unpackGSM7 converts GSM 7-bit packed data to a UTF-8 string

decode.go

package main

import (
	"encoding/hex"
	"fmt"
)

func main() {
	pdu := "b11a" // Example PDU hex string
	decodedBytes, err := hex.DecodeString(pdu)
	if err != nil {
		fmt.Println("Error decoding hex:", err)
		return
	}

	// Convert GSM-7 encoded data to string
	result := unpackGSM7(decodedBytes)
	fmt.Println("Decoded string:", result)
}

// unpackGSM7 converts GSM 7-bit packed data to a UTF-8 string
func unpackGSM7(data []byte) string {
	// Calculate the number of 7-bit characters
	length := len(data) * 8 / 7
	result := make([]byte, length)

	var value uint
	var bits int

	for i, b := range data {
		// Place new byte into value, shifting left by 8 bits
		value |= uint(b) << bits
		bits += 8

		// Extract as many 7-bit groups as possible
		for bits >= 7 {
			result[i] = byte(value & 0x7F) // Extract the 7-bit group
			value >>= 7                    // Remove the extracted bits
			bits -= 7                      // We've used 7 bits
		}
	}

	return string(result)
}