local config = require('todo.config')
local parse = require('todo.parse')
local store = require('todo.store')

---@class todo.ParsedEntry
---@field type 'task'|'header'|'blank'
---@field id? integer
---@field description? string
---@field priority? integer
---@field category? string
---@field due? string
---@field lnum integer

---@class todo.diff
local M = {}

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

---@param lines string[]
---@return todo.ParsedEntry[]
function M.parse_buffer(lines)
  local result = {}
  local current_category = nil

  for i, line in ipairs(lines) do
    local id, body = line:match('^/(%d+)/(  .+)$')
    if not id then
      body = line:match('^(  .+)$')
    end
    if line == '' then
      table.insert(result, { type = 'blank', lnum = i })
    elseif id or body then
      local stripped = body:match('^  (.+)$') or body
      local priority = 0
      if stripped:match('^! ') then
        priority = 1
        stripped = stripped:sub(3)
      end
      local description, metadata = parse.body(stripped)
      if description and description ~= '' then
        table.insert(result, {
          type = 'task',
          id = id and tonumber(id) or nil,
          description = description,
          priority = priority,
          category = metadata.cat or current_category or config.get().default_category,
          due = metadata.due,
          lnum = i,
        })
      end
    elseif line:match('^%S') then
      current_category = line
      table.insert(result, { type = 'header', category = line, lnum = i })
    end
  end

  return result
end

---@param lines string[]
function M.apply(lines)
  local parsed = M.parse_buffer(lines)
  local now = timestamp()
  local data = store.data()

  local old_by_id = {}
  for _, task in ipairs(data.tasks) do
    if task.status ~= 'deleted' then
      old_by_id[task.id] = task
    end
  end

  local seen_ids = {}
  local order_counter = 0

  for _, entry in ipairs(parsed) do
    if entry.type ~= 'task' then
      goto continue
    end

    order_counter = order_counter + 1

    if entry.id and old_by_id[entry.id] then
      if seen_ids[entry.id] then
        store.add({
          description = entry.description,
          category = entry.category,
          priority = entry.priority,
          due = entry.due,
          order = order_counter,
        })
      else
        seen_ids[entry.id] = true
        local task = old_by_id[entry.id]
        local changed = false
        if task.description ~= entry.description then
          task.description = entry.description
          changed = true
        end
        if task.category ~= entry.category then
          task.category = entry.category
          changed = true
        end
        if task.priority ~= entry.priority then
          task.priority = entry.priority
          changed = true
        end
        if task.due ~= entry.due then
          task.due = entry.due
          changed = true
        end
        if task.order ~= order_counter then
          task.order = order_counter
          changed = true
        end
        if changed then
          task.modified = now
        end
      end
    else
      store.add({
        description = entry.description,
        category = entry.category,
        priority = entry.priority,
        due = entry.due,
        order = order_counter,
      })
    end

    ::continue::
  end

  for id, task in pairs(old_by_id) do
    if not seen_ids[id] then
      task.status = 'deleted'
      task['end'] = now
      task.modified = now
    end
  end

  store.save()
end

return M