[Day 20] Optimize

Author: goggle

README.md

@@ -29,7 +29,7 @@ This Julia package contains my solutions for [Advent of Code 2024](https://adven
 | 17 | [:white_check_mark:](https://adventofcode.com/2024/day/17) | 6.921 ms | 706.30 KiB | [:white_check_mark:](https://github.com/goggle/AdventOfCode2024.jl/blob/main/src/day17.jl) |
 | 18 | [:white_check_mark:](https://adventofcode.com/2024/day/18) | 3.007 ms | 5.51 MiB | [:white_check_mark:](https://github.com/goggle/AdventOfCode2024.jl/blob/main/src/day18.jl) |
 | 19 | [:white_check_mark:](https://adventofcode.com/2024/day/19) | 57.908 ms | 3.41 MiB | [:white_check_mark:](https://github.com/goggle/AdventOfCode2024.jl/blob/main/src/day19.jl) |
-| 20 | [:white_check_mark:](https://adventofcode.com/2024/day/20) | 227.453 ms | 224.97 MiB| [:white_check_mark:](https://github.com/goggle/AdventOfCode2024.jl/blob/main/src/day20.jl) |
+| 20 | [:white_check_mark:](https://adventofcode.com/2024/day/20) | 49.638 ms | 701.11 KiB| [:white_check_mark:](https://github.com/goggle/AdventOfCode2024.jl/blob/main/src/day20.jl) |
 | 21 | [:white_check_mark:](https://adventofcode.com/2024/day/21) | 296.609 μs | 78.89 KiB | [:white_check_mark:](https://github.com/goggle/AdventOfCode2024.jl/blob/main/src/day21.jl) |
 | 22 | [:white_check_mark:](https://adventofcode.com/2024/day/22) | 25.522 ms | 1.18 MiB | [:white_check_mark:](https://github.com/goggle/AdventOfCode2024.jl/blob/main/src/day22.jl) |
 | 23 | [:white_check_mark:](https://adventofcode.com/2024/day/23) | 4.197 ms | 3.82 MiB | [:white_check_mark:](https://github.com/goggle/AdventOfCode2024.jl/blob/main/src/day23.jl) |

src/day20.jl

@@ -1,65 +1,76 @@
 module Day20
 
 using AdventOfCode2024
-
+using DataStructures
 
 function day20(input::String = readInput(joinpath(@__DIR__, "..", "data", "day20.txt")))
-    data = map(x -> x[1], reduce(vcat, permutedims.(map(x -> split(x, ""), split(input)))))
+    data = stack(split(rstrip(input), '\n'))
     tte = time_to_end(data)
     return [count_cheats(tte, 100), count_cheats(tte, 100; max_time=20)]
 end
 
 function time_to_end(data::Matrix{Char})
-    tte = -1 * ones(Int, size(data)...)
-    goal = findall(x -> x == 'E', data)[1]
-    current = [goal]
-    time = 0
-    while !isempty(current)
-        for elem ∈ current
-            tte[elem] = time
-        end
-        next = Vector{CartesianIndex{2}}()
-        while !isempty(current)
-            elem = popfirst!(current)
-            for neigh ∈ [elem + x for x ∈ CartesianIndex.((1,-1,0,0), (0,0,1,-1))]
-                if data[neigh] != '#' && tte[neigh] == -1
-                    push!(next, neigh)
+    rows, cols = size(data)
+    tte = fill(-1, rows, cols)
+    goal = findfirst(==('E'), data)
+    queue = Deque{CartesianIndex{2}}()
+    push!(queue, goal)
+    tte[goal] = 0
+    time = 1
+    
+    while !isempty(queue)
+        for _ in 1:length(queue)
+            elem = popfirst!(queue)
+            for dir in (CartesianIndex(-1, 0), CartesianIndex(1, 0), CartesianIndex(0, -1), CartesianIndex(0, 1))
+                neigh = elem + dir
+                if checkbounds(Bool, data, neigh) && data[neigh] != '#' && tte[neigh] == -1
+                    tte[neigh] = time
+                    push!(queue, neigh)
                 end
             end
         end
-        current = next
         time += 1
     end
     return tte
 end
 
 function count_cheats(tte::Matrix{Int}, threshold::Int; max_time::Int=2)
+    rows, cols = size(tte)
     count = 0
-    for c ∈ findall(x -> x >= 0, tte)
-        for e ∈ reachable_within(tte, c, max_time)
-            time_saved = tte[e] < tte[c] ? tte[c] - tte[e] - sum(abs.((c-e).I)) : -1
-            if time_saved >= threshold
-                count += 1
+    offsets = generate_offsets(max_time)
+    
+    for c in findall(>=(0), tte)
+        ci, cj = Tuple(c)
+        tte_c = tte[c]
+        
+        for (di, dj) in offsets
+            ni, nj = ci + di, cj + dj
+            if 1 <= ni <= rows && 1 <= nj <= cols
+                e = CartesianIndex(ni, nj)
+                if tte[e] >= 0 && tte[e] < tte_c
+                    time_saved = tte_c - tte[e] - (abs(di) + abs(dj))
+                    time_saved >= threshold && (count += 1)
+                end
             end
         end
     end
     return count
 end
 
-function reachable_within(tte::Matrix{Int}, c::CartesianIndex{2}, picoseconds::Int)
-    reachable = Set{CartesianIndex{2}}()
-    for p ∈ 1:picoseconds
-        for i ∈ 0:p
-            j = p - i
-            for (s1, s2) ∈ zip((1, 1, -1, -1), (1, -1, 1, -1))
-                n = CartesianIndex(c[1] + s1*i, c[2] + s2*j)
-                if checkbounds(Bool, tte, n) && tte[n] >= 0
-                    push!(reachable, n)
+@inline function generate_offsets(max_dist::Int)
+    offsets = Tuple{Int,Int}[]
+    for d in 0:max_dist
+        for i in 0:d
+            j = d - i
+            for s1 in (-1, 1)
+                for s2 in (-1, 1)
+                    push!(offsets, (s1*i, s2*j))
                 end
             end
         end
     end
-    return reachable
+    unique!(offsets)
+    return offsets
 end
 
 end # module