Advent of Code day 15 in R

solution.r

test_cases_desc = R'(
Given the starting numbers 1,3,2, the 2020th number spoken is 1.
Given the starting numbers 2,1,3, the 2020th number spoken is 10.
Given the starting numbers 1,2,3, the 2020th number spoken is 27.
Given the starting numbers 2,3,1, the 2020th number spoken is 78.
Given the starting numbers 3,2,1, the 2020th number spoken is 438.
Given the starting numbers 3,1,2, the 2020th number spoken is 1836.
)'

`%>%` = magrittr::`%>%`

test_cases = stringr::str_match_all(
    test_cases_desc,
    'Given the starting numbers ([^ ]*), the 2020th number spoken is ([^.]*)'
) %>%
    .[[1L]] %>%
    .[, -1L] %>%
    {list(
        Start = lapply(strsplit(.[, 1L], ','), as.integer),
        Result = as.integer(.[, 2L])
    )} %>%
    purrr::transpose()

###################################################################

inc = function (var) {
    var = deparse(substitute(var))
    old = get(var, envir = parent.frame())
    assign(var, old + 1L, envir = parent.frame())
    invisible(old)
}

dict = function (...) {
    structure(list2env(list(...)), class = 'dict')
}

`[[.dict` = function (dict, key) {
    key = as.character(key)
    NextMethod()
}

`[[<-.dict` = function (dict, key, value) {
    key = as.character(key)
    NextMethod()
}

play_game = function (start, until = 2020L, verbose = FALSE) {
    log = function (msg) if (verbose) message(glue::glue(msg))

    seen = dict()
    turn = 1L

    for (num in start) {
        log('{turn}: {num}')
        last_seen = seen[[num]]
        seen[[num]] = inc(turn)
    }

    while (turn <= until) {
        num = if (is.null(last_seen)) 0L else turn - last_seen - 1L
        log('{turn}: {num}')
        last_seen = seen[[num]]
        seen[[num]] = inc(turn)
    }

    num
}

play_game(c(0L, 3L, 6L), until = 10L, verbose = TRUE)
stopifnot(play_game(c(0L, 3L, 6L), until = 2020L) == 436L)

for (test in test_cases) local({
    result = play_game(test$Start)
    if (result != test$Result) {
        message(glue::glue('Failed test: play_game({test$Start}) = {result}, expected {test$Result}'))
    }
})