initial commit

c/difference-of-squares/src/difference_of_squares.c

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+#include "difference_of_squares.h"
+
+/* See GNUmakefile in following link for explanation
+ * https://exercism.io/my/solutions/103b2f7d92db42309c1988030f5202c7
+ */
+#if defined UNIT_TEST || defined DEBUG
+#include <stdio.h>
+#include <stdlib.h>
+#endif
+
+unsigned int sum_of_squares(unsigned int number)
+{
+    /* the sum of 1st n integers squares is:
+     *  S = 1² + 2² + 3² ..... + (n-1)² + n²
+     *    = [ n * (n+1) * (2n+1) ] / 6
+     * some visual explanations on:
+     *   http://www.takayaiwamoto.com/Sums_and_Series/sumsqr_1.html
+     */
+    return number * (number+1) * (2*number + 1) / 6;
+}
+
+unsigned int square_of_sum(unsigned int number)
+{
+    register int res;
+    /* The sum of n 1st integers is:
+     *  S  = 1 + 2 + 3 ... + (n-1) + n
+     *     = [ n * (n+1) ] / 2
+     * demonstration is trivial for this one.
+     */
+    res=number * (number+1) / 2;
+    return res*res;
+}
+
+unsigned int difference_of_squares(unsigned int number)
+{
+    return square_of_sum(number) - sum_of_squares(number);
+}
+
+#ifdef UNIT_TEST
+int main(int ac, char **av)
+{
+    int arg=1;
+    int i;
+
+    for (; arg<ac; ++arg) {
+        i=atoi(av[arg]);
+        printf("sumsq(%d)=%d\n", i, sum_of_squares(i));
+        printf("sqsum(%d)=%d\n", i, square_of_sum(i));
+        printf("diff(%d)=%d\n", i, difference_of_squares(i));
+    }
+}
+#endif

c/difference-of-squares/src/difference_of_squares.h

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+#ifndef DIFFERENCE_OF_SQUARES_H
+#define DIFFERENCE_OF_SQUARES_H
+
+unsigned int sum_of_squares(unsigned int number);
+unsigned int square_of_sum(unsigned int number);
+unsigned int difference_of_squares(unsigned int number);
+
+/* See GNUmakefile in following link for explanation
+ * https://exercism.io/my/solutions/103b2f7d92db42309c1988030f5202c7
+ */
+#ifdef  TESTALL
+#undef  TEST_IGNORE
+#define TEST_IGNORE() {}
+#endif
+
+#endif