1 files changed, 90 insertions, 0 deletions
diff --git a/2021/day17/src/main.rs b/2021/day17/src/main.rs
new file mode 100644
index 0000000..641962e
--- /dev/null
+++ b/2021/day17/src/main.rs
@@ -0,0 +1,90 @@
+use regex::Regex;
+use std::{fs::read_to_string, ops::Range};
+
+#[derive(Debug, Clone, Copy)]
+struct Coord(isize, isize);
+
+#[derive(Debug, Clone)]
+struct TargetArea(pub Range<isize>, pub Range<isize>);
+impl TargetArea {
+    fn contains(&self, c: Coord) -> bool {
+        self.0.contains(&c.0) && self.1.contains(&c.1)
+    }
+}
+
+fn parse_target_area(line: &str) -> TargetArea {
+    let x_caps = Regex::new(r"x=([-\d]*)..([-\d]*)")
+        .unwrap()
+        .captures(line)
+        .unwrap();
+    let y_caps = Regex::new(r"y=([-\d]*)..([-\d]*)")
+        .unwrap()
+        .captures(line)
+        .unwrap();
+
+    TargetArea(
+        x_caps[1].parse().unwrap()..x_caps[2].parse::<isize>().unwrap() + 1isize,
+        y_caps[1].parse().unwrap()..y_caps[2].parse::<isize>().unwrap() + 1isize,
+    )
+}
+
+fn does_intersect(mut vx: isize, mut vy: isize, area: &TargetArea) -> bool {
+    let mut x = 0;
+    let mut y = 0;
+    for _ in 0.. {
+        x += vx;
+        y += vy;
+
+        vx = if vx > 0 {
+            vx - 1
+        } else if vx < 0 {
+            vx + 1
+        } else {
+            0
+        };
+        vy -= 1;
+        if area.contains(Coord(x, y)) {
+            return true;
+        } else if x > area.0.end || y < area.1.start {
+            return false;
+        }
+    }
+
+    unreachable!()
+}
+
+fn max_y(mut vy: isize) -> isize {
+    let mut y = 0;
+    for _ in 0.. {
+        y += vy;
+        vy -= 1;
+
+        if vy <= 0 {
+            return y;
+        }
+    }
+
+    unreachable!()
+}
+
+fn main() {
+    let area = parse_target_area(&read_to_string("./input").unwrap());
+
+    let mut max_y_found = 0;
+    let mut max_y_vel = Coord(0, 0);
+    let mut num_valid = 0;
+    for vx in 0..=area.0.end * 2 {
+        for vy in -area.1.start.abs() * 2..=area.1.start.abs() * 2 {
+            if does_intersect(vx, vy, &area) {
+                if max_y(vy) > max_y_found {
+                    max_y_found = max_y(vy);
+                    max_y_vel = Coord(vx, vy);
+                }
+                num_valid += 1;
+            }
+        }
+    }
+
+    println!("part 1: {} with velocity {:?}", max_y_found, max_y_vel);
+    println!("part 2: {} possible velocities", num_valid);
+}