package aml

import (
	"errors"
	"io"
	"reflect"
	"unsafe"
)

var (
	errInvalidUnreadByte = errors.New("amlStreamReader: invalid use of UnreadByte")
)

type amlStreamReader struct {
	offset uint32
	data   []byte
}

// Init sets up the reader so it can read up to dataLen bytes from the virtual
// memory address dataAddr. If a non-zero initialOffset is specified, it will
// be used as the current offset in the stream.
func (r *amlStreamReader) Init(dataAddr uintptr, dataLen, initialOffset uint32) {
	// Overlay a byte slice on top of the memory block to be accessed.
	r.data = *(*[]byte)(unsafe.Pointer(&reflect.SliceHeader{
		Len:  int(dataLen),
		Cap:  int(dataLen),
		Data: dataAddr,
	}))

	r.SetOffset(initialOffset)
}

// EOF returns true if the end of the stream has been reached.
func (r *amlStreamReader) EOF() bool {
	return r.offset == uint32(len(r.data))
}

// ReadByte returns the next byte from the stream.
func (r *amlStreamReader) ReadByte() (byte, error) {
	if r.EOF() {
		return 0, io.EOF
	}

	r.offset++
	return r.data[r.offset-1], nil
}

// PeekByte returns the next byte from the stream without advancing the read pointer.
func (r *amlStreamReader) PeekByte() (byte, error) {
	if r.EOF() {
		return 0, io.EOF
	}

	return r.data[r.offset], nil
}

// LastByte returns the last byte read off the stream
func (r *amlStreamReader) LastByte() (byte, error) {
	if r.offset == 0 {
		return 0, io.EOF
	}

	return r.data[r.offset-1], nil
}

// UnreadByte moves back the read pointer by one byte.
func (r *amlStreamReader) UnreadByte() error {
	if r.offset == 0 {
		return errInvalidUnreadByte
	}

	r.offset--
	return nil
}

// Offset returns the current offset.
func (r *amlStreamReader) Offset() uint32 {
	return r.offset
}

// SetOffset sets the reader offset to the supplied value.
func (r *amlStreamReader) SetOffset(off uint32) {
	r.offset = off
}