feat: Add another hexadecimal to octal conversion (#658)

* Add another hexadecimal to octal conversion

* Apply suggestions

Also changed the return type of `hex_to_oct` to `const char *`, as it
returns an address of static variable.

* Update comment of hexadecimal_to_octal2.c

* updating DIRECTORY.md

* Apply suggestions

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>

DIRECTORY.md

@@ -17,6 +17,7 @@
   * [Decimal To Octal](https://github.com/TheAlgorithms/C/blob/master/conversions/decimal_to_octal.c)
   * [Decimal To Octal Recursion](https://github.com/TheAlgorithms/C/blob/master/conversions/decimal_to_octal_recursion.c)
   * [Hexadecimal To Octal](https://github.com/TheAlgorithms/C/blob/master/conversions/hexadecimal_to_octal.c)
+  * [Hexadecimal To Octal2](https://github.com/TheAlgorithms/C/blob/master/conversions/hexadecimal_to_octal2.c)
   * [Int To String](https://github.com/TheAlgorithms/C/blob/master/conversions/int_to_string.c)
   * [Octal To Binary](https://github.com/TheAlgorithms/C/blob/master/conversions/octal_to_binary.c)
   * [Octal To Decimal](https://github.com/TheAlgorithms/C/blob/master/conversions/octal_to_decimal.c)

conversions/hexadecimal_to_octal2.c

@@ -0,0 +1,119 @@
+/**
+ * @file
+ * @brief Convert hexadecimal number to octal number (with decimal intermediary)
+ * @details
+ * The input is valid from 0 to 0xFFFF_FFFF_FFFF_FFFF.
+ *
+ * At first, this program converts a hex string to an unsigned long long
+ * decimal, and then to an octal string.
+ *
+ * When there is an invalid character in input string, this program stops
+ * parsing and converts the string until that character.
+ *
+ * @see hexadecimal_to_octal.c
+ */
+
+#include <stdio.h>  /// for printf() and fgets()
+#include <string.h> /// for memset()
+
+/**
+ * @brief Convert a hexadecimal number to octal number.
+ * @param hex Hexadecimal number to convert.
+ * @returns A pointer to the converted octal string.
+ */
+const char *hex_to_oct(const char *hex)
+{
+#define MAX_OCT_STR_LEN 23 /* 17_7777_7777_7777_7777_7777 */
+    static char octal[MAX_OCT_STR_LEN];
+    memset(octal, '\0', MAX_OCT_STR_LEN); // Initialize as NULL string
+
+    unsigned long long decimal = 0;
+    int i = 0;
+    int len;
+
+    if (hex == NULL)
+    {
+        // Return an empty string
+        return octal;
+    }
+
+    /* Hexadecimal to decimal conversion */
+    while (*hex != '\n' && *hex != '\0')
+    {
+        char ch = *hex;
+
+        if (ch >= '0' && ch <= '9')
+        {
+            ch -= '0';
+        }
+        else if (ch >= 'a' && ch <= 'f')
+        {
+            ch = ch - 'a' + 10;
+        }
+        else if (ch >= 'A' && ch <= 'F')
+        {
+            ch = ch - 'A' + 10;
+        }
+        else
+        {
+            printf("Invalid hexadecimal input: %c\n", ch);
+            break;
+        }
+
+        decimal *= 16;
+        decimal += ch;
+        hex++;
+    }
+
+    /* Decimal to octal conversion */
+    if (decimal == 0)
+    {
+        octal[0] = '0';
+        len = 1;
+    }
+    else
+    {
+        i = 0;
+        while (decimal > 0)
+        {
+            octal[i] = '0' + decimal % 8;
+            i++;
+            decimal /= 8;
+        }
+
+        len = i;
+    }
+
+    octal[len] = '\0';
+
+    /* Reverse the octal string */
+    for (i = 0; i < len / 2; i++)
+    {
+        char tmp = octal[i];
+        octal[i] = octal[len - i - 1];
+        octal[len - i - 1] = tmp;
+    }
+
+    return octal;
+}
+
+/**
+ * @brief Main function
+ * @returns 0 on exit
+ */
+int main()
+{
+#define MAX_HEX_STR_LEN 17 /* FFFF_FFFF_FFFF_FFFF */
+    char hex[MAX_HEX_STR_LEN];
+
+    /* Input hexadecimal number from user */
+    printf("Enter any hexadecimal number: ");
+    fgets(hex, MAX_HEX_STR_LEN, stdin);
+
+    const char *octal = hex_to_oct(hex);
+
+    printf("Hexadecimal number = %s\n", hex);
+    printf("Octal number = %s\n", octal);
+
+    return 0;
+}