Merge commit 'd581e9b58dbd841707deb2469c649b423358b657' into prompt/random-selector

README.md

@@ -10,6 +10,7 @@ A templated C++20 Wave Function Collapse engine that can work with 2D grids, 3D
 - **Multiple World Types**: Built-in support for 2D/3D arrays, graphs, and Sudoku grids
 - **Coordinate Agnostic**: Works without coordinate systems using only cell IDs
 - **C++20**: Uses modern C++ features like concepts, ranges, and smart pointers
+- **Custom Random Selectors**: Pluggable random selection strategies for cell collapse, including compile-time compatible options
 
 ## Project Structure
 
@@ -31,6 +32,78 @@ nd-wfc/
 └── build/                     # Build directory (generated)
 ```
 
+## Custom Random Selectors
+
+The WFC library now supports customizable random selection strategies for choosing cell values during the collapse process. This allows users to implement different randomization algorithms while maintaining compile-time compatibility.
+
+### Features
+
+- **Default Random Selector**: A constexpr-compatible seed-based randomizer using linear congruential generator
+- **Advanced Random Selector**: High-quality randomization using `std::mt19937` and `std::uniform_int_distribution`
+- **Custom Selectors**: Support for user-defined selector classes that can capture state and maintain behavior between calls
+- **Lambda Support**: Selectors can be implemented as lambdas with captured variables for flexible behavior
+
+### Usage
+
+#### Basic Usage with Default Selector
+
+```cpp
+using MyWFC = WFC::Builder<MyWorld, int, VariableMap>
+    ::SetRandomSelector<WFC::DefaultRandomSelector<int>>
+    ::Build;
+
+MyWorld world(size);
+MyWFC::Run(world, seed);
+```
+
+#### Advanced Random Selector
+
+```cpp
+using MyWFC = WFC::Builder<MyWorld, int, VariableMap>
+    ::SetRandomSelector<WFC::AdvancedRandomSelector<int>>
+    ::Build;
+
+MyWorld world(size);
+MyWFC::Run(world, seed);
+```
+
+#### Custom Selector Implementation
+
+```cpp
+class CustomSelector {
+private:
+    mutable int callCount = 0;
+
+public:
+    size_t operator()(std::span<const int> possibleValues) const {
+        // Round-robin selection
+        return callCount++ % possibleValues.size();
+    }
+};
+
+using MyWFC = WFC::Builder<MyWorld, int, VariableMap>
+    ::SetRandomSelector<CustomSelector>
+    ::Build;
+```
+
+#### Stateful Lambda Selector
+
+```cpp
+int counter = 0;
+auto selector = [&counter](std::span<const int> values) mutable -> size_t {
+    return counter++ % values.size();
+};
+
+// Use with WFC (would need to wrap in a class for template parameter)
+```
+
+### Key Benefits
+
+1. **Compile-time Compatible**: The default selector works in constexpr contexts for compile-time WFC solving
+2. **Stateful Selection**: Selectors can maintain state between calls for deterministic or custom behaviors
+3. **Flexible Interface**: Simple function signature `(std::span<const VarT>) -> size_t` makes implementation easy
+4. **Performance**: Custom selectors can optimize for specific use cases (e.g., always pick first, weighted selection)
+
 ## Building the Project
 
 ### Prerequisites

demos/random_selector_example.cpp

@@ -0,0 +1,191 @@
+/**
+ * @brief Example demonstrating custom random selectors in WFC
+ *
+ * This example shows how to use different random selection strategies
+ * in the Wave Function Collapse algorithm, including:
+ * - Default constexpr random selector
+ * - Advanced random selector with std::mt19937
+ * - Custom lambda-based selectors
+ */
+
+#include <iostream>
+#include <vector>
+#include <array>
+#include <random>
+
+#include "../include/nd-wfc/wfc.hpp"
+
+// Simple test world for demonstration
+struct SimpleWorld {
+    using ValueType = int;
+    std::vector<int> data;
+    size_t grid_size;
+
+    SimpleWorld(size_t size) : data(size * size, 0), grid_size(size) {}
+    size_t size() const { return data.size(); }
+    void setValue(size_t id, int value) { data[id] = value; }
+    int getValue(size_t id) const { return data[id]; }
+
+    void print() const {
+        for (size_t i = 0; i < grid_size; ++i) {
+            for (size_t j = 0; j < grid_size; ++j) {
+                std::cout << data[i * grid_size + j] << " ";
+            }
+            std::cout << std::endl;
+        }
+    }
+};
+
+int main() {
+    std::cout << "=== WFC Random Selector Examples ===\n\n";
+
+    // Example 1: Using the default constexpr random selector
+    std::cout << "1. Default Random Selector (constexpr-friendly):\n";
+    {
+        WFC::DefaultRandomSelector<int> selector(0x12345678);
+
+        // Test with some sample values
+        std::array<int, 4> values = {1, 2, 3, 4};
+        std::span<const int> span(values.data(), values.size());
+
+        std::cout << "Possible values: ";
+        for (int v : values) std::cout << v << " ";
+        std::cout << "\nSelected indices: ";
+
+        for (int i = 0; i < 8; ++i) {
+            size_t index = selector(span);
+            std::cout << index << "(" << values[index] << ") ";
+        }
+        std::cout << "\n\n";
+    }
+
+    // Example 2: Using the advanced random selector
+    std::cout << "2. Advanced Random Selector (std::mt19937):\n";
+    {
+        std::mt19937 rng(54321);
+        WFC::AdvancedRandomSelector<int> selector(rng);
+
+        std::array<int, 5> values = {10, 20, 30, 40, 50};
+        std::span<const int> span(values.data(), values.size());
+
+        std::cout << "Possible values: ";
+        for (int v : values) std::cout << v << " ";
+        std::cout << "\nSelected values: ";
+
+        for (int i = 0; i < 10; ++i) {
+            size_t index = selector(span);
+            std::cout << values[index] << " ";
+        }
+        std::cout << "\n\n";
+    }
+
+    // Example 3: Custom lambda selector that always picks the first element
+    std::cout << "3. Custom Lambda Selector (always first):\n";
+    {
+        auto firstSelector = [](std::span<const int> values) -> size_t {
+            return 0; // Always pick first
+        };
+
+        std::array<int, 3> values = {100, 200, 300};
+        std::span<const int> span(values.data(), values.size());
+
+        std::cout << "Possible values: ";
+        for (int v : values) std::cout << v << " ";
+        std::cout << "\nSelected values: ";
+
+        for (int i = 0; i < 5; ++i) {
+            size_t index = firstSelector(span);
+            std::cout << values[index] << " ";
+        }
+        std::cout << "\n\n";
+    }
+
+    // Example 4: Stateful lambda selector with captured variables
+    std::cout << "4. Stateful Lambda Selector (round-robin):\n";
+    {
+        int callCount = 0;
+        auto roundRobinSelector = [&callCount](std::span<const int> values) mutable -> size_t {
+            return callCount++ % values.size();
+        };
+
+        std::array<int, 3> values = {1, 2, 3};
+        std::span<const int> span(values.data(), values.size());
+
+        std::cout << "Possible values: ";
+        for (int v : values) std::cout << v << " ";
+        std::cout << "\nRound-robin selection: ";
+
+        for (int i = 0; i < 9; ++i) {
+            size_t index = roundRobinSelector(span);
+            std::cout << values[index] << " ";
+        }
+        std::cout << "\n\n";
+    }
+
+    // Example 5: Custom selector with probability weighting
+    std::cout << "5. Weighted Random Selector:\n";
+    {
+        std::mt19937 rng(99999);
+        auto weightedSelector = [&rng](std::span<const int> values) -> size_t {
+            // Simple weighted selection - favor earlier indices
+            std::vector<double> weights;
+            for (size_t i = 0; i < values.size(); ++i) {
+                weights.push_back(1.0 / (i + 1.0)); // Higher weight for lower indices
+            }
+
+            std::discrete_distribution<size_t> dist(weights.begin(), weights.end());
+            return dist(rng);
+        };
+
+        std::array<int, 4> values = {1, 2, 3, 4};
+        std::span<const int> span(values.data(), values.size());
+
+        std::cout << "Possible values: ";
+        for (int v : values) std::cout << v << " ";
+        std::cout << "\nWeighted selection ( favors lower values): ";
+
+        for (int i = 0; i < 12; ++i) {
+            size_t index = weightedSelector(span);
+            std::cout << values[index] << " ";
+        }
+        std::cout << "\n\n";
+    }
+
+    // Example 6: Integration with WFC Builder
+    std::cout << "6. WFC Builder Integration:\n";
+    {
+        // Define a simple WFC setup with custom random selector
+        using VariableMap = WFC::VariableIDMap<int, 0, 1, 2, 3>;
+
+        // Using default random selector
+        using DefaultWFC = WFC::Builder<SimpleWorld, int, VariableMap>
+            ::SetRandomSelector<WFC::DefaultRandomSelector<int>>
+            ::Build;
+
+        // Using advanced random selector
+        using AdvancedWFC = WFC::Builder<SimpleWorld, int, VariableMap>
+            ::SetRandomSelector<WFC::AdvancedRandomSelector<int>>
+            ::Build;
+
+        // Note: Lambda types cannot be used directly as template parameters
+        // Instead, you would create a custom selector class:
+        // class CustomSelector {
+        // public:
+        //     size_t operator()(std::span<const int> values) const {
+        //         return values.size() > 1 ? 1 : 0;
+        //     }
+        // };
+        // using CustomWFC = WFC::Builder<SimpleWorld, int, VariableMap>
+        //     ::SetRandomSelector<CustomSelector>
+        //     ::Build;
+
+        std::cout << "Successfully created WFC types with different random selectors:\n";
+        std::cout << "- DefaultWFC with DefaultRandomSelector\n";
+        std::cout << "- AdvancedWFC with AdvancedRandomSelector\n";
+        std::cout << "- Custom selector classes can be created for lambda-like behavior\n";
+    }
+
+    std::cout << "\n=== Examples completed successfully! ===\n";
+
+    return 0;
+}

include/nd-wfc/wfc.hpp

@@ -345,7 +345,8 @@ struct Callbacks
 template<typename WorldT, typename VarT,
     typename VariableIDMapT = VariableIDMap<VarT>,
     typename ConstrainerFunctionMapT = ConstrainerFunctionMap<void*>,
-    typename CallbacksT = Callbacks<WorldT>>
+    typename CallbacksT = Callbacks<WorldT>,
+    typename RandomSelectorT = DefaultRandomSelector<VarT>>
 class WFC {
 public:
     static_assert(WorldType<WorldT>, "WorldT must satisfy World type requirements");
@@ -359,14 +360,16 @@ public:
         WFCQueue<size_t> propagationQueue;
         Wave<MaskType> wave;
         std::mt19937& rng;
+        RandomSelectorT& randomSelector;
         WFCStackAllocator& allocator;
         size_t& iterations;
 
-        SolverState(WorldT& world, size_t variableAmount, std::mt19937& rng, WFCStackAllocator& allocator, size_t& iterations)
+        SolverState(WorldT& world, size_t variableAmount, std::mt19937& rng, RandomSelectorT& randomSelector, WFCStackAllocator& allocator, size_t& iterations)
             : world(world)
             , propagationQueue{ WFCStackAllocatorAdapter<size_t>(allocator) }
             , wave{ world.size(), variableAmount, allocator }
             , rng(rng)
+            , randomSelector(randomSelector)
             , allocator(allocator)
             , iterations(iterations)
         {}
@@ -391,11 +394,13 @@ public:
 
         size_t iterations = 0;
         auto random = std::mt19937{ seed };
+        RandomSelectorT randomSelector{ seed };
         SolverState state
         {
             world,
             ConstrainerFunctionMapT::size(),
             random,
+            randomSelector,
             allocator,
             iterations
         };
@@ -532,8 +537,13 @@ private:
         // randomly select a value from possible values
         while (availableValues)
         {
-            std::uniform_int_distribution<size_t> dist(0, availableValues - 1);
-            size_t randomIndex = dist(state.rng);
+            // Create a span of the actual variable values for the random selector
+            std::array<VarT, VariableIDMapT::ValuesRegisteredAmount> valueArray;
+            for (size_t i = 0; i < availableValues; ++i) {
+                valueArray[i] = VariableIDMapT::GetValue(possibleValues[i]);
+            }
+            std::span<const VarT> currentPossibleValues(valueArray.data(), availableValues);
+            size_t randomIndex = state.randomSelector(currentPossibleValues);
             size_t selectedValue = possibleValues[randomIndex];
             
             {
@@ -621,15 +631,65 @@ concept ConstrainerFunction = requires(T func, WorldT& world, size_t index, Worl
     func(world, index, value, constrainer);
 };
 
+/**
+ * @brief Concept to validate random selector function signature
+ * The function must be callable with parameters: (std::span<const VarT>) and return size_t
+ */
+template <typename T, typename VarT>
+concept RandomSelectorFunction = requires(T func, std::span<const VarT> possibleValues) {
+    { func(possibleValues) } -> std::convertible_to<size_t>;
+};
+
+/**
+ * @brief Default constexpr random selector using a simple seed-based algorithm
+ * This provides a compile-time random selection that maintains state between calls
+ */
+template <typename VarT>
+class DefaultRandomSelector {
+private:
+    mutable uint32_t m_seed;
+
+public:
+    constexpr explicit DefaultRandomSelector(uint32_t seed = 0x12345678) : m_seed(seed) {}
+
+    constexpr size_t operator()(std::span<const VarT> possibleValues) const {
+        if (possibleValues.empty()) return 0;
+
+        // Simple linear congruential generator for constexpr compatibility
+        m_seed = m_seed * 1103515245 + 12345;
+        return static_cast<size_t>(m_seed) % possibleValues.size();
+    }
+};
+
+/**
+ * @brief Advanced random selector using std::mt19937 and std::uniform_int_distribution
+ * This provides high-quality randomization for runtime use
+ */
+template <typename VarT>
+class AdvancedRandomSelector {
+private:
+    std::mt19937& m_rng;
+
+public:
+    explicit AdvancedRandomSelector(std::mt19937& rng) : m_rng(rng) {}
+
+    size_t operator()(std::span<const VarT> possibleValues) const {
+        if (possibleValues.empty()) return 0;
+
+        std::uniform_int_distribution<size_t> dist(0, possibleValues.size() - 1);
+        return dist(m_rng);
+    }
+};
+
 /**
  * @brief Builder class for creating WFC instances
  */
-template<typename WorldT, typename VarT = typename WorldT::ValueType, typename VariableIDMapT = VariableIDMap<VarT>, typename ConstrainerFunctionMapT = ConstrainerFunctionMap<void*>, typename CallbacksT = Callbacks<WorldT>>
+template<typename WorldT, typename VarT = typename WorldT::ValueType, typename VariableIDMapT = VariableIDMap<VarT>, typename ConstrainerFunctionMapT = ConstrainerFunctionMap<void*>, typename CallbacksT = Callbacks<WorldT>, typename RandomSelectorT = DefaultRandomSelector<VarT>>
 class Builder {
 public:
 
     template <VarT ... Values>
-    using DefineIDs = Builder<WorldT, VarT, typename VariableIDMapT::template Merge<Values...>, ConstrainerFunctionMapT, CallbacksT>;
+    using DefineIDs = Builder<WorldT, VarT, typename VariableIDMapT::template Merge<Values...>, ConstrainerFunctionMapT, CallbacksT, RandomSelectorT>;
 
     template <typename ConstrainerFunctionT, VarT ... CorrespondingValues>
         requires ConstrainerFunction<ConstrainerFunctionT, WorldT, VarT, VariableIDMapT>
@@ -640,17 +700,21 @@ public:
             ConstrainerFunctionT,
             VariableIDMap<VarT, CorrespondingValues...>,
             decltype([](WorldT&, size_t, WorldValue<VarT>, Constrainer<VariableIDMapT>&) {})
-            >, CallbacksT
+            >, CallbacksT, RandomSelectorT
         >;
 
     template <typename NewCellCollapsedCallbackT>
-    using SetCellCollapsedCallback = Builder<WorldT, VarT, VariableIDMapT, ConstrainerFunctionMapT, typename CallbacksT::template SetCellCollapsedCallbackT<NewCellCollapsedCallbackT>>;
+    using SetCellCollapsedCallback = Builder<WorldT, VarT, VariableIDMapT, ConstrainerFunctionMapT, typename CallbacksT::template SetCellCollapsedCallbackT<NewCellCollapsedCallbackT>, RandomSelectorT>;
     template <typename NewContradictionCallbackT>
-    using SetContradictionCallback = Builder<WorldT, VarT, VariableIDMapT, ConstrainerFunctionMapT, typename CallbacksT::template SetContradictionCallbackT<NewContradictionCallbackT>>;
+    using SetContradictionCallback = Builder<WorldT, VarT, VariableIDMapT, ConstrainerFunctionMapT, typename CallbacksT::template SetContradictionCallbackT<NewContradictionCallbackT>, RandomSelectorT>;
     template <typename NewBranchCallbackT>
-    using SetBranchCallback = Builder<WorldT, VarT, VariableIDMapT, ConstrainerFunctionMapT, typename CallbacksT::template SetBranchCallbackT<NewBranchCallbackT>>;
+    using SetBranchCallback = Builder<WorldT, VarT, VariableIDMapT, ConstrainerFunctionMapT, typename CallbacksT::template SetBranchCallbackT<NewBranchCallbackT>, RandomSelectorT>;
 
-    using Build = WFC<WorldT, VarT, VariableIDMapT, ConstrainerFunctionMapT, CallbacksT>;
+    template <typename NewRandomSelectorT>
+        requires RandomSelectorFunction<NewRandomSelectorT, VarT>
+    using SetRandomSelector = Builder<WorldT, VarT, VariableIDMapT, ConstrainerFunctionMapT, CallbacksT, NewRandomSelectorT>;
+
+    using Build = WFC<WorldT, VarT, VariableIDMapT, ConstrainerFunctionMapT, CallbacksT, RandomSelectorT>;
 };
 
 } // namespace WFC

tests/test_main.cpp

@@ -1,4 +1,125 @@
 #include <gtest/gtest.h>
+#include <array>
+#include <vector>
+#include <algorithm>
+#include "nd-wfc/wfc.hpp"
+#include "nd-wfc/worlds.hpp"
+
+// Test world for demonstration
+struct TestWorld {
+    using ValueType = int;
+    std::vector<int> data;
+
+    TestWorld(size_t size) : data(size, 0) {}
+    size_t size() const { return data.size(); }
+    void setValue(size_t id, int value) { data[id] = value; }
+    int getValue(size_t id) const { return data[id]; }
+};
+
+// Test random selectors
+TEST(RandomSelectorTest, DefaultRandomSelector) {
+    using namespace WFC;
+
+    // Test default random selector with constexpr capabilities
+    DefaultRandomSelector<int> selector(12345);
+
+    std::array<int, 3> testValues = {1, 2, 3};
+    std::span<const int> span(testValues.data(), testValues.size());
+
+    // Test that selector returns valid indices
+    size_t index1 = selector(span);
+    size_t index2 = selector(span);
+
+    EXPECT_LT(index1, testValues.size());
+    EXPECT_LT(index2, testValues.size());
+    EXPECT_GE(index1, 0);
+    EXPECT_GE(index2, 0);
+}
+
+TEST(RandomSelectorTest, AdvancedRandomSelector) {
+    using namespace WFC;
+
+    std::mt19937 rng(54321);
+    AdvancedRandomSelector<int> selector(rng);
+
+    std::array<int, 4> testValues = {10, 20, 30, 40};
+    std::span<const int> span(testValues.data(), testValues.size());
+
+    // Test that selector returns valid indices
+    size_t index = selector(span);
+    EXPECT_LT(index, testValues.size());
+    EXPECT_GE(index, 0);
+
+    // Verify the selected value matches
+    EXPECT_EQ(testValues[index], span[index]);
+}
+
+TEST(RandomSelectorTest, CustomLambdaSelector) {
+    using namespace WFC;
+
+    // Custom selector that always picks the first element
+    auto firstSelector = [](std::span<const int> values) -> size_t {
+        return 0;
+    };
+
+    std::array<int, 3> testValues = {100, 200, 300};
+    std::span<const int> span(testValues.data(), testValues.size());
+
+    size_t index = firstSelector(span);
+    EXPECT_EQ(index, 0);
+    EXPECT_EQ(span[index], 100);
+}
+
+TEST(RandomSelectorTest, StatefulLambdaSelector) {
+    using namespace WFC;
+
+    // Stateful selector that cycles through options
+    uint32_t callCount = 0;
+    auto cyclingSelector = [&callCount](std::span<const int> values) mutable -> size_t {
+        return callCount++ % values.size();
+    };
+
+    std::array<int, 3> testValues = {1, 2, 3};
+    std::span<const int> span(testValues.data(), testValues.size());
+
+    // Test multiple calls
+    EXPECT_EQ(cyclingSelector(span), 0);
+    EXPECT_EQ(cyclingSelector(span), 1);
+    EXPECT_EQ(cyclingSelector(span), 2);
+    EXPECT_EQ(cyclingSelector(span), 0); // Should cycle back
+}
+
+TEST(RandomSelectorTest, WFCIntegration) {
+    using namespace WFC;
+
+    // Create a simple WFC setup with custom random selector
+    using VariableMap = VariableIDMap<int, 0, 1, 2>;
+    using CustomWFC = Builder<TestWorld, int, VariableMap>
+        ::SetRandomSelector<DefaultRandomSelector<int>>
+        ::Build;
+
+    TestWorld world(4);
+    uint32_t seed = 12345;
+
+    // This should compile and run without errors
+    // (Note: This is a basic integration test - full WFC solving would require proper constraints)
+    SUCCEED();
+}
+
+TEST(RandomSelectorTest, ConstexprDefaultSelector) {
+    using namespace WFC;
+
+    // Test that default selector can be used in constexpr context
+    constexpr DefaultRandomSelector<int> selector(0xDEADBEEF);
+
+    constexpr std::array<int, 2> testValues = {42, 84};
+    constexpr auto span = std::span<const int>(testValues.data(), testValues.size());
+
+    // This should compile in constexpr context
+    constexpr size_t index = selector(span);
+    EXPECT_LT(index, testValues.size());
+    EXPECT_GE(index, 0);
+}
 
 int main(int argc, char **argv) {
     ::testing::InitGoogleTest(&argc, argv);