James H has implemented a new `Tricky' difficulty level in Light Up:

a non-recursive level above Easy, which therefore moves the
recursive Hard mode further up still. Play-testing suggests that in
fact Tricky is often _harder_ than the old Hard mode, since the
latter had limited depth of recursion and would therefore spot
complex deductions only if it happened to start a recursion on the
right square; Tricky may be limited in the sophistication of its
complex deductions, but it never misses one, so its puzzles tend to
be hard all over.

Also in this checkin, a new source file `nullfe.c', containing all
the annoying stub functions required to make command-line solvers
link successfully. James wrote this for (the new) lightupsolver, and
I've used it to simplify the other stand-alone solvers.

[originally from svn r6254]

combi.c

@@ -0,0 +1,110 @@
+#include <assert.h>
+#include <string.h>
+
+#include "puzzles.h"
+
+/* horrific and doesn't check overflow. */
+static long factx(long x, long y)
+{
+    long acc = 1, i;
+
+    for (i = y; i <= x; i++)
+        acc *= i;
+    return acc;
+}
+
+void reset_combi(combi_ctx *combi)
+{
+    int i;
+    combi->nleft = combi->total;
+    for (i = 0; i < combi->r; i++)
+        combi->a[i] = i;
+}
+
+combi_ctx *new_combi(int r, int n)
+{
+    long nfr, nrf;
+    combi_ctx *combi;
+
+    assert(r <= n);
+    assert(n >= 1);
+
+    combi = snew(combi_ctx);
+    memset(combi, 0, sizeof(combi_ctx));
+    combi->r = r;
+    combi->n = n;
+
+    combi->a = snewn(r, int);
+    memset(combi->a, 0, r * sizeof(int));
+
+    nfr = factx(n, r+1);
+    nrf = factx(n-r, 1);
+    combi->total = (int)(nfr / nrf);
+
+    reset_combi(combi);
+    return combi;
+}
+
+/* returns NULL when we're done otherwise returns input. */
+combi_ctx *next_combi(combi_ctx *combi)
+{
+    int i = combi->r - 1, j;
+
+    if (combi->nleft == combi->total)
+        goto done;
+    else if (combi->nleft <= 0)
+        return NULL;
+
+    while (combi->a[i] == combi->n - combi->r + i)
+        i--;
+    combi->a[i] += 1;
+    for (j = i+1; j < combi->r; j++)
+        combi->a[j] = combi->a[i] + j - i;
+
+    done:
+    combi->nleft--;
+    return combi;
+}
+
+void free_combi(combi_ctx *combi)
+{
+    sfree(combi->a);
+    sfree(combi);
+}
+
+/* compile this with:
+ *   gcc -o combi.exe -DSTANDALONE_COMBI_TEST combi.c malloc.c
+ */
+#ifdef STANDALONE_COMBI_TEST
+
+#include <stdio.h>
+
+void fatal(char *fmt, ...)
+{
+    abort();
+}
+
+int main(int argc, char *argv[])
+{
+    combi_ctx *c;
+    int i, r, n;
+
+    if (argc < 3) {
+        fprintf(stderr, "Usage: combi R N\n");
+        exit(1);
+    }
+
+    r = atoi(argv[1]); n = atoi(argv[2]);
+    c = new_combi(r, n);
+    printf("combi %d of %d, %d elements.\n", c->r, c->n, c->total);
+
+    while (next_combi(c)) {
+        for (i = 0; i < c->r; i++) {
+            printf("%d ", c->a[i]);
+        }
+        printf("\n");
+    }
+    free_combi(c);
+}
+
+#endif