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5322e66a12
Implement a real Seek() method in the slicereader helper (prior to this it had a simplified SeekTo function), so that slicereader's will satisfy the ReadSeeker interface (needed in a subsequent patch). Change-Id: I832e3ec1e34d0f8c6b5edf390470f6f943c6ece0 Reviewed-on: https://go-review.googlesource.com/c/go/+/495438 Run-TryBot: Than McIntosh <thanm@google.com> TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: David Chase <drchase@google.com>
123 lines
2.5 KiB
Go
123 lines
2.5 KiB
Go
// Copyright 2021 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package slicereader
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import (
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"encoding/binary"
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"fmt"
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"io"
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"unsafe"
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)
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// This file contains the helper "SliceReader", a utility for
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// reading values from a byte slice that may or may not be backed
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// by a read-only mmap'd region.
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type Reader struct {
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b []byte
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readonly bool
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off int64
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}
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func NewReader(b []byte, readonly bool) *Reader {
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r := Reader{
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b: b,
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readonly: readonly,
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}
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return &r
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}
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func (r *Reader) Read(b []byte) (int, error) {
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amt := len(b)
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toread := r.b[r.off:]
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if len(toread) < amt {
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amt = len(toread)
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}
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copy(b, toread)
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r.off += int64(amt)
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return amt, nil
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}
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func (r *Reader) Seek(offset int64, whence int) (ret int64, err error) {
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switch whence {
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case io.SeekStart:
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if offset < 0 || offset > int64(len(r.b)) {
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return 0, fmt.Errorf("invalid seek: new offset %d (out of range [0 %d]", offset, len(r.b))
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}
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r.off = offset
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return offset, nil
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case io.SeekCurrent:
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newoff := r.off + offset
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if newoff < 0 || newoff > int64(len(r.b)) {
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return 0, fmt.Errorf("invalid seek: new offset %d (out of range [0 %d]", newoff, len(r.b))
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}
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r.off = newoff
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return r.off, nil
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case io.SeekEnd:
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newoff := int64(len(r.b)) + offset
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if newoff < 0 || newoff > int64(len(r.b)) {
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return 0, fmt.Errorf("invalid seek: new offset %d (out of range [0 %d]", newoff, len(r.b))
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}
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r.off = newoff
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return r.off, nil
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}
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// other modes are not supported
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return 0, fmt.Errorf("unsupported seek mode %d", whence)
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}
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func (r *Reader) Offset() int64 {
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return r.off
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}
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func (r *Reader) ReadUint8() uint8 {
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rv := uint8(r.b[int(r.off)])
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r.off += 1
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return rv
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}
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func (r *Reader) ReadUint32() uint32 {
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end := int(r.off) + 4
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rv := binary.LittleEndian.Uint32(r.b[int(r.off):end:end])
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r.off += 4
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return rv
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}
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func (r *Reader) ReadUint64() uint64 {
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end := int(r.off) + 8
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rv := binary.LittleEndian.Uint64(r.b[int(r.off):end:end])
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r.off += 8
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return rv
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}
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func (r *Reader) ReadULEB128() (value uint64) {
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var shift uint
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for {
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b := r.b[r.off]
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r.off++
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value |= (uint64(b&0x7F) << shift)
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if b&0x80 == 0 {
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break
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}
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shift += 7
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}
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return
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}
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func (r *Reader) ReadString(len int64) string {
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b := r.b[r.off : r.off+len]
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r.off += len
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if r.readonly {
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return toString(b) // backed by RO memory, ok to make unsafe string
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}
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return string(b)
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}
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func toString(b []byte) string {
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if len(b) == 0 {
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return ""
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}
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return unsafe.String(&b[0], len(b))
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}
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