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2 Commits
c3929a56d8 ... 4240f8fc8c

Author SHA1 Message Date
Bert Peters
4240f8fc8c Avoid queueing worse routes 2021-12-15 18:40:20 +01:00
Bert Peters
9a4ac427e0 Use Dijkstra instead of A* 
The available distance heuristic doesn't save enough steps to offset its
overhead.
 2021-12-15 18:29:08 +01:00
1 changed files with 8 additions and 14 deletions
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22
2021/src/day15.rs
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@@ -69,23 +69,21 @@ impl Map {
pub fn shortest_path(&self, start: Point, end: Point) -> u32 { pub fn shortest_path(&self, start: Point, end: Point) -> u32 {
let mut todo = BinaryHeap::new(); let mut todo = BinaryHeap::new();
todo.push(Reverse((Self::manhattan(start, end), 0, start))); todo.push(Reverse((0, start)));
let mut visited = vec![false; self.data.len()]; let mut best = vec![u32::MAX; self.data.len()];
let height = self.height() as i32; let height = self.height() as i32;
while let Some(Reverse((_, distance, pos))) = todo.pop() { while let Some(Reverse((distance, pos))) = todo.pop() {
if pos == end { if pos == end {
return distance; return distance;
} }
if visited[self.index(pos)] { if best[self.index(pos)] < distance {
continue; continue;
} }
visited[self.index(pos)] = true;
let (x, y) = pos; let (x, y) = pos;
for dy in -1..=1 { for dy in -1..=1 {
@@ -100,15 +98,15 @@ impl Map {
let new = (x + dx, y + dy); let new = (x + dx, y + dy);
let index = self.index(new); let index = self.index(new);
let new_distance = distance + self.data[index] as u32;
if visited[index] { if best[index] <= new_distance {
continue; continue;
} }
let new_distance = distance + self.data[index] as u32; best[index] = new_distance;
let new_guess = Self::manhattan(new, end) + new_distance;
todo.push(Reverse((new_guess, new_distance, new))); todo.push(Reverse((new_distance, new)));
} }
} }
} }
@@ -116,10 +114,6 @@ impl Map {
panic!("No route found from {:?} to {:?}", start, end); panic!("No route found from {:?} to {:?}", start, end);
} }
fn manhattan((xa, ya): Point, (xb, yb): Point) -> u32 {
(xa - xb).abs() as u32 + (ya - yb).abs() as u32
}
pub fn height(&self) -> usize { pub fn height(&self) -> usize {
self.data.len() / self.width self.data.len() / self.width
} }