13 files changed, 668 insertions, 145 deletions

diff --git a/.gitignore b/.gitignore
index d9b6f50..2a53474 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1 +1,2 @@
 ttt3d
+*.o
diff --git a/Makefile b/Makefile
index 3652a5d..59e28d0 100644
--- a/Makefile
+++ b/Makefile
@@ -1,5 +1,5 @@
-CC = gcc
-CFLAGS = -Wall -Wextra -std=c11 -O3 -fwrapv -flto
+CXX = g++
+CXXFLAGS = -Wall -Wextra -std=c++17 -O3 -fwrapv -flto
 LDFLAGS =
 TARGET = ttt3d
 
@@ -8,8 +8,11 @@ TARGET = ttt3d
 all: $(TARGET)
 
 clean:
-	rm -f $(TARGET)
+	rm -f $(TARGET) *.o
 
 
-$(TARGET): $(wildcard *.c *.h)
-	$(CC) $(CFLAGS) -o $@ $(filter %.c,$^) $(LDFLAGS)
+$(TARGET): $(patsubst %.cpp,%.o,$(wildcard *.cpp))
+	$(CXX) $(CXXFLAGS) -o $@ $^ $(LDFLAGS)
+
+%.o: %.cpp $(wildcard *.h)
+	$(CXX) $(CXXFLAGS) -c -o $@ $<
diff --git a/board.c b/board.c
deleted file mode 100644
index 91cee2a..0000000
--- a/board.c
+++ /dev/null
@@ -1,48 +0,0 @@
-#include "board.h"
-
-
-void b_set(board_t B, cboard_t C) {
-	B[0] = C[0];
-	B[1] = C[1];
-}
-
-void b_drop(board_t B, int xy, int v) {
-	u64 mask = 0xfULL << (4 * xy), stkbot = 0x1ULL << (4 * xy);
-	u64 bit = ((B[0] | B[1]) & mask) + stkbot;
-	B[v] = B[v] | bit;
-}
-
-bool b_stk_full(cboard_t B, int xy) {
-	u64 mask = 0xfULL << (4 * xy);
-	return ((B[0] | B[1]) & mask) == mask;
-}
-
-static u64 winmasks[61] = {
-	0xfULL, 0xf0ULL, 0xf00ULL, 0x1111ULL,
-	0x2222ULL, 0x4444ULL, 0x8421ULL, 0x8888ULL,
-	0xf000ULL, 0xf0000ULL, 0xf00000ULL, 0xf000000ULL,
-	0x11110000ULL, 0x22220000ULL, 0x44440000ULL, 0x84210000ULL,
-	0x88880000ULL, 0xf0000000ULL, 0xf00000000ULL, 0xf000000000ULL,
-	0xf0000000000ULL, 0x111100000000ULL, 0x222200000000ULL, 0x444400000000ULL,
-	0x842100000000ULL, 0x888800000000ULL, 0xf00000000000ULL, 0x1000100010001ULL,
-	0x2000200020002ULL, 0x4000400040004ULL, 0x8000400020001ULL, 0x8000800080008ULL,
-	0xf000000000000ULL, 0x10001000100010ULL, 0x20002000200020ULL, 0x40004000400040ULL,
-	0x80004000200010ULL, 0x80008000800080ULL, 0xf0000000000000ULL, 0x100010001000100ULL,
-	0x200020002000200ULL, 0x400040004000400ULL, 0x800040002000100ULL, 0x800080008000800ULL,
-	0xf00000000000000ULL, 0x1000010000100001ULL, 0x1000100010001000ULL, 0x1111000000000000ULL,
-	0x2000020000200002ULL, 0x2000200020002000ULL, 0x2222000000000000ULL, 0x4000040000400004ULL,
-	0x4000400040004000ULL, 0x4444000000000000ULL, 0x8000040000200001ULL, 0x8000080000800008ULL,
-	0x8000400020001000ULL, 0x8000800080008000ULL, 0x8421000000000000ULL, 0x8888000000000000ULL,
-	0xf000000000000000ULL,
-};
-
-enum win_t b_win(cboard_t B) {
-	if ((B[0] | B[1]) == ~0ULL) return WIN_DRAW;
-
-	for (int i = 0; i < (int)(sizeof(winmasks) / sizeof(winmasks[0])); i++) {
-		if ((B[0] & winmasks[i]) == winmasks[i]) return WIN_P0;
-		if ((B[1] & winmasks[i]) == winmasks[i]) return WIN_P1;
-	}
-
-	return WIN_NONE;
-}
diff --git a/board.cpp b/board.cpp
new file mode 100644
index 0000000..5afcea1
--- /dev/null
+++ b/board.cpp
@@ -0,0 +1,81 @@
+#include <array>
+#include "board.h"
+
+using namespace std;
+
+
+Board Board::empty() {
+	Board B;
+	B.bd[0] = B.bd[1] = 0;
+	return B;
+}
+
+Board Board::uninitialised() {
+	return Board();
+}
+
+void Board::drop(int xy, int v) {
+	uint64_t mask = 0xfULL << (4 * xy), stkbot = 0x1ULL << (4 * xy);
+	uint64_t bit = ((bd[0] | bd[1]) & mask) + stkbot;
+	bd[v] |= bit;
+}
+
+bool Board::stkFull(int xy) const {
+	uint64_t mask = 0xfULL << (4 * xy);
+	return ((bd[0] | bd[1]) & mask) == mask;
+}
+
+static array<uint64_t, 61> winmasks = {
+	0xfULL, 0xf0ULL, 0xf00ULL, 0x1111ULL,
+	0x2222ULL, 0x4444ULL, 0x8421ULL, 0x8888ULL,
+	0xf000ULL, 0xf0000ULL, 0xf00000ULL, 0xf000000ULL,
+	0x11110000ULL, 0x22220000ULL, 0x44440000ULL, 0x84210000ULL,
+	0x88880000ULL, 0xf0000000ULL, 0xf00000000ULL, 0xf000000000ULL,
+	0xf0000000000ULL, 0x111100000000ULL, 0x222200000000ULL, 0x444400000000ULL,
+	0x842100000000ULL, 0x888800000000ULL, 0xf00000000000ULL, 0x1000100010001ULL,
+	0x2000200020002ULL, 0x4000400040004ULL, 0x8000400020001ULL, 0x8000800080008ULL,
+	0xf000000000000ULL, 0x10001000100010ULL, 0x20002000200020ULL, 0x40004000400040ULL,
+	0x80004000200010ULL, 0x80008000800080ULL, 0xf0000000000000ULL, 0x100010001000100ULL,
+	0x200020002000200ULL, 0x400040004000400ULL, 0x800040002000100ULL, 0x800080008000800ULL,
+	0xf00000000000000ULL, 0x1000010000100001ULL, 0x1000100010001000ULL, 0x1111000000000000ULL,
+	0x2000020000200002ULL, 0x2000200020002000ULL, 0x2222000000000000ULL, 0x4000040000400004ULL,
+	0x4000400040004000ULL, 0x4444000000000000ULL, 0x8000040000200001ULL, 0x8000080000800008ULL,
+	0x8000400020001000ULL, 0x8000800080008000ULL, 0x8421000000000000ULL, 0x8888000000000000ULL,
+	0xf000000000000000ULL,
+};
+
+win_t Board::checkWin() const {
+	if ((bd[0] | bd[1]) == ~0ULL) return WIN_DRAW;
+
+	for (int i = 0; i < (int)winmasks.size(); i++) {
+		if ((bd[0] & winmasks[i]) == winmasks[i]) return WIN_P0;
+		if ((bd[1] & winmasks[i]) == winmasks[i]) return WIN_P1;
+	}
+
+	return WIN_NONE;
+}
+
+uint64_t Board::hash() const {
+	return bd[0] ^ ((bd[1] << 29) | (bd[1] >> (64 - 29)));
+}
+
+int Board::numFilled() const {
+	return __builtin_popcountll(bd[0] | bd[1]);
+}
+
+bool Board::isEmpty() const {
+	return bd[0] == 0 && bd[1] == 0;
+}
+
+int Board::numValidMoves() const {
+	uint64_t topsMask = 0x8888'8888'8888'8888ULL;
+	return 16 - __builtin_popcountll((bd[0] | bd[1]) & topsMask);
+}
+
+int Board::playerToMove() const {
+	return numFilled() % 2;
+}
+
+bool Board::operator==(const Board &other) const {
+	return bd[0] == other.bd[0] && bd[1] == other.bd[1];
+}
diff --git a/board.h b/board.h
index f795d03..f21403b 100644
--- a/board.h
+++ b/board.h
@@ -1,16 +1,7 @@
 #pragma once
 
-#include <stdbool.h>
-#include <stdint.h>
+#include <cstdint>
 
-typedef uint64_t u64;
-typedef uint8_t u8;
-
-
-// as bit array: [16 * y + 4 * x + z]
-// (x,y) = floor position, z = height with z=0 on the bottom
-typedef u64 board_t[2];
-typedef const u64 cboard_t[2];
 
 enum win_t {
 	WIN_NONE,
@@ -19,7 +10,25 @@ enum win_t {
 	WIN_DRAW,
 };
 
-void b_set(board_t B, cboard_t C);
-void b_drop(board_t B, int xy, int v);
-bool b_stk_full(cboard_t B, int xy);
-enum win_t b_win(cboard_t B);
+struct Board {
+	// as bit array: [16 * y + 4 * x + z]
+	// (x,y) = floor position, z = height with z=0 on the bottom
+	uint64_t bd[2];
+
+	static Board empty();
+	static Board uninitialised();
+
+	void drop(int xy, int v);
+	bool stkFull(int xy) const;
+	win_t checkWin() const;
+	uint64_t hash() const;
+	int numFilled() const;
+	bool isEmpty() const;
+	int numValidMoves() const;
+	int playerToMove() const;
+
+	bool operator==(const Board &other) const;
+
+protected:
+	Board() = default;
+};
diff --git a/genwinmasks.py b/genwinmasks.py
index 45a9f58..9d118be 100755
--- a/genwinmasks.py
+++ b/genwinmasks.py
@@ -25,7 +25,7 @@ for x in range(4):
 
 masks.sort()
 
-print("static u64 winmasks[{}] = {{".format(len(masks)))
+print("static array<uint64_t, 61> winmasks[{}] = {{".format(len(masks)))
 for i in range((len(masks) + 3) // 4):
     print("\t" + " ".join(hex(n) + "ULL," for n in masks[4*i:4*i+4]))
 print("};")
diff --git a/listalloc.h b/listalloc.h
new file mode 100644
index 0000000..cdb512d
--- /dev/null
+++ b/listalloc.h
@@ -0,0 +1,132 @@
+#pragma once
+
+#include <stdexcept>
+#include <vector>
+#include <stack>
+#include <memory>
+
+using namespace std;
+
+
+template <typename T, typename Index>
+class ListAlloc {
+	static_assert(is_integral_v<Index> && is_unsigned_v<Index>);
+
+public:
+	const size_t capacity;
+
+	ListAlloc(size_t capacity);
+	~ListAlloc();
+
+	template <typename... Args>
+	Index allocate(Args... args);
+	void deallocate(Index index);
+
+	T* at(Index index);
+
+private:
+	char *buffer;
+	stack<Index> freeStack;
+	size_t cursor = 0;
+};
+
+template <typename T, typename Index>
+class ListAllocRef {
+	static_assert(is_integral_v<Index> && is_unsigned_v<Index>);
+
+public:
+	ListAllocRef() : index(-1) {}
+	ListAllocRef(Index index) : index(index) {}
+
+	~ListAllocRef() {
+		if (index != (Index)-1) {
+			assert(false && "Non-empty ListAllocRef upon destruction");
+		}
+	}
+
+	// No copying
+	ListAllocRef(const ListAllocRef<T, Index> &other) = delete;
+	ListAllocRef<T, Index>& operator=(const ListAllocRef<T, Index> &other) = delete;
+
+	operator bool() const {
+		return index != (Index)-1;
+	}
+
+	ListAllocRef<T, Index>& operator=(Index newIndex) {
+		if (index != (Index)-1) {
+			throw logic_error("operator= on non-empty ListAllocRef");
+		}
+
+		index = newIndex;
+		return *this;
+	}
+
+	T* get(ListAlloc<T, Index> &allocator) {
+		return allocator.at(index);
+	}
+
+	const T* get(ListAlloc<T, Index> &allocator) const {
+		return allocator.at(index);
+	}
+
+	void deallocate(ListAlloc<T, Index> &allocator) {
+		if (index != (Index)-1) {
+			allocator.deallocate(index);
+			index = -1;
+		}
+	}
+
+private:
+	Index index;
+};
+
+template <typename T, typename Index>
+ListAlloc<T, Index>::ListAlloc(size_t capacity)
+		: capacity(capacity)
+		, buffer(new char[capacity * sizeof(T)]) {
+
+	size_t largestIndex = capacity - 1;
+	if (capacity != 0 && (size_t)(Index)largestIndex != largestIndex) {
+		throw logic_error("Capacity too large for index type in ListAlloc");
+	}
+
+	fprintf(stderr, "ListAlloc with capacity=%zu, size=%zu\n", capacity, capacity * sizeof(T));
+}
+
+template <typename T, typename Index>
+ListAlloc<T, Index>::~ListAlloc() {
+	if (freeStack.size() != cursor) {
+		assert(false && "Not all entries deallocated in ~ListAlloc");
+	}
+
+	delete[] buffer;
+}
+
+template <typename T, typename Index>
+template <typename... Args>
+Index ListAlloc<T, Index>::allocate(Args... args) {
+	Index index;
+
+	if (!freeStack.empty()) {
+		index = freeStack.top();
+		freeStack.pop();
+	} else if (cursor < capacity) {
+		index = cursor++;
+	} else {
+		throw runtime_error("Out of memory in ListAlloc");
+	}
+
+	new(at(index)) T(args...);
+	return index;
+}
+
+template <typename T, typename Index>
+void ListAlloc<T, Index>::deallocate(Index index) {
+	at(index)->~T();
+	freeStack.push(index);
+}
+
+template <typename T, typename Index>
+T* ListAlloc<T, Index>::at(Index index) {
+	return (T*)&buffer[index * sizeof(T)];
+}
diff --git a/main.c b/main.c
deleted file mode 100644
index 6b5cac8..0000000
--- a/main.c
+++ /dev/null
@@ -1,77 +0,0 @@
-#include <stdio.h>
-#include <stdlib.h>
-#include "board.h"
-
-#define LARGE 100000
-
-
-static inline int min(int a, int b) {
-	return a < b ? a : b;
-}
-
-static inline int max(int a, int b) {
-	return a < b ? b : a;
-}
-
-
-static int win_score[4] = {
-	[WIN_NONE] = 0,
-	[WIN_P0] = 1000,
-	[WIN_P1] = -1000,
-	[WIN_DRAW] = 0,
-};
-
-static int evaluate(cboard_t B) {
-	(void)B;
-	return 0;
-}
-
-static int minimax_p0(cboard_t B, int alpha, int beta, int depth);
-
-static int minimax_p1(cboard_t B, int alpha, int beta, int depth) {
-	if (depth == 0) return evaluate(B);
-	enum win_t win = b_win(B);
-	if (win != WIN_NONE) return win_score[win];
-
-	int best = LARGE;
-	for (int i = 0; i < 16; i++) {
-		if (b_stk_full(B, i)) continue;
-		board_t C; b_set(C, B);
-		b_drop(C, i, 1);
-		int sc = minimax_p0(C, alpha, beta, depth - 1);
-		best = min(sc, best);
-		beta = min(best, beta);
-		if (beta <= alpha) break;
-	}
-
-	return best;
-}
-
-static int minimax_p0(cboard_t B, int alpha, int beta, int depth) {
-	if (depth == 0) return evaluate(B);
-	enum win_t win = b_win(B);
-	if (win != WIN_NONE) return win_score[win];
-
-	if (depth >= 36) {
-		printf("depth=%d alpha=%d beta=%d\n", depth, alpha, beta);
-	}
-
-	int best = -LARGE;
-	for (int i = 0; i < 16; i++) {
-		if (b_stk_full(B, i)) continue;
-		board_t C; b_set(C, B);
-		b_drop(C, i, 0);
-		int sc = minimax_p1(C, alpha, beta, depth - 1);
-		best = max(sc, best);
-		alpha = max(best, alpha);
-		if (beta <= alpha) break;
-	}
-
-	return best;
-}
-
-
-int main(void) {
-	board_t B = {0, 0};
-	printf("%d\n", minimax_p0(B, -LARGE, LARGE, 64));
-}
diff --git a/main.cpp b/main.cpp
new file mode 100644
index 0000000..0a9c2d2
--- /dev/null
+++ b/main.cpp
@@ -0,0 +1,26 @@
+#define USE_PN_SEARCH
+
+#include <array>
+#include <memory>
+#include <cstdio>
+#include <cstdlib>
+#include "board.h"
+
+#ifdef USE_PN_SEARCH
+#include "pnsearch.h"
+#else
+#include "minimax.h"
+#endif
+
+using namespace std;
+
+
+int main(void) {
+	Board B = Board::empty();
+
+#ifdef USE_PN_SEARCH
+	printf("%d\n", pnsearch(B));
+#else
+	printf("%d\n", minimax<0>(B, -MINIMAX_LARGE, MINIMAX_LARGE, 64));
+#endif
+}
diff --git a/minimax.cpp b/minimax.cpp
new file mode 100644
index 0000000..c4011da
--- /dev/null
+++ b/minimax.cpp
@@ -0,0 +1,91 @@
+#include <array>
+#include <memory>
+#include "minimax.h"
+
+using namespace std;
+
+#define LARGE MINIMAX_LARGE
+
+
+static int win_score[4] = {
+	[WIN_NONE] = 0,
+	[WIN_P0] = 1,
+	[WIN_P1] = -1,
+	[WIN_DRAW] = -1,
+};
+
+struct TransItem {
+	Board B = Board::empty();
+	int score = 0;
+};
+
+static unique_ptr<array<TransItem, 10000019>> transTable =
+	make_unique<decltype(transTable)::element_type>();
+
+static void transTableStore(uint64_t index, const Board &B, int score) {
+	TransItem &entry = (*transTable)[index];
+	// if (entry.B.isEmpty() || B.numFilled() < entry.B.numFilled()) {
+		entry.B = B;
+		entry.score = score;
+	// }
+}
+
+static uint64_t transHits = 0, transTotal = 0;
+static uint64_t transHitDepthSum = 0;
+
+template <int player>
+int minimax(const Board &B, int alpha, int beta, int depth) {
+	uint64_t transIndex = B.hash() % transTable->size();
+	transTotal++;
+	if (!B.isEmpty() && (*transTable)[transIndex].B == B) {
+		transHits++;
+		transHitDepthSum += depth;
+		return (*transTable)[transIndex].score;
+	}
+
+	win_t win = B.checkWin();
+	if (win != WIN_NONE) {
+		transTableStore(transIndex, B, win_score[win]);
+		return win_score[win];
+	}
+
+	if (depth == 0) return win_score[WIN_NONE];
+
+	if (depth >= 42) {
+		printf("depth=%d alpha=%d beta=%d transHitrate=%lf (avg depth %lf)\n",
+				depth, alpha, beta,
+				(double)transHits / transTotal, (double)transHitDepthSum / transTotal);
+	}
+
+	int best = player == 0 ? -LARGE : LARGE;
+
+	// Returns true if iteration should be stopped
+	auto tryMove = [&](int i) {
+		if (B.stkFull(i)) return false;
+
+		Board C(B);
+		C.drop(i, player);
+
+		int sc = minimax<1 - player>(C, alpha, beta, depth - 1);
+
+		if constexpr (player == 0) {
+			best = max(sc, best);
+			alpha = max(best, alpha);
+		} else {
+			best = min(sc, best);
+			beta = min(best, beta);
+		}
+
+		return beta <= alpha;
+	};
+
+	for (int i = 0; i < 16; i++) {
+		if (tryMove(i)) break;
+	}
+
+	transTableStore(transIndex, B, best);
+	return best;
+}
+
+template int minimax<0>(const Board &B, int alpha, int beta, int depth);
+template int minimax<1>(const Board &B, int alpha, int beta, int depth);
diff --git a/minimax.h b/minimax.h
new file mode 100644
index 0000000..8c77acd
--- /dev/null
+++ b/minimax.h
@@ -0,0 +1,13 @@
+#pragma once
+
+#include "board.h"
+
+#define MINIMAX_LARGE 10000
+
+
+// Returns:
+// - 1 if player 0 wins
+// - -1 if player 1 wins or can force a draw
+// - 0 if neither player can force a win within the given depth
+template <int player>
+int minimax(const Board &B, int alpha, int beta, int depth);
diff --git a/pnsearch.cpp b/pnsearch.cpp
new file mode 100644
index 0000000..a09d362
--- /dev/null
+++ b/pnsearch.cpp
@@ -0,0 +1,286 @@
+#include <array>
+#include <memory>
+#include <algorithm>
+#include <cstdio>
+#include <cstring>
+#include <cassert>
+#include "pnsearch.h"
+#include "minimax.h"
+#include "listalloc.h"
+
+using namespace std;
+
+
+static uint64_t gNodesAllocated = 0;
+static uint64_t gEvalWins = 0;
+static uint64_t gNodesPruned = 0;
+
+struct AOTree {
+	enum type_t { AND, OR };
+	enum eval_t { TRUE, FALSE, UNKNOWN };
+
+	AOTree *const parent;
+	const Board B;
+	const type_t type;
+
+	bool expanded = false;
+	int pnum = 1, dnum = 1;
+	array<ListAllocRef<AOTree, uint32_t>, 16> children;
+
+	AOTree(AOTree *parent, Board B, type_t type) noexcept;
+	~AOTree() noexcept;
+
+	void writeDOT(FILE *f, bool root = true) const;
+	uint64_t treeSize() const;
+};
+
+static ListAlloc<AOTree, uint32_t> gAllocator(6ULL * 1024 * 1024 * 1024 / sizeof(AOTree));
+
+AOTree::AOTree(AOTree *parent, Board B, type_t type) noexcept
+		: parent(parent), B(B), type(type) {
+
+	gNodesAllocated++;
+}
+
+AOTree::~AOTree() noexcept {
+	gNodesAllocated--;
+
+	for (int i = 0; i < 16; i++) {
+		children[i].deallocate(gAllocator);
+	}
+}
+
+void AOTree::writeDOT(FILE *f, bool root) const {
+	if (root) fprintf(f, "digraph G {\n");
+
+	fprintf(f, "\t\"%p\" [shape=%s, label=\"%d %d\"];\n", 
+			this, type == AND ? "ellipse" : "rectangle",
+			pnum, dnum);
+
+	for (auto &child : children) {
+		if (child) {
+			fprintf(f, "\t\"%p\" -> \"%p\"\n", this, child.get(gAllocator));
+			child.get(gAllocator)->writeDOT(f, false);
+		}
+	}
+
+	if (root) fprintf(f, "}\n");
+}
+
+uint64_t AOTree::treeSize() const {
+	uint64_t s = 1;
+	for (auto &child : children) {
+		if (child) s += child.get(gAllocator)->treeSize();
+	}
+	return s;
+}
+
+
+template <typename F>
+static int pnum_sum(const array<ListAllocRef<AOTree, uint32_t>, 16> &arr, F func) {
+	int sum = 0;
+
+	for (int i = 0; i < 16; i++) {
+		if (arr[i]) {
+			int value = func(*arr[i].get(gAllocator));
+			if (value == -1) return -1;
+			sum += value;
+		}
+	}
+
+	return sum;
+}
+
+template <typename F>
+static int pnum_min(const array<ListAllocRef<AOTree, uint32_t>, 16> &arr, F func) {
+	int lowest = -1;
+
+	for (int i = 0; i < 16; i++) {
+		if (arr[i]) {
+			int value = func(*arr[i].get(gAllocator));
+			if (value == -1) continue;
+			if (lowest == -1) lowest = value;
+			else lowest = min(lowest, value);
+		}
+	}
+
+	return lowest;
+}
+
+
+static AOTree::eval_t evaluate(const Board &B) {
+#if 0
+	// Basic win check
+	switch (B.checkWin()) {
+		case WIN_NONE: return AOTree::UNKNOWN;
+		case WIN_P0:   return AOTree::TRUE;
+		case WIN_P1:   return AOTree::FALSE;
+		case WIN_DRAW: return AOTree::FALSE;
+	}
+	assert(false);
+#else
+	// Shallow minimax
+	auto func = B.playerToMove() == 0 ? minimax<0> : minimax<1>;
+	switch (func(B, -MINIMAX_LARGE, MINIMAX_LARGE, 4)) {
+		case 1: return AOTree::TRUE;
+		case -1: return AOTree::FALSE;
+		case 0: return AOTree::UNKNOWN;
+	}
+	assert(false);
+#endif
+}
+
+void generateAllChildren(AOTree &node) {
+	int onturn = node.B.playerToMove();
+	int onturn_child = 1 - onturn;
+	AOTree::type_t child_type = onturn_child == 0 ? AOTree::OR : AOTree::AND;
+
+	for (int i = 0; i < 16; i++) {
+		if (node.B.stkFull(i)) continue;
+
+		Board C(node.B);
+		C.drop(i, onturn);
+		node.children[i] = gAllocator.allocate(&node, C, child_type);
+	}
+}
+
+static void developNode(AOTree &node) {
+	generateAllChildren(node);
+
+	node.expanded = true;
+
+	for (int i = 0; i < 16; i++) {
+		if (!node.children[i]) continue;
+		AOTree &child = *node.children[i].get(gAllocator);
+
+		AOTree::eval_t eval = evaluate(child.B);
+		switch (eval) {
+			case AOTree::FALSE:   child.pnum = -1; child.dnum =  0; break;
+			case AOTree::TRUE:    child.pnum =  0; child.dnum = -1; break;
+			case AOTree::UNKNOWN: {
+				// ENHANCEMENT: initial proof/disproof numbers according to number of children
+				int numSubChildren = child.B.numValidMoves();
+				switch (child.type) {
+					case AOTree::OR:  child.pnum = 1; child.dnum = numSubChildren; break;
+					case AOTree::AND: child.pnum = numSubChildren; child.dnum = 1; break;
+				}
+				break;
+			}
+		}
+		gEvalWins += eval == AOTree::TRUE || eval == AOTree::FALSE;
+	}
+}
+
+static void updateAncestors(AOTree &node_) {
+	AOTree *node = &node_;
+
+	while (node) {
+		assert(node->expanded);
+
+		switch (node->type) {
+			case AOTree::AND:
+				node->pnum = pnum_sum(node->children, [](const AOTree &n) { return n.pnum; });
+				node->dnum = pnum_min(node->children, [](const AOTree &n) { return n.dnum; });
+				break;
+
+			case AOTree::OR:
+				node->pnum = pnum_min(node->children, [](const AOTree &n) { return n.pnum; });
+				node->dnum = pnum_sum(node->children, [](const AOTree &n) { return n.dnum; });
+				break;
+		}
+
+		for (int i = 0; i < 16; i++) {
+			if (!node->children[i]) continue;
+
+			AOTree &child = *node->children[i].get(gAllocator);
+
+			// ENHANCEMENT: Delete solved subtrees
+			if (child.pnum == 0 || child.dnum == 0) {
+				gNodesPruned += child.treeSize();
+				node->children[i].deallocate(gAllocator);
+			}
+		}
+
+		node = node->parent;
+	}
+}
+
+static AOTree& selectMostProving(AOTree &node_) {
+	AOTree *node = &node_;
+
+	while (node->expanded) {
+		switch (node->type) {
+			case AOTree::OR: {
+				auto &laref = *find_if(node->children.begin(), node->children.end(),
+									[&node](ListAllocRef<AOTree, uint32_t> &child) {
+										if (!child) return false;
+										AOTree &obj = *child.get(gAllocator);
+										return obj.pnum == node->pnum;
+									});
+				node = laref.get(gAllocator);
+				break;
+			}
+
+			case AOTree::AND: {
+				auto &laref = *find_if(node->children.begin(), node->children.end(),
+									[&node](ListAllocRef<AOTree, uint32_t> &child) {
+										if (!child) return false;
+										AOTree &obj = *child.get(gAllocator);
+										return obj.dnum == node->dnum;
+									});
+				node = laref.get(gAllocator);
+				break;
+			}
+		}
+	}
+
+	return *node;
+}
+
+static AOTree::eval_t proofNumberSearch(AOTree &root) {
+	switch (evaluate(root.B)) {
+		case AOTree::TRUE:    root.pnum = 0; root.dnum = -1; break;
+		case AOTree::FALSE:   root.pnum = -1; root.dnum = 0; break;
+		case AOTree::UNKNOWN: break;
+	}
+
+	for (int iter = 0; root.pnum != 0 && root.dnum != 0; iter++) {
+		// char filename[128];
+		// sprintf(filename, "pntree-%d.dot", iter);
+		// FILE *f = fopen(filename, "w");
+		// root.writeDOT(f);
+		// fclose(f);
+
+		// if (iter == 20) break;
+
+		if (iter % 50000 == 0) {
+			printf("%d iterations done, %lu nodes allocated, %lu terminals, %lu pruned, root %d,%d\n", iter, gNodesAllocated, gEvalWins, gNodesPruned, root.pnum, root.dnum);
+		}
+
+		AOTree &mostProvingNode = selectMostProving(root);
+		developNode(mostProvingNode);
+		updateAncestors(mostProvingNode);
+	}
+
+	if (root.pnum == 0) return AOTree::TRUE;
+	else if (root.dnum == 0) return AOTree::FALSE;
+	else return AOTree::UNKNOWN;
+}
+
+int pnsearch(const Board &B) {
+	// printf("\x1B[1mWARNING: THIS IS GOING TO CONSUME A LOT OF MEMORY VERY QUICKLY. PLEASE WATCH IN htop AND BE READY TO ^C.\x1B[0m\n");
+
+	AOTree root(nullptr, B, B.numFilled() % 2 == 0 ? AOTree::OR : AOTree::AND);
+	AOTree::eval_t eval = proofNumberSearch(root);
+
+	switch (eval) {
+		case AOTree::TRUE: return 0;
+		case AOTree::FALSE: return 1;
+		case AOTree::UNKNOWN:
+			printf("UNKNOWN from proofNumberSearch!\n");
+			exit(0);
+			assert(false);
+	}
+
+	assert(false);
+}
diff --git a/pnsearch.h b/pnsearch.h
new file mode 100644
index 0000000..e0023bf
--- /dev/null
+++ b/pnsearch.h
@@ -0,0 +1,6 @@
+#pragma once
+
+#include "board.h"
+
+
+int pnsearch(const Board &B);