Day 5 part 2

2021/src/day5.rs

@@ -1,3 +1,4 @@
+//!
 //! --- Day 5: Hydrothermal Venture ---
 //! You come across a field of hydrothermal vents on the ocean floor! These vents constantly produce large, opaque clouds, so it would be best to avoid them if possible.
 //!
@@ -36,6 +37,29 @@
 //! To avoid the most dangerous areas, you need to determine the number of points where at least two lines overlap. In the above example, this is anywhere in the diagram with a 2 or larger - a total of 5 points.
 //!
 //! Consider only horizontal and vertical lines. At how many points do at least two lines overlap?
+//!
+//! --- Part Two ---
+//! Unfortunately, considering only horizontal and vertical lines doesn't give you the full picture; you need to also consider diagonal lines.
+//!
+//! Because of the limits of the hydrothermal vent mapping system, the lines in your list will only ever be horizontal, vertical, or a diagonal line at exactly 45 degrees. In other words:
+//!
+//! An entry like 1,1 -> 3,3 covers points 1,1, 2,2, and 3,3.
+//! An entry like 9,7 -> 7,9 covers points 9,7, 8,8, and 7,9.
+//! Considering all lines from the above example would now produce the following diagram:
+//!
+//! 1.1....11.
+//! .111...2..
+//! ..2.1.111.
+//! ...1.2.2..
+//! .112313211
+//! ...1.2....
+//! ..1...1...
+//! .1.....1..
+//! 1.......1.
+//! 222111....
+//! You still need to determine the number of points where at least two lines overlap. In the above example, this is still anywhere in the diagram with a 2 or larger - now a total of 12 points.
+//!
+//! Consider all of the lines. At how many points do at least two lines overlap?
 
 use std::{
     fmt::{Debug, Error, Formatter},
@@ -54,7 +78,6 @@ struct Point {
     y: i32,
 }
 
-#[derive(Debug)]
 struct Line {
     p0: Point,
     p1: Point,
@@ -68,6 +91,16 @@ enum LineError {
     PrematureEOL,
 }
 
+impl Debug for Line {
+    fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error> {
+        writeln!(
+            f,
+            "{},{} -> {},{}",
+            self.p0.x, self.p0.y, self.p1.x, self.p1.y,
+        )
+    }
+}
+
 impl FromStr for Line {
     type Err = LineError;
 
@@ -113,13 +146,13 @@ impl Image {
 impl Index<(usize, usize)> for Image {
     type Output = u32;
     fn index(&self, (x, y): (usize, usize)) -> &Self::Output {
-        &self.pixels[x + y * (self.width - 1)]
+        &self.pixels[x + y * self.width]
     }
 }
 
 impl IndexMut<(usize, usize)> for Image {
     fn index_mut(&mut self, (x, y): (usize, usize)) -> &mut Self::Output {
-        &mut self.pixels[x + y * (self.width - 1)]
+        &mut self.pixels[x + y * self.width]
     }
 }
 
@@ -128,7 +161,12 @@ impl Debug for Image {
         writeln!(f, "({}, {})", self.width, self.height)?;
         for y in 0..self.height {
             for x in 0..self.width {
-                write!(f, "{:2}", self[(x, y)])?;
+                let v = self[(x, y)];
+                if v == 0 {
+                    write!(f, ".")?;
+                } else {
+                    write!(f, "{}", v)?;
+                }
             }
             writeln!(f)?;
         }
@@ -146,8 +184,8 @@ fn parse(input: &str) -> Result<Vec<Line>> {
 }
 
 fn draw(im: &mut Image, l: &Line) {
-    let dx = l.p0.x - l.p1.x;
-    let dy = l.p0.y - l.p1.y;
+    let dx = l.p1.x - l.p0.x;
+    let dy = l.p1.y - l.p0.y;
 
     if dx == 0 {
         let x = l.p0.x as usize;
@@ -157,8 +195,7 @@ fn draw(im: &mut Image, l: &Line) {
         for y in sy..=ey {
             im[(x, y as usize)] += 1;
         }
-    }
-    if dy == 0 {
+    } else if dy == 0 {
         let y = l.p0.y as usize;
         let sx = l.p0.x;
         let ex = l.p1.x;
@@ -166,6 +203,20 @@ fn draw(im: &mut Image, l: &Line) {
         for x in sx..=ex {
             im[(x as usize, y)] += 1;
         }
+    } else {
+        // We only support 45 degree angles.
+        assert_eq!(dx.abs(), dy.abs());
+        let dx = dx / dx.abs();
+        let dy = dy / dy.abs();
+
+        let mut x = l.p0.x;
+        let mut y = l.p0.y;
+        while x != l.p1.x && y != l.p1.y {
+            im[(x as usize, y as usize)] += 1;
+            x += dx;
+            y += dy;
+        }
+        im[(x as usize, y as usize)] += 1;
     }
 }
 
@@ -175,12 +226,14 @@ fn part1(lines: &[Line]) -> Result<u32> {
         .iter()
         .map(|l| l.p0.x.max(l.p1.x) as usize)
         .max()
-        .expect("couldn't find max width");
+        .expect("couldn't find max width")
+        + 1;
     let height = lines
         .iter()
         .map(|l| l.p0.y.max(l.p1.y) as usize)
         .max()
-        .expect("couldn't find max height");
+        .expect("couldn't find max height")
+        + 1;
     let mut im = Image::new(width, height);
     for l in lines
         .iter()
@@ -191,13 +244,26 @@ fn part1(lines: &[Line]) -> Result<u32> {
     Ok(im.answer())
 }
 
-/*
 #[aoc(day5, part2)]
-fn part2(depths: &[u32]) -> Result<u32> {
-todo!("part2")
-Ok(())
+fn part2(lines: &[Line]) -> Result<u32> {
+    let width = lines
+        .iter()
+        .map(|l| l.p0.x.max(l.p1.x) as usize)
+        .max()
+        .expect("couldn't find max width")
+        + 1;
+    let height = lines
+        .iter()
+        .map(|l| l.p0.y.max(l.p1.y) as usize)
+        .max()
+        .expect("couldn't find max height")
+        + 1;
+    let mut im = Image::new(width, height);
+    for l in lines {
+        draw(&mut im, l);
+    }
+    Ok(im.answer())
 }
-*/
 
 #[cfg(test)]
 mod tests {
@@ -222,14 +288,22 @@ mod tests {
         Ok(())
     }
 
-    /*
     #[test]
-    fn test_part2()->Result<()> {
-    let input = r#"
+    fn test_part2() -> Result<()> {
+        let input = r#"
+0,9 -> 5,9
+8,0 -> 0,8
+9,4 -> 3,4
+2,2 -> 2,1
+7,0 -> 7,4
+6,4 -> 2,0
+0,9 -> 2,9
+3,4 -> 1,4
+0,0 -> 8,8
+5,5 -> 8,2
     "#
-    .trim();
-    assert_eq!(part2(&parse(input)?)?, TODO);
-    Ok(())
+        .trim();
+        assert_eq!(part2(&parse(input)?)?, 12);
+        Ok(())
     }
-    */
 }