[Day 24] Add solution

README.md

@@ -32,8 +32,8 @@ This Julia package contains my solutions for [Advent of Code 2023](https://adven
 | 20 | [:white_check_mark:](https://adventofcode.com/2023/day/20) | 65.710 ms | 28.76 MiB | [:white_check_mark:](https://github.com/goggle/AdventOfCode2023.jl/blob/master/src/day20.jl) |
 | 21 | [:white_check_mark:](https://adventofcode.com/2023/day/21) | 73.583 ms | 187.11 MiB | [:white_check_mark:](https://github.com/goggle/AdventOfCode2023.jl/blob/master/src/day21.jl) |
 | 22 | [:white_check_mark:](https://adventofcode.com/2023/day/22) | 790.712 ms | 631.26 MiB | [:white_check_mark:](https://github.com/goggle/AdventOfCode2023.jl/blob/master/src/day22.jl) |
-| 23 | [:white_check_mark:](https://adventofcode.com/2023/day/23) | 2.136 s | 9.69 MiB | [:white_check_mark:](https://github.com/goggle/AdventOfCode2023.jl/blob/master/src/day23.jl) |
-<!-- | 24 | [:white_check_mark:](https://adventofcode.com/2023/day/24) |  |  | [:white_check_mark:](https://github.com/goggle/AdventOfCode2023.jl/blob/master/src/day24.jl) | -->
+| 23 | [:white_check_mark:](https://adventofcode.com/2023/day/23) | 2.979 s | 9.69 MiB | [:white_check_mark:](https://github.com/goggle/AdventOfCode2023.jl/blob/master/src/day23.jl) |
+| 24 | [:white_check_mark:](https://adventofcode.com/2023/day/24) | 41.181 ms | 49.71 MiB | [:white_check_mark:](https://github.com/goggle/AdventOfCode2023.jl/blob/master/src/day24.jl) |
 <!-- | 25 | [:white_check_mark:](https://adventofcode.com/2023/day/25) |  |  | [:white_check_mark:](https://github.com/goggle/AdventOfCode2023.jl/blob/master/src/day25.jl) | -->
 
 

src/day24.jl

@@ -3,35 +3,36 @@ module Day24
 using AdventOfCode2023
 using LinearAlgebra
 
-struct Hailstone
+struct Line
     p::Vector{Int}
     v::Vector{Int}
 end
 
+eval_at(h::Line, t::Number) = h.p + t * h.v
 
 function day24(input::String = readInput(joinpath(@__DIR__, "..", "data", "day24.txt")))
     hailstones = parse_input(input)
-    part1(hailstones)
+    return [part1(hailstones), part2(hailstones)]
 end
 
 function parse_input(input::AbstractString)
-    data = Hailstone[]
+    data = Line[]
     for line ∈ eachsplit(rstrip(input), "\n")
         left, right = strip.(split(line, "@"))
-        push!(data, Hailstone(parse.(Int, split(left, ",")), parse.(Int, split(right, ","))))
+        push!(data, Line(parse.(Int, split(left, ",")), parse.(Int, split(right, ","))))
     end
     return data
 end
 
-function part1(data::Vector{Hailstone}; leftbound::Int64=20_0000_000_000_000, rightbound::Int64=400_000_000_000_000)
+function part1(lines::Vector{Line}; leftbound::Int64=20_0000_000_000_000, rightbound::Int64=400_000_000_000_000)
     c = 0
-    for i ∈ firstindex(data):lastindex(data)
-        for j ∈ i + 1: lastindex(data)
-            A = hcat(data[i].v[1:2], -data[j].v[1:2])
-            rhs = data[j].p[1:2] - data[i].p[1:2]
+    for i ∈ firstindex(lines):lastindex(lines)
+        for j ∈ i + 1: lastindex(lines)
+            A = hcat(lines[i].v[1:2], -lines[j].v[1:2])
+            rhs = lines[j].p[1:2] - lines[i].p[1:2]
             A[1] * A[4] - A[2] * A[3] == 0  && continue  # parallel
             t, s = A \ rhs
-            x, y = data[i].p[1:2] + t * data[i].v[1:2]
+            x, y = lines[i].p[1:2] + t * lines[i].v[1:2]
             if leftbound <= x <= rightbound && leftbound <= y <= rightbound && t >= 0 && s >= 0
                 c += 1
             end
@@ -40,4 +41,24 @@ function part1(data::Vector{Hailstone}; leftbound::Int64=20_0000_000_000_000, ri
     return c
 end
 
+function part2(lines::Vector{Line})
+    for lind=firstindex(lines):lastindex(lines)-3
+        i, j, k = lind, lind + 1, lind + 2
+        cij, sij = _coeffs_rhsscalar(lines[i], lines[j])
+        cik, sik = _coeffs_rhsscalar(lines[i], lines[k])
+        cjk, sjk = _coeffs_rhsscalar(lines[j], lines[k])
+        A = vcat(cij', cik', cjk')
+        det(A) ≈ 0 && continue
+        rhs = [sij, sik, sjk]
+        w = round.(Int, A \ rhs)
+        A = hcat((lines[i].v - w)[1:2], (w - lines[j].v)[1:2])
+        rhs = (lines[j].p - lines[i].p)[1:2]
+        t, _ = A \ rhs
+        return round.(Int, eval_at(Line(lines[i].p, lines[i].v - w), t)) |> sum
+    end
+end
+
+_coeffs_rhsscalar(l1::Line, l2::Line) = (float(l1.p) - l2.p) × (float(l1.v) - l2.v), dot(float(l1.p) - l2.p, float(l1.v) × l2.v)
+
+
 end # module

test/runtests.jl

@@ -322,4 +322,6 @@ end
              "20, 19, 15 @  1, -5, -3\n"
     hailstones_sample = AdventOfCode2023.Day24.parse_input(sample)
     @test AdventOfCode2023.Day24.part1(hailstones_sample; leftbound=7, rightbound=27) == 2
+    @test AdventOfCode2023.Day24.part2(hailstones_sample) == 47
+    @test AdventOfCode2023.Day24.day24() == [20336, 677656046662770]
 end