diff --git a/2018/15.cpp b/2018/15.cpp
index c26c781..a1d43fc 100644
--- a/2018/15.cpp
+++ b/2018/15.cpp
@@ -1,12 +1,14 @@
 #include <cstdint>
+#include <deque>
 #include <iostream>
 #include <map>
+#include <numeric>
+#include <optional>
+#include <set>
 #include <sstream>
 #include <string>
 #include <tuple>
 #include <vector>
-#include <deque>
-#include <optional>
 
 #include <doctest.h>
 
@@ -16,8 +18,6 @@ using namespace aocpp;
 
 namespace {
 
-int const initial_hp = 200;
-
 auto IsUnit(char c) -> bool {
   return c == 'G' || c == 'E';
 }
@@ -26,183 +26,461 @@ auto IsOpposed(char x, char y) -> bool {
   return x == 'G' && y == 'E' || x == 'E' && y == 'G';
 }
 
-auto PickTarget(
-  Grid const& grid,
-  std::map<Coord, int> & hp,
-  Coord my_coord, char my_team
-) -> std::optional<Coord>
+struct IsBefore {
+  auto operator()(Coord const& a, Coord const& b) const -> bool {
+    return a.y < b.y || a.y == b.y && a.x < b.x;
+  }
+};
+
+int const initial_hp = 200;
+Coord const reading_order[] {{0,-1}, {-1,0}, {1,0}, {0,1}};
+
+struct Game {
+  Grid grid_;
+  std::map<Coord, int> hp_;
+  int e_damage_;
+  int g_damage_;
+  bool no_elves_can_die;
+  std::ostream * debug_out_;
+
+  Game(Grid grid)
+  : grid_(std::move(grid))
+  , hp_{}
+  , e_damage_(3)
+  , g_damage_(3)
+  , no_elves_can_die{false}
+  , debug_out_{}
+  {
+    // set each unit to its initial hit points
+    grid_.each([&](auto k, auto v) {
+      if (IsUnit(v)) {
+        hp_[k] = initial_hp;
+      }
+    });
+  }
+
+  auto Simulate() -> std::optional<int>;
+  auto PickTarget(Coord my_coord, char my_team)
+    -> std::optional<std::map<Coord, int>::iterator>;
+  auto Move(Coord my_coord, char my_team) -> std::optional<Coord>;
+};
+
+auto operator<<(std::ostream & out, Game const& game) -> std::ostream & {
+  for (std::size_t y = 0; y < game.grid_.rows.size(); ++y) {
+    out << game.grid_.rows[y];
+    bool first = true;
+    for (std::size_t x = 0; x < game.grid_.rows[y].size(); ++x) {
+      auto c = game.grid_.rows[y][x];
+      if (IsUnit(c)) {
+        if (first) {
+          first = false;
+          out << "   ";
+        } else {
+          out << ", ";
+        }
+        Coord coord {std::int64_t(x),std::int64_t(y)};
+        out << c << "(" << game.hp_.at(coord) << ")";
+      }
+    }
+    out << std::endl;
+  }
+  return out;
+}
+
+// Determine the best target to attack from the current position, if there is one.
+auto Game::PickTarget(Coord my_coord, char my_team)
+-> std::optional<std::map<Coord, int>::iterator>
 {
-  std::optional<Coord> best;
+  std::optional<std::map<Coord, int>::iterator> best;
   int best_hp;
-  for (auto const dir : {Coord{0,-1}, Coord{-1,0}, Coord{1,0}, Coord{0,1}}) { //reading order
+  for (auto const dir : reading_order) {
     auto here = dir + my_coord;
-    if (IsOpposed(my_team, grid[here]) &&
-        (!best || best_hp > hp[here])) {
-          best = here;
-          best_hp = hp[here];
+    if (IsOpposed(my_team, grid_[here])) {
+      auto it = hp_.find(here);
+      if (!best || best_hp > it->second) {
+          best = it;
+          best_hp = it->second;
+      }
     }
   }
   return best;
 }
 
-auto Before(Coord a, Coord b) {
-  return a.y < b.y || a.y == b.y && a.x < b.x;
-}
-
-auto Move(
-  Grid const& grid,
-  std::map<Coord, int> & hp,
-  Coord my_coord, char my_team
-) -> std::optional<Coord>
+// Determine the best location to move to, if there is one.
+auto Game::Move(Coord my_coord, char my_team) -> std::optional<Coord>
 {
-
   int best_distance;
-  std::optional<Coord> best_start, best_end;
-  
-  std::map<Coord, std::pair<Coord, int>> seen;
+  std::optional<Coord> best_start;
+  Coord best_end;
+
+  std::set<Coord> seen;
   std::deque<std::tuple<Coord, Coord, int>> todo;
 
-  for (auto const dir : {Coord{0,-1}, Coord{-1,0}, Coord{1,0}, Coord{0,1}}) {
+  for (auto const dir : reading_order) {
     auto start = my_coord + dir;
-    todo.push_back({start,start,1});
+    if (grid_[start] == '.') {
+      todo.push_back({start,start,1});
+    }
   }
 
   while (!todo.empty()) {
     auto [start, cursor, steps] = todo.front();
     todo.pop_front();
-    
-    if (grid[cursor] != '.') {
-      continue;
-    }
 
-    if (best_start && best_distance < steps) break;
-
-    auto [it,success] = seen.insert({cursor, {start, steps}});
-    if (!success) {
-      if (Before(start, it->second.first)) continue;
-      it->second.first = start;
-    }
-    
-    for (auto const dir : {Coord{0,-1}, Coord{-1,0}, Coord{1,0}, Coord{0,1}}) {
-      auto next = cursor + dir;
-      if (seen.insert({next, {start, steps+1}}).second) {
-        todo.push_back({start, next, steps+1});
+    if (best_start) {
+      if (best_distance < steps) break;
+      if (best_start && !IsBefore()(cursor, best_end)) continue;
+    } else {
+      if (!seen.insert(cursor).second) { continue; }
+      for (auto const dir : reading_order) {
+        auto const next = cursor + dir;
+        if (grid_[next] == '.') {
+          todo.push_back({start, cursor + dir, steps+1});
+        }
       }
     }
-      
-    if (PickTarget(grid, hp, cursor, my_team) &&
-        (!best_start ||
-        steps < best_distance ||
-        steps == best_distance && Before(cursor, *best_end) ||
-        steps == best_distance && cursor == *best_end && Before(start, *best_start)))
-    {
+    if (PickTarget(cursor, my_team)) {
+      best_start = start;
       best_end = cursor;
       best_distance = steps;
-      best_start = start;
     }
   }
   return best_start;
 }
 
-auto Simulate(Grid & grid) {
-
-  // hit points for unit at coordinate
-  std::map<Coord, int> hp;
-
-  // set each unit to its initial hit points
-  grid.each([&](auto k, auto v) {
-    if (IsUnit(v)) {
-      hp[k] = initial_hp;
-    }
-  });
+auto Game::Simulate() -> std::optional<int> {
 
   for (int rounds = 0;; rounds++) {
-    std::cout << "round " << rounds << std::endl;
-    for (auto const& row : grid.rows) {
-      std::cout << row << std::endl;
+    if (debug_out_) {
+      *debug_out_ << "After " << rounds << " rounds:" << std::endl
+                  << *this << std::endl;
     }
 
     // determine order of unit updates
-    std::deque<Coord> turn_order;
-    grid.each([&](auto k, auto v) {
-      if (IsUnit(v)) {
-        turn_order.push_back(k);
-      }
-    });
+    std::set<Coord, IsBefore> turn_order;
+    for (auto const& [k,_] : hp_) {
+      turn_order.insert(k);
+    }
 
-    while (!turn_order.empty()) {
-      auto active_coord = turn_order.front();
-      char active_team = grid[active_coord];
-      turn_order.pop_front();
+    for (auto active_coord : turn_order) {
+      char active_team = grid_[active_coord];
 
       // Detect when no targets remain for this unit and end the game
       bool game_over = true;
-      grid.each([&](auto k, auto v) {
+      grid_.each([&](auto k, auto v) {
         if (IsOpposed(active_team, v)) game_over = false;
       });
       if (game_over) {
         int total_hp = 0;
-        for (auto const& [_,v] : hp) { total_hp += v; }
-        return rounds * total_hp;
+        for (auto const& [_,v] : hp_) { total_hp += v; }
+        return {rounds * total_hp};
       }
 
       // Try to pick a target without moving
-      auto target = PickTarget(grid, hp, active_coord, active_team);
+      auto target = PickTarget(active_coord, active_team);
 
       // only move when necessary
       if (!target) {
         // move if we can
-        if (auto const destination = Move(grid, hp, active_coord, active_team)) {
-          std::swap(grid[*destination], grid[active_coord]);
+        if (auto const destination = Move(active_coord, active_team)) {
+          std::swap(grid_[*destination], grid_[active_coord]);
 
-          auto const it = hp.find(active_coord);
-          hp[*destination] = it->second;
-          hp.erase(it);
+          auto const it = hp_.find(active_coord);
+          hp_[*destination] = it->second;
+          hp_.erase(it);
 
           active_coord = *destination;
         }
 
-        target = PickTarget(grid, hp, active_coord, active_team);
+        target = PickTarget(active_coord, active_team);
       }
 
       // target in range, do the attack
       if (target) {
+        auto & [target_coord, target_hp] = **target;
+        auto damage = active_team == 'G' ? g_damage_ : e_damage_;
 
-        // apply damage
-        auto remaining = hp[*target] -= 3;
+        // lethal
+        if (target_hp <= damage) {
+          if (no_elves_can_die && grid_[target_coord] == 'E') return {};
 
-        // handle death
-        if (remaining <= 0) {
-          hp.erase(*target);
-
-          // free up location on the map
-          grid[*target] = '.';
-
-          // possibly remove dead unit from turn queue
-          auto it = std::find(turn_order.begin(), turn_order.end(), *target);
-          if (it != turn_order.end()) {
-          turn_order.erase(it);
-          }
-          
-          
+          grid_[target_coord] = '.';
+          turn_order.erase(target_coord);
+          hp_.erase(*target); // invalidates target, target_coord, target_hp
+        } else {
+          target_hp -= damage;
         }
       }
     }
   }
 }
 
+auto Part2(Game game) {
+  game.no_elves_can_die = true;
+  int best;
 
+  auto const attempt = [&](int const power) {
+    auto g = game;
+    g.e_damage_ = power;
+    return g.Simulate();
+  };
+
+  int too_low = 3;
+  int high = 4;
+  for(;;) {
+    if (auto outcome = attempt(high)) {
+      best = *outcome; break;
+    }
+    too_low = high;
+    high *= 2;
+  }
+
+  while (too_low+1 < high) {
+    auto x = std::midpoint(too_low, high);
+    if (auto outcome = attempt(x)) {
+      best = *outcome; high = x;
+    } else {
+      too_low = x;
+    }
+  }
+
+  return std::make_pair(best, high);
+}
 
 } // namespace
 
 TEST_SUITE("2018-15 examples") {
-  TEST_CASE("example") {
+
+  auto case1(int expect, std::string start, std::string end) -> void {
+    std::istringstream in { std::move(start) };
+    std::ostringstream out;
+    Game game {Grid::Parse(in)};
+    auto outcome = game.Simulate();
+    out << game;
+    CHECK(out.str() == end);
+    CHECK(outcome == std::optional<int>{expect});
+  }
+
+  TEST_CASE("example 1") {
+    case1(27730,
+      "#######\n"
+      "#.G...#\n"
+      "#...EG#\n"
+      "#.#.#G#\n"
+      "#..G#E#\n"
+      "#.....#\n"
+      "#######\n",
+      "#######\n"
+      "#G....#   G(200)\n"
+      "#.G...#   G(131)\n"
+      "#.#.#G#   G(59)\n"
+      "#...#.#\n"
+      "#....G#   G(200)\n"
+      "#######\n");
+  }
+  TEST_CASE("example 2") {
+    case1(36334,
+      "#######\n"
+      "#G..#E#\n"
+      "#E#E.E#\n"
+      "#G.##.#\n"
+      "#...#E#\n"
+      "#...E.#\n"
+      "#######\n",
+      "#######\n"
+      "#...#E#   E(200)\n"
+      "#E#...#   E(197)\n"
+      "#.E##.#   E(185)\n"
+      "#E..#E#   E(200), E(200)\n"
+      "#.....#\n"
+      "#######\n");
+  }
+  TEST_CASE("example 3") {
+    case1(39514,
+      "#######\n"
+      "#E..EG#\n"
+      "#.#G.E#\n"
+      "#E.##E#\n"
+      "#G..#.#\n"
+      "#..E#.#\n"
+      "#######\n",
+      "#######\n"
+      "#.E.E.#   E(164), E(197)\n"
+      "#.#E..#   E(200)\n"
+      "#E.##.#   E(98)\n"
+      "#.E.#.#   E(200)\n"
+      "#...#.#\n"
+      "#######\n");
+  }
+  TEST_CASE("example 4") {
+    case1(27755,
+      "#######\n"
+      "#E.G#.#\n"
+      "#.#G..#\n"
+      "#G.#.G#\n"
+      "#G..#.#\n"
+      "#...E.#\n"
+      "#######\n",
+      "#######\n"
+      "#G.G#.#   G(200), G(98)\n"
+      "#.#G..#   G(200)\n"
+      "#..#..#\n"
+      "#...#G#   G(95)\n"
+      "#...G.#   G(200)\n"
+      "#######\n");
+  }
+  TEST_CASE("example 5") {
+    case1(28944,
+      "#######\n"
+      "#.E...#\n"
+      "#.#..G#\n"
+      "#.###.#\n"
+      "#E#G#G#\n"
+      "#...#G#\n"
+      "#######\n",
+      "#######\n"
+      "#.....#\n"
+      "#.#G..#   G(200)\n"
+      "#.###.#\n"
+      "#.#.#.#\n"
+      "#G.G#G#   G(98), G(38), G(200)\n"
+      "#######\n");
+  }
+  TEST_CASE("example 6") {
+    case1(18740,
+      "#########\n"
+      "#G......#\n"
+      "#.E.#...#\n"
+      "#..##..G#\n"
+      "#...##..#\n"
+      "#...#...#\n"
+      "#.G...G.#\n"
+      "#.....G.#\n"
+      "#########\n",
+      "#########\n"
+      "#.G.....#   G(137)\n"
+      "#G.G#...#   G(200), G(200)\n"
+      "#.G##...#   G(200)\n"
+      "#...##..#\n"
+      "#.G.#...#   G(200)\n"
+      "#.......#\n"
+      "#.......#\n"
+      "#########\n"
+    );
+  }
+
+  auto case2(int expect, std::string start, std::string end) -> void {
+    std::istringstream in { std::move(start) };
+    std::ostringstream out;
+    Game game {Grid::Parse(in)};
+    auto [outcome, e_damage] = Part2(game);
+    game.e_damage_ = e_damage;
+    game.Simulate();
+    out << game;
+    CHECK(out.str() == end);
+    CHECK(outcome == expect);
+  }
+
+  TEST_CASE("example 7") {
+    case2(4988,
+      "#######\n"
+      "#.G...#\n"
+      "#...EG#\n"
+      "#.#.#G#\n"
+      "#..G#E#\n"
+      "#.....#\n"
+      "#######\n",
+      "#######\n"
+      "#..E..#   E(158)\n"
+      "#...E.#   E(14)\n"
+      "#.#.#.#\n"
+      "#...#.#\n"
+      "#.....#\n"
+      "#######\n"
+    );
+  }
+
+  TEST_CASE("example 8") {
+    case2(31284,
+      "#######\n"
+      "#E..EG#\n"
+      "#.#G.E#\n"
+      "#E.##E#\n"
+      "#G..#.#\n"
+      "#..E#.#\n"
+      "#######\n",
+      "#######\n"
+      "#.E.E.#   E(200), E(23)\n"
+      "#.#E..#   E(200)\n"
+      "#E.##E#   E(125), E(200)\n"
+      "#.E.#.#   E(200)\n"
+      "#...#.#\n"
+      "#######\n");
+  }
+
+  TEST_CASE("example 9") {
+    case2(3478,
+      "#######\n"
+      "#E.G#.#\n"
+      "#.#G..#\n"
+      "#G.#.G#\n"
+      "#G..#.#\n"
+      "#...E.#\n"
+      "#######\n",
+      "#######\n"
+      "#.E.#.#   E(8)\n"
+      "#.#E..#   E(86)\n"
+      "#..#..#\n"
+      "#...#.#\n"
+      "#.....#\n"
+      "#######\n");
+  }
+
+  TEST_CASE("example 10") {
+    case2(6474,
+      "#######\n"
+      "#.E...#\n"
+      "#.#..G#\n"
+      "#.###.#\n"
+      "#E#G#G#\n"
+      "#...#G#\n"
+      "#######\n",
+      "#######\n"
+      "#...E.#   E(14)\n"
+      "#.#..E#   E(152)\n"
+      "#.###.#\n"
+      "#.#.#.#\n"
+      "#...#.#\n"
+      "#######\n");
+  }
+
+  TEST_CASE("example 11") {
+    case2(1140,
+      "#########\n"
+      "#G......#\n"
+      "#.E.#...#\n"
+      "#..##..G#\n"
+      "#...##..#\n"
+      "#...#...#\n"
+      "#.G...G.#\n"
+      "#.....G.#\n"
+      "#########\n",
+      "#########\n"
+      "#.......#\n"
+      "#.E.#...#   E(38)\n"
+      "#..##...#\n"
+      "#...##..#\n"
+      "#...#...#\n"
+      "#.......#\n"
+      "#.......#\n"
+      "#########\n");
   }
 }
 
 auto main(int argc, char** argv) -> int {
   auto grid = Grid::Parse(*aocpp::Startup(argc, argv));
-  auto part1 = Simulate(grid);
-  std::cout << "Part 1: " << part1 << std::endl;
-  //std::cout << "Part 2: " << Run2(std::move(input)) << std::endl;
+  Game game {std::move(grid)};
+  //game.debug_out_ = &std::cout;
+  std::cout << "Part 1: " << *Game{game}.Simulate() << std::endl;
+  std::cout << "Part 2: " << Part2(std::move(game)).first << std::endl;
 }