Add solution for 2022 day 17 Part 2

2022/puzzle-17-02.cc

@@ -0,0 +1,161 @@
+//
+// Created by Matthew Gretton-Dann on 17/12/2022.
+//
+
+#include <array>
+#include <iostream>
+#include <list>
+#include <map>
+#include <numeric>
+#include <regex>
+#include <stdexcept>
+#include <utility>
+#include <vector>
+
+using Int = std::int64_t;
+using UInt = std::uint64_t;
+
+using namespace std::string_literals;
+using Shape = std::vector<std::string>;
+
+auto shapes_intersect(Shape const& tower, Shape const& shape, UInt x, UInt y) -> bool
+{
+  for (auto const& row : shape) {
+    UInt cx{x};
+    for (auto const& c : row) {
+      if (c != ' ' && tower[y][cx] != '.') {
+        return true;
+      }
+      ++cx;
+    }
+    ++y;
+  }
+  return false;
+}
+
+auto tower_height(Shape const& tower) -> UInt
+{
+  UInt height{tower.size() - 1};
+  while (tower[height].find('#') == std::string::npos) {
+    --height;
+  }
+  return height;
+}
+
+auto main() -> int
+{
+  std::string instructions;
+  if (!std::getline(std::cin, instructions)) {
+    std::cerr << "No input\n";
+    return EXIT_FAILURE;
+  }
+
+  std::string const floor{"#-------#"};
+  std::string const wall{"|.......|"};
+  Shape const horiz = {{"oooo"}};
+  Shape const plus = {{" o "}, {"ooo"}, {" o "}};
+  Shape const corner = {{"ooo"}, {"  o"}, {"  o"}};
+  Shape const vert = {{"o"}, {"o"}, {"o"}, {"o"}};
+  Shape const square = {{"oo"}, {"oo"}};
+  std::vector<Shape> const shapes{horiz, plus, corner, vert, square};
+
+  // Numbers we want to store for later
+  std::vector<UInt> heights;  // heights[i] gives height of tower before piece i falls.
+  std::map<UInt, UInt>
+    wind_iter_map;  // Maps place in wind string vs iteration.  Used to see when we start repeating.
+
+  // We have a ridiculous number of iterations to do - more than we can do on this machine so we
+  // are going to assume that we have three phases: Setup, repeated, Teradown
+  // We identify the repeated section by being at the same place in the instructions for Shape 0 as
+  // we have been before.
+  UInt previous_repeated_height{0};
+
+  Shape tower{floor};
+  auto wind_it{instructions.begin()};
+  auto shape_it{shapes.begin()};
+  UInt constexpr max_iter{1000000000000};
+  for (UInt i{0}; i < max_iter; ++i) {
+    UInt const height{tower_height(tower)};
+    heights.push_back(height);
+
+    if (shape_it == shapes.begin()) {
+      // See if we've seen this wind position before for shape 0.
+      auto [it, success] = wind_iter_map.insert({wind_it - instructions.begin(), i});
+      if (!success) {
+        // We have - what is the delta in height since the last time?  If it is the same delta as
+        // the previous repeat we saw, then we can stop iterating.
+        UInt const repeated_height(heights[it->second] - heights[i]);
+        std::cout << "Repeated wind instruction " << it->first << ": Iters [" << it->second << ", "
+                  << i << ").  Heights: [" << heights[it->second] << ", " << heights[i]
+                  << "].  Difference: " << heights[i] - heights[it->second] << ".\n";
+        if (repeated_height == previous_repeated_height) {
+          break;
+        }
+        previous_repeated_height = repeated_height;
+        it->second = i;
+      }
+    }
+
+    // Where should the block appear?  (2, 3 + height).
+    UInt x{3};
+    UInt y{4 + height};
+
+    // Now make sure we have enough wall in the tower.
+    while (tower.size() < y + shape_it->size() + 1) {
+      tower.push_back(wall);
+    }
+
+    // Now do the instructions in turn:
+    while (true) {
+      // Wind movement
+      UInt const trial_x{x + (*wind_it == '>' ? 1 : -1)};
+      if (!shapes_intersect(tower, *shape_it, trial_x, y)) {
+        x = trial_x;
+      }
+
+      ++wind_it;
+      if (wind_it == instructions.end()) {
+        wind_it = instructions.begin();
+      }
+
+      // Downward movement
+      UInt const trial_y{y - 1};
+      if (!shapes_intersect(tower, *shape_it, x, trial_y)) {
+        y = trial_y;
+      }
+      else {
+        break;
+      }
+    }
+
+    // Insert the piece.
+    for (auto const& r : *shape_it) {
+      UInt cx{x};
+      for (auto c : r) {
+        if (c != ' ') {
+          tower[y][cx] = '#';
+        }
+        ++cx;
+      }
+      ++y;
+    }
+
+    // Next shape
+    ++shape_it;
+    if (shape_it == shapes.end()) {
+      shape_it = shapes.begin();
+    }
+  }
+
+  UInt const repeat_start{wind_iter_map.find(wind_it - instructions.begin())->second};
+  UInt const repeat_end{heights.size() - 1};
+  // [0, repeat_start) is the prologue. [repeat_start, repeat_end) is repeated heights added.
+  // [repeat_end, max_iter) is the epilogue.
+  UInt const repeat_length{repeat_end - repeat_start};
+  UInt const num_repeats{(max_iter - repeat_start) / repeat_length};
+  UInt const num_left = max_iter - repeat_length * num_repeats;
+  UInt const height =
+    heights[num_left] + (heights[repeat_end] - heights[repeat_start]) * num_repeats;
+  std::cout << "Maximum height: " << height << "\n";
+  return EXIT_SUCCESS;
+}