Hashtable: Extend nextPrime() to support 64-bit size range

Extend the nextPrime() function so that it can report primes up to
(2^63 - 25).

The old prime number sequence stops at (2^37 - 25). Although it's
possible to extend the sequence https://oeis.org/A014234 to support
up to 2^63, the code size can be smaller by encoding the differences to
the prime numbers (i.e. the sequence https://oeis.org/A013603 ) as the
array instead.

The next prime in the sequence, (2^64 - 59), is not useful for the
Hashtable purpose (and it exceeds SSIZE_MAX) and so is left out.

Signed-off-by: Kang-Che Sung <explorer09@gmail.com>

Author: Explorer09

Hashtable.c

@@ -10,6 +10,7 @@ in the source distribution for its full text.
 #include "Hashtable.h"
 
 #include <assert.h>
+#include <limits.h>
 #include <stdint.h>
 #include <stdlib.h>
 #include <string.h>
@@ -88,21 +89,21 @@ size_t Hashtable_count(const Hashtable* this) {
 
 #endif /* NDEBUG */
 
-/* https://oeis.org/A014234 */
-static const uint64_t OEISprimes[] = {
-   7, 13, 31, 61, 127, 251, 509, 1021, 2039, 4093, 8191,
-   16381, 32749, 65521, 131071, 262139, 524287, 1048573,
-   2097143, 4194301, 8388593, 16777213, 33554393,
-   67108859, 134217689, 268435399, 536870909, 1073741789,
-   2147483647, 4294967291, 8589934583, 17179869143,
-   34359738337, 68719476731, 137438953447
-};
-
 static size_t nextPrime(size_t n) {
-   /* on 32-bit make sure we do not return primes not fitting in size_t */
-   for (size_t i = 0; i < ARRAYSIZE(OEISprimes) && OEISprimes[i] < SIZE_MAX; i++) {
-      if (n <= OEISprimes[i])
-         return OEISprimes[i];
+   // Table of differences so that (2^m - primeDiffs[m]) is a prime.
+   // This is OEIS sequence https://oeis.org/A013603 except for
+   // entry 0 (2^0 = 1 as a non-prime special case).
+   static const uint8_t primeDiffs[64] = {
+      0, 0, 1, 1, 3, 1, 3, 1, 5, 3, 3, 9, 3, 1, 3, 19,
+      15, 1, 5, 1, 3, 9, 3, 15, 3, 39, 5, 39, 57, 3, 35, 1,
+      5, 9, 41, 31, 5, 25, 45, 7, 87, 21, 11, 57, 17, 55, 21, 115,
+      59, 81, 27, 129, 47, 111, 33, 55, 5, 13, 27, 55, 93, 1, 57, 25,
+   };
+
+   for (uint8_t shift = 3; shift < sizeof(n) * CHAR_BIT && shift < ARRAYSIZE(primeDiffs); shift++) {
+      size_t prime = ((size_t)1 << shift) - primeDiffs[shift];
+      if (n <= prime)
+         return prime;
    }
 
    CRT_fatalError("Hashtable: no prime found");