Day 17 part 2

2021/src/day17.rs

@@ -9,6 +9,31 @@ struct Target {
     y_max: isize,
 }
 
+impl Target {
+    fn hit(&self, x: isize, y: isize) -> bool {
+        x >= self.x_min && x <= self.x_max && y >= self.y_min && y <= self.y_max
+    }
+}
+
+fn shoot(x: isize, y: isize, tgt: &Target) -> bool {
+    let mut x_inc = x;
+    let mut y_inc = y;
+    let mut x_cur = 0;
+    let mut y_cur = 0;
+    while x_cur <= tgt.x_max && y_cur >= tgt.y_min {
+        x_cur += x_inc;
+        y_cur += y_inc;
+        if x_inc > 0 {
+            x_inc -= 1;
+        }
+        y_inc -= 1;
+        if tgt.hit(x_cur, y_cur) {
+            return true;
+        }
+    }
+    false
+}
+
 impl FromStr for Target {
     type Err = Infallible;
 
@@ -35,19 +60,26 @@ impl FromStr for Target {
 }
 
 #[aoc(day17, part1)]
-fn part1(input: &str) -> Result<usize> {
+fn part1(input: &str) -> Result<isize> {
     let tgt: Target = input.parse()?;
-    dbg!(&tgt);
-    Ok((0..(tgt.y_min).abs() as usize).sum())
+    let n = tgt.y_min.abs() - 1;
+    Ok(n * (n + 1) / 2)
 }
 
-/*
 #[aoc(day17, part2)]
 fn part2(input: &str) -> Result<usize> {
-    todo!("part2");
-    Ok(0)
+    let tgt: Target = input.parse()?;
+    let mut cnt = 0;
+    let y_range = tgt.y_min.abs().max(tgt.y_min.abs());
+    for y in -y_range..=y_range {
+        for x in 1..=tgt.x_max {
+            if shoot(x, y, &tgt) {
+                cnt += 1;
+            }
+        }
+    }
+    Ok(cnt)
 }
-*/
 
 #[cfg(test)]
 mod tests {
@@ -63,14 +95,13 @@ target area: x=20..30, y=-10..-5
         Ok(())
     }
 
-    /*
     #[test]
     fn test_part2() -> Result<()> {
         let input = r#"
+target area: x=20..30, y=-10..-5
 "#
         .trim();
-        assert_eq!(part2(input)?, usize::MAX);
+        assert_eq!(part2(input)?, 112);
         Ok(())
     }
-    */
 }