solve day 21

data/day_21.txt

@@ -0,0 +1,2 @@
+Player 1 starting position: 7
+Player 2 starting position: 8

src/day_21.jl

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+include(joinpath(readchomp(`git rev-parse --show-toplevel`), "aoc.jl"))
+import .Aoc: @aoc
+using Pipe: @pipe
+
+function prepare_input(input)
+	p1 = parse(Int, match(r"position: (\d)", input[1]).captures[1])
+	p2 = parse(Int, match(r"position: (\d)", input[2]).captures[1])
+	[p1, p2]
+end
+
+function p1(input::Vector{String})
+	p = prepare_input(input)
+	score = [0,0]
+	d = 1
+	while true
+		for i in 1:2
+			np = (p[i] + sum(d:d+2)) % 10 
+			d += 3
+			score[i] += p[i] = np
+			score[i] >= 1000 && return (score[i==1 ? 2 : 1]*(d-1))
+		end
+	end
+end
+
+function simulate(position, score, mul, scores)
+	length(scores) == 0 && return
+	s = popfirst!(scores)
+	for (throw, chances) in [(3, 1),(4, 3),(5, 6),(6, 7),(7, 6),(8, 3),(9, 1)]
+		position2 = (position + throw) % 10
+		score2 = score + (position2 == 0 ? 10 : position2)
+		paths = mul * chances
+		if score2 >= 21
+			s[1] += paths
+		else
+			s[2] += paths
+			simulate(position2, score2, paths, scores)
+		end
+	end
+	pushfirst!(scores, s)
+end
+
+function p2(input::Vector{String})
+	p = prepare_input(input)
+	
+	s1 = [ [0,0] for _ in 1:21 ]
+	simulate(p[1], 0, 1, s1)
+	s2 = [ [0,0] for _ in 1:21 ]
+	simulate(p[2], 0, 1, s2)
+
+	w1 = sum(map(x -> x[1][1] * x[2][2], zip(s1[2:end], s2)))
+	w2 = sum(map(x -> x[1][1] * x[2][2], zip(s2, s1)))
+	return w1 > w2 ? w1 : w2
+end
+
+@aoc(2021, 21)