local config = require('todo.config')

---@class todo.LineMeta
---@field type 'task'|'header'|'blank'
---@field id? integer
---@field due? string
---@field raw_due? string
---@field status? string
---@field category? string

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

---@param due? string
---@return string?
local function format_due(due)
  if not due then
    return nil
  end
  local y, m, d = due:match('^(%d%d%d%d)-(%d%d)-(%d%d)$')
  if not y then
    return due
  end
  local t = os.time({ year = tonumber(y), month = tonumber(m), day = tonumber(d) })
  return os.date(config.get().date_format, t)
end

---@param tasks todo.Task[]
local function sort_tasks(tasks)
  table.sort(tasks, function(a, b)
    if a.priority ~= b.priority then
      return a.priority > b.priority
    end
    if a.order ~= b.order then
      return a.order < b.order
    end
    return a.id < b.id
  end)
end

---@param tasks todo.Task[]
local function sort_tasks_priority(tasks)
  table.sort(tasks, function(a, b)
    if a.priority ~= b.priority then
      return a.priority > b.priority
    end
    local a_due = a.due or ''
    local b_due = b.due or ''
    if a_due ~= b_due then
      if a_due == '' then
        return false
      end
      if b_due == '' then
        return true
      end
      return a_due < b_due
    end
    if a.order ~= b.order then
      return a.order < b.order
    end
    return a.id < b.id
  end)
end

---@param tasks todo.Task[]
---@return string[] lines
---@return todo.LineMeta[] meta
function M.category_view(tasks)
  local by_cat = {}
  local cat_order = {}
  local cat_seen = {}
  local done_by_cat = {}

  for _, task in ipairs(tasks) do
    local cat = task.category or config.get().default_category
    if not cat_seen[cat] then
      cat_seen[cat] = true
      table.insert(cat_order, cat)
      by_cat[cat] = {}
      done_by_cat[cat] = {}
    end
    if task.status == 'done' then
      table.insert(done_by_cat[cat], task)
    else
      table.insert(by_cat[cat], task)
    end
  end

  for _, cat in ipairs(cat_order) do
    sort_tasks(by_cat[cat])
    sort_tasks(done_by_cat[cat])
  end

  local lines = {}
  local meta = {}

  for i, cat in ipairs(cat_order) do
    if i > 1 then
      table.insert(lines, '')
      table.insert(meta, { type = 'blank' })
    end
    table.insert(lines, cat)
    table.insert(meta, { type = 'header', category = cat })

    local all = {}
    for _, t in ipairs(by_cat[cat]) do
      table.insert(all, t)
    end
    for _, t in ipairs(done_by_cat[cat]) do
      table.insert(all, t)
    end

    for _, task in ipairs(all) do
      local prefix = '/' .. task.id .. '/'
      local indent = '  '
      local prio = task.priority == 1 and '! ' or ''
      local line = prefix .. indent .. prio .. task.description
      table.insert(lines, line)
      table.insert(meta, {
        type = 'task',
        id = task.id,
        due = format_due(task.due),
        raw_due = task.due,
        status = task.status,
        category = cat,
      })
    end
  end

  return lines, meta
end

---@param tasks todo.Task[]
---@return string[] lines
---@return todo.LineMeta[] meta
function M.priority_view(tasks)
  local pending = {}
  local done = {}

  for _, task in ipairs(tasks) do
    if task.status == 'done' then
      table.insert(done, task)
    else
      table.insert(pending, task)
    end
  end

  sort_tasks_priority(pending)
  sort_tasks_priority(done)

  local lines = {}
  local meta = {}

  local all = {}
  for _, t in ipairs(pending) do
    table.insert(all, t)
  end
  for _, t in ipairs(done) do
    table.insert(all, t)
  end

  for _, task in ipairs(all) do
    local prefix = '/' .. task.id .. '/'
    local indent = '  '
    local prio = task.priority == 1 and '! ' or ''
    local line = prefix .. indent .. prio .. task.description
    table.insert(lines, line)
    table.insert(meta, {
      type = 'task',
      id = task.id,
      due = format_due(task.due),
      raw_due = task.due,
      status = task.status,
      category = task.category,
    })
  end

  return lines, meta
end

return M