 // be not nil. One special read fail is actually a normal situation: end
 // of file reached. In that case, the returned error wille be io.EOF.
+// Once a read error is encountered, that same read error will guaranteed
+// be return on every subsequent read at or beyond the provided offset.
 func (r *Reader) RuneAt(offset int) (rune, error) {
 	// Re-issue a previously seen read error.
 	if r.err != nil && offset >= r.errOffset {
@@ -116,40 +117,92 @@ func (r *Reader) RuneAt(offset int) (rune, error) {
 
 	// Rune at provided offset is not yet available in the input buffer.
 	// Read runes until we have enough runes to satisfy the offset.
-	for r.bufferLen <= offset {
-		readRune, _, err := r.bufio.ReadRune()
+	l := len(r.buffer)
+	n := offset - l + 1 // nr of runes to add to the buffer to get to offset
+	if n > 0 {
+		r.grow(n)
+		for writeAt := l; writeAt <= offset; writeAt++ {
+			readRune, _, err := r.bufio.ReadRune()
 
-		// Handle errors.
-		if err != nil {
-			r.err = err
-			r.errOffset = r.bufferLen
-			return utf8.RuneError, err
+			// Skip BOM.
+			if !r.firstReadDone {
+				r.firstReadDone = true
+				if readRune == '\uFEFF' {
+					writeAt--
+					continue
+				}
+			}
+
+			// Handle errors.
+			if err != nil {
+				r.err = err
+				r.errOffset = writeAt
+				return utf8.RuneError, err
+			}
+
+			r.buffer[writeAt] = readRune
 		}
-
-		// Skip BOM.
-		if readRune == '\uFEFF' && r.bufferOffset == 0 {
-			r.bufferOffset++
-			continue
-		}
-
-		r.buffer = append(r.buffer, readRune)
-		r.bufferLen++
 	}
+
 	return r.buffer[offset], nil
 }
 
+// The upcoming code was inspired heavily by the Go built-in 'bytes' package.
+
+// smallBufferSize is an initial allocation minimal capacity.
+const smallBufferSize = 64
+
+// ErrTooLarge is passed to panic if memory cannot be allocated to store data in a buffer.
+var ErrTooLarge = errors.New("parsekit.reader: too large")
+
+// grow grows the buffer to guarantee space for n more bytes.
+// It returns the index where bytes should be written.
+// If the buffer can't grow it will panic with ErrTooLarge.
+func (r *Reader) grow(n int) {
+	// Instantiate new buffer.
+	if r.buffer == nil {
+		b := smallBufferSize
+		if b < n {
+			b = n
+		}
+		r.buffer = make([]rune, n, b)
+		return
+	}
+	l := len(r.buffer)
+	c := cap(r.buffer)
+	// Grow the buffer by reslicing within the available capacity.
+	if n <= c-l {
+		r.buffer = r.buffer[:l+n]
+		return
+	}
+	// Grow the buffer by allocating a new one and copying the data.
+	buf := makeSlice(2*c + n)
+	copy(buf, r.buffer)
+	r.buffer = buf[:l+n]
+}
+
+// makeSlice allocates a slice of size n. If the allocation fails, it panics
+// with ErrTooLarge.
+func makeSlice(n int) []rune {
+	// If the make fails, give a known error.
+	defer func() {
+		if recover() != nil {
+			panic(ErrTooLarge)
+		}
+	}()
+	return make([]rune, n)
+}
+
 // Flush deletes the provided number of runes from the start of the
 // reader buffer. After flushing the buffer, offset 0 as used by RuneAt()
 // will point to the rune that comes after the flushed runes.
 // So what this basically does is turn the Reader into a sliding window.
 func (r *Reader) Flush(numberOfRunes int) {
-	if numberOfRunes > r.bufferLen {
+	if numberOfRunes > len(r.buffer) {
 		panic(fmt.Sprintf(
 			"parsekit.Input.Reader.Flush(): number of runes to flush (%d) "+
-				"exceeds size of the buffer (%d)", numberOfRunes, r.bufferLen))
+				"exceeds size of the buffer (%d)", numberOfRunes, len(r.buffer)))
 	}
-	r.bufferOffset += numberOfRunes
-	r.bufferLen -= numberOfRunes
 	r.buffer = r.buffer[numberOfRunes:]
 	if r.err != nil {
 		r.errOffset -= numberOfRunes

reader/reader_test.go

@@ -231,6 +231,48 @@ func TestGivenErrorFromReader_ErrorIsCached(t *testing.T) {
 	assert.Equal(t, io.EOF, err)
 }
 
+func TestInputLargerThanDefaultBufSize64(t *testing.T) {
+	input, size := makeLargeStubReader()
+	r := reader.New(input)
+
+	readRune, err := r.RuneAt(0)
+	assert.Equal(t, 'X', readRune)
+	readRune, err = r.RuneAt(size - 1)
+	assert.Equal(t, 'Y', readRune)
+	readRune, err = r.RuneAt(size)
+	assert.Equal(t, io.EOF, err)
+	readRune, err = r.RuneAt(10)
+	assert.Equal(t, 'X', readRune)
+}
+
+func TestInputLargerThanDefaultBufSize64_WithFirstReadLargerThanBufSize64(t *testing.T) {
+	input, size := makeLargeStubReader()
+	r := reader.New(input)
+
+	readRune, _ := r.RuneAt(size - 200)
+	assert.Equal(t, 'X', readRune)
+	readRune, _ = r.RuneAt(size - 1)
+	assert.Equal(t, 'Y', readRune)
+}
+
+func TestInputLargerThanDefaultBufSize64_WithFirstReadToLastByte(t *testing.T) {
+	input, size := makeLargeStubReader()
+	r := reader.New(input)
+
+	readRune, _ := r.RuneAt(size - 1)
+	assert.Equal(t, 'Y', readRune)
+}
+
+func makeLargeStubReader() (*StubReader, int) {
+	size := utf8.UTFMax * 64 * 5
+	bytes := make([]byte, size)
+	for i := range bytes {
+		bytes[i] = 'X'
+	}
+	bytes[size-1] = 'Y'
+	return &StubReader{bytes: bytes, errors: []error{io.EOF}}, size
+}
+
 type StubReader struct {
 	bytes  []byte
 	errors []error