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 {
@@ -62,6 +63,7 @@ func (c *GSM7Coder) Encode(s string, buf *bytes.Buffer) error {
 			offset = 1
 		}
 	}
+	log.Println(buf.Cap(), buf.Len())
 	return nil
 }
 
@@ -72,7 +74,7 @@ func (c *GSM7Coder) Decode(buf *bytes.Buffer) (string, error) {
 		bitshift    byte = 0
 		leap, shift bool
 	)
-	outLength := len(gsm7)*8/7
+	outLength := DecodesInto(buf)
 	lengthDiff := outLength - len(gsm7)
 	gsm7 = append(gsm7, make([]byte, lengthDiff)...)
 
@@ -141,3 +143,17 @@ func InsertAt(data *[]byte, index int, value byte) {
 	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
+}

encoding/gsm7_test.go

@@ -219,7 +219,62 @@ func TestDeletesLastValue(t *testing.T) {
 	}
 }
 
-// benchmark
+// region misc tests
+func TestGSM7EncodesIntoSmallString(t *testing.T) {
+	input := "Sunshine"
+	expected := 7
+	actual := EncodesInto(&input)
+	if actual != expected {
+		t.Errorf("Expected %d, but got %d", expected, actual)
+	}
+}
+
+func TestGSM7EncodesIntoLargerNot8nString(t *testing.T) {
+	input := "Golden rays play, Chasing night away."
+	expected := 33
+	actual := EncodesInto(&input)
+	if actual != expected {
+		t.Errorf("Expected %d, but got %d", expected, actual)
+	}
+}
+
+func TestGSM7EncodesIntoLarger8nString(t *testing.T) {
+	input := "Ducks are fucking great, they quacks, O quackers, what the fuck."
+	expected := 56
+	actual := EncodesInto(&input)
+	if actual != expected {
+		t.Errorf("Expected %d, but got %d", expected, actual)
+	}
+}
+
+func TestGSM7DecodesIntoSmallString(t *testing.T) {
+	input := []byte{0b11010011, 0b10111010, 0b01111011, 0b10001110, 0b01001110, 0b10111011, 0b11001011}
+	expected := 8
+	actual := DecodesInto(bytes.NewBuffer(input))
+	if actual != expected {
+		t.Errorf("Expected %d, but got %d", expected, actual)
+	}
+}
+
+func TestGSM7DecodesIntoLargerNot8nString(t *testing.T) {
+	input := []byte{0b11000111, 0b00110111, 0b10011011, 0b01011100, 0b01110110, 0b10000011, 0b11100100, 0b11100001, 0b11111100, 0b00011100, 0b00000100, 0b01100111, 0b10000111, 0b11110011, 0b00101100, 0b11010000, 0b00010000, 0b00011101, 0b10011110, 0b10100111, 0b11011101, 0b01100111, 0b10010000, 0b00111011, 0b01111101, 0b01000110, 0b11010011, 0b01000001, 0b11100001, 0b01111011, 0b00111000, 0b11101111, 0b00000010}
+	expected := 37
+	actual := DecodesInto(bytes.NewBuffer(input))
+	if actual != expected {
+		t.Errorf("Expected %d, but got %d", expected, actual)
+	}
+}
+
+func TestGSM7DecodesIntoLarger8nString(t *testing.T) {
+	input := []byte{0b11000100, 0b11111010, 0b01111000, 0b00111101, 0b00000111, 0b10000101, 0b11100101, 0b01100101, 0b10010000, 0b10111001, 0b00111110, 0b01011110, 0b10100111, 0b11011101, 0b01100111, 0b11010000, 0b01011001, 0b01011110, 0b00001110, 0b11010011, 0b01011001, 0b00100000, 0b00111010, 0b10111010, 0b10011100, 0b00000111, 0b11000101, 0b11101011, 0b11100001, 0b11110001, 0b01111010, 0b11001110, 0b00000010, 0b00111101, 0b01000001, 0b11110001, 0b01111010, 0b01111000, 0b10111100, 0b00101110, 0b11001011, 0b11100111, 0b00101100, 0b11010000, 0b00011101, 0b00011101, 0b10100110, 0b10000011, 0b11101000, 0b11101000, 0b00110010, 0b11001000, 0b01011100, 0b00011111, 0b10101111, 0b01011101}
+	expected := 64
+	actual := DecodesInto(bytes.NewBuffer(input))
+	if actual != expected {
+		t.Errorf("Expected %d, but got %d", expected, actual)
+	}
+}
+
+// region benchmark
 func BenchmarkGSM7EncodeSimpleASCIIString(b *testing.B) {
 	coder := &GSM7Coder{}
 	var buf bytes.Buffer