package navicat

import (
	"encoding/binary"
	"fmt"
	"io"
)

type reader struct {
	buf []byte
	off int
}

func newReader(b []byte) *reader {
	return &reader{buf: b}
}

func (r *reader) remaining() int {
	return len(r.buf) - r.off
}

func (r *reader) peek() (byte, error) {
	if r.off >= len(r.buf) {
		return 0, io.EOF
	}
	return r.buf[r.off], nil
}

func (r *reader) readByte() (byte, error) {
	if r.off >= len(r.buf) {
		return 0, io.ErrUnexpectedEOF
	}
	b := r.buf[r.off]
	r.off++
	return b, nil
}

func (r *reader) readN(n int) ([]byte, error) {
	if n < 0 {
		return nil, fmt.Errorf("negative length")
	}
	if r.off+n > len(r.buf) {
		return nil, io.ErrUnexpectedEOF
	}
	s := r.buf[r.off : r.off+n]
	r.off += n
	return s, nil
}

func (r *reader) u16() (uint16, error) {
	b, err := r.readN(2)
	if err != nil {
		return 0, err
	}
	return binary.BigEndian.Uint16(b), nil
}

func (r *reader) u32() (uint32, error) {
	b, err := r.readN(4)
	if err != nil {
		return 0, err
	}
	return binary.BigEndian.Uint32(b), nil
}

// block reads GetBlock. nullable allows 0xFF as SQL NULL (EchoData only).
func (r *reader) block(nullable bool) (val []byte, isNull bool, err error) {
	if r.off >= len(r.buf) {
		return nil, false, io.ErrUnexpectedEOF
	}
	first := r.buf[r.off]
	if nullable && first == markNull {
		r.off++
		return nil, true, nil
	}
	r.off++
	var n int
	if first < 254 {
		n = int(first)
	} else if first == markLong {
		ln, err := r.u32()
		if err != nil {
			return nil, false, err
		}
		if ln > 64<<20 {
			return nil, false, fmt.Errorf("block length %d exceeds 64MiB limit", ln)
		}
		n = int(ln)
	} else {
		return nil, false, fmt.Errorf("invalid block marker 0x%02x", first)
	}
	b, err := r.readN(n)
	if err != nil {
		return nil, false, err
	}
	return b, false, nil
}

func (r *reader) blockBytes() ([]byte, error) {
	b, isNull, err := r.block(false)
	if err != nil {
		return nil, err
	}
	if isNull {
		return nil, fmt.Errorf("unexpected NULL block")
	}
	return b, nil
}

func (r *reader) blockString() (string, error) {
	b, err := r.blockBytes()
	if err != nil {
		return "", err
	}
	return string(b), nil
}