2024 day 14: solve part 1

2024/answer_tests/day14/example1.txt

@@ -0,0 +1,2 @@
+Day 14:
+12

2024/input/day14/example1.txt

@@ -0,0 +1,12 @@
+p=0,4 v=3,-3
+p=6,3 v=-1,-3
+p=10,3 v=-1,2
+p=2,0 v=2,-1
+p=0,0 v=1,3
+p=3,0 v=-2,-2
+p=7,6 v=-1,-3
+p=3,0 v=-1,-2
+p=9,3 v=2,3
+p=7,3 v=-1,2
+p=2,4 v=2,-3
+p=9,5 v=-3,-3

2024/src/day14.cpp

@@ -0,0 +1,30 @@
+/******************************************************************************
+ * File:        day14.cpp
+ *
+ * Author:      Eric T. Johnson (yut23)
+ * Created:     2024-12-14
+ *****************************************************************************/
+
+#include "day14.hpp"
+#include "lib.hpp"
+#include <fstream>  // for ifstream
+#include <iostream> // for cout
+
+int main(int argc, char **argv) {
+    std::ifstream infile = aoc::parse_args(argc, argv);
+    const bool is_example =
+        std::string{argv[1]}.find("example") != std::string::npos;
+    aoc::Pos bounds = is_example ? aoc::Pos{11, 7} : aoc::Pos{101, 103};
+
+    auto robots = aoc::day14::read_input(infile);
+
+    for (int time = 0; time < 100; ++time) {
+        for (auto &robot : robots) {
+            robot.update(bounds);
+        }
+    }
+
+    std::cout << aoc::day14::safety_factor(robots, bounds) << "\n";
+
+    return 0;
+}

2024/src/day14.hpp

@@ -0,0 +1,82 @@
+/******************************************************************************
+ * File:        day14.hpp
+ *
+ * Author:      Eric T. Johnson (yut23)
+ * Created:     2024-12-14
+ *****************************************************************************/
+
+#ifndef DAY14_HPP_5W9MWUPG
+#define DAY14_HPP_5W9MWUPG
+
+#include "lib.hpp"  // for Pos, Delta, expect_input
+#include <array>    // for array
+#include <iostream> // for istream
+#include <string>   // for string, getline
+#include <utility>  // for move
+#include <vector>   // for vector
+
+namespace aoc::day14 {
+
+struct Robot {
+    Pos pos{0, 0};
+    Delta vel{0, 0};
+
+    void update(const Pos &bounds);
+};
+
+std::istream &operator>>(std::istream &is, Robot &robot) {
+    is >> std::noskipws;
+    is >> aoc::expect_input("p=") >> robot.pos.x >> aoc::expect_input(',') >>
+        robot.pos.y >> aoc::expect_input(" v=") >> robot.vel.dx >>
+        aoc::expect_input(',') >> robot.vel.dy;
+    is >> std::ws;
+    return is;
+}
+
+auto read_input(std::istream &is) {
+    Robot robot;
+    std::vector<Robot> robots;
+    while (is >> robot) {
+        robots.push_back(std::move(robot));
+    }
+    return robots;
+}
+
+void Robot::update(const Pos &bounds) {
+    pos += vel;
+    if (pos.x < 0) {
+        pos.x += bounds.x;
+    } else if (pos.x >= bounds.x) {
+        pos.x -= bounds.x;
+    }
+    if (pos.y < 0) {
+        pos.y += bounds.y;
+    } else if (pos.y >= bounds.y) {
+        pos.y -= bounds.y;
+    }
+}
+
+int safety_factor(const std::vector<Robot> &robots, const Pos &bounds) {
+    std::array<int, 4> quadrants{};
+    for (const auto &robot : robots) {
+        int quad_idx = 0;
+        if (robot.pos.x == bounds.x / 2 || robot.pos.y == bounds.y / 2) {
+            // robot is exactly in the middle, so it doesn't count for any
+            // quadrant
+            continue;
+        }
+        if (robot.pos.x > bounds.x / 2) {
+            quad_idx |= 1;
+        }
+        if (robot.pos.y > bounds.y / 2) {
+            quad_idx |= 2;
+        }
+        ++quadrants[quad_idx];
+    }
+
+    return quadrants[0] * quadrants[1] * quadrants[2] * quadrants[3];
+}
+
+} // namespace aoc::day14
+
+#endif /* end of include guard: DAY14_HPP_5W9MWUPG */