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// Copyright 2021 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package slicereader
import (
"encoding/binary"
"fmt"
"io"
"unsafe"
)
// This file contains the helper "SliceReader", a utility for
// reading values from a byte slice that may or may not be backed
// by a read-only mmap'd region.
type Reader struct {
b []byte
readonly bool
off int64
}
func NewReader(b []byte, readonly bool) *Reader {
r := Reader{
b: b,
readonly: readonly,
}
return &r
}
func (r *Reader) Read(b []byte) (int, error) {
amt := len(b)
toread := r.b[r.off:]
if len(toread) < amt {
amt = len(toread)
}
copy(b, toread)
r.off += int64(amt)
return amt, nil
}
func (r *Reader) Seek(offset int64, whence int) (ret int64, err error) {
switch whence {
case io.SeekStart:
if offset < 0 || offset > int64(len(r.b)) {
return 0, fmt.Errorf("invalid seek: new offset %d (out of range [0 %d]", offset, len(r.b))
}
r.off = offset
return offset, nil
case io.SeekCurrent:
newoff := r.off + offset
if newoff < 0 || newoff > int64(len(r.b)) {
return 0, fmt.Errorf("invalid seek: new offset %d (out of range [0 %d]", newoff, len(r.b))
}
r.off = newoff
return r.off, nil
case io.SeekEnd:
newoff := int64(len(r.b)) + offset
if newoff < 0 || newoff > int64(len(r.b)) {
return 0, fmt.Errorf("invalid seek: new offset %d (out of range [0 %d]", newoff, len(r.b))
}
r.off = newoff
return r.off, nil
}
// other modes are not supported
return 0, fmt.Errorf("unsupported seek mode %d", whence)
}
func (r *Reader) Offset() int64 {
return r.off
}
func (r *Reader) ReadUint8() uint8 {
rv := uint8(r.b[int(r.off)])
r.off += 1
return rv
}
func (r *Reader) ReadUint32() uint32 {
end := int(r.off) + 4
rv := binary.LittleEndian.Uint32(r.b[int(r.off):end:end])
r.off += 4
return rv
}
func (r *Reader) ReadUint64() uint64 {
end := int(r.off) + 8
rv := binary.LittleEndian.Uint64(r.b[int(r.off):end:end])
r.off += 8
return rv
}
func (r *Reader) ReadULEB128() (value uint64) {
var shift uint
for {
b := r.b[r.off]
r.off++
value |= (uint64(b&0x7F) << shift)
if b&0x80 == 0 {
break
}
shift += 7
}
return
}
func (r *Reader) ReadString(len int64) string {
b := r.b[r.off : r.off+len]
r.off += len
if r.readonly {
return toString(b) // backed by RO memory, ok to make unsafe string
}
return string(b)
}
func toString(b []byte) string {
if len(b) == 0 {
return ""
}
return unsafe.String(&b[0], len(b))
}
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