// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
// SPDX-License-Identifier: MIT

// Package xdr implements the primitive encoding of the External Data
// Representation Standard, RFC 4506, on which ONC RPC and the NFS protocols
// are built.
//
// Values are encoded in big endian byte order and every encoding occupies a
// multiple of four bytes: a fixed or variable length opaque value and a
// string of n bytes are followed by zero to three zero padding bytes.
package xdr

import "errors"

// ErrTruncated is returned when a decode runs past the end of the input.
var ErrTruncated = errors.New("xdr: unexpected end of input")

// ErrBadLength is returned when a declared length cannot be honoured.
var ErrBadLength = errors.New("xdr: impossible length")

// AppendUint32 appends v to b as four bytes in big endian order.
func AppendUint32(b []byte, v uint32) []byte {
	return append(b, byte(v>>24), byte(v>>16), byte(v>>8), byte(v))
}

// AppendInt32 appends v to b in the XDR integer encoding, which is the
// two's complement of the value in four big endian bytes.
func AppendInt32(b []byte, v int32) []byte {
	return AppendUint32(b, uint32(v))
}

// AppendUint64 appends v to b as eight bytes in big endian order, most
// significant word first.
func AppendUint64(b []byte, v uint64) []byte {
	return AppendUint32(AppendUint32(b, uint32(v>>32)), uint32(v))
}

// AppendInt64 appends v to b in the XDR hyper integer encoding, which is the
// two's complement of the value in eight big endian bytes.
func AppendInt64(b []byte, v int64) []byte {
	return AppendUint64(b, uint64(v))
}

// AppendBool appends v as the XDR boolean, which is the number zero for
// false and one for true.
func AppendBool(b []byte, v bool) []byte {
	if v {
		return AppendUint32(b, 1)
	}
	return AppendUint32(b, 0)
}

// appendPad appends the zero padding that brings an n byte body up to a
// multiple of four bytes.
func appendPad(b []byte, n int) []byte {
	var pad [3]byte
	return append(b, pad[:(4-n%4)%4]...)
}

// AppendFixedOpaque appends v followed by zero padding to a four byte
// boundary. The length is known from the surrounding structure and is not
// part of the encoding.
func AppendFixedOpaque(b []byte, v []byte) []byte {
	b = append(b, v...)
	return appendPad(b, len(v))
}

// AppendVarOpaque appends v as a count of bytes followed by the bytes and
// their zero padding.
func AppendVarOpaque(b []byte, v []byte) []byte {
	b = AppendUint32(b, uint32(len(v)))
	b = append(b, v...)
	return appendPad(b, len(v))
}

// AppendString appends s as a variable length opaque value holding UTF-8
// bytes.
func AppendString(b []byte, s string) []byte {
	b = AppendUint32(b, uint32(len(s)))
	b = append(b, s...)
	return appendPad(b, len(s))
}

// A Decoder reads XDR values from a byte slice. Its methods return the zero
// value and an error when the input does not hold the value; the input is
// never modified.
type Decoder struct {
	b   []byte
	off int
}

// NewDecoder returns a decoder over b.
func NewDecoder(b []byte) *Decoder { return &Decoder{b: b} }

// Remaining reports how many bytes of the input are still unread.
func (d *Decoder) Remaining() int { return len(d.b) - d.off }

// Uint32 reads four bytes in big endian order.
func (d *Decoder) Uint32() (uint32, error) {
	if d.Remaining() < 4 {
		return 0, ErrTruncated
	}
	v := uint32(d.b[d.off])<<24 | uint32(d.b[d.off+1])<<16 |
		uint32(d.b[d.off+2])<<8 | uint32(d.b[d.off+3])
	d.off += 4
	return v, nil
}

// Int32 reads an XDR integer.
func (d *Decoder) Int32() (int32, error) {
	v, err := d.Uint32()
	return int32(v), err
}

// Uint64 reads two words, most significant first.
func (d *Decoder) Uint64() (uint64, error) {
	hi, err := d.Uint32()
	if err != nil {
		return 0, err
	}
	lo, err := d.Uint32()
	if err != nil {
		return 0, err
	}
	return uint64(hi)<<32 | uint64(lo), nil
}

// Int64 reads an XDR hyper integer.
func (d *Decoder) Int64() (int64, error) {
	v, err := d.Uint64()
	return int64(v), err
}

// Bool reads an XDR boolean. Any nonzero word decodes as true, because the
// standard constrains what a sender writes and not what a receiver accepts.
func (d *Decoder) Bool() (bool, error) {
	v, err := d.Uint32()
	return v != 0, err
}

// skipPad consumes the zero padding after an n byte body.
func (d *Decoder) skipPad(n int) error {
	if p := (4 - n%4) % 4; p > 0 {
		if d.Remaining() < p {
			return ErrTruncated
		}
		d.off += p
	}
	return nil
}

// Raw reads exactly n bytes with no padding. The returned slice aliases
// the decoder's input.
func (d *Decoder) Raw(n int) ([]byte, error) {
	if n < 0 {
		return nil, ErrBadLength
	}
	if n > d.Remaining() {
		return nil, ErrTruncated
	}
	v := d.b[d.off : d.off+n]
	d.off += n
	return v, nil
}

// FixedOpaque reads exactly n bytes and skips their padding.
func (d *Decoder) FixedOpaque(n int) ([]byte, error) {
	if n < 0 {
		return nil, ErrBadLength
	}
	if n > d.Remaining() {
		return nil, ErrTruncated
	}
	v := make([]byte, n)
	copy(v, d.b[d.off:d.off+n])
	d.off += n
	if err := d.skipPad(n); err != nil {
		return nil, err
	}
	return v, nil
}

// VarOpaque reads a count of bytes followed by the bytes and their padding.
// A count beyond the remaining input returns an error before any allocation.
func (d *Decoder) VarOpaque() ([]byte, error) {
	n, err := d.Uint32()
	if err != nil {
		return nil, err
	}
	if uint64(int(n)) != uint64(n) {
		// The count does not fit an int on this platform.
		return nil, ErrBadLength
	}
	return d.FixedOpaque(int(n))
}

// String reads a variable length opaque value as a string.
func (d *Decoder) String() (string, error) {
	v, err := d.VarOpaque()
	if err != nil {
		return "", err
	}
	return string(v), nil
}