Add new sub-package HexUtils

This new sub-package contains two utility methods for converting byte
array from/to hex string.

HexUtils/README.md

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+# HexUtils
+
+## Introduction
+
+This is a simple sample code for converting hex string to/from byte array.
+
+## Usage
+
+You can easily understand how to use the following 2 utility methods:
+
+- unsigned char *util_hex_string_to_byte_array(const char *string, size_t length)
+- char *util_byte_array_to_hex_string(const unsigned char* array, size_t length)
+
+## Dependency
+
+There is no special dependency on other external libraries.
+
+## Licence
+
+This software is released under the MIT License, see LICENSE.
+
+## Author
+
+Cheerio (cheerio.the.bear@gmail.com)

HexUtils/hex_utils.c

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+/*
+ *  Copyright (C) 2021 Cheerio <cheerio.the.bear@gmail.com>
+ *
+ *  This program is free software; you can redistribute it and/or modify it
+ *  under the terms of the MIT license.
+ *  See the license information described in LICENSE file.
+ */
+
+#include <assert.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+static const char *UTIL_CONVERT_TO_HEX_CHAR = "0123456789ABCDEF";
+
+unsigned char *util_hex_string_to_byte_array(const char *string, size_t length)
+{
+    unsigned char *array;
+    int index;
+    unsigned char octet;
+    char nibble;
+
+    if ((array = (unsigned char *) malloc(length / 2)) == NULL) {
+        return NULL;
+    }
+
+    for (index = 0; index < length; index++) {
+        nibble = string[index];
+
+        if (('0' <= nibble) && (nibble <= '9')) {
+            nibble = nibble - '0';
+        } else if (('a' <= nibble) && (nibble <= 'f')) {
+            nibble = 0x0A + (nibble - 'a');
+        } else if (('A' <= nibble) && (nibble <= 'F')) {
+            nibble = 0x0A + (nibble - 'A');
+        } else {
+            free(array);
+            return NULL;
+        }
+
+        if ((index % 2) == 0) {
+            octet = nibble;
+        } else {
+            array[index / 2] = (octet << 4) + nibble;
+        }
+    }
+
+    return array;
+}
+
+char *util_byte_array_to_hex_string(const unsigned char* array, size_t length)
+{
+    char *string;
+    char *destination;
+    int index;
+    unsigned char octet;
+
+    if (length == 0) {
+        return NULL;
+    }
+
+    if ((string = destination = (char *) malloc(length * 2 + 1)) == NULL) {
+        return NULL;
+    }
+
+    for (index = 0; index < length; index++) {
+        octet = array[index];
+        *destination++ = UTIL_CONVERT_TO_HEX_CHAR[octet / 0x10];
+        *destination++ = UTIL_CONVERT_TO_HEX_CHAR[octet % 0x10];
+    }
+
+    *destination = '\0';
+    return string;
+}
+
+int main(int argc, char *argv[]) {
+
+    const char *data = "00017F8081FEFF";
+    unsigned char *array;
+    char *string;
+
+    array = util_hex_string_to_byte_array(data, strlen(data));
+    assert(array[0] == 0x00);
+    assert(array[1] == 0x01);
+    assert(array[2] == 0x7F);
+    assert(array[3] == 0x80);
+    assert(array[4] == 0x81);
+    assert(array[5] == 0xFE);
+    assert(array[6] == 0xFF);
+
+    string = util_byte_array_to_hex_string(array, strlen(data) / 2);
+    assert(strcmp(data, string) == 0);
+
+    free(array);
+    free(string);
+
+    printf("OK\n");
+
+    exit(0);
+}