2024 day 13: solve part 1

2024/answer_tests/day13/example1.txt

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+Day 13:
+480

2024/input/day13/example1.txt

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+Button A: X+94, Y+34
+Button B: X+22, Y+67
+Prize: X=8400, Y=5400
+
+Button A: X+26, Y+66
+Button B: X+67, Y+21
+Prize: X=12748, Y=12176
+
+Button A: X+17, Y+86
+Button B: X+84, Y+37
+Prize: X=7870, Y=6450
+
+Button A: X+69, Y+23
+Button B: X+27, Y+71
+Prize: X=18641, Y=10279

2024/src/day13.cpp

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+/******************************************************************************
+ * File:        day13.cpp
+ *
+ * Author:      Eric T. Johnson (yut23)
+ * Created:     2024-12-13
+ *****************************************************************************/
+
+#include "day13.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);
+
+    auto claw_machines = aoc::day13::read_input(infile);
+
+    int part_1 = 0;
+    for (const auto &claw : claw_machines) {
+        if constexpr (aoc::DEBUG) {
+            std::cerr << claw;
+        }
+        const auto &[a_presses, b_presses] = claw.min_presses();
+        if (a_presses > 0 && b_presses > 0) {
+            int tokens = a_presses * 3 + b_presses * 1;
+            if constexpr (aoc::DEBUG) {
+                std::cerr << "  winnable with " << tokens << " tokens\n";
+            }
+            part_1 += tokens;
+        }
+    }
+    std::cout << part_1 << "\n";
+
+    return 0;
+}

2024/src/day13.hpp

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+/******************************************************************************
+ * File:        day13.hpp
+ *
+ * Author:      Eric T. Johnson (yut23)
+ * Created:     2024-12-13
+ *****************************************************************************/
+
+#ifndef DAY13_HPP_WCDIDLRZ
+#define DAY13_HPP_WCDIDLRZ
+
+#include "lib.hpp"  // for Pos, expect_input, DEBUG
+#include <iostream> // for istream, ostream, cerr, noskipws, ws
+#include <utility>  // for pair
+#include <vector>   // for vector
+
+namespace aoc::day13 {
+
+struct ClawMachine {
+    Pos a{0, 0};
+    Pos b{0, 0};
+    Pos prize{0, 0};
+
+    std::pair<int, int> min_presses() const;
+};
+
+std::istream &operator>>(std::istream &is, ClawMachine &claw) {
+    ClawMachine tmp;
+    is >> std::noskipws;
+    is >> aoc::expect_input("Button A: X+") >> tmp.a.x >>
+        aoc::expect_input(", Y+") >> tmp.a.y >> aoc::expect_input('\n') >>
+        aoc::expect_input("Button B: X+") >> tmp.b.x >>
+        aoc::expect_input(", Y+") >> tmp.b.y >> aoc::expect_input('\n') >>
+        aoc::expect_input("Prize: X=") >> tmp.prize.x >>
+        aoc::expect_input(", Y=") >> tmp.prize.y >> aoc::expect_input('\n');
+
+    if (is) {
+        // only store if the entire stream was read correctly
+        claw = std::move(tmp);
+    }
+    return is;
+}
+
+std::ostream &operator<<(std::ostream &os, const ClawMachine &claw) {
+    os << "Button A: X+" << claw.a.x << ", Y+" << claw.b.x << "\n"
+       << "Button B: X+" << claw.b.x << ", Y+" << claw.b.x << "\n"
+       << "Prize: X=" << claw.prize.x << ", Y=" << claw.prize.y << "\n";
+    return os;
+}
+
+// this is just a 2x2 matrix solve, use Cramer's rule
+std::pair<int, int> ClawMachine::min_presses() const {
+    const auto determinant = [](const Pos &p, const Pos &q) -> int {
+        return p.x * q.y - q.x * p.y;
+    };
+    int denom = determinant(a, b);
+    int a_numer = determinant(prize, b);
+    int b_numer = determinant(a, prize);
+    if constexpr (aoc::DEBUG) {
+        std::cerr << "  a = " << a_numer << "/" << denom << " = "
+                  << static_cast<double>(a_numer) / denom << ", b = " << b_numer
+                  << "/" << denom << " = "
+                  << static_cast<double>(b_numer) / denom << "\n";
+    }
+    if (a_numer % denom == 0 && b_numer % denom == 0) {
+        return {a_numer / denom, b_numer / denom};
+    }
+    return {-1, -1};
+}
+
+auto read_input(std::istream &is) {
+    std::vector<ClawMachine> claw_machines;
+    ClawMachine claw;
+    while (is >> claw) {
+        claw_machines.push_back(std::move(claw));
+        // consume extra newline if present
+        is >> std::ws;
+    }
+    return claw_machines;
+}
+
+} // namespace aoc::day13
+
+#endif /* end of include guard: DAY13_HPP_WCDIDLRZ */