local input = arg[1] or '/var/log/tutorial-access.log'
local output = arg[2] or '/var/log/access-summary.txt'
local function fail(message)
  io.stderr:write(message, '\n')
  os.exit(1)
end
if input == output then fail('Input and output must be different files') end
local source, err = io.open(input, 'r')
if not source then fail('Cannot read ' .. input .. ': ' .. tostring(err)) end
local requests, skipped, bytes, errors4, errors5 = 0, 0, 0, 0, 0
local statuses, paths = {}, {}
for line in source:lines() do
  if line:match('%S') then
    local request, code, size = line:match('^%S+%s+%S+%s+%S+%s+%b[]%s+"([^"]*)"%s+(%d%d%d)%s+(%S+)')
    local method, path = (request or ''):match('^([A-Z]+)%s+(%S+)%s+HTTP/%d+%.%d+$')
    local status = tonumber(code)
    if method and status and status >= 100 and status <= 599
        and (size == '-' or size:match('^%d+$')) then
      requests = requests + 1
      bytes = bytes + (tonumber(size) or 0)
      statuses[code] = (statuses[code] or 0) + 1
      paths[path] = (paths[path] or 0) + 1
      if status >= 400 and status < 500 then errors4 = errors4 + 1 end
      if status >= 500 then errors5 = errors5 + 1 end
    else
      skipped = skipped + 1
    end
  end
end
source:close()
local lines = {
  'Source: ' .. input,
  'Generated: ' .. os.date('!%Y-%m-%d %H:%M:%S UTC'),
  'Requests: ' .. requests,
  'Skipped: ' .. skipped,
  'Response bytes: ' .. bytes,
  '4xx: ' .. errors4,
  '5xx: ' .. errors5,
  '', 'Statuses:'
}
local codes = {}
for code in pairs(statuses) do codes[#codes + 1] = code end
table.sort(codes)
for _, code in ipairs(codes) do lines[#lines + 1] = code .. ': ' .. statuses[code] end
lines[#lines + 1] = ''
lines[#lines + 1] = 'Top paths:'
local ranked = {}
for path, count in pairs(paths) do ranked[#ranked + 1] = {path = path, count = count} end
table.sort(ranked, function(a, b)
  if a.count ~= b.count then return a.count > b.count end
  return a.path < b.path
end)
for i = 1, math.min(5, #ranked) do
  lines[#lines + 1] = ranked[i].count .. ' ' .. ranked[i].path
end
local temporary = output .. '.tmp'
local report, open_err = io.open(temporary, 'w')
if not report then fail('Cannot write ' .. temporary .. ': ' .. tostring(open_err)) end
local written, write_err = report:write(table.concat(lines, '\n'), '\n')
local closed, close_err = report:close()
if not written or not closed then
  os.remove(temporary)
  fail('Cannot save report: ' .. tostring(write_err or close_err))
end
local renamed, rename_err = os.rename(temporary, output)
if not renamed then
  os.remove(temporary)
  fail('Cannot replace ' .. output .. ': ' .. tostring(rename_err))
end
print('Saved: ' .. output)