Add "len" methods to encode and decode

To get length of encoded/decoded string

encoding/gsm7.go

@@ -3,10 +3,15 @@ package encoding
 import (
 	"bytes"
 	"fmt"
+	"log"
 )
 
 type GSM7Coder struct{}
 
+// Make sure buffer can fit EncodesInto bytes
+// Otherwise Encode will allocate memory as it sees fit
+// Which is fine but not optimal
+// Preallocate the buffer with the size of EncodesInto bytes
 func (c *GSM7Coder) Encode(s string, buf *bytes.Buffer) error {
 	// utf8 := *(*[]byte)(unsafe.Pointer(&s))
 	utf8 := []byte(s)
@@ -15,10 +20,6 @@ func (c *GSM7Coder) Encode(s string, buf *bytes.Buffer) error {
 		bitshift    byte = 0
 		leap, shift bool
 	)
-	tbw := len(utf8) * 7 / 8
-	if buf.Available() < tbw {
-		buf.Grow(tbw)
-	}		
 
 	for index, septet := range utf8 {
 		if septet > 0b01111111 {
@@ -62,17 +63,18 @@ func (c *GSM7Coder) Encode(s string, buf *bytes.Buffer) error {
 			offset = 1
 		}
 	}
+	log.Println(buf.Cap(), buf.Len())
 	return nil
 }
 
 func (c *GSM7Coder) Decode(buf *bytes.Buffer) (string, error) {
 	gsm7 := buf.Bytes()
 	var (
-		offset      int    = (len(gsm7) / 8) + 1
-		bitshift    byte   = 0
+		offset      int  = (len(gsm7) / 8) + 1
+		bitshift    byte = 0
 		leap, shift bool
 	)
-	outLength := len(gsm7)*8/7
+	outLength := DecodesInto(buf)
 	lengthDiff := outLength - len(gsm7)
 	gsm7 = append(gsm7, make([]byte, lengthDiff)...)
 
@@ -130,7 +132,7 @@ func (c *GSM7Coder) Decode(buf *bytes.Buffer) (string, error) {
 	}
 	// log.Printf("Result: %+v", gsm7)
 	// for _, v := range gsm7 {
-		// log.Printf("%08b", v)
+	// log.Printf("%08b", v)
 	// }
 	return string(gsm7), nil
 }
@@ -138,6 +140,20 @@ func (c *GSM7Coder) Decode(buf *bytes.Buffer) (string, error) {
 // Allocation free
 // Which means data MUST have space for value
 func InsertAt(data *[]byte, index int, value byte) {
-    copy((*data)[index+1:], (*data)[index:])
-    (*data)[index] = value
+	copy((*data)[index+1:], (*data)[index:])
+	(*data)[index] = value
+}
+
+func EncodesInto(s *string) int {
+	slen := len(*s)
+	enclen := slen * 7 / 8
+	if slen%8 != 0 {
+		enclen++
+	}
+	return enclen
+}
+func DecodesInto(buf *bytes.Buffer) int {
+	blen := buf.Len()
+	declen := blen * 8 / 7
+	return declen
 }