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source.go
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source.go
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// Copyright (c) Roman Atachiants and contributors. All rights reserved.
// Licensed under the MIT license. See LICENSE file in the project root for details.
package iostream
import (
"bufio"
"bytes"
"encoding/binary"
"errors"
"io"
"reflect"
"unsafe"
)
// MaxVarintLenN is the maximum length of a varint-encoded N-bit integer.
const (
maxVarintLen64 = 10 * 7
)
var overflow = errors.New("binary: varint overflows a 64-bit integer")
// source represents a required contract for a decoder to work properly
type source interface {
io.Reader
io.ByteReader
Slice(n int) (buffer []byte, err error)
ReadUvarint() (uint64, error)
ReadVarint() (int64, error)
Offset() int64
}
// newSource figures out the most efficient source to use for the provided type
func newSource(r io.Reader) source {
switch v := r.(type) {
case nil:
return newSliceSource(nil)
case *bytes.Buffer:
return newSliceSource(v.Bytes())
case *sliceSource:
return v
default:
rdr, ok := r.(source)
if !ok {
rdr = newStreamSource(r)
}
return rdr
}
}
// --------------------------- Slice Reader ---------------------------
// sliceSource implements a source that only reads from a slice
type sliceSource struct {
buffer []byte
offset int64
}
// newSliceSource returns a new source reading from b.
func newSliceSource(b []byte) *sliceSource {
return &sliceSource{b, 0}
}
// Offset returns the number of bytes read through this reader.
func (r *sliceSource) Offset() int64 {
return r.offset
}
// Read implements the io.Reader interface.
func (r *sliceSource) Read(b []byte) (n int, err error) {
if r.offset >= int64(len(r.buffer)) {
return 0, io.EOF
}
n = copy(b, r.buffer[r.offset:])
r.offset += int64(n)
return
}
// ReadByte implements the io.ByteReader interface.
func (r *sliceSource) ReadByte() (byte, error) {
if r.offset >= int64(len(r.buffer)) {
return 0, io.EOF
}
b := r.buffer[r.offset]
r.offset++
return b, nil
}
// Slice selects a sub-slice of next bytes. This is similar to Read() but does not
// actually perform a copy, but simply uses the underlying slice (if available) and
// returns a sub-slice pointing to the same array. Since this requires access
// to the underlying data, this is only available for our default source.
func (r *sliceSource) Slice(n int) ([]byte, error) {
if r.offset+int64(n) > int64(len(r.buffer)) {
return nil, io.EOF
}
cur := r.offset
r.offset += int64(n)
return r.buffer[cur:r.offset], nil
}
// ReadUvarint reads an encoded unsigned integer from r and returns it as a uint64.
func (r *sliceSource) ReadUvarint() (uint64, error) {
var x uint64
for s := 0; s < maxVarintLen64; s += 7 {
if r.offset >= int64(len(r.buffer)) {
return 0, io.EOF
}
b := r.buffer[r.offset]
r.offset++
if b < 0x80 {
if s == maxVarintLen64-7 && b > 1 {
return x, overflow
}
return x | uint64(b)<<s, nil
}
x |= uint64(b&0x7f) << s
}
return x, overflow
}
// ReadVarint reads an encoded signed integer from r and returns it as an int64.
func (r *sliceSource) ReadVarint() (int64, error) {
ux, err := r.ReadUvarint() // ok to continue in presence of error
x := int64(ux >> 1)
if ux&1 != 0 {
x = ^x
}
return x, err
}
// --------------------------- Stream Reader ---------------------------
// streamSource represents a source implementation for a generic source (i.e. streams)
type streamSource struct {
io.Reader
io.ByteReader
scratch []byte
offset int64
}
// newStreamSource returns a new stream source
func newStreamSource(r io.Reader) *streamSource {
src := &streamSource{
Reader: r,
}
// If we can already read byte at a time, we're done
if br, ok := r.(io.ByteReader); ok {
src.ByteReader = br
return src
}
// If stream doesn't have a byte reader, wrap it with a buffered reader
buffered := bufio.NewReader(r)
src.Reader = buffered
src.ByteReader = buffered
return src
}
// Offset returns the number of bytes read through this reader.
func (r *streamSource) Offset() int64 {
return r.offset
}
// Read implements the io.Reader interface.
func (r *streamSource) Read(b []byte) (int, error) {
n, err := r.Reader.Read(b)
r.offset += int64(n)
return n, err
}
// ReadByte implements the io.ByteReader interface.
func (r *streamSource) ReadByte() (byte, error) {
v, err := r.ByteReader.ReadByte()
r.offset++
return v, err
}
// Slice selects a sub-slice of next bytes.
func (r *streamSource) Slice(n int) ([]byte, error) {
if len(r.scratch) < n {
r.scratch = make([]byte, capacityFor(uint(n+1)))
}
// Read from the stream into our scratch buffer
n, err := io.ReadAtLeast(r.Reader, r.scratch[:n], n)
r.offset += int64(n)
return r.scratch[:n], err
}
// ReadUvarint reads an encoded unsigned integer from r and returns it as a uint64.
func (r *streamSource) ReadUvarint() (uint64, error) {
return binary.ReadUvarint(r)
}
// ReadVarint reads a variable-length Int64 from the buffer.
func (r *streamSource) ReadVarint() (int64, error) {
return binary.ReadVarint(r)
}
// --------------------------- Convert Funcs ---------------------------
// toString converts byte slice to a string without allocating.
func toString(b *[]byte) string {
return *(*string)(unsafe.Pointer(b))
}
// toBytes converts a string to a byte slice without allocating.
func toBytes(v string) (b []byte) {
strHeader := (*reflect.StringHeader)(unsafe.Pointer(&v))
byteHeader := (*reflect.SliceHeader)(unsafe.Pointer(&b))
byteHeader.Data = strHeader.Data
l := len(v)
byteHeader.Len = l
byteHeader.Cap = l
return
}
// capacityFor computes the next power of 2 for a given index
func capacityFor(v uint) int {
v--
v |= v >> 1
v |= v >> 2
v |= v >> 4
v |= v >> 8
v |= v >> 16
v++
return int(v)
}