local fmt = require "fmt"

local anchor_day = |y| -> (2 + 5 * (y % 4) + 4 *(y % 100) + 6 * (y % 400)) % 7

local is_leap_year = |y| -> y % 4 == 0 and (y % 100 != 0 or y % 400 == 0)

local first_days_common = {3, 7, 7, 4, 2, 6, 4, 1, 5, 3, 7, 5}
local first_days_leap   = {4, 1, 7, 4, 2, 6, 4, 1, 5, 3, 7, 5}

local dates = {
    "1800-01-06",
    "1875-03-29",
    "1915-12-07",
    "1970-12-23",
    "2043-05-14",
    "2077-02-12",
    "2101-04-02"
}

local function parse(date)
    local y = tonumber(date:sub(1, 4))
    local m = tonumber(date:sub(6, 7))
    local d = tonumber(date:sub(9, 10))
    return y, m, d
end

print("Days of week given by Doomsday rule:")
for dates as date do
    local y, m, d = parse(date)
    local a = anchor_day(y)
    local w = d - (is_leap_year(y) ? first_days_leap[m] : first_days_common[m])
    if w < 0 then w += 7 end
    local dow = (a + w) % 7
    print($"{date} -> {fmt.weekday(dow)}")
end

print("\nDays of week given by standard library functions:")
for dates as date do
    local y, m, d = parse(date)
    local t = os.time({year = y, month = m, day = d})
    print($"{date} -> {os.date("%A", t)}")
end