Data commit

Task/Combinations/C/combinations-1.c

@@ -0,0 +1,29 @@
+#include <stdio.h>
+
+/* Type marker stick: using bits to indicate what's chosen.  The stick can't
+ * handle more than 32 items, but the idea is there; at worst, use array instead */
+typedef unsigned long marker;
+marker one = 1;
+
+void comb(int pool, int need, marker chosen, int at)
+{
+	if (pool < need + at) return; /* not enough bits left */
+
+	if (!need) {
+		/* got all we needed; print the thing.  if other actions are
+		 * desired, we could have passed in a callback function. */
+		for (at = 0; at < pool; at++)
+			if (chosen & (one << at)) printf("%d ", at);
+		printf("\n");
+		return;
+	}
+	/* if we choose the current item, "or" (|) the bit to mark it so. */
+	comb(pool, need - 1, chosen | (one << at), at + 1);
+	comb(pool, need, chosen, at + 1);  /* or don't choose it, go to next */
+}
+
+int main()
+{
+	comb(5, 3, 0, 0);
+	return 0;
+}

Task/Combinations/C/combinations-2.c

@@ -0,0 +1,27 @@
+#include <stdio.h>
+
+void comb(int m, int n, unsigned char *c)
+{
+	int i;
+	for (i = 0; i < n; i++) c[i] = n - i;
+
+	while (1) {
+		for (i = n; i--;)
+			printf("%d%c", c[i], i ? ' ': '\n');
+
+		/* this check is not strictly necessary, but if m is not close to n,
+		   it makes the whole thing quite a bit faster */
+		i = 0;
+		if (c[i]++ < m) continue;
+
+		for (; c[i] >= m - i;) if (++i >= n) return;
+		for (c[i]++; i; i--) c[i-1] = c[i] + 1;
+	}
+}
+
+int main()
+{
+	unsigned char buf[100];
+	comb(5, 3, buf);
+	return 0;
+}