From f6aed5387fadb97f149c6cc95e85c11090b0e9d5 Mon Sep 17 00:00:00 2001
From: Tom Smeding <tom.smeding@gmail.com>
Date: Sat, 1 Sep 2018 10:10:24 +0200
Subject: Add backtracking solver

---
 .gitignore   |   1 +
 Makefile     |   2 +-
 solve_bt.cpp | 248 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 250 insertions(+), 1 deletion(-)
 create mode 100644 solve_bt.cpp

diff --git a/.gitignore b/.gitignore
index 94491a3..0e0fdf2 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,5 +1,6 @@
 solve
 solve2
+solve_bt
 *.o
 *.dSYM
 .DS_Store
diff --git a/Makefile b/Makefile
index 89c0b26..5d17a4c 100644
--- a/Makefile
+++ b/Makefile
@@ -1,6 +1,6 @@
 CXX = g++
 CXXFLAGS = -Wall -Wextra -std=c++11 -fwrapv -O2
-CXXTARGETS = solve solve2
+CXXTARGETS = solve solve2 solve_bt
 
 .PHONY: all clean remake
 
diff --git a/solve_bt.cpp b/solve_bt.cpp
new file mode 100644
index 0000000..418e286
--- /dev/null
+++ b/solve_bt.cpp
@@ -0,0 +1,248 @@
+#include <iostream>
+#include <iomanip>
+#include <string>
+#include <vector>
+#include <cstring>
+#include <cstdint>
+#include <cassert>
+
+using namespace std;
+
+
+struct Sudoku {
+	int8_t bd[81];
+	uint16_t poss[81];
+	int nfull = 0;
+
+	Sudoku() {
+		for (int i = 0; i < 81; i++) {
+			poss[i] = 0x1ff;
+		}
+	}
+
+	inline int8_t& operator[](int8_t i) {return bd[i];}
+	inline const int8_t& operator[](int8_t i) const {return bd[i];}
+};
+
+static istream& operator>>(istream &in, Sudoku &su) {
+	su.nfull = 0;
+
+	for (int i = 0; i < 81; i++) {
+		char c;
+		in >> c;
+		if (c == '.') {
+			su[i] = -1;
+		} else {
+			assert('1' <= c && c <= '9');
+			su[i] = c - '1';
+			su.nfull++;
+		}
+	}
+
+	return in;
+}
+
+static ostream& operator<<(ostream &os, const Sudoku &su) {
+	for (int y = 0; y < 9; y++) {
+		if (y != 0) {
+			os << endl;
+			if (y % 3 == 0) os << endl;
+		}
+
+		for (int x = 0; x < 9; x++) {
+			if (x != 0) {
+				os << ' ';
+				if (x % 3 == 0) os << ' ';
+			}
+			int8_t v = su[9 * y + x];
+			if (v == -1) os << '.';
+			else os << (int)v + 1;
+			// string s;
+			// for (int i = 0; i < 9; i++) {
+			//     if (su.poss[9 * y + x] & (1<<i)) s += '1' + i;
+			// }
+			// os << '(' << setw(9) << s << ')';
+		}
+	}
+
+	return os;
+}
+
+static bool isValid(const Sudoku &su) {
+	for (int y = 0; y < 9; y++) {
+		int i = 9 * y;
+		uint8_t seen[9];
+		memset(seen, 0, 9);
+		for (int x = 0; x < 9; x++, i++) {
+			int v = su[i];
+			if (v != -1) {
+				if (seen[v]) return false;
+				else seen[v] = 1;
+			}
+		}
+	}
+
+	for (int x = 0; x < 9; x++) {
+		int i = x;
+		uint8_t seen[9];
+		memset(seen, 0, 9);
+		for (int y = 0; y < 9; y++, i += 9) {
+			int v = su[i];
+			if (v != -1) {
+				if (seen[v]) return false;
+				else seen[v] = 1;
+			}
+		}
+	}
+
+	int i = 0;
+	for (int by = 0; by < 9; by += 3) {
+		for (int bx = 0; bx < 9; bx += 3) {
+			uint8_t seen[9];
+			memset(seen, 0, 9);
+
+			int j = i;
+			for (int y = 0; y < 3; y++) {
+				for (int x = 0; x < 3; x++) {
+					int v = su[j];
+					if (v != -1) {
+						if (seen[v]) return false;
+						else seen[v] = 1;
+					}
+					j++;
+				}
+				j += 6;
+			}
+
+			i += 3;
+		}
+		i += 18;
+	}
+
+	return true;
+}
+
+vector<Sudoku> solutions;
+
+static void scratchAround(Sudoku &su, int idx, int x, int y) {
+	uint16_t mask = 0x1ff & ~(1 << su[idx]);
+
+	for (int i = 0; i < 9; i++) {
+		su.poss[9 * y + i] &= mask;
+		su.poss[9 * i + x] &= mask;
+	}
+
+	int bx = x / 3 * 3, by = y / 3 * 3;
+	for (int yi = 0; yi < 3; yi++) {
+		for (int xi = 0; xi < 3; xi++) {
+			su.poss[9 * (by + yi) + bx + xi] &= mask;
+		}
+	}
+}
+
+static void scratchAround(Sudoku &su, int idx) {
+	scratchAround(su, idx, idx % 9, idx / 9);
+}
+
+static void scratchAround(Sudoku &su, int x, int y) {
+	scratchAround(su, 9 * y + x, x, y);
+}
+
+static void solveDestructive(Sudoku &su) {
+	// cerr << "solveDestructive(" << su.nfull << ")" << endl;
+
+	for (int y = 0, i = 0; y < 9; y++) {
+		for (int x = 0; x < 9; x++, i++) {
+			if (su[i] != -1) continue;
+
+			if (__builtin_popcount(su.poss[i]) == 1) {
+				su[i] = __builtin_ctz(su.poss[i]);
+				su.nfull++;
+				scratchAround(su, i);
+			}
+		}
+	}
+
+	if (su.nfull == 81) {
+		if (isValid(su)) {
+			solutions.push_back(su);
+		}
+		return;
+	}
+
+	int mincount = 99, minat = -1;
+
+	for (int y = 0, i = 0; y < 9; y++) {
+		for (int x = 0; x < 9; x++, i++) {
+			if (su[i] != -1) continue;
+
+			int count = __builtin_popcount((uint32_t)su.poss[i]);
+			if (count < mincount) {
+				mincount = count;
+				minat = i;
+			}
+		}
+	}
+
+	if (mincount == 1) {
+		// do a tail call
+		solveDestructive(su);
+		return;
+	}
+
+	if (mincount == 0) {
+		// invalid
+		return;
+	}
+
+	// cerr << "minat = " << minat << " ";
+	// cerr << "mincount = " << mincount << endl;
+
+	int i = minat;
+	for (int v = 0; v < 9; v++) {
+		if ((su.poss[i] & (1 << v)) == 0) continue;
+
+		Sudoku su2 = su;
+		su2[i] = v;
+		su2.nfull++;
+		// cerr << "try " << v << endl;
+		if (!isValid(su2)) {
+			// cerr << "  invalid" << endl;
+			continue;
+		}
+
+		scratchAround(su2, i);
+		solveDestructive(su2);
+	}
+}
+
+static void solve(const Sudoku &su) {
+	Sudoku su2 = su;
+	solveDestructive(su2);
+}
+
+
+static void cleanPoss(Sudoku &su) {
+	for (int y = 0; y < 9; y++) {
+		for (int x = 0; x < 9; x++) {
+			scratchAround(su, x, y);
+		}
+	}
+}
+
+int main() {
+	Sudoku su;
+	cin >> su;
+
+	solutions.clear();
+	cleanPoss(su);
+	solve(su);
+
+	if (solutions.size() != 1) {
+		cout << solutions.size() << " solutions found:" << endl;
+	}
+
+	for (const Sudoku &su2 : solutions) {
+		cout << su2 << endl << endl;
+	}
+}
-- 
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