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

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

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

---@param text string
---@return string|nil
function M.resolve_date(text)
  local lower = text:lower()
  local today = os.date('*t') --[[@as osdate]]

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

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

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

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

  return nil
end

---@param text string
---@return string description
---@return { due?: string, cat?: string } 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 date_pattern_strict = '^' .. vim.pesc(dk) .. ':(%d%d%d%d%-%d%d%-%d%d)$'
  local date_pattern_any = '^' .. vim.pesc(dk) .. ':(.+)$'

  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_date(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
          break
        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 } 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

return M