* mpz/bin_uiui.c: Fix result for n==0 and n==k.

This is per a bug-gmp report by Hans J Lahne <hjlas@online.no>, but
not using the patch offerred, instead taking the opportunity to
flatten out the nested conditionals.

The diff for this revision is a bit horrible, but only the initial
setups have changed, the main loop has just been reindented.

1 files changed, 63 insertions, 58 deletions

diff --git a/mpz/bin_uiui.c b/mpz/bin_uiui.c
index 8035303c8..813e35e25 100644
--- a/mpz/bin_uiui.c
+++ b/mpz/bin_uiui.c
@@ -33,73 +33,78 @@ mpz_bin_uiui (r, n, k)
      unsigned long int k;
 #endif
 {
+  unsigned long int  i, j;
+  mp_limb_t          nacc, kacc;
+  unsigned long int  cnt;
+
+  /* bin(n,k) = 0 if k>n. */
   if (n < k)
-    /* This special case is a good idea since the unsigned variable n would
-       else become negative.  It is not an exception that is mathematically
-       necessary.  */
-    mpz_set_ui (r, 0);
-  else
     {
-      unsigned long int i, j;
-      mp_limb_t nacc, kacc;
-      unsigned long int cnt;
+      mpz_set_ui (r, 0);
+      return;
+    }
 
-      /* Rewrite bin(n,k) as bin(n,n-k) if that is simpler to compute.  */
-      if (k > n / 2)
-	k = n - k;
+  /* Rewrite bin(n,k) as bin(n,n-k) if that is smaller. */
+  k = MIN (k, n-k);
+
+  /* bin(n,0) = 1 */
+  if (k == 0)
+    {
+      mpz_set_ui (r, 1);
+      return;
+    }
 
-      j = n - k + 1;
-      mpz_set_ui (r, j);
+  j = n - k + 1;
+  mpz_set_ui (r, j);
 
-      /* Initialize accumulators.  */
-      nacc = 1;
-      kacc = 1;
+  /* Initialize accumulators.  */
+  nacc = 1;
+  kacc = 1;
 
-      cnt = 0;
-      for (i = 2; i <= k; i++)
-	{
-	  mp_limb_t n1, n0, k1, k0;
+  cnt = 0;
+  for (i = 2; i <= k; i++)
+    {
+      mp_limb_t n1, n0, k1, k0;
 
-	  j++;
+      j++;
 #if 0
-	  /* Remove common multiples of 2.  This will allow us to accumulate
-	     more in nacc and kacc before we need a bignum step.  It would make
-	     sense to cancel factors of 3, 5, etc too, but this would be best
-	     handled by sieving out factors.  Alternatively, we could perform a
-	     gcd of the accumulators just as they have overflown, and keep
-	     accumulating until the gcd doesn't remove a significant factor.  */
-	  while (((nacc | kacc) & 1) == 0)
-	    {
-	      nacc >>= 1;
-	      kacc >>= 1;
-	    }
+      /* Remove common multiples of 2.  This will allow us to accumulate
+         more in nacc and kacc before we need a bignum step.  It would make
+         sense to cancel factors of 3, 5, etc too, but this would be best
+         handled by sieving out factors.  Alternatively, we could perform a
+         gcd of the accumulators just as they have overflown, and keep
+         accumulating until the gcd doesn't remove a significant factor.  */
+      while (((nacc | kacc) & 1) == 0)
+        {
+          nacc >>= 1;
+          kacc >>= 1;
+        }
 #else
-	  cnt = ((nacc | kacc) & 1) ^ 1;
-	  nacc >>= cnt;
-	  kacc >>= cnt;
+      cnt = ((nacc | kacc) & 1) ^ 1;
+      nacc >>= cnt;
+      kacc >>= cnt;
 #endif
-	  /* Accumulate next multiples.  */
-	  umul_ppmm (n1, n0, nacc, j);
-	  umul_ppmm (k1, k0, kacc, i);
-	  if (n1 != 0)
-	    {
-	      /* Accumulator overflow.  Perform bignum step.  */
-	      mpz_mul_ui (r, r, nacc);
-	      nacc = j;
-	      mpz_tdiv_q_ui (r, r, kacc);
-	      kacc = i;
-	    }
-	  else
-	    {
-	      if (k1 != 0) abort ();
-	      /* Save new products in accumulators to keep accumulating.  */
-	      nacc = n0;
-	      kacc = k0;
-	    }
-	}
-
-      /* Take care of whatever is left in accumulators.  */
-      mpz_mul_ui (r, r, nacc);
-      mpz_tdiv_q_ui (r, r, kacc);
+      /* Accumulate next multiples.  */
+      umul_ppmm (n1, n0, nacc, j);
+      umul_ppmm (k1, k0, kacc, i);
+      if (n1 != 0)
+        {
+          /* Accumulator overflow.  Perform bignum step.  */
+          mpz_mul_ui (r, r, nacc);
+          nacc = j;
+          mpz_tdiv_q_ui (r, r, kacc);
+          kacc = i;
+        }
+      else
+        {
+          if (k1 != 0) abort ();
+          /* Save new products in accumulators to keep accumulating.  */
+          nacc = n0;
+          kacc = k0;
+        }
     }
+
+  /* Take care of whatever is left in accumulators.  */
+  mpz_mul_ui (r, r, nacc);
+  mpz_tdiv_q_ui (r, r, kacc);
 }