ヤギは引きたくないんじゃ

montyhall.jl

using Random

using StaticArrays

function selectrest(i::T, j::T) where T <: Integer
    (i, j) == (T(1), T(2)) && return T(3)
    (i, j) == (T(2), T(1)) && return T(3)

    (i, j) == (T(2), T(3)) && return T(1)
    (i, j) == (T(3), T(2)) && return T(1)

    (i, j) == (T(1), T(3)) && return T(2)
    (i, j) == (T(3), T(1)) && return T(2)
    
    return zero(T)
end

function simulate(ntrial)
    N = 3
    doors = SVector{N, UInt8}(1:N)
    result = fill(false, N)
    cnt = 0

    not = Dict{UInt8, SVector{(N-1), UInt8}}()
    for n in 1:N
        not[n] = SVector{N-1, UInt8}([i for i in 1:N if i != n])
    end
    
    for _ in 1:ntrial
        # the chairperson sets prize door
        prize_door = rand(doors)
        result[prize_door] = true
        # you choose a door
        guest_door = rand(doors)

        lose_door = ifelse(guest_door == prize_door,
                        rand(not[prize_door]),
                        selectrest(guest_door, prize_door)
                    )

        new_door = selectrest(lose_door, guest_door)
        cnt += ifelse(result[new_door], 1, 0)
        # reset state
        fill!(result, false)
    end

    "$(100cnt/ntrial) %"
end

ntrial = 100000000
@time simulate(ntrial)