Merge pull request #4 from LTLA/rng_scope

Implement the RNGScope class for RAII-based RNG control.

examples/CMakeLists.txt

@@ -12,4 +12,5 @@ make_test(ex_list)
 make_test(ex_decompose)
 make_test(ex_error)
 make_test(ex_views)
+make_test(ex_rng_scope)
 make_test(ex_igraph_tutorial)

examples/ex_rng_scope.cpp

@@ -0,0 +1,31 @@
+#include <iostream>
+#include <igraph.hpp>
+#include "ex_vector_print.hpp"
+
+int main() {
+    // Check that we can influence the RNG.
+    {
+        ig::RNGScope filled(1000);
+        std::cout << "New RNG scope:" << std::endl;
+        std::cout << "  Draw from [0, 100]: " << igraph_rng_get_integer(igraph_rng_default(), 0, 100) << std::endl;
+        std::cout << "  Draw from [0, 100]: " << igraph_rng_get_integer(igraph_rng_default(), 0, 100) << std::endl;
+    }
+
+    // Check that the destructor correctly unsets the RNG.
+    {
+        ig::RNGScope filled(1000);
+        std::cout << "New RNG scope:" << std::endl;
+        std::cout << "  Draw from [0, 100]: " << igraph_rng_get_integer(igraph_rng_default(), 0, 100) << std::endl;
+
+        {
+            ig::RNGScope nested; // uses a default seed and type.
+            std::cout << "  New RNG scope:" << std::endl;
+            std::cout << "    Draw from [0, 100]: " << igraph_rng_get_integer(igraph_rng_default(), 0, 100) << std::endl;
+            std::cout << "    Draw from [0, 100]: " << igraph_rng_get_integer(igraph_rng_default(), 0, 100) << std::endl;
+        }
+
+        std::cout << "  Draw from [0, 100]: " << igraph_rng_get_integer(igraph_rng_default(), 0, 100) << std::endl;
+    }
+
+    return 0;
+}

include/igraph.hpp

@@ -86,6 +86,8 @@ using igIntVecList = igVecList<igraph_integer_t>;
 using igBoolVec = igVec<igraph_bool_t>;
 using igBoolMat = igMat<igraph_bool_t>;
 
+#include "rng_scope.hpp"
+
 class igGraphList;
 
 class igGraph {

include/rng_scope.hpp

@@ -0,0 +1,41 @@
+class RNGScope {
+public:
+    RNGScope(igraph_uint_t seed) : RNGScope(seed, &igraph_rngtype_pcg32) {}
+
+    RNGScope(igraph_uint_t seed, const igraph_rng_type_t* type) {
+        igCheck(igraph_rng_init(&current, type));
+
+        auto errcode = igraph_rng_seed(&current, seed);
+        if (errcode != IGRAPH_SUCCESS) {
+            igraph_rng_destroy(&current);
+            throw igException(errcode);
+        }
+
+        previous = *(igraph_rng_default());
+        igraph_rng_set_default(&current);
+    }
+
+    RNGScope(const igraph_rng_type_t* type) {
+        igCheck(igraph_rng_init(&current, type));
+        previous = *(igraph_rng_default());
+        igraph_rng_set_default(&current);
+    }
+
+    RNGScope() : RNGScope(&igraph_rngtype_pcg32) {}
+
+public:
+    // We shouldn't be copying or moving an RNGscope object, as this defeats the logic provided by scoping.
+    RNGScope(const RNGScope&) = delete;
+    RNGScope& operator=(const RNGScope&) = delete;
+    RNGScope(RNGScope&&) = delete;
+    RNGScope& operator=(RNGScope&&) = delete;
+
+    ~RNGScope() {
+        igraph_rng_set_default(&previous);
+        igraph_rng_destroy(&current);
+    }
+
+private:
+    igraph_rng_t previous;
+    igraph_rng_t current;
+};