1 files changed, 125 insertions, 0 deletions

diff --git a/src/utils.h b/src/utils.h
new file mode 100644
index 0000000..0c0b4d1
--- /dev/null
+++ b/src/utils.h
@@ -0,0 +1,125 @@
+_static int64_t factorial(int64_t);
+_static bool isperm(uint8_t *, int64_t);
+_static int64_t permtoindex(uint8_t *, int64_t);
+_static void indextoperm(int64_t, int64_t, uint8_t *);
+_static int permsign(uint8_t *, int64_t);
+
+_static int64_t
+factorial(int64_t n)
+{
+        int64_t i, ret;
+
+        if (n > _max_factorial) {
+                LOG("Error: won't compute factorial for n=%" PRId64 " because"
+                    " it is larger than %" PRId64 "\n", n, _max_factorial);
+                return -1;
+        }
+
+        if (n < 0)
+                return 0;
+
+        for (i = 1, ret = 1; i <= n; i++)
+                ret *= i;
+
+        return ret;
+}
+
+_static bool
+isperm(uint8_t *a, int64_t n)
+{
+        int64_t i;
+        bool aux[_max_factorial+1];
+
+        if (n > _max_factorial) {
+                LOG("Error: won't compute 'isperm()' for n=%" PRId64 " because"
+                    " it is larger than %" PRId64 "\n", n, _max_factorial);
+                return false;
+        }
+
+        memset(aux, false, n);
+        
+        for (i = 0; i < n; i++) {
+                if (a[i] < 0 || a[i] >= n)
+                        return false;
+                else
+                        aux[a[i]] = true;
+        }
+
+        for (i = 0; i < n; i++)
+                if (!aux[i])
+                        return false;
+
+        return true;
+}
+
+_static int64_t
+permtoindex(uint8_t *a, int64_t n)
+{
+        int64_t i, j, c, ret;
+
+        if (n > _max_factorial) {
+                LOG("Error: won't compute 'permtoindex()' for n=%" PRId64
+                    " because it is larger than %" PRId64 "\n",
+                    n, _max_factorial);
+                return -1;
+        }
+
+        if (!isperm(a, n))
+                return -1;
+
+        for (i = 0, ret = 0; i < n; i++) {
+                for (j = i+1, c = 0; j < n; j++)
+                        c += (a[i] > a[j]) ? 1 : 0;
+                ret += factorial(n-i-1) * c;
+        }
+
+        return ret;
+}
+
+_static void
+indextoperm(int64_t p, int64_t n, uint8_t *r)
+{
+        int64_t i, j, c;
+        uint8_t a[_max_factorial+1];
+
+        if (n > _max_factorial) {
+                LOG("Error: won't compute 'permtoindex()' for n=%" PRId64
+                    " because it is larger than %" PRId64 "\n",
+                    n, _max_factorial);
+                goto indextoperm_error;
+        }
+
+        memset(a, 0, n);
+
+        if (p < 0 || p >= factorial(n))
+                goto indextoperm_error;
+
+        for (i = 0; i < n; i++) {
+                for (j = 0, c = 0; c <= p / factorial(n-i-1); j++)
+                        c += a[j] ? 0 : 1;
+                r[i] = j-1;
+                a[j-1] = 1;
+                p %= factorial(n-i-1);
+        }
+
+        if (!isperm(r, n))
+                goto indextoperm_error;
+
+        return;
+
+indextoperm_error:
+        memset(r, _error, n);
+}
+
+_static int
+permsign(uint8_t *a, int64_t n)
+{
+        int i, j;
+        uint8_t ret;
+
+        for (i = 0, ret = 0; i < n; i++)
+                for (j = i+1; j < n; j++)
+                        ret += a[i] > a[j] ? 1 : 0;
+
+        return ret % 2;
+}