tasks a-s

Task/Non-continuous-subsequences/C/non-continuous-subsequences-3.c

@@ -0,0 +1,51 @@
+#include <stdio.h>
+
+typedef unsigned char sint;
+enum states { s_blnk = 0, s_tran, s_cont, s_disj };
+
+/* Recursively look at each item in list, taking both choices of
+   picking the item or not.  The state at each step depends on prvious
+   pickings, with the state transition table:
+	blank + no pick -> blank
+	blank + pick -> contiguous
+	transitional + no pick -> transitional
+	transitional + pick -> disjoint
+	contiguous + no pick -> transitional
+	contiguous + pick -> contiguous
+	disjoint + pick -> disjoint
+	disjoint + no pick -> disjoint
+   At first step, before looking at any item, state is blank.
+   Because state is known at each step and needs not be calculated,
+   it can be quite fast.
+*/
+unsigned char tbl[][2] = {
+	{ s_blnk, s_cont },
+	{ s_tran, s_disj },
+	{ s_tran, s_cont },
+	{ s_disj, s_disj },
+};
+
+void pick(sint n, sint step, sint state, char **v, unsigned long bits)
+{
+	int i, b;
+	if (step == n) {
+		if (state != s_disj) return;
+		for (i = 0, b = 1; i < n; i++, b <<= 1)
+			if ((b & bits)) printf("%s ", v[i]);
+		putchar('\n');
+		return;
+	}
+
+	bits <<= 1;
+	pick(n, step + 1, tbl[state][0], v, bits); /* no pick */
+	pick(n, step + 1, tbl[state][1], v, bits | 1); /* pick */
+}
+
+int main(int c, char **v)
+{
+	if (c - 1 >= sizeof(unsigned long) * 4)
+		printf("Too many items");
+	else
+		pick(c - 1, 0, s_blnk, v + 1, 0);
+	return 0;
+}