From 8f2d613931c56e11652968db19b8d2fff3062f70 Mon Sep 17 00:00:00 2001
From: Miles Bader <miles@gnu.org>
Date: Fri, 31 Mar 1995 20:31:10 +0000
Subject: Initial revision

---
 libihash/primes.c | 135 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 135 insertions(+)
 create mode 100644 libihash/primes.c

(limited to 'libihash/primes.c')

diff --git a/libihash/primes.c b/libihash/primes.c
new file mode 100644
index 00000000..d0fc4d0c
--- /dev/null
+++ b/libihash/primes.c
@@ -0,0 +1,135 @@
+/* Prime number generation
+   Copyright (C) 1994 Free Software Foundation
+
+   This program is free software; you can redistribute it and/or
+   modify it under the terms of the GNU General Public License as
+   published by the Free Software Foundation; either version 2, or (at
+   your option) any later version.
+
+   This program is distributed in the hope that it will be useful, but
+   WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program; if not, write to the Free Software
+   Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. */
+
+#include <stdlib.h>
+#include <limits.h>
+#include <string.h>
+#include <assert.h>
+
+#define BITS_PER_UNSIGNED (8 * sizeof (unsigned))
+#define SQRT_INT_MAX (1 << (BITS_PER_UNSIGNED / 2))
+
+/* Return the next prime greater than or equal to N. */
+int 
+nextprime (unsigned n)
+{
+  /* Among other things, We guarantee that, for all i (0 <= i < primes_len), 
+     primes[i] is a prime,
+     next_multiple[i] is a multiple of primes[i],
+     next_multiple[i] > primes[primes_len - 1],
+     next_multiple[i] is not a multiple of two unless primes[i] == 2, and
+     next_multiple[i] is the smallest such value.  */
+  static unsigned *primes, *next_multiple;
+  static int primes_len;
+  static int primes_size;
+  static unsigned next_sieve;	/* always even */
+  unsigned max_prime;
+
+  if (! primes)
+    {
+      primes_size = 128;
+      primes        = (unsigned *) malloc (primes_size * sizeof (*primes));
+      next_multiple = (unsigned *) malloc (primes_size
+					   * sizeof (*next_multiple));
+
+      primes[0] = 2;		next_multiple[0] = 6;
+      primes[1] = 3;		next_multiple[1] = 9;
+      primes[2] = 5;		next_multiple[2] = 15;
+      primes_len = 3;
+
+      next_sieve = primes[primes_len - 1] + 1;
+    }
+
+  if (n <= primes[0])
+    return primes[0];
+
+  while (n > (max_prime = primes[primes_len - 1])) 
+    {
+      /* primes doesn't contain any prime large enough.  Sieve from
+         max_prime + 1 to 2 * max_prime, looking for more primes.  */
+      unsigned start = next_sieve;
+      unsigned end   = start + max_prime + 1;
+      char *sieve = (char *) alloca ((end - start) * sizeof (*sieve));
+      int i;
+
+      assert (sieve);
+
+      bzero (sieve, (end - start) * sizeof (*sieve));
+
+      /* Make the sieve indexed by prime number, rather than
+	 distance-from-start-to-the-prime-number.  When we're done,
+	 sieve[P] will be zero iff P is prime.
+
+	 ANSI C doesn't define what this means.  Fuck them.  */
+      sieve -= start;
+
+      /* Set sieve[i] for all composites i, start <= i < end.
+	 Ignore multiples of 2.  */
+      for (i = 1; i < primes_len; i++)
+	{
+	  unsigned twice_prime = 2 * primes[i];
+	  unsigned multiple;
+
+	  for (multiple = next_multiple[i];
+	       multiple < end;
+	       multiple += twice_prime)
+	    sieve[multiple] = 1;
+	  next_multiple[i] = multiple;
+	}
+
+      for (i = start + 1; i < end; i += 2)
+	if (! sieve[i])
+	  {
+	    if (primes_len >= primes_size)
+	      {
+		primes_size *= 2;
+		primes = (int *) realloc (primes,
+					  primes_size * sizeof (*primes));
+		next_multiple
+		  = (int *) realloc (next_multiple,
+				     primes_size * sizeof (*next_multiple));
+	      }
+	    primes[primes_len] = i;
+	    if (i >= SQRT_INT_MAX)
+	      next_multiple[primes_len] = INT_MAX;
+	    else
+	      next_multiple[primes_len] = i * i;
+	    primes_len++;
+	  }
+
+      next_sieve = end;
+    }
+
+  /* Now we have at least one prime >= n.  Find the smallest such.  */
+  {
+    int bottom = 0;
+    int top = primes_len;
+
+    while (bottom < top)
+      {
+	int mid = (bottom + top) / 2;
+
+	if (primes[mid] < n)
+	  bottom = mid + 1;
+	else
+	  top = mid;
+      }
+    
+    return primes[top];
+  }
+}
+
-- 
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