days 5 and 6

day05/src/main.rs

@@ -48,7 +48,8 @@ fn main() {
             .filter(|p| pages.contains(p))
             .collect();
 
-        let middle: i32 = *halfway(&true_pages).expect("error finding the halfway point");
+        let middle: i32 = true_pages.get((true_pages.len() - 1) / 2)
+            .expect("error finding the halfway point");
 
         if iter::zip(pages.iter(), true_pages.iter()).all(|(a, b)| a==b) {
             // it's good!
@@ -63,8 +64,3 @@ fn main() {
     println!("the sum of middle pages of good updates is {good_sum}");
     println!("and the bad updates is {bad_sum}");
 }
-
-fn halfway<T>(v: &[T]) -> Option<&T> {
-    if v.len() == 0 { return None; }
-    return v.get((v.len() - 1) / 2)
-}

day06/src/main.rs

@@ -1,3 +1,161 @@
-fn main() {
+use std::collections::HashSet;
-    println!("Hello, world!");
+use std::collections::HashMap;
+use std::fs;
+use std::env;
+use crate::Dir::{North, South, East, West};
+
+// nodes are (coord, direction) pairs.
+// neighbors are step1+turnleft, step2+turnleft, step3+turnleft, step1+turnright, etc
+#[derive(Clone, Copy, PartialEq, Eq, Hash, Debug)]
+enum Dir { North, South, East, West }
+
+fn step(start: &Coord, d: Dir, steps: i32) -> Coord {
+    match d {
+        North => (start.0, start.1 - steps),
+        South => (start.0, start.1 + steps),
+        East => (start.0 + steps, start.1),
+        West => (start.0 - steps, start.1)
+    }
+}
+
+fn rotate_right(direction: Dir) -> Dir {
+    match direction {
+        North => East,
+        East => South,
+        South => West,
+        West => North
+    }
+}
+
+type Coord = (i32, i32);
+
+fn main() {
+    println!("Hello, AoC day 06!");
+
+    let args: Vec<String> = env::args().collect();
+    if args.len() != 2 {
+        println!("wrong number of arguments!");
+        std::process::exit(1);
+    }
+
+    let file_path = &args[1];
+
+    let contents = fs::read_to_string(file_path).expect("Should have been able to read the file");
+    let mut grid: HashMap<Coord, char> = HashMap::new();
+
+    let mut guard_position = (0,0);
+    let mut x;
+    let mut y = 0;
+    // build our grid!
+    for line in contents.lines() {
+        x = 0;
+        for c in line.chars() {
+            if c == '^' {
+                guard_position = (x,y);
+                grid.insert((x,y), '.');
+                x += 1;
+            } else {
+                grid.insert((x,y), c);
+                x += 1;
+            }  
+        }
+        y += 1;
+    }
+
+    if let Ok(count) = patrol(guard_position, &grid) {
+        println!("visited {count} locations!");
+    } else {
+        println!("found a loop!");
+    }
+
+    if let Ok(potential_loops) = patrol_and_count_potential_loops(guard_position, &grid) {
+        println!("there were {potential_loops} locations where we could make a loop.")
+    }
+}
+
+struct Looped;
+fn patrol(start: Coord, lab: &HashMap<Coord, char>) -> Result<usize, Looped> {
+    let mut guard_position = start;
+    let mut guard_facing = North;
+
+    let mut visited: HashSet<Coord> = HashSet::new();
+    let mut conditions: HashSet<(Coord, Dir)> = HashSet::new();
+    loop {
+        visited.insert(guard_position);
+        if conditions.contains(&(guard_position, guard_facing)) {
+            return Err(Looped);
+        }
+        conditions.insert((guard_position, guard_facing));
+        let next = step(&guard_position, guard_facing, 1);
+        match lab.get(&next) {
+            Some('.') => guard_position = next,
+            Some('#') => guard_facing = rotate_right(guard_facing),
+            Some(_) => panic!("encountered an unexpected character!"),
+            None => break,
+        }
+    }
+
+    return Ok(visited.len());
+}
+
+fn patrol_and_count_potential_loops(start: Coord, lab: &HashMap<Coord, char>) -> Result<usize, Looped> {
+    let mut guard_position = start;
+    let mut guard_facing = North;
+
+    let mut candidates: HashSet<Coord> = HashSet::new();
+    let mut conditions: HashSet<(Coord, Dir)> = HashSet::new();
+    loop {
+        if conditions.contains(&(guard_position, guard_facing)) {
+            return Err(Looped);
+        }
+
+        conditions.insert((guard_position, guard_facing));
+
+        let next = step(&guard_position, guard_facing, 1);
+        
+        if obstacle_to_the_right(guard_position, guard_facing, lab) {
+            candidates.insert(next);
+        }
+
+        match lab.get(&next) {
+            Some('.') => guard_position = next,
+            Some('#') => guard_facing = rotate_right(guard_facing),
+            Some(_) => panic!("encountered an unexpected character!"),
+            None => break,
+        }
+    }
+
+    let mut loop_spots: HashSet<Coord> = HashSet::new();
+    for candidate_location in candidates {
+        let mut dream_lab = lab.clone();
+        
+        if dream_lab.get(&candidate_location) != Some(&'.') {
+            continue;
+        }
+
+        dream_lab.insert(candidate_location, '#');
+
+        if let Ok(_) = patrol(start, &dream_lab){
+            continue;
+        }
+
+        loop_spots.insert(candidate_location);
+    }
+
+    return Ok(loop_spots.len());
+}
+
+fn obstacle_to_the_right(start: Coord, facing: Dir, lab: &HashMap<Coord, char>) -> bool {
+    let direction = rotate_right(facing);
+
+    let mut position = start;
+    while let Some(next) = lab.get(&position) {
+        if *next == '.' {
+            position = step(&position, direction, 1);
+        } else if *next == '#' {
+            return true;
+        }
+    }
+
+    return false;
 }