---@class pending.RecurSpec
---@field freq 'daily'|'weekly'|'monthly'|'yearly'
---@field interval integer
---@field byday? string[]
---@field from_completion boolean
---@field _raw? string

---@class pending.recur
local M = {}

---@type table<string, pending.RecurSpec>
local named = {
  daily = { freq = 'daily', interval = 1, from_completion = false },
  weekdays = {
    freq = 'weekly',
    interval = 1,
    byday = { 'MO', 'TU', 'WE', 'TH', 'FR' },
    from_completion = false,
  },
  weekly = { freq = 'weekly', interval = 1, from_completion = false },
  biweekly = { freq = 'weekly', interval = 2, from_completion = false },
  monthly = { freq = 'monthly', interval = 1, from_completion = false },
  quarterly = { freq = 'monthly', interval = 3, from_completion = false },
  yearly = { freq = 'yearly', interval = 1, from_completion = false },
  annual = { freq = 'yearly', interval = 1, from_completion = false },
}

---@param spec string
---@return pending.RecurSpec?
function M.parse(spec)
  local from_completion = false
  local s = spec

  if s:sub(1, 1) == '!' then
    from_completion = true
    s = s:sub(2)
  end

  local lower = s:lower()

  local base = named[lower]
  if base then
    return {
      freq = base.freq,
      interval = base.interval,
      byday = base.byday,
      from_completion = from_completion,
    }
  end

  local n, unit = lower:match('^(%d+)([dwmy])$')
  if n then
    local num = tonumber(n) --[[@as integer]]
    if num < 1 then
      return nil
    end
    local freq_map = { d = 'daily', w = 'weekly', m = 'monthly', y = 'yearly' }
    return {
      freq = freq_map[unit],
      interval = num,
      from_completion = from_completion,
    }
  end

  if s:match('^FREQ=') then
    return {
      freq = 'daily',
      interval = 1,
      from_completion = from_completion,
      _raw = s,
    }
  end

  return nil
end

---@param spec string
---@return boolean
function M.validate(spec)
  return M.parse(spec) ~= nil
end

---@param base_date string
---@param freq string
---@param interval integer
---@return string
local function advance_date(base_date, freq, interval)
  local y, m, d = base_date:match('^(%d+)-(%d+)-(%d+)$')
  local yn = tonumber(y) --[[@as integer]]
  local mn = tonumber(m) --[[@as integer]]
  local dn = tonumber(d) --[[@as integer]]

  if freq == 'daily' then
    return os.date('%Y-%m-%d', os.time({ year = yn, month = mn, day = dn + interval })) --[[@as string]]
  elseif freq == 'weekly' then
    return os.date('%Y-%m-%d', os.time({ year = yn, month = mn, day = dn + interval * 7 })) --[[@as string]]
  elseif freq == 'monthly' then
    local new_m = mn + interval
    local new_y = yn
    while new_m > 12 do
      new_m = new_m - 12
      new_y = new_y + 1
    end
    local last_day = os.date('*t', os.time({ year = new_y, month = new_m + 1, day = 0 })) --[[@as osdate]]
    local clamped_d = math.min(dn, last_day.day --[[@as integer]])
    return os.date('%Y-%m-%d', os.time({ year = new_y, month = new_m, day = clamped_d })) --[[@as string]]
  elseif freq == 'yearly' then
    local new_y = yn + interval
    local last_day = os.date('*t', os.time({ year = new_y, month = mn + 1, day = 0 })) --[[@as osdate]]
    local clamped_d = math.min(dn, last_day.day --[[@as integer]])
    return os.date('%Y-%m-%d', os.time({ year = new_y, month = mn, day = clamped_d })) --[[@as string]]
  end
  return base_date
end

---@param base_date string
---@param spec string
---@param mode 'scheduled'|'completion'
---@return string
function M.next_due(base_date, spec, mode)
  local parsed = M.parse(spec)
  if not parsed then
    return base_date
  end

  local today = os.date('%Y-%m-%d') --[[@as string]]

  if mode == 'completion' then
    return advance_date(today, parsed.freq, parsed.interval)
  end

  local next_date = advance_date(base_date, parsed.freq, parsed.interval)
  while next_date <= today do
    next_date = advance_date(next_date, parsed.freq, parsed.interval)
  end
  return next_date
end

---@param spec string
---@return string
function M.to_rrule(spec)
  local parsed = M.parse(spec)
  if not parsed then
    return ''
  end

  if parsed._raw then
    return 'RRULE:' .. parsed._raw
  end

  local parts = { 'FREQ=' .. parsed.freq:upper() }
  if parsed.interval > 1 then
    table.insert(parts, 'INTERVAL=' .. parsed.interval)
  end
  if parsed.byday then
    table.insert(parts, 'BYDAY=' .. table.concat(parsed.byday, ','))
  end
  return 'RRULE:' .. table.concat(parts, ';')
end

---@return string[]
function M.shorthand_list()
  return { 'daily', 'weekdays', 'weekly', 'biweekly', 'monthly', 'quarterly', 'yearly', 'annual' }
end

return M