1 files changed, 104 insertions, 0 deletions
diff --git a/src/bin/day17.rs b/src/bin/day17.rs
new file mode 100644
index 0000000..78c2e9e
--- /dev/null
+++ b/src/bin/day17.rs
@@ -0,0 +1,104 @@
+fn main() {
+    let (x1, x2) = (269, 292);
+    let (y1, y2) = (-68, -44);
+    println!("17a: {}", highest_y(x1, x2, y1, y2));
+    println!("17b: {}", count_velocities(x1, x2, y1, y2));
+}
+
+fn hit_target_y(velocity: i64, y1: i64, y2: i64, steps: i64) -> bool {
+    let mut pos = 0;
+    let mut velocity = velocity;
+
+    /* steps already done until 0 is reached again */
+    let initial_steps = velocity * 2 + 1;
+
+    for _ in initial_steps .. steps {
+        velocity += 1;
+        pos -= velocity;
+    }
+
+    pos >= y1 && pos <= y2
+}
+
+fn hit_target_x(velocity: i64, x1: i64, x2: i64, steps: i64) -> bool {
+    let mut pos = 0;
+    let mut velocity = velocity;
+
+    for _ in 0 .. steps {
+        pos += velocity;
+        velocity -= 1;
+        velocity = std::cmp::max(0, velocity);
+    }
+
+    pos >= x1 && pos <= x2
+}
+
+fn peak_y(velocity: i64) -> i64 {
+    velocity * (velocity + 1) / 2
+}
+
+fn highest_y(x1: i64, x2: i64, y1: i64, y2: i64) -> i64 {
+    let mut max = 0;
+    for vel_x in 1 .. 1000 {
+        for vel_y in 0 .. 1000 {
+            for steps in 1 .. 150 {
+                if hit_target_x(vel_x, x1, x2, steps) && hit_target_y(vel_y, y1, y2, steps) {
+                    max = std::cmp::max(max, peak_y(vel_y));
+                    break;
+                }
+            }
+        }
+    }
+    max
+}
+
+fn count_velocities(x1: i64, x2: i64, y1: i64, y2: i64) -> i64 {
+    let mut count = 0;
+    for vel_x in 1 .. 1000 {
+        for vel_y in -1000 .. 1000 {
+            for steps in 1 .. 150 {
+                if hit_target_x(vel_x, x1, x2, steps) && hit_target_y(vel_y, y1, y2, steps) {
+                    count += 1;
+                    break;
+                }
+            }
+        }
+    }
+    count
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test() {
+        let (x1, x2) = (20, 30);
+        let (y1, y2) = (-10, -5);
+
+        assert!(!hit_target_x(7, x1, x2, 3));
+        assert!(hit_target_x(7, x1, x2, 4));
+        assert!(hit_target_x(7, x1, x2, 5));
+        assert!(hit_target_x(7, x1, x2, 6));
+        assert!(hit_target_x(7, x1, x2, 7));
+
+        assert!(!hit_target_y(2, y1, y2, 6));
+        assert!(hit_target_y(2, y1, y2, 7));
+        assert!(!hit_target_y(2, y1, y2, 8));
+
+        assert!(!hit_target_x(6, x1, x2, 4));
+        assert!(hit_target_x(6, x1, x2, 5));
+        assert!(hit_target_x(6, x1, x2, 6));
+        assert!(hit_target_x(6, x1, x2, 7));
+        assert!(hit_target_x(6, x1, x2, 8));
+        assert!(hit_target_x(6, x1, x2, 9));
+        assert!(hit_target_x(6, x1, x2, 10));
+
+        assert!(!hit_target_y(3, y1, y2, 8));
+        assert!(hit_target_y(3, y1, y2, 9));
+        assert!(!hit_target_y(3, y1, y2, 10));
+
+        assert_eq!(highest_y(x1, x2, y1, y2), 45);
+        assert_eq!(count_velocities(x1, x2, y1, y2), 112);
+    }
+}