Advent of Code 2025 - Solutions

This year we’re doing it in R. Libraries I used can be found below.

library(stringr)
library(magrittr)
library(roperators)

Day 1

Part 1

day1_input = readLines('inputs/day1.txt')

turn = function(init, direction, steps) {
    if (direction  == 'R') {
        return((init + steps) %% 100)
    } else {
        tempstep = init - steps
        if (tempstep >= 0) {
            return(tempstep)
        } else {
            nextstep = tempstep
            while (nextstep < 0) {
                nextstep = nextstep + 100
            }
            return(nextstep)
        }
    }
}

parse_combination = function(com) {
    direction = substr(com, 1, 1)
    steps = substr(com, 2, nchar(com)) %>%
        as.integer()

    return(
        list(dir = direction, step = steps)
    )
}

decode = function(init, combinations) {
    final_password = 0
    for (com in combinations) {
        next_step = parse_combination(com)
        next_step = turn(init, next_step$dir, next_step$step)
        if (next_step == 0) {
            final_password %+=% 1
        }
        init = next_step
    }
    return(final_password)
}

decode(50, day1_input)

[1] 982

Part 2

turn2 = function(init, direction, steps) {
    if (direction  == 'R') {
        if (init + steps > 99) {
            newstep = init + steps
            crossed = 0
            if (newstep %% 100 == 0) {
                crossed = -1
            }
            while (newstep > 99) {
                newstep %-=% 100
                crossed %+=% 1
            }
            return(
                list(newstep = newstep, crossed = crossed)
            )
        } else {
            return(
                list(newstep = init + steps, crossed = 0)
            )
        }
    } else {
        tempstep = init - steps
        if (tempstep >= 0) {
            return(
                list(newstep = tempstep, crossed = 0)
            )
        } else {
            nextstep = tempstep
            crossed = 0
            if (init == 0) {
                crossed = -1
            }
            while (nextstep < 0) {
                nextstep %+=% 100
                crossed %+=% 1
            }
            return(
                list(newstep = nextstep, crossed = crossed)
            )
        }
    }
}

decode2 = function(init, combinations) {
    final_password = 0
    for (com in combinations) {
        next_step = parse_combination(com)
        next_step = turn2(init, next_step$dir, next_step$step)
        if (next_step$newstep == 0) {
            final_password %+=% 1
        }
        final_password %+=% next_step$crossed
        init = next_step$newstep
    }
    return(final_password)
}

decode2(50, day1_input)

[1] 6106