Solve Day 15 Part 2

Author: zeichensystem

.vscode/settings.json

@@ -90,7 +90,8 @@
  "unordered_set": "cpp",
  "ranges": "cpp",
  "shared_mutex": "cpp",
- "list": "cpp"
+ "list": "cpp",
+ "map": "cpp"
  },
  "cmake.configureOnOpen": true,
  "cmake.buildDirectory": "${workspaceFolder}/build/${buildType}"

day-15/day-15.cpp

@@ -1,41 +1,43 @@
-#include <unordered_map>
+#include <array>
+#include <numeric>
 #include "../aocio/aocio.hpp"
 
 /*
  Problem: https://adventofcode.com/2023/day/15
  
  Solutions: 
  - Part 1: 512283
- - Part 2: 
+ - Part 2: 215827
  Notes: 
-
+ - Part 2: Naive solution, but it still runs instantly. 
 */
 
 bool is_ascii(char c)
 {
  return c >= 0 && c <= 127; 
 }
 
-size_t hash(std::string_view s)
+uint8_t hash(std::string_view s)
 {
- size_t h = 0; 
+ uint8_t h = 0; 
  for (char c : s) {
  if (!is_ascii(c)) {
  throw "hash: not an ASCII-character";
  }
- h += (uint8_t)c; 
- h *= 17; 
- h = h % 256; 
+ // ((h + c) * 17) % 256 <=> ((h + c) % 256 * 17 % 256) % 256 <=> ((h + c) % 256 * 17) % 256
+ h = (uint8_t)(h + (uint8_t)c); 
+ h = (uint8_t)(h * 17);
  }
  return h; 
 }
 
-size_t part_one(const std::vector<std::string>& lines)
+int part_one(const std::vector<std::string>& lines)
 {
- size_t total_hash = 0; 
+ int total_hash = 0; 
  std::string_view line = lines.at(0);
  size_t start_idx = 0; 
- do {
+ 
+ while (start_idx < line.size()) {
  size_t end_idx = line.find(",", start_idx); 
  if (end_idx == std::string::npos) {
  end_idx = line.size();
@@ -44,14 +46,97 @@ size_t part_one(const std::vector<std::string>& lines)
  std::string_view word = line.substr(start_idx, end_idx - start_idx); 
  total_hash += hash(word); 
  start_idx = end_idx + 1; 
- } while (start_idx < line.size());
+ }
 
  return total_hash;
 }
 
-size_t part_two(const std::vector<std::string>& lines)
+struct Lens {
+ std::string lbl; 
+ int focal_len; 
+};
+
+inline bool operator==(const Lens& lens, std::string_view lbl) {
+ return lens.lbl == lbl; 
+}
+
+inline bool operator==(std::string_view lbl, const Lens& lens) {
+ return lens == lbl; 
+}
+
+struct Box 
+{
+ std::vector<Lens> lenses; 
+ uint8_t idx; 
+
+ void insert_or_update_lens(const std::string& lbl, int focal_len)
+ {
+ if (auto lens = std::find(lenses.begin(), lenses.end(), lbl); lens != lenses.end()) { 
+ lens->focal_len = focal_len;
+ } else {
+ lenses.push_back(Lens{lbl, focal_len});
+ }
+ }
+ 
+ void remove_lens(const std::string& lbl)
+ {
+ if (auto lens = std::find(lenses.begin(), lenses.end(), lbl); lens != lenses.end()) {
+ lenses.erase(lens); 
+ }
+ }
+
+ int calc_focusing_power() const
+ {
+ int total = 0, slot_num = 1;
+ for (const Lens& lens : lenses) {
+ int focusing_pwr = (idx + 1) * slot_num++ * lens.focal_len; 
+ total += focusing_pwr; 
+ }
+ return total;
+ }
+}; 
+
+int part_two(const std::vector<std::string>& lines)
 {
- return 0;
+ std::array<Box, 256> boxes; 
+ for (size_t i = 0; i < boxes.size(); ++i) {
+ boxes.at(i).idx = (uint8_t)i; 
+ }
+ 
+ std::string_view line = lines.at(0);
+ size_t start_idx = 0; 
+
+ while (start_idx < line.size()) {
+ size_t end_idx = line.find(",", start_idx); 
+ if (end_idx == std::string::npos) {
+ end_idx = line.size();
+ }
+ assert(end_idx >= start_idx);
+ const std::string_view word = line.substr(start_idx, end_idx - start_idx); 
+
+ size_t cmd_idx = word.find_first_of("-="); 
+ if (cmd_idx == std::string::npos) {
+ throw "Missing command";
+ }
+ const char cmd = word.at(cmd_idx); 
+
+ const std::string lens_lbl = std::string{word.substr(0, cmd_idx)};
+ Box &box = boxes.at(hash(lens_lbl));
+
+ if (cmd == '-') {
+ box.remove_lens(lens_lbl); 
+ } else if (cmd == '=') {
+ const std::string focal_len_str = std::string{word.substr(cmd_idx + 1, word.size())}; 
+ int focal_len = aocio::parse_num(focal_len_str); 
+ box.insert_or_update_lens(lens_lbl, focal_len);
+ } else {
+ throw "Invalid command";
+ }
+
+ start_idx = end_idx + 1; 
+ }
+
+ return std::accumulate(boxes.begin(), boxes.end(), 0, [](int total, const Box& box) {return total + box.calc_focusing_power();});
 }
 
 int main()
@@ -65,9 +150,9 @@ int main()
  return EXIT_FAILURE;
  } 
  try {
- size_t p1 = part_one(lines); 
+ int p1 = part_one(lines); 
  std::cout << "Part 1: " << p1 << "\n";
- size_t p2 = part_two(lines);
+ int p2 = part_two(lines);
  std::cout << "Part 2: " << p2 << "\n";
  } catch (const char* err) {
  std::cerr << "Error: " << err << "\n";