Implement GSM7 packing

encoding/gsm7.go

@@ -2,17 +2,52 @@ package encoding
 
 import (
 	"bytes"
-	"log"
+	"fmt"
 )
 
 type GSM7Coder struct{}
 
-func (c *GSM7Coder) Encode(s string, buf *bytes.Buffer) {
-	utf8 := []byte(s)
-	log.Println(utf8)
-	buf.Write(utf8)
+var masks = []byte{
+	0b00000000,
+	0b00000001,
+	0b00000011,
+	0b00000111,
+	0b00001111,
+	0b00011111,
+	0b00111111,
+	0b01111111,
+	0b11111111,
 }
 
-func (c *GSM7Coder) Decode(buf *bytes.Buffer) string {
-	return buf.String()
+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
 }