9

Author: gogsbread

cpp/9.cpp

@@ -0,0 +1,40 @@
+#include "common.h"
+
+namespace {
+using Pos = std::complex<long>;
+
+template <size_t N> long p1(const auto &input) {
+  auto Hasher = [](const Pos &k) -> std::size_t {
+    return std::hash<long>()(k.real()) ^ std::hash<long>()(k.imag());
+  };
+  auto offset = [](const Pos &p) {
+    return Pos{(std::abs(p.real()) >= 2 ? p.real() / 2L : p.real()),
+               (std::abs(p.imag()) >= 2 ? p.imag() / 2L : p.imag())};
+  };
+  std::unordered_set<Pos, decltype(Hasher)> v{Pos{0, 0}};
+  std::array<Pos, N> rope{};
+  std::unordered_map<char, Pos> dir{
+      {'R', Pos{1, 0}}, {'L', Pos{-1, 0}}, {'U', Pos{0, 1}}, {'D', Pos{0, -1}}};
+  for (auto [d, l, prev] = std::tuple{' ', 0L, Pos{0, 0}};
+       const auto &s : input) {
+    std::istringstream ss{s};
+    ss >> d >> l;
+    while (l--) {
+      rope[0] += dir[d];
+      for (const auto i : std::ranges::views::iota(1UL, N)) {
+        const auto diff = rope[i - 1] - rope[i];
+        if (std::abs(diff) >= 2)
+          rope[i] += offset(diff);
+      }
+      v.emplace(rope[N - 1]);
+    }
+  }
+  return v.size();
+}
+} // namespace
+
+int main() {
+  const auto &input = gb::advent2021::readIn();
+  gb::advent2021::writeOut(std::to_string(p1<2>(input)));
+  gb::advent2021::writeOut(std::to_string(p1<10>(input)));
+}

cpp/common.h

@@ -4,6 +4,7 @@
 #include <charconv>
 #include <climits>
 #include <cmath>
+#include <complex>
 #include <cstdint>
 #include <cstdio>
 #include <cstring>

cpp/meson.build

@@ -34,9 +34,9 @@ executable('8',
     '8.cpp',
     dependencies: [])
 
-# executable('9',
-#     '9.cpp',
-#     dependencies: [])
+executable('9',
+    '9.cpp',
+    dependencies: [])
 
 # executable('10',
 #     '10.cpp',