day10 task 2

src/tasks/day10.rs

@@ -1,3 +1,4 @@
+use itertools::Itertools;
 use std::collections::HashSet;
 
 #[allow(dead_code)]
@@ -15,13 +16,88 @@ pub fn run() {
         .flatten()
         .collect();
     task1(&asteroids);
+    task2(&asteroids);
 }
 
 fn task1(asteroids: &HashSet<(i32, i32)>) {
-    let max = asteroids
+    let (max, _) = get_best_station(&asteroids);
+    println!(
+        "Task 1: maximum visible asteroids from another asteroid are {}",
+        max
+    );
+}
+
+fn task2(asteroids: &HashSet<(i32, i32)>) {
+    let (_, station) = get_best_station(&asteroids);
+    println!("Task 2: The station is at {:?}", station);
+    let mut others = asteroids.clone();
+    others.remove(&station);
+    // map to polar coordinate
+    let mut others: Vec<((i32, i32), (f32, f32))> = others
+        .into_iter()
+        .map(|other| (other, to_polar(station, other)))
+        .collect();
+    use std::cmp::Ordering;
+    others.sort_by(|(_, a), (_, b)| {
+        // sort by angle
+        if a.0 < b.0 {
+            Ordering::Less
+        } else {
+            Ordering::Greater
+        }
+    });
+    let mut v: Vec<(f32, Vec<_>)> = others
+        .iter()
+        .group_by(|(_, (alpha, _))| alpha)
+        .into_iter()
+        .map(|(angle, group)| (*angle, group.collect()))
+        .collect();
+
+    let mut i = 0;
+    let mut shot = Vec::new();
+    while shot.len() < 200 {
+        while v[i].1.is_empty() {
+            i = (i + 1) % v.len();
+        }
+        shot.push(v[i].1.remove(0));
+        i = (i + 1) % v.len();
+    }
+
+    let ((x, y), _) = shot.last().unwrap();
+    println!("Task 2: {}", x * 100 + y);
+}
+
+fn d(a: (i32, i32), b: (i32, i32)) -> f32 {
+    ((a.0 - b.0) as f32).powi(2) + ((a.1 - b.1) as f32).powi(2).sqrt()
+}
+
+/**
+ * generate polar coordinate relative to POV.
+ * rotate angles such that "up" is 0.
+*/
+fn to_polar(pov: (i32, i32), other: (i32, i32)) -> (f32, f32) {
+    let mut alpha =
+        ((other.1 - pov.1) as f32).atan2((other.0 - pov.0) as f32) + std::f32::consts::FRAC_PI_2;
+    if alpha < 0.0 {
+        alpha += 2.0 * std::f32::consts::PI;
+    }
+    (alpha, d(pov, other))
+}
+
+fn blocks_view(pov: (i32, i32), target: (i32, i32), occluder: (i32, i32)) -> bool {
+    if occluder == target {
+        return false;
+    }
+    // using polar coordinates
+    let (ao, d_o) = to_polar(pov, occluder);
+    let (at, d_t) = to_polar(pov, target);
+    (ao - at).abs() <= std::f32::EPSILON && d_t > d_o
+}
+
+fn get_best_station(asteroids: &HashSet<(i32, i32)>) -> (usize, (i32, i32)) {
+    asteroids
         .iter()
         .map(|asteroid| {
-            println!("inspecting {:?}", asteroid);
             // map to number of other asteroids it sees
             let mut others = asteroids.clone();
             others.remove(&asteroid);
@@ -32,35 +108,13 @@ fn task1(asteroids: &HashSet<(i32, i32)>) {
                     let blocker = others
                         .iter()
                         .find(|occluder| blocks_view(*asteroid, **target, **occluder));
-                    if let Some(occluder) = blocker {
-                        println!("{:?} -> X{:?}X -> {:?}", asteroid, occluder, target);
-                    }
                     !blocker.is_some()
                 })
-                .inspect(|target| println!("sees {:?}", target))
                 .count();
-            println!("{:?}: {}", asteroid, count);
-            count
+            (count, *asteroid)
         })
-        .max()
-        .unwrap();
-    println!(
-        "Task 1: maximum visible asteroids from another asteroid are {}",
-        max
-    );
-}
-
-fn blocks_view(pov: (i32, i32), target: (i32, i32), occluder: (i32, i32)) -> bool {
-    if occluder == target {
-        return false;
-    }
-    fn d(a: (i32, i32), b: (i32, i32)) -> f32 {
-        ((a.0 - b.0) as f32).powi(2) + ((a.1 - b.1) as f32).powi(2).sqrt()
-    }
-    // using polar coordinates
-    let alpha_occluder = ((occluder.1 - pov.1) as f32).atan2((occluder.0 - pov.0) as f32);
-    let alpha_target = ((target.1 - pov.1) as f32).atan2((target.0 - pov.0) as f32);
-    (alpha_occluder - alpha_target).abs() <= std::f32::EPSILON && d(pov, target) > d(pov, occluder)
+        .max_by_key(|(count, _)| *count)
+        .unwrap()
 }
 
 mod test {