local config = require('pending.config')

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

---@param s string
---@return boolean
local function is_valid_date(s)
  local y, m, d = s:match('^(%d%d%d%d)-(%d%d)-(%d%d)$')
  if not y then
    return false
  end
  local yn = tonumber(y) --[[@as integer]]
  local mn = tonumber(m) --[[@as integer]]
  local dn = tonumber(d) --[[@as integer]]
  if mn < 1 or mn > 12 then
    return false
  end
  if dn < 1 or dn > 31 then
    return false
  end
  local t = os.time({ year = yn, month = mn, day = dn })
  local check = os.date('*t', t) --[[@as osdate]]
  return check.year == yn and check.month == mn and check.day == dn
end

---@param s string
---@return boolean
local function is_valid_time(s)
  local h, m = s:match('^(%d%d):(%d%d)$')
  if not h then
    return false
  end
  local hn = tonumber(h) --[[@as integer]]
  local mn = tonumber(m) --[[@as integer]]
  return hn >= 0 and hn <= 23 and mn >= 0 and mn <= 59
end

---@param s string
---@return string|nil
local function normalize_time(s)
  local h, m, period

  h, m, period = s:match('^(%d+):(%d%d)([ap]m)$')
  if not h then
    h, period = s:match('^(%d+)([ap]m)$')
    if h then
      m = '00'
    end
  end
  if not h then
    h, m = s:match('^(%d%d):(%d%d)$')
  end
  if not h then
    h, m = s:match('^(%d):(%d%d)$')
  end
  if not h then
    h = s:match('^(%d+)$')
    if h then
      m = '00'
    end
  end

  if not h then
    return nil
  end

  local hn = tonumber(h) --[[@as integer]]
  local mn = tonumber(m) --[[@as integer]]

  if period then
    if hn < 1 or hn > 12 then
      return nil
    end
    if period == 'am' then
      hn = hn == 12 and 0 or hn
    else
      hn = hn == 12 and 12 or hn + 12
    end
  else
    if hn < 0 or hn > 23 then
      return nil
    end
  end

  if mn < 0 or mn > 59 then
    return nil
  end

  return string.format('%02d:%02d', hn, mn)
end

---@param s string
---@return boolean
local function is_valid_datetime(s)
  local date_part, time_part = s:match('^(.+)T(.+)$')
  if not date_part then
    return is_valid_date(s)
  end
  return is_valid_date(date_part) and is_valid_time(time_part)
end

---@return string
local function date_key()
  return config.get().date_syntax or 'due'
end

---@return string
local function recur_key()
  return config.get().recur_syntax or 'rec'
end

local weekday_map = {
  sun = 1,
  mon = 2,
  tue = 3,
  wed = 4,
  thu = 5,
  fri = 6,
  sat = 7,
}

local month_map = {
  jan = 1,
  feb = 2,
  mar = 3,
  apr = 4,
  may = 5,
  jun = 6,
  jul = 7,
  aug = 8,
  sep = 9,
  oct = 10,
  nov = 11,
  dec = 12,
}

---@param today osdate
---@return string
local function today_str(today)
  return os.date('%Y-%m-%d', os.time({ year = today.year, month = today.month, day = today.day })) --[[@as string]]
end

---@param date_part string
---@param time_suffix? string
---@return string
local function append_time(date_part, time_suffix)
  if time_suffix then
    return date_part .. 'T' .. time_suffix
  end
  return date_part
end

---@param text string
---@return string|nil
function M.resolve_date(text)
  local date_input, time_suffix = text:match('^(.+)@(.+)$')
  if time_suffix then
    time_suffix = normalize_time(time_suffix)
    if not time_suffix then
      return nil
    end
  else
    date_input = text
  end

  local dt = date_input:match('^(%d%d%d%d%-%d%d%-%d%dT%d%d:%d%d)$')
  if dt then
    local dp, tp = dt:match('^(.+)T(.+)$')
    if is_valid_date(dp) and is_valid_time(tp) then
      return dt
    end
    return nil
  end

  if is_valid_date(date_input) then
    return append_time(date_input, time_suffix)
  end

  local lower = date_input:lower()
  local today = os.date('*t') --[[@as osdate]]

  if lower == 'today' or lower == 'eod' then
    return append_time(today_str(today), time_suffix)
  end

  if lower == 'yesterday' then
    return append_time(
      os.date('%Y-%m-%d', os.time({ year = today.year, month = today.month, day = today.day - 1 })) --[[@as string]],
      time_suffix
    )
  end

  if lower == 'tomorrow' then
    return append_time(
      os.date('%Y-%m-%d', os.time({ year = today.year, month = today.month, day = today.day + 1 })) --[[@as string]],
      time_suffix
    )
  end

  if lower == 'sow' then
    local delta = -((today.wday - 2) % 7)
    return append_time(
      os.date(
        '%Y-%m-%d',
        os.time({ year = today.year, month = today.month, day = today.day + delta })
      ) --[[@as string]],
      time_suffix
    )
  end

  if lower == 'eow' then
    local delta = (1 - today.wday) % 7
    return append_time(
      os.date(
        '%Y-%m-%d',
        os.time({ year = today.year, month = today.month, day = today.day + delta })
      ) --[[@as string]],
      time_suffix
    )
  end

  if lower == 'som' then
    return append_time(
      os.date('%Y-%m-%d', os.time({ year = today.year, month = today.month, day = 1 })) --[[@as string]],
      time_suffix
    )
  end

  if lower == 'eom' then
    return append_time(
      os.date('%Y-%m-%d', os.time({ year = today.year, month = today.month + 1, day = 0 })) --[[@as string]],
      time_suffix
    )
  end

  if lower == 'soq' then
    local q = math.ceil(today.month / 3)
    local first_month = (q - 1) * 3 + 1
    return append_time(
      os.date('%Y-%m-%d', os.time({ year = today.year, month = first_month, day = 1 })) --[[@as string]],
      time_suffix
    )
  end

  if lower == 'eoq' then
    local q = math.ceil(today.month / 3)
    local last_month = q * 3
    return append_time(
      os.date('%Y-%m-%d', os.time({ year = today.year, month = last_month + 1, day = 0 })) --[[@as string]],
      time_suffix
    )
  end

  if lower == 'soy' then
    return append_time(
      os.date('%Y-%m-%d', os.time({ year = today.year, month = 1, day = 1 })) --[[@as string]],
      time_suffix
    )
  end

  if lower == 'eoy' then
    return append_time(
      os.date('%Y-%m-%d', os.time({ year = today.year, month = 12, day = 31 })) --[[@as string]],
      time_suffix
    )
  end

  if lower == 'later' or lower == 'someday' then
    return append_time(config.get().someday_date, time_suffix)
  end

  local n = lower:match('^%+(%d+)d$')
  if n then
    return append_time(
      os.date(
        '%Y-%m-%d',
        os.time({
          year = today.year,
          month = today.month,
          day = today.day + (
              tonumber(n) --[[@as integer]]
            ),
        })
      ) --[[@as string]],
      time_suffix
    )
  end

  n = lower:match('^%+(%d+)w$')
  if n then
    return append_time(
      os.date(
        '%Y-%m-%d',
        os.time({
          year = today.year,
          month = today.month,
          day = today.day + (
              tonumber(n) --[[@as integer]]
            ) * 7,
        })
      ) --[[@as string]],
      time_suffix
    )
  end

  n = lower:match('^%+(%d+)m$')
  if n then
    return append_time(
      os.date(
        '%Y-%m-%d',
        os.time({
          year = today.year,
          month = today.month + (
              tonumber(n) --[[@as integer]]
            ),
          day = today.day,
        })
      ) --[[@as string]],
      time_suffix
    )
  end

  n = lower:match('^%-(%d+)d$')
  if n then
    return append_time(
      os.date(
        '%Y-%m-%d',
        os.time({
          year = today.year,
          month = today.month,
          day = today.day - (
              tonumber(n) --[[@as integer]]
            ),
        })
      ) --[[@as string]],
      time_suffix
    )
  end

  n = lower:match('^%-(%d+)w$')
  if n then
    return append_time(
      os.date(
        '%Y-%m-%d',
        os.time({
          year = today.year,
          month = today.month,
          day = today.day - (
              tonumber(n) --[[@as integer]]
            ) * 7,
        })
      ) --[[@as string]],
      time_suffix
    )
  end

  local ord = lower:match('^(%d+)[snrt][tdh]$')
  if ord then
    local day_num = tonumber(ord) --[[@as integer]]
    if day_num >= 1 and day_num <= 31 then
      local m, y = today.month, today.year
      if today.day >= day_num then
        m = m + 1
        if m > 12 then
          m = 1
          y = y + 1
        end
      end
      local t = os.time({ year = y, month = m, day = day_num })
      local check = os.date('*t', t) --[[@as osdate]]
      if check.day == day_num then
        return append_time(os.date('%Y-%m-%d', t) --[[@as string]], time_suffix)
      end
      m = m + 1
      if m > 12 then
        m = 1
        y = y + 1
      end
      t = os.time({ year = y, month = m, day = day_num })
      check = os.date('*t', t) --[[@as osdate]]
      if check.day == day_num then
        return append_time(os.date('%Y-%m-%d', t) --[[@as string]], time_suffix)
      end
      return nil
    end
  end

  local target_month = month_map[lower]
  if target_month then
    local y = today.year
    if today.month >= target_month then
      y = y + 1
    end
    return append_time(
      os.date('%Y-%m-%d', os.time({ year = y, month = target_month, day = 1 })) --[[@as string]],
      time_suffix
    )
  end

  local target_wday = weekday_map[lower]
  if target_wday then
    local current_wday = today.wday
    local delta = (target_wday - current_wday) % 7
    return append_time(
      os.date(
        '%Y-%m-%d',
        os.time({ year = today.year, month = today.month, day = today.day + delta })
      ) --[[@as string]],
      time_suffix
    )
  end

  return nil
end

---@param text string
---@return string description
---@return { due?: string, cat?: string, rec?: string, rec_mode?: 'scheduled'|'completion' } metadata
function M.body(text)
  local tokens = {}
  for token in text:gmatch('%S+') do
    table.insert(tokens, token)
  end

  local metadata = {}
  local i = #tokens
  local dk = date_key()
  local rk = recur_key()
  local date_pattern_strict = '^' .. vim.pesc(dk) .. ':(%d%d%d%d%-%d%d%-%d%d[T%d:]*)$'
  local date_pattern_any = '^' .. vim.pesc(dk) .. ':(.+)$'
  local rec_pattern = '^' .. vim.pesc(rk) .. ':(%S+)$'

  while i >= 1 do
    local token = tokens[i]
    local due_val = token:match(date_pattern_strict)
    if due_val then
      if metadata.due then
        break
      end
      if not is_valid_datetime(due_val) then
        break
      end
      metadata.due = due_val
      i = i - 1
    else
      local raw_val = token:match(date_pattern_any)
      if raw_val then
        if metadata.due then
          break
        end
        local resolved = M.resolve_date(raw_val)
        if not resolved then
          break
        end
        metadata.due = resolved
        i = i - 1
      else
        local cat_val = token:match('^cat:(%S+)$')
        if cat_val then
          if metadata.cat then
            break
          end
          metadata.cat = cat_val
          i = i - 1
        else
          local rec_val = token:match(rec_pattern)
          if rec_val then
            if metadata.rec then
              break
            end
            local recur = require('pending.recur')
            local raw_spec = rec_val
            if raw_spec:sub(1, 1) == '!' then
              metadata.rec_mode = 'completion'
              raw_spec = raw_spec:sub(2)
            end
            if not recur.validate(raw_spec) then
              break
            end
            metadata.rec = raw_spec
            i = i - 1
          else
            break
          end
        end
      end
    end
  end

  local desc_tokens = {}
  for j = 1, i do
    table.insert(desc_tokens, tokens[j])
  end
  local description = table.concat(desc_tokens, ' ')

  return description, metadata
end

---@param text string
---@return string description
---@return { due?: string, cat?: string, rec?: string, rec_mode?: 'scheduled'|'completion' } metadata
function M.command_add(text)
  local cat_prefix = text:match('^(%S.-):%s')
  if cat_prefix then
    local first_char = cat_prefix:sub(1, 1)
    if first_char == first_char:upper() and first_char ~= first_char:lower() then
      local rest = text:sub(#cat_prefix + 2):match('^%s*(.+)$')
      if rest then
        local desc, meta = M.body(rest)
        meta.cat = meta.cat or cat_prefix
        return desc, meta
      end
    end
  end
  return M.body(text)
end

---@param due string
---@return boolean
function M.is_overdue(due)
  local now = os.date('*t') --[[@as osdate]]
  local today = os.date('%Y-%m-%d') --[[@as string]]
  local date_part, time_part = due:match('^(.+)T(.+)$')
  if not date_part then
    return due < today
  end
  if date_part < today then
    return true
  end
  if date_part > today then
    return false
  end
  local current_time = string.format('%02d:%02d', now.hour, now.min)
  return time_part < current_time
end

---@param due string
---@return boolean
function M.is_today(due)
  local now = os.date('*t') --[[@as osdate]]
  local today = os.date('%Y-%m-%d') --[[@as string]]
  local date_part, time_part = due:match('^(.+)T(.+)$')
  if not date_part then
    return due == today
  end
  if date_part ~= today then
    return false
  end
  local current_time = string.format('%02d:%02d', now.hour, now.min)
  return time_part >= current_time
end

return M