78 lines
1.9 KiB
C++
78 lines
1.9 KiB
C++
#include <iostream>
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#include <regex>
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#include <string>
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template<typename T>
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constexpr auto triangle_sum(T t) noexcept -> T
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{
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return t * (t + 1) / 2;
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}
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auto main() -> int
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{
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static std::regex re{R"(target area: x=(-?\d+)\.\.(-?\d+), y=(-?\d+)\.\.(-?\d+))"};
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std::smatch m;
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std::string line;
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if (!std::getline(std::cin, line)) {
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std::cerr << "Unable to read line";
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return 1;
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}
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if (!std::regex_search(line, m, re)) {
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std::cerr << "Unable to interpret: " << line << '\n';
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return 1;
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}
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auto x0{std::stol(m.str(1))};
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auto x1{std::stol(m.str(2))};
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auto y0{std::stol(m.str(3))};
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auto y1{std::stol(m.str(4))};
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auto [x_min, x_max] = std::minmax(std::abs(x0), std::abs(x1));
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auto [y_min, y_max] = std::minmax(std::abs(y0), std::abs(y1));
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/* Find a minimum vx0. */
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long vx0_min{1};
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while (triangle_sum(vx0_min) < x_min) {
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++vx0_min;
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}
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unsigned count_success{0};
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/* For each of the possible starting velocities. */
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for (long vy0_start{-y_max}; vy0_start <= y_max; ++vy0_start) {
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for (long vx0_start{vx0_min}; vx0_start <= x_max; ++vx0_start) {
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auto vx0{vx0_start};
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auto vy0{vy0_start};
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long x{0};
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long y{0};
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/* Iterate until we're in the target range in the x direction. We know this will happen as
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* vx0_start is at least vx0_min.
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*/
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while (x < x_min) {
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x += vx0;
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vx0 = std::max(vx0 - 1, long{0});
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y += (vy0--);
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}
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/* Now check to see where whether the y co-ordinate will end up in the target zone whilst
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* we're still in the zone horizontally.
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*/
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while (x <= x_max && -y <= y_max) {
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if (y_min <= -y && -y <= y_max) {
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++count_success;
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break;
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}
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x += vx0;
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vx0 = std::max(vx0 - 1, long{0});
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y += (vy0--);
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}
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}
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}
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std::cout << "Count of successful trajectories:" << count_success << '\n';
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return 0;
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}
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