local config = require('pending.config')

---@class pending.Task
---@field id integer
---@field description string
---@field status 'pending'|'done'|'deleted'
---@field category? string
---@field priority integer
---@field due? string
---@field recur? string
---@field recur_mode? 'scheduled'|'completion'
---@field entry string
---@field modified string
---@field end? string
---@field order integer
---@field _extra? table<string, any>

---@class pending.Data
---@field version integer
---@field next_id integer
---@field tasks pending.Task[]
---@field undo pending.Task[][]

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

local SUPPORTED_VERSION = 1

---@type pending.Data?
local _data = nil

---@return pending.Data
local function empty_data()
  return {
    version = SUPPORTED_VERSION,
    next_id = 1,
    tasks = {},
    undo = {},
  }
end

---@param path string
local function ensure_dir(path)
  local dir = vim.fn.fnamemodify(path, ':h')
  if vim.fn.isdirectory(dir) == 0 then
    vim.fn.mkdir(dir, 'p')
  end
end

---@return string
local function timestamp()
  return os.date('!%Y-%m-%dT%H:%M:%SZ') --[[@as string]]
end

---@type table<string, true>
local known_fields = {
  id = true,
  description = true,
  status = true,
  category = true,
  priority = true,
  due = true,
  recur = true,
  recur_mode = true,
  entry = true,
  modified = true,
  ['end'] = true,
  order = true,
}

---@param task pending.Task
---@return table
local function task_to_table(task)
  local t = {
    id = task.id,
    description = task.description,
    status = task.status,
    entry = task.entry,
    modified = task.modified,
  }
  if task.category then
    t.category = task.category
  end
  if task.priority and task.priority ~= 0 then
    t.priority = task.priority
  end
  if task.due then
    t.due = task.due
  end
  if task.recur then
    t.recur = task.recur
  end
  if task.recur_mode then
    t.recur_mode = task.recur_mode
  end
  if task['end'] then
    t['end'] = task['end']
  end
  if task.order and task.order ~= 0 then
    t.order = task.order
  end
  if task._extra then
    for k, v in pairs(task._extra) do
      t[k] = v
    end
  end
  return t
end

---@param t table
---@return pending.Task
local function table_to_task(t)
  local task = {
    id = t.id,
    description = t.description,
    status = t.status or 'pending',
    category = t.category,
    priority = t.priority or 0,
    due = t.due,
    recur = t.recur,
    recur_mode = t.recur_mode,
    entry = t.entry,
    modified = t.modified,
    ['end'] = t['end'],
    order = t.order or 0,
    _extra = {},
  }
  for k, v in pairs(t) do
    if not known_fields[k] then
      task._extra[k] = v
    end
  end
  if next(task._extra) == nil then
    task._extra = nil
  end
  return task
end

---@return pending.Data
function M.load()
  local path = config.get().data_path
  local f = io.open(path, 'r')
  if not f then
    _data = empty_data()
    return _data
  end
  local content = f:read('*a')
  f:close()
  if content == '' then
    _data = empty_data()
    return _data
  end
  local ok, decoded = pcall(vim.json.decode, content)
  if not ok then
    error('pending.nvim: failed to parse ' .. path .. ': ' .. tostring(decoded))
  end
  if decoded.version and decoded.version > SUPPORTED_VERSION then
    error(
      'pending.nvim: data file version '
        .. decoded.version
        .. ' is newer than supported version '
        .. SUPPORTED_VERSION
        .. '. Please update the plugin.'
    )
  end
  _data = {
    version = decoded.version or SUPPORTED_VERSION,
    next_id = decoded.next_id or 1,
    tasks = {},
    undo = {},
  }
  for _, t in ipairs(decoded.tasks or {}) do
    table.insert(_data.tasks, table_to_task(t))
  end
  for _, snapshot in ipairs(decoded.undo or {}) do
    if type(snapshot) == 'table' then
      local tasks = {}
      for _, raw in ipairs(snapshot) do
        table.insert(tasks, table_to_task(raw))
      end
      table.insert(_data.undo, tasks)
    end
  end
  return _data
end

---@return nil
function M.save()
  if not _data then
    return
  end
  local path = config.get().data_path
  ensure_dir(path)
  local out = {
    version = _data.version,
    next_id = _data.next_id,
    tasks = {},
    undo = {},
  }
  for _, task in ipairs(_data.tasks) do
    table.insert(out.tasks, task_to_table(task))
  end
  for _, snapshot in ipairs(_data.undo) do
    local serialized = {}
    for _, task in ipairs(snapshot) do
      table.insert(serialized, task_to_table(task))
    end
    table.insert(out.undo, serialized)
  end
  local encoded = vim.json.encode(out)
  local tmp = path .. '.tmp'
  local f = io.open(tmp, 'w')
  if not f then
    error('pending.nvim: cannot write to ' .. tmp)
  end
  f:write(encoded)
  f:close()
  local ok, rename_err = os.rename(tmp, path)
  if not ok then
    os.remove(tmp)
    error('pending.nvim: cannot rename ' .. tmp .. ' to ' .. path .. ': ' .. tostring(rename_err))
  end
end

---@return pending.Data
function M.data()
  if not _data then
    M.load()
  end
  return _data --[[@as pending.Data]]
end

---@return pending.Task[]
function M.tasks()
  return M.data().tasks
end

---@return pending.Task[]
function M.active_tasks()
  local result = {}
  for _, task in ipairs(M.tasks()) do
    if task.status ~= 'deleted' then
      table.insert(result, task)
    end
  end
  return result
end

---@param id integer
---@return pending.Task?
function M.get(id)
  for _, task in ipairs(M.tasks()) do
    if task.id == id then
      return task
    end
  end
  return nil
end

---@param fields { description: string, status?: string, category?: string, priority?: integer, due?: string, recur?: string, recur_mode?: string, order?: integer, _extra?: table }
---@return pending.Task
function M.add(fields)
  local data = M.data()
  local now = timestamp()
  local task = {
    id = data.next_id,
    description = fields.description,
    status = fields.status or 'pending',
    category = fields.category or config.get().default_category,
    priority = fields.priority or 0,
    due = fields.due,
    recur = fields.recur,
    recur_mode = fields.recur_mode,
    entry = now,
    modified = now,
    ['end'] = nil,
    order = fields.order or 0,
    _extra = fields._extra,
  }
  data.next_id = data.next_id + 1
  table.insert(data.tasks, task)
  return task
end

---@param id integer
---@param fields table<string, any>
---@return pending.Task?
function M.update(id, fields)
  local task = M.get(id)
  if not task then
    return nil
  end
  local now = timestamp()
  for k, v in pairs(fields) do
    if k ~= 'id' and k ~= 'entry' then
      if v == vim.NIL then
        task[k] = nil
      else
        task[k] = v
      end
    end
  end
  task.modified = now
  if fields.status == 'done' or fields.status == 'deleted' then
    task['end'] = task['end'] or now
  end
  return task
end

---@param id integer
---@return pending.Task?
function M.delete(id)
  return M.update(id, { status = 'deleted', ['end'] = timestamp() })
end

---@param id integer
---@return integer?
function M.find_index(id)
  for i, task in ipairs(M.tasks()) do
    if task.id == id then
      return i
    end
  end
  return nil
end

---@param tasks pending.Task[]
---@return nil
function M.replace_tasks(tasks)
  M.data().tasks = tasks
end

---@return pending.Task[]
function M.snapshot()
  local result = {}
  for _, task in ipairs(M.active_tasks()) do
    local copy = {}
    for k, v in pairs(task) do
      if k ~= '_extra' then
        copy[k] = v
      end
    end
    if task._extra then
      copy._extra = {}
      for k, v in pairs(task._extra) do
        copy._extra[k] = v
      end
    end
    table.insert(result, copy --[[@as pending.Task]])
  end
  return result
end

---@return pending.Task[][]
function M.undo_stack()
  return M.data().undo
end

---@param stack pending.Task[][]
---@return nil
function M.set_undo_stack(stack)
  M.data().undo = stack
end

---@param id integer
---@return nil
function M.set_next_id(id)
  M.data().next_id = id
end

---@return nil
function M.unload()
  _data = nil
end

return M