package encoding

import (
	"bytes"
	"fmt"
)

type GSM7Coder struct{}

func (c *GSM7Coder) Encode(s string, buf *bytes.Buffer) error {
	// utf8 := *(*[]byte)(unsafe.Pointer(&s))
	utf8 := []byte(s)
	var offset int = 1
	var bitshift byte = 0
	var leap, shift bool

	for index, septet := range utf8 {
		if septet > 0b01111111 {
			return fmt.Errorf("invalid character at index %d", index)
		}
		if index == 0 {
			continue
		}
		bitshift++
		// log.Printf("Index:%-3d Offset:%-3d Bitshift:%-3d CurrentByte:%08b (%-3d) OffsetByte:%08b (%-3d) Leap:%5v", index, offset, bitshift, utf8[index], utf8[index], utf8[index-offset], utf8[index-offset], leap)
		mask := byte(255 >> (8 - bitshift))
		masked := (mask & septet) << (8 - bitshift)
		// log.Printf("Index:%-3d Offset:%-3d Bitshift:%-3d Mask:%08b Masked:%08b", index, offset, bitshift, mask, masked)
		if leap {
			masked >>= 1
		}
		utf8[index-offset] |= masked
		utf8[index] >>= bitshift

		if !leap {
			buf.WriteByte(utf8[index-offset])
		}
		if index == len(utf8)-1 && utf8[index] > 0 {
			buf.WriteByte(utf8[index])
		}
		// log.Printf("Index:%-3d Offset:%-3d Bitshift:%-3d CurrentByte:%08b (%-3d) OffsetByte:%08b (%-3d) Leap:%5v", index, offset, bitshift, utf8[index], utf8[index], utf8[index-offset], utf8[index-offset], leap)
		if bitshift >= 7 {
			if leap {
				// log.Printf("Shift at Index:%-3d Offset:%-3d Bitshift:%-3d", index, offset, bitshift)
				leap = false
				bitshift = 0
				offset++
				shift = true
				continue
			}
			// log.Printf("Leap at Index:%-3d Offset:%-3d Bitshift:%-3d", index, offset, bitshift)
			leap = true
			bitshift = 6
		}
		if shift {
			offset = 1
		}
	}
	return nil
}

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