Add 2021 day 17 puzzles

2021/puzzle-17-02.cc

@@ -0,0 +1,77 @@
+#include <iostream>
+#include <regex>
+#include <string>
+
+template<typename T>
+constexpr auto triangle_sum(T t) noexcept -> T
+{
+  return t * (t + 1) / 2;
+}
+
+auto main() -> int
+{
+  static std::regex re{R"(target area: x=(-?\d+)\.\.(-?\d+), y=(-?\d+)\.\.(-?\d+))"};
+  std::smatch m;
+  std::string line;
+
+  if (!std::getline(std::cin, line)) {
+    std::cerr << "Unable to read line";
+    return 1;
+  }
+  if (!std::regex_search(line, m, re)) {
+    std::cerr << "Unable to interpret: " << line << '\n';
+    return 1;
+  }
+
+  auto x0{std::stol(m.str(1))};
+  auto x1{std::stol(m.str(2))};
+  auto y0{std::stol(m.str(3))};
+  auto y1{std::stol(m.str(4))};
+
+  auto [x_min, x_max] = std::minmax(std::abs(x0), std::abs(x1));
+  auto [y_min, y_max] = std::minmax(std::abs(y0), std::abs(y1));
+
+  /* Find a minimum vx0.  */
+  long vx0_min{1};
+  while (triangle_sum(vx0_min) < x_min) {
+    ++vx0_min;
+  }
+
+  unsigned count_success{0};
+
+  /* For each of the possible starting velocities.  */
+  for (long vy0_start{-y_max}; vy0_start <= y_max; ++vy0_start) {
+    for (long vx0_start{vx0_min}; vx0_start <= x_max; ++vx0_start) {
+      auto vx0{vx0_start};
+      auto vy0{vy0_start};
+      long x{0};
+      long y{0};
+
+      /* Iterate until we're in the target range in the x direction.  We know this will happen as
+       * vx0_start is at least vx0_min.
+       */
+      while (x < x_min) {
+        x += vx0;
+        vx0 = std::max(vx0 - 1, long{0});
+        y += (vy0--);
+      }
+
+      /* Now check to see where whether the y co-ordinate will end up in the target zone whilst
+       * we're still in the zone horizontally.
+       */
+      while (x <= x_max && -y <= y_max) {
+        if (y_min <= -y && -y <= y_max) {
+          ++count_success;
+          break;
+        }
+        x += vx0;
+        vx0 = std::max(vx0 - 1, long{0});
+        y += (vy0--);
+      }
+    }
+  }
+  
+  std::cout << "Count of successful trajectories:" << count_success << '\n';
+
+  return 0;
+}