1 files changed, 12 insertions, 10 deletions

diff --git a/src/utils/math.h b/src/utils/math.h
index 9936558..ebbd63e 100644
--- a/src/utils/math.h
+++ b/src/utils/math.h
@@ -49,7 +49,7 @@ permtoindex(size_t n, const uint8_t *a)
 
         for (i = 0, ret = 0; i < n; i++) {
                 for (j = i+1, c = 0; j < n; j++)
-                        c += (a[i] > a[j]) ? 1 : 0;
+                        c += a[i] > a[j];
                 ret += factorial[n-i-1] * c;
         }
 
@@ -59,21 +59,23 @@ permtoindex(size_t n, const uint8_t *a)
 STATIC void
 indextoperm(int64_t p, size_t n, uint8_t *r)
 {
-        int64_t c;
-        size_t i, j;
-        uint8_t a[FACTORIAL_MAX+1];
+        int64_t c, k;
+        size_t i, j, used;
 
         DBG_ASSERT(n <= FACTORIAL_MAX, "Error: cannot compute indextoperm() "
             "for set of size %zu > %" PRId64 "\n", n, FACTORIAL_MAX);
         DBG_ASSERT(p >= 0 && p < factorial[n], "Error: invalid permutation "
             "index %" PRId64 " for set of size %zu\n", p, n);
 
-        memset(a, 0, n);
-        for (i = 0; i < n; i++) {
-                for (j = 0, c = 0; c <= p / factorial[n-i-1]; j++)
-                        c += a[j] ? 0 : 1;
+        for (i = 0, used = 0; i < n; i++) {
+                k = p / factorial[n-i-1];
+
+                /* Find k-th unused number */
+                for (j = 0, c = 0; c <= k; j++)
+                        c += 1 - ((used & (1<<j)) >> j);
+
                 r[i] = j-1;
-                a[j-1] = 1;
+                used |= 1 << (j-1);
                 p %= factorial[n-i-1];
         }
 
@@ -87,7 +89,7 @@ permsign(size_t n, const uint8_t *a)
 
         for (i = 0, ret = 0; i < n; i++)
                 for (j = i+1; j < n; j++)
-                        ret += a[i] > a[j] ? 1 : 0;
+                        ret += a[i] > a[j];
 
         return ret % 2;
 }