local strftime = require"luatz.strftime".strftime
local strformat = string.format
local floor = math.floor
local idiv
do
    -- Try and use actual integer division when available (Lua 5.3+)
    local idiv_loader = (loadstring or load)([[return function(n,d) return n//d end]], "idiv") -- luacheck: ignore 113
    if idiv_loader then
        idiv = idiv_loader()
    else
        idiv = function(n, d)
            return floor(n / d)
        end
    end
end

local mon_lengths = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}
-- Number of days in year until start of month; not corrected for leap years
local months_to_days_cumulative = {0}
for i = 2, 12 do
    months_to_days_cumulative[i] = months_to_days_cumulative[i - 1] + mon_lengths[i - 1]
end
-- For Sakamoto's Algorithm (day of week)
local sakamoto = {0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4};

local function is_leap(y)
    if (y % 4) ~= 0 then
        return false
    elseif (y % 100) ~= 0 then
        return true
    else
        return (y % 400) == 0
    end
end

local function month_length(m, y)
    if m == 2 then
        return is_leap(y) and 29 or 28
    else
        return mon_lengths[m]
    end
end

local function leap_years_since(year)
    return idiv(year, 4) - idiv(year, 100) + idiv(year, 400)
end

local function day_of_year(day, month, year)
    local yday = months_to_days_cumulative[month]
    if month > 2 and is_leap(year) then
        yday = yday + 1
    end
    return yday + day
end

local function day_of_week(day, month, year)
    if month < 3 then
        year = year - 1
    end
    return (year + leap_years_since(year) + sakamoto[month] + day) % 7 + 1
end

local function borrow(tens, units, base)
    local frac = tens % 1
    units = units + frac * base
    tens = tens - frac
    return tens, units
end

local function carry(tens, units, base)
    if units >= base then
        tens = tens + idiv(units, base)
        units = units % base
    elseif units < 0 then
        tens = tens + idiv(units, base)
        units = (base + units) % base
    end
    return tens, units
end

-- Modify parameters so they all fit within the "normal" range
local function normalise(year, month, day, hour, min, sec)
    -- `month` and `day` start from 1, need -1 and +1 so it works modulo
    month, day = month - 1, day - 1

    -- Convert everything (except seconds) to an integer
    -- by propagating fractional components down.
    year, month = borrow(year, month, 12)
    -- Carry from month to year first, so we get month length correct in next line around leap years
    year, month = carry(year, month, 12)
    month, day = borrow(month, day, month_length(floor(month + 1), year))
    day, hour = borrow(day, hour, 24)
    hour, min = borrow(hour, min, 60)
    min, sec = borrow(min, sec, 60)

    -- Propagate out of range values up
    -- e.g. if `min` is 70, `hour` increments by 1 and `min` becomes 10
    -- This has to happen for all columns after borrowing, as lower radixes may be pushed out of range
    min, sec = carry(min, sec, 60) -- TODO: consider leap seconds?
    hour, min = carry(hour, min, 60)
    day, hour = carry(day, hour, 24)
    -- Ensure `day` is not underflowed
    -- Add a whole year of days at a time, this is later resolved by adding months
    -- TODO[OPTIMIZE]: This could be slow if `day` is far out of range
    while day < 0 do
        month = month - 1
        if month < 0 then
            year = year - 1
            month = 11
        end
        day = day + month_length(month + 1, year)
    end
    year, month = carry(year, month, 12)

    -- TODO[OPTIMIZE]: This could potentially be slow if `day` is very large
    while true do
        local i = month_length(month + 1, year)
        if day < i then
            break
        end
        day = day - i
        month = month + 1
        if month >= 12 then
            month = 0
            year = year + 1
        end
    end

    -- Now we can place `day` and `month` back in their normal ranges
    -- e.g. month as 1-12 instead of 0-11
    month, day = month + 1, day + 1

    return year, month, day, hour, min, sec
end

local leap_years_since_1970 = leap_years_since(1970)
local function timestamp(year, month, day, hour, min, sec)
    year, month, day, hour, min, sec = normalise(year, month, day, hour, min, sec)

    local days_since_epoch = day_of_year(day, month, year) + 365 * (year - 1970) -- Each leap year adds one day
    + (leap_years_since(year - 1) - leap_years_since_1970) - 1

    return days_since_epoch * (60 * 60 * 24) + hour * (60 * 60) + min * 60 + sec
end

local timetable_methods = {}

function timetable_methods:unpack()
    return assert(self.year, "year required"), assert(self.month, "month required"), assert(self.day, "day required"),
        self.hour or 12, self.min or 0, self.sec or 0, self.yday, self.wday
end

function timetable_methods:normalise()
    local year, month, day
    year, month, day, self.hour, self.min, self.sec = normalise(self:unpack())

    self.day = day
    self.month = month
    self.year = year
    self.yday = day_of_year(day, month, year)
    self.wday = day_of_week(day, month, year)

    return self
end
timetable_methods.normalize = timetable_methods.normalise -- American English

function timetable_methods:timestamp()
    return timestamp(self:unpack())
end

function timetable_methods:rfc_3339()
    local year, month, day, hour, min, fsec = self:unpack()
    local sec, msec = borrow(fsec, 0, 1000)
    msec = math.floor(msec)
    return strformat("%04u-%02u-%02uT%02u:%02u:%02d.%03d", year, month, day, hour, min, sec, msec)
end

function timetable_methods:strftime(format_string)
    return strftime(format_string, self)
end

local timetable_mt

local function coerce_arg(t)
    if getmetatable(t) == timetable_mt then
        return t:timestamp()
    end
    return t
end

timetable_mt = {
    __index = timetable_methods,
    __tostring = timetable_methods.rfc_3339,
    __eq = function(a, b)
        return a:timestamp() == b:timestamp()
    end,
    __lt = function(a, b)
        return a:timestamp() < b:timestamp()
    end,
    __sub = function(a, b)
        return coerce_arg(a) - coerce_arg(b)
    end,
}

local function cast_timetable(tm)
    return setmetatable(tm, timetable_mt)
end

local function new_timetable(year, month, day, hour, min, sec, yday, wday)
    return cast_timetable {
        year = year,
        month = month,
        day = day,
        hour = hour,
        min = min,
        sec = sec,
        yday = yday,
        wday = wday,
    }
end

function timetable_methods:clone()
    return new_timetable(self:unpack())
end

local function new_from_timestamp(ts)
    if type(ts) ~= "number" then
        error("bad argument #1 to 'new_from_timestamp' (number expected, got " .. type(ts) .. ")", 2)
    end
    return new_timetable(1970, 1, 1, 0, 0, ts):normalise()
end

return {
    is_leap = is_leap,
    day_of_year = day_of_year,
    day_of_week = day_of_week,
    normalise = normalise,
    timestamp = timestamp,

    new = new_timetable,
    new_from_timestamp = new_from_timestamp,
    cast = cast_timetable,
    timetable_mt = timetable_mt,
}
