day 18.

This one was easy, but I kinda phoned it in. At least I learned about
iota_view.

Author: peadar

18/Makefile

@@ -0,0 +1,2 @@
+#CXXSTD = c++23
+include ../Makefile.inc

18/eg.txt

@@ -0,0 +1,25 @@
+5,4
+4,2
+4,5
+3,0
+2,1
+6,3
+2,4
+1,5
+0,6
+3,3
+2,6
+5,1
+1,2
+5,5
+2,5
+6,5
+1,4
+0,4
+6,4
+1,1
+6,1
+1,0
+0,5
+1,6
+2,0

18/solve.cc

@@ -0,0 +1,100 @@
+#include "aoc.h"
+
+namespace {
+enum Direction { NORTH, EAST, SOUTH, WEST };
+
+using Scalar = unsigned;
+using Int = unsigned short;
+constexpr Int MAX = std::numeric_limits<Int>::max();
+
+struct Point {
+   Scalar row{}, col{};
+   Point operator+ (const Point &rhs) const noexcept { return { Scalar(row + rhs.row), Scalar(col + rhs.col) }; }
+   auto operator <=> (const Point &rhs) const = default;
+};
+
+struct Cell {
+   unsigned short wall:1{};
+   unsigned short epoch:15{};
+   Int cost;
+};
+
+constexpr Point velocity(Direction d) noexcept {
+   switch (d) {
+      case NORTH: return {Scalar(-1), 0};
+      case SOUTH: return {1, 0};
+      case EAST: return {0, 1};
+      case WEST: return {0, Scalar(-1)};
+      default: __builtin_unreachable();
+   }
+}
+
+struct Node {
+   unsigned long cost { MAX };
+   Point pos{};
+   auto operator <=> (const Node &rhs) const = default;
+};
+
+static constexpr Scalar ROWS = 71;
+static constexpr Scalar COLS = 71;
+
+struct MemorySpace {
+   std::vector<Cell> map;
+   unsigned age{};
+   Point s{0,0}, e{ROWS-1, COLS-1};
+
+   Cell & at(Point p) { return map[p.row * COLS + p.col]; }
+   std::vector<Point> walls;
+   const Cell & at(Point p) const { return map[p.row * COLS + p.col]; }
+
+   Int dijkstra(unsigned maxage) {
+      for (auto &cell : map)
+         cell.cost = MAX;
+      std::set<Node> Q;
+      Q.insert({.cost=0, .pos = {0,0}});
+      while (Q.begin()->pos != e) {
+         auto head = Q.begin();
+         auto item = *head;
+         Q.erase(head);
+         auto try_add = [&](Point p, Int c) {
+            if (p.row >= ROWS || p.col >= COLS)
+               return;
+            auto &cell = at(p);
+            if ((cell.wall && cell.epoch < maxage) || cell.cost < c )
+               return;
+            cell.cost = c;
+            Q.insert({.cost=c, .pos=p });
+         };
+         try_add(item.pos + velocity(NORTH), item.cost + 1);
+         try_add(item.pos + velocity(SOUTH), item.cost + 1);
+         try_add(item.pos + velocity(EAST), item.cost + 1);
+         try_add(item.pos + velocity(WEST), item.cost + 1);
+         if (Q.empty())
+            return MAX;
+      }
+      return Q.begin()->cost;
+   }
+
+   MemorySpace(std::istream &is) noexcept : map ( ROWS * COLS ) {
+      for (std::string line; getline(is, line); ) {
+         std::string_view v = line;
+         Scalar col = aoc::parsetoken<int>(v, ",");
+         Scalar row = aoc::parsetoken<int>(v, "\n");
+         walls.emplace_back(row, col);
+         auto &cell = at({row, col});
+         cell.wall = true;
+         cell.epoch = age++;
+      }
+   }
+};
+aoc::Case case1{"part1", [](std::istream &is, std::ostream &os) { os << MemorySpace(is).dijkstra(1024); } };
+
+struct Part2 : MemorySpace {
+   Part2( std::istream &is ) : MemorySpace(is) {}
+   void part2(std::ostream &os) {
+      auto res = std::ranges::partition_point( std::ranges::iota_view(unsigned(0), age) , [this] ( unsigned value ) { auto v = dijkstra(value); return v != MAX; });
+      os << walls[*res - 1].col << "," << walls[*res - 1].row << "\n";
+   }
+};
+aoc::Case case2{"part2", [](std::istream &is, std::ostream &os) { Part2(is).part2(os); } };
+}

Makefile.inc

@@ -1,6 +1,7 @@
 OPT ?= -O0 -pipe
-CXXFLAGS += -g --std=c++2a -Wall -Wextra $(OPT) -I../include
-ASFLAGS += -g --std=c++2a -Wall -Wextra $(OPT) -I../include
+CXXSTD ?= c++2a
+CXXFLAGS += -g --std=$(CXXSTD) -Wall -Wextra $(OPT) -I../include
+ASFLAGS += -g --std=$(CXXSTD) -Wall -Wextra $(OPT) -I../include
 
 .PHONY: all clean
 VPATH = ../lib