add string new from slice

examples/string_example.c

@@ -0,0 +1,10 @@
+#include <stdio.h>
+#include "../string.h"
+
+
+int main()
+{
+    char *substr = "Hello, World!";
+    struct string_t *str = string_new_slice(substr, 0, 4);
+    printf("%s\n", str->data);
+}

string.c

@@ -40,6 +40,24 @@ void string_free(struct string_t *s)
     free(s);
 }
 
+struct string_t *string_new_slice(const char *data, size_t start, size_t end)
+{
+    /* why +2? 
+     * 1. Null termination.
+     * 2. We the entire data from the start position to the end position to be present.
+     *    say start = 0 and end = 1, we want both the 0th char and the 1st char, i.e end - start + 1.
+     */
+    size_t len = end - start + 2;
+    struct string_t *s = malloc(sizeof(struct string_t));
+    s->capacity = __STRING_DEFAULT_CAPACITY__ >= len ? __STRING_DEFAULT_CAPACITY__ : len;
+    s->data = (char *)(malloc(__CHAR_BIT__ * s->capacity));
+    s->length = len;
+
+    memcpy(s->data, data + start, len);
+    s->data[s->length - 1] = 0;
+    return s;
+}
+
 size_t string_length(const struct string_t *s)
 {
     if (s == NULL)

string.h

@@ -36,6 +36,8 @@ struct string_t *string_new(const char *data);
  */
 void string_free(struct string_t *str);
 
+struct string_t *string_new_slice(const char *data, size_t start, size_t end);
+
 /**
  * Gets the length of a string.
  * @param str The string.
@@ -77,4 +79,4 @@ int string_compare(const struct string_t *src, const struct string_t *dest);
 bool string_equal(const struct string_t *str1, const struct string_t *str2);
 
 
-#endif
+#endif