[Day 24] Add solution for part 1

Project.toml

@@ -6,6 +6,7 @@ version = "0.1.0"
 [deps]
 BenchmarkTools = "6e4b80f9-dd63-53aa-95a3-0cdb28fa8baf"
 DataStructures = "864edb3b-99cc-5e75-8d2d-829cb0a9cfe8"
+LinearAlgebra = "37e2e46d-f89d-539d-b4ee-838fcccc9c8e"
 Memoize = "c03570c3-d221-55d1-a50c-7939bbd78826"
 Printf = "de0858da-6303-5e67-8744-51eddeeeb8d7"
 

src/AdventOfCode2023.jl

@@ -3,7 +3,7 @@ module AdventOfCode2023
 using BenchmarkTools
 using Printf
 
-solvedDays = 1:23
+solvedDays = 1:24
 # Include the source files:
 for day in solvedDays
     ds = @sprintf("%02d", day)

src/day24.jl

@@ -0,0 +1,43 @@
+module Day24
+
+using AdventOfCode2023
+using LinearAlgebra
+
+struct Hailstone
+    p::Vector{Int}
+    v::Vector{Int}
+end
+
+
+function day24(input::String = readInput(joinpath(@__DIR__, "..", "data", "day24.txt")))
+    hailstones = parse_input(input)
+    part1(hailstones)
+end
+
+function parse_input(input::AbstractString)
+    data = Hailstone[]
+    for line ∈ eachsplit(rstrip(input), "\n")
+        left, right = strip.(split(line, "@"))
+        push!(data, Hailstone(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)
+    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]
+            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]
+            if leftbound <= x <= rightbound && leftbound <= y <= rightbound && t >= 0 && s >= 0
+                c += 1
+            end
+        end
+    end
+    return c
+end
+
+end # module