anotherthingbytesthedust.lua

-- very helpful library
local _ = require("luadash")

-- helper split function.
-- returns an iterator, like pairs(...) / ipairs(...)
function string:split(pat)
    pat = pat or '%s+'
    local st, g = 1, self:gmatch("()("..pat..")")
    local function getter(segs, seps, sep, cap1, ...)
        st = sep and seps + #sep
        return self:sub(segs, (seps or 0) - 1), cap1 or sep, ...
    end
    return function() if st then return getter(st, g()) end end
end

local input = {}

-- read and parse each line of the input
for line in io.lines("input.txt") do
    -- rip leading "[", and split by "] " to remove brackets
    local m = line:sub(2):split("] ")
    local timestamp, command = m(), m()

    -- timestamp ==> relative time
    m = timestamp:split(" ")
    local date, clock = m(), m()

    m = clock:split(":")
    local hour, minute = tonumber((m())), tonumber((m()))

    -- add command to input list
    input[#input + 1] = {
        ["date"] = date .. " " .. clock,
        ["minute"] = minute,
        ["command"] = command
    }
end

-- sort input by date (string comparison ftw, "1518-11-23 00:20" < "1518-11-23 00:27")
input =_.sort_by(input, function(log)
    return log.date
end)

local guard = -1
local sleep = {}

for k, log in pairs(input) do
    local m = log.command:split(" ")
    local sw = m()

    if sw == "Guard" then
        guard = tonumber(m():sub(2))
        if not sleep[guard] then
            sleep[guard] = {
                ["guard"] = guard,
                ["sleep"] = {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},
                ["asleep"] = false
            }
        end
    elseif sw == "falls" then
        if not sleep[guard].asleep then
            for i = log.minute+1, 60 do
                sleep[guard].sleep[i] = sleep[guard].sleep[i] + 1
            end
            sleep[guard].asleep = true
        end
    elseif sw == "wakes" then
        if sleep[guard].asleep then
            for i = log.minute+1, 60 do
                sleep[guard].sleep[i] = math.max(sleep[guard].sleep[i] - 1, 0)
            end
        end
        sleep[guard].asleep = false
    end
end

-- part 1

local flat = {}

for k,v in pairs(sleep) do
    flat[#flat+1] = v
end

local sorted = _.sort_by(flat, function(guard)
    local sum = 0
    for k, sleep in pairs(guard.sleep) do
        sum = sum + sleep
    end

    return sum
end)

local target = sorted[#sorted]

local best_key = 0
local best_val = -1

for k, v in pairs(target.sleep) do
    if v > best_val then
        best_key = k
        best_val = v
    end
end

print("Solution #1:", target.guard * (best_key - 1))

-- part 2

local best_guard = -1
local best_key = -1
local best_val = -1

for k, guard in pairs(sleep) do
    for minute = 1,60 do
        if guard.sleep[minute] > best_val then
            best_guard = guard.guard
            best_key = minute
            best_val = guard.sleep[minute]
        end
    end
end

print("Solution #2:", sleep[best_guard].guard * (best_key - 1))