Day 17

Cargo.toml

@@ -66,3 +66,7 @@ path = "src/day15.rs"
 [[bin]]
 name = "day16"
 path = "src/day16.rs"
+
+[[bin]]
+name = "day17"
+path = "src/day17.rs"

src/day17.rs

@@ -0,0 +1,111 @@
+use std::ops::RangeInclusive;
+
+#[derive(Clone, Debug)]
+struct Area {
+    x: RangeInclusive<i32>,
+    y: RangeInclusive<i32>,
+}
+
+impl Area {
+    fn contains(&self, x: i32, y: i32) -> bool {
+        self.x.contains(&x) && self.y.contains(&y)
+    }
+}
+
+fn simulate(area: &Area, dx: i32, dy: i32) -> Option<i32> {
+    let mut hit = false;
+    let mut max_y = 0;
+    for step in 0..1000 {
+        let x = if step < dx {
+            // General solution for $ x_{t+1} - x_t = -t - dx + 1 $.
+            ((2 * dx + 1) * step - step * step) / 2
+        } else {
+            // Special case when $ x_{t+1} - x_t = 0 $ is reached.
+            (dx * dx + dx) / 2
+        };
+        let y = ((2 * dy + 1) * step - step * step) / 2;
+
+        if y > max_y {
+            max_y = y;
+        }
+
+        if area.contains(x, y) {
+            hit = true;
+        }
+
+        if y < *area.y.start() {
+            break;
+        }
+    }
+
+    hit.then(|| max_y)
+}
+
+fn find_max_y(area: Area) -> i32 {
+    let mut max_y = 0;
+    for dx in 1..=*area.x.end() {
+        for dy in 1..1000 {
+            if let Some(y) = simulate(&area, dx, dy) {
+                if y > max_y {
+                    max_y = y;
+                }
+            }
+        }
+    }
+    max_y
+}
+
+fn count_hits(area: Area) -> i32 {
+    let mut counter = 0;
+    for dx in 1..=*area.x.end() {
+        for dy in *area.y.start()..1000 {
+            if simulate(&area, dx, dy).is_some() {
+                counter += 1;
+            }
+        }
+    }
+    counter
+}
+
+fn main() {
+    let area = Area {
+        x: (179..=201),
+        y: (-109..=-63),
+    };
+    println!("solution {}", find_max_y(area.clone()));
+    println!("solution {}", count_hits(area));
+}
+
+#[test]
+fn test_find_max_y() {
+    assert_eq!(
+        find_max_y(Area {
+            x: (20..=30),
+            y: (-10..=-5)
+        }),
+        45
+    );
+}
+
+#[test]
+fn test_simulate() {
+    let area = Area {
+        x: (20..=30),
+        y: (-10..=-5),
+    };
+    assert_eq!(simulate(&area, 7, 2), Some(3));
+    assert_eq!(simulate(&area, 6, 3), Some(6));
+    assert_eq!(simulate(&area, 17, -4), None);
+    assert_eq!(simulate(&area, 6, 9), Some(45));
+}
+
+#[test]
+fn test_count_hits() {
+    assert_eq!(
+        count_hits(Area {
+            x: (20..=30),
+            y: (-10..=-5)
+        }),
+        112
+    );
+}