Added Solution for 2021 day 17

2021/day17_trick_shot/Cargo.toml

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+[package]
+name = "day17_trick_shot"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

2021/day17_trick_shot/challenge.txt

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+You finally decode the Elves' message. `HI`, the message says. You continue searching for the sleigh keys.
+
+Ahead of you is what appears to be a large [ocean trench](https://en.wikipedia.org/wiki/Oceanic_trench). Could the keys have fallen into it? You'd better send a probe to investigate.
+
+The probe launcher on your submarine can fire the probe with any [integer](https://en.wikipedia.org/wiki/Integer) velocity in the `x` (forward) and `y` (upward, or downward if negative) directions. For example, an initial `x,y` velocity like `0,10` would fire the probe straight up, while an initial velocity like `10,-1` would fire the probe forward at a slight downward angle.
+
+The probe's `x,y` position starts at `0,0`. Then, it will follow some trajectory by moving in *steps*. On each step, these changes occur in the following order:
+
+* The probe's `x` position increases by its `x` velocity.
+* The probe's `y` position increases by its `y` velocity.
+* Due to drag, the probe's `x` velocity changes by `1` toward the value `0`; that is, it decreases by `1` if it is greater than `0`, increases by `1` if it is less than `0`, or does not change if it is already `0`.
+* Due to gravity, the probe's `y` velocity decreases by `1`.
+
+For the probe to successfully make it into the trench, the probe must be on some trajectory that causes it to be within a *target area* after any step. The submarine computer has already calculated this target area (your puzzle input). For example:
+
+```
+target area: x=20..30, y=-10..-5
+```
+
+This target area means that you need to find initial `x,y` velocity values such that after any step, the probe's `x` position is at least `20` and at most `30`, *and* the probe's `y` position is at least `-10` and at most `-5`.
+
+Given this target area, one initial velocity that causes the probe to be within the target area after any step is `7,2`:
+
+```
+.............#....#............
+.......#..............#........
+...............................
+S........................#.....
+...............................
+...............................
+...........................#...
+...............................
+....................TTTTTTTTTTT
+....................TTTTTTTTTTT
+....................TTTTTTTT#TT
+....................TTTTTTTTTTT
+....................TTTTTTTTTTT
+....................TTTTTTTTTTT
+
+```
+
+In this diagram, `S` is the probe's initial position, `0,0`. The `x` coordinate increases to the right, and the `y` coordinate increases upward. In the bottom right, positions that are within the target area are shown as `T`. After each step (until the target area is reached), the position of the probe is marked with `#`. (The bottom-right `#` is both a position the probe reaches and a position in the target area.)
+
+Another initial velocity that causes the probe to be within the target area after any step is `6,3`:
+
+```
+...............#..#............
+...........#........#..........
+...............................
+......#..............#.........
+...............................
+...............................
+S....................#.........
+...............................
+...............................
+...............................
+.....................#.........
+....................TTTTTTTTTTT
+....................TTTTTTTTTTT
+....................TTTTTTTTTTT
+....................TTTTTTTTTTT
+....................T#TTTTTTTTT
+....................TTTTTTTTTTT
+
+```
+
+Another one is `9,0`:
+
+```
+S........#.....................
+.................#.............
+...............................
+........................#......
+...............................
+....................TTTTTTTTTTT
+....................TTTTTTTTTT#
+....................TTTTTTTTTTT
+....................TTTTTTTTTTT
+....................TTTTTTTTTTT
+....................TTTTTTTTTTT
+
+```
+
+One initial velocity that *doesn't* cause the probe to be within the target area after any step is `17,-4`:
+
+```
+S..............................................................
+...............................................................
+...............................................................
+...............................................................
+.................#.............................................
+....................TTTTTTTTTTT................................
+....................TTTTTTTTTTT................................
+....................TTTTTTTTTTT................................
+....................TTTTTTTTTTT................................
+....................TTTTTTTTTTT..#.............................
+....................TTTTTTTTTTT................................
+...............................................................
+...............................................................
+...............................................................
+...............................................................
+................................................#..............
+...............................................................
+...............................................................
+...............................................................
+...............................................................
+...............................................................
+...............................................................
+..............................................................#
+
+```
+
+The probe appears to pass through the target area, but is never within it after any step. Instead, it continues down and to the right - only the first few steps are shown.
+
+If you're going to fire a highly scientific probe out of a super cool probe launcher, you might as well do it with *style*. How high can you make the probe go while still reaching the target area?
+
+In the above example, using an initial velocity of `6,9` is the best you can do, causing the probe to reach a maximum `y` position of `*45*`. (Any higher initial `y` velocity causes the probe to overshoot the target area entirely.)
+
+Find the initial velocity that causes the probe to reach the highest `y` position and still eventually be within the target area after any step. *What is the highest `y` position it reaches on this trajectory?*
+
+To begin, [get your puzzle input](17/input).
+
+Answer:

2021/day17_trick_shot/src/lib.rs

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+use core::fmt::Display;
+use std::num::ParseIntError;
+
+#[derive(Debug, PartialEq, Eq)]
+pub enum ParseError {
+    ParseIntError(std::num::ParseIntError),
+    LineMalformed(String),
+}
+
+impl From<ParseIntError> for ParseError {
+    fn from(value: ParseIntError) -> Self {
+        Self::ParseIntError(value)
+    }
+}
+
+impl Display for ParseError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Self::ParseIntError(e) => write!(f, "Unable to parse into integer: {e}"),
+            Self::LineMalformed(v) => write!(f, "Line is malformed: {v}"),
+        }
+    }
+}
+
+struct Area {
+    x_min: isize,
+    x_max: isize,
+    y_min: isize,
+    y_max: isize,
+}
+
+impl TryFrom<&str> for Area {
+    type Error = ParseError;
+
+    fn try_from(value: &str) -> Result<Self, Self::Error> {
+        let components: Vec<_> = value.split(&[' ', ',']).collect();
+        if components.len() == 5 {
+            let x: Vec<_> = components[2].split(&['.', '=']).collect();
+            let y: Vec<_> = components[4].split(&['.', '=']).collect();
+            if x.len() == 4 && y.len() == 4 {
+                Ok(Self {
+                    x_min: x[1].parse()?, 
+                    x_max: x[3].parse()?,
+                    y_min: y[1].parse()?,
+                    y_max: y[3].parse()?,
+                })
+            } else {
+                Err(Self::Error::LineMalformed(value.to_string()))
+            }
+        } else {
+            Err(Self::Error::LineMalformed(value.to_string()))
+        }
+    }
+}
+
+pub fn run(input: &str) -> Result<(isize, usize), ParseError> {
+    let target = Area::try_from(input)?;
+    let mut max_y = 0;
+    let mut hits = 0;
+    for y in target.y_min..-target.y_min {
+        for x in 1..=target.x_max {
+            let attempt = launch((0, 0), (x, y), &target, 0);
+            if attempt.0 {
+                max_y = max_y.max(attempt.1);
+                hits += 1;
+            }
+        }
+    }
+    let first = max_y;
+    let second = hits;
+    Ok((first, second))
+}
+
+fn launch((x, y): (isize, isize), (x_vel, y_vel): (isize, isize), target: &Area, max_y: isize) -> (bool, isize) {
+    if (target.x_min..=target.x_max).contains(&x) && (target.y_min..=target.y_max).contains(&y) {
+        (true, max_y)
+    } else if x_vel >= 0 && target.x_max < x ||
+              x_vel <= 0 && target.x_min > x ||
+              y_vel <= 0 && target.y_min > y {
+        (false, 0)
+    } else if x_vel > 0 {
+        launch((x+x_vel, y+y_vel), (x_vel-1, y_vel-1), target, max_y.max(y+y_vel))
+    } else if x_vel < 0 {
+        launch((x+x_vel, y+y_vel), (x_vel+1, y_vel-1), target, max_y.max(y+y_vel))
+    } else {
+        launch((x+x_vel, y+y_vel), (0, y_vel-1), target, max_y.max(y+y_vel))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::fs::read_to_string;
+
+    fn read_file(name: &str) -> String {
+        read_to_string(name).expect(&format!("Unable to read file: {name}")[..]).trim().to_string()
+    }
+
+    #[test]
+    fn test_sample() {
+        let sample_input = read_file("tests/sample_input");
+        let target = Area::try_from(&sample_input[..]).unwrap();
+        let should_hit = [
+            (7, 2),
+            (6, 3),
+            (9, 0),
+        ];
+        let should_miss = [ (17, -4) ];
+        for vel in should_hit {
+            assert!(launch((0, 0), vel, &target, 0).0);
+        }
+        for vel in should_miss {
+            assert!(!launch((0, 0), vel, &target, 0).0);
+        }
+        assert_eq!(run(&sample_input), Ok((45, 112)));
+    }
+
+    #[test]
+    fn test_challenge() {
+        let challenge_input = read_file("tests/challenge_input");
+        assert_eq!(run(&challenge_input), Ok((6903, 2351)));
+    }
+}

2021/day17_trick_shot/tests/challenge_input

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+target area: x=235..259, y=-118..-62

2021/day17_trick_shot/tests/sample_input

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+target area: x=20..30, y=-10..-5