Add pcg-advance-128.

ghc-mod is complaining it isn't there. Don't know why.

c/pcg-advance-128.c

@@ -0,0 +1,64 @@
+/*
+ * PCG Random Number Generation for C.
+ *
+ * Copyright 2014 Melissa O'Neill <oneill@pcg-random.org>
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * For additional information about the PCG random number generation scheme,
+ * including its license and other licensing options, visit
+ *
+ *       http://www.pcg-random.org
+ */
+
+/*
+ * This code is derived from the canonical C++ PCG implementation, which
+ * has many additional features and is preferable if you can use C++ in
+ * your project.
+ *
+ * Repetative C code is derived using C preprocessor metaprogramming
+ * techniques.
+ */
+
+#include "pcg_variants.h"
+
+/* Multi-step advance functions (jump-ahead, jump-back)
+ *
+ * The method used here is based on Brown, "Random Number Generation
+ * with Arbitrary Stride,", Transactions of the American Nuclear
+ * Society (Nov. 1994).  The algorithm is very similar to fast
+ * exponentiation.
+ *
+ * Even though delta is an unsigned integer, we can pass a
+ * signed integer to go backwards, it just goes "the long way round".
+ */
+
+#if PCG_HAS_128BIT_OPS
+pcg128_t pcg_advance_lcg_128(pcg128_t state, pcg128_t delta, pcg128_t cur_mult,
+                             pcg128_t cur_plus)
+{
+    pcg128_t acc_mult = 1u;
+    pcg128_t acc_plus = 0u;
+    while (delta > 0) {
+        if (delta & 1) {
+            acc_mult *= cur_mult;
+            acc_plus = acc_plus * cur_mult + cur_plus;
+        }
+        cur_plus = (cur_mult + 1) * cur_plus;
+        cur_mult *= cur_mult;
+        delta /= 2;
+    }
+    return acc_mult * state + acc_plus;
+}
+#endif
+

pcg-random.cabal

@@ -55,6 +55,7 @@ library
     -std=c99 -fPIC
 
   c-sources:
+    c/pcg-advance-128.c
     c/pcg-advance-64.c
     c/pcg-global-64.c
     c/pcg-output-64.c