local config = require('pending.config')
local parse = require('pending.parse')

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

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

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

---@param lines string[]
---@return pending.ParsedEntry[]
function M.parse_buffer(lines)
  local result = {}
  local current_category = nil
  local start = 1
  if lines[1] and lines[1]:match('^FILTER:') then
    start = 2
  end

  for i = start, #lines do
    local line = lines[i]
    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('^- %[.%] (.*)$') or body
      local state_char = body:match('^- %[(.-)%]') or ' '
      local priority = state_char == '!' and 1 or 0
      local status = state_char == 'x' and 'done' or 'pending'
      local description, metadata = parse.body(stripped)
      if description and description ~= '' then
        table.insert(result, {
          type = 'task',
          id = id and tonumber(id) or nil,
          description = description,
          priority = priority,
          status = status,
          category = metadata.cat or current_category or config.get().default_category,
          due = metadata.due,
          rec = metadata.rec,
          rec_mode = metadata.rec_mode,
          lnum = i,
        })
      end
    elseif line:match('^# (.+)$') then
      current_category = line:match('^# (.+)$')
      table.insert(result, { type = 'header', category = current_category, lnum = i })
    end
  end

  return result
end

---@param lines string[]
---@param s pending.Store
---@param hidden_ids? table<integer, true>
---@return nil
function M.apply(lines, s, hidden_ids)
  local parsed = M.parse_buffer(lines)
  local now = timestamp()
  local data = s: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
        s:add({
          description = entry.description,
          category = entry.category,
          priority = entry.priority,
          due = entry.due,
          recur = entry.rec,
          recur_mode = entry.rec_mode,
          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 entry.due ~= nil and task.due ~= entry.due then
          task.due = entry.due
          changed = true
        end
        if entry.rec ~= nil then
          if task.recur ~= entry.rec then
            task.recur = entry.rec
            changed = true
          end
          if task.recur_mode ~= entry.rec_mode then
            task.recur_mode = entry.rec_mode
            changed = true
          end
        end
        if entry.status and task.status ~= entry.status then
          task.status = entry.status
          if entry.status == 'done' then
            task['end'] = now
          else
            task['end'] = nil
          end
          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
      s:add({
        description = entry.description,
        category = entry.category,
        priority = entry.priority,
        due = entry.due,
        recur = entry.rec,
        recur_mode = entry.rec_mode,
        order = order_counter,
      })
    end

    ::continue::
  end

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

  s:save()
end

return M