Hello world and puzzle program.

combinations/src/combinations.c

@@ -0,0 +1,30 @@
+/* calculates all subsets of the string comb by assigning eatch letter a position in a binary number and counting up. */
+#include <stdio.h>
+#include <string.h>
+
+// #define DEBUG
+
+int main() {
+  char* comb = "ABCD";
+  int count = strlen(comb);
+  // that bitshifting is the same is squaring count
+  int permutations = (1 << count);
+
+  printf("string is %s\nthere are %d permutations", comb, permutations);
+
+  for (unsigned char i = 0; i < permutations; i++) {
+    for (unsigned char j = 0; j < 4; j++) {
+      // when j=0, will result in 0b0001, when j=1, 0b0010, j=2 0b0100.
+      unsigned char mask = 1 << j;
+#ifdef DEBUG
+      printf(" j=%x mask=%x, postmask=%x, i=%x", j, mask, (i & mask), i);
+#endif /* ifdef DEBUG */
+      if ((i & mask) != 0) {
+        printf(" %c", comb[j]);
+      }
+    }
+    printf("\n");
+  }
+
+  return 0;
+}