Update Sudoku Solver to check for naked singles.

solver.cpp

@@ -6,6 +6,11 @@ int main(int argc, char** argv) {
   Puzzle puzzle = Puzzle::FromString(
       "3..4.162.1...8.4....5.2.83..578........7..5.3..29.4..748.53..1.2.3.9...."
       ".7...6.9.");
+  while (puzzle.ApplyNextStep())
+    ;
+  if (!puzzle.IsSolved()) {
+    std::cout << "Error! Couldn't Solve" << std::endl;
+  }
   std::cout << "https://tiramisu.one/sudoku.html?p=" << puzzle.CurrentState()
             << "&m=" << puzzle.PencilMarkState() << std::endl;
 }

solver/cell.cpp

@@ -1,5 +1,6 @@
 #include "cell.h"
 
+#include <algorithm>
 #include <cassert>
 
 Cell::Cell()
@@ -15,3 +16,7 @@ void Cell::Restrict(uint8_t value) {
   assert(value >= 1 && value <= 9);
   possibilities_[value - 1] = false;
 }
+
+uint8_t Cell::NumPossibilities() {
+  return std::count(possibilities_.begin(), possibilities_.end(), true);
+}

solver/cell.h

@@ -15,6 +15,8 @@ class Cell {
 
   void Restrict(uint8_t value);
 
+  uint8_t NumPossibilities();
+
   bool IsSolved() const { return state_ == Solved; }
   uint8_t value() const { return value_; }
   bool IsPossible(uint8_t v) const { return possibilities_[v - 1]; }

solver/puzzle.cpp

@@ -1,5 +1,6 @@
 #include "puzzle.h"
 
+#include <algorithm>
 #include <cassert>
 #include <iostream>
 #include <sstream>
@@ -63,6 +64,30 @@ std::string Puzzle::PencilMarkState() {
   return temp;
 }
 
+bool Puzzle::IsSolved() {
+  return std::all_of(cells_.begin(), cells_.end(),
+                     [](const Cell& c) { return c.IsSolved(); });
+}
+
+bool Puzzle::ApplyNextStep() {
+  // Search for a naked single.
+  for (int i = 0; i < 81; i++) {
+    if (cells_[i].IsSolved()) {
+      continue;
+    }
+
+    if (cells_[i].NumPossibilities() == 1) {
+      for (uint8_t v = 1; v <= 9; v++) {
+        if (cells_[i].IsPossible(v)) {
+          AssignSquare(i, v);
+        }
+      }
+      return true;
+    }
+  }
+  return false;
+}
+
 void Puzzle::AssignSquare(uint8_t id, uint8_t value) {
   assert(id < 81);
   assert(value >= 0 && value <= 9);

solver/puzzle.h

@@ -12,6 +12,9 @@ class Puzzle {
   std::string CurrentState();
   std::string PencilMarkState();
 
+  bool IsSolved();
+  bool ApplyNextStep();
+
   void AssignSquare(uint8_t id, uint8_t value);
 
  private: