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// Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause
// Package cstruct provides a helper for decoding binary data that is in the
// form of a padded C structure.
package cstruct
import (
"errors"
"io"
"github.com/josharian/native"
)
// Size of a pointer-typed value, in bits
const pointerSize = 32 << (^uintptr(0) >> 63)
// We assume that non-64-bit platforms are 32-bit; we don't expect Go to run on
// a 16- or 8-bit architecture any time soon.
const is64Bit = pointerSize == 64
// Decoder reads and decodes padded fields from a slice of bytes. All fields
// are decoded with native endianness.
//
// Methods of a Decoder do not return errors, but rather store any error within
// the Decoder. The first error can be obtained via the Err method; after the
// first error, methods will return the zero value for their type.
type Decoder struct {
b []byte
off int
err error
dbuf [8]byte // for decoding
}
// NewDecoder creates a Decoder from a byte slice.
func NewDecoder(b []byte) *Decoder {
return &Decoder{b: b}
}
var errUnsupportedSize = errors.New("unsupported size")
func padBytes(offset, size int) int {
if offset == 0 || size == 1 {
return 0
}
remainder := offset % size
return size - remainder
}
func (d *Decoder) getField(b []byte) error {
size := len(b)
// We only support fields that are multiples of 2 (or 1-sized)
if size != 1 && size&1 == 1 {
return errUnsupportedSize
}
// Fields are aligned to their size
padBytes := padBytes(d.off, size)
if d.off+size+padBytes > len(d.b) {
return io.EOF
}
d.off += padBytes
copy(b, d.b[d.off:d.off+size])
d.off += size
return nil
}
// Err returns the first error that was encountered by this Decoder.
func (d *Decoder) Err() error {
return d.err
}
// Offset returns the current read offset for data in the buffer.
func (d *Decoder) Offset() int {
return d.off
}
// Byte returns a single byte from the buffer.
func (d *Decoder) Byte() byte {
if d.err != nil {
return 0
}
if err := d.getField(d.dbuf[0:1]); err != nil {
d.err = err
return 0
}
return d.dbuf[0]
}
// Byte returns a number of bytes from the buffer based on the size of the
// input slice. No padding is applied.
//
// If an error is encountered or this Decoder has previously encountered an
// error, no changes are made to the provided buffer.
func (d *Decoder) Bytes(b []byte) {
if d.err != nil {
return
}
// No padding for byte slices
size := len(b)
if d.off+size >= len(d.b) {
d.err = io.EOF
return
}
copy(b, d.b[d.off:d.off+size])
d.off += size
}
// Uint16 returns a uint16 decoded from the buffer.
func (d *Decoder) Uint16() uint16 {
if d.err != nil {
return 0
}
if err := d.getField(d.dbuf[0:2]); err != nil {
d.err = err
return 0
}
return native.Endian.Uint16(d.dbuf[0:2])
}
// Uint32 returns a uint32 decoded from the buffer.
func (d *Decoder) Uint32() uint32 {
if d.err != nil {
return 0
}
if err := d.getField(d.dbuf[0:4]); err != nil {
d.err = err
return 0
}
return native.Endian.Uint32(d.dbuf[0:4])
}
// Uint64 returns a uint64 decoded from the buffer.
func (d *Decoder) Uint64() uint64 {
if d.err != nil {
return 0
}
if err := d.getField(d.dbuf[0:8]); err != nil {
d.err = err
return 0
}
return native.Endian.Uint64(d.dbuf[0:8])
}
// Uintptr returns a uintptr decoded from the buffer.
func (d *Decoder) Uintptr() uintptr {
if d.err != nil {
return 0
}
if is64Bit {
return uintptr(d.Uint64())
} else {
return uintptr(d.Uint32())
}
}
// Int16 returns a int16 decoded from the buffer.
func (d *Decoder) Int16() int16 {
return int16(d.Uint16())
}
// Int32 returns a int32 decoded from the buffer.
func (d *Decoder) Int32() int32 {
return int32(d.Uint32())
}
// Int64 returns a int64 decoded from the buffer.
func (d *Decoder) Int64() int64 {
return int64(d.Uint64())
}
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