package encoding

import (
	"bytes"
	"fmt"
)

type GSM7Coder struct{}

var masks = []byte{
	0b00000000,
	0b00000001,
	0b00000011,
	0b00000111,
	0b00001111,
	0b00011111,
	0b00111111,
	0b01111111,
	0b11111111,
}

func (c *GSM7Coder) Encode(s string, buf *bytes.Buffer) error {
	// utf8 := *(*[]byte)(unsafe.Pointer(&s))
	utf8 := []byte(s)
	var offset byte = 1
	var bitshift byte = 1

	for index, septet := range utf8 {
		if septet > 0b01111111 {
			return fmt.Errorf("invalid character at index %d", index)
		}
		bindex := byte(index)
		if bindex == 0 {
			continue
		}
		mask := masks[bitshift]
		masked := (mask & septet) << (8 - bitshift)
		utf8[bindex-offset] |= masked
		utf8[bindex] >>= bitshift

		buf.WriteByte(utf8[bindex-offset])
		bitshift++
		if utf8[bindex] == 0 {
			offset++
			bitshift = 1
		}
	}
	return nil
}

func (c *GSM7Coder) Decode(buf *bytes.Buffer) (string, error) {
	return buf.String(), nil
}