-- Unicode library

local pattern = '[%z\1-\127\194-\244][\128-\191]*'

-- helper function
local function posrelat(pos, len)
  if pos < 0 then
    pos = len + pos + 1
  end
  return pos
end

local utf8 = {}

-- THE MEAT

function utf8.map(s, f, no_subs)
  local i = 0

  if no_subs then
    for b, e in s:gmatch('()' .. pattern .. '()') do
      i = i + 1
      local c = e - b
      f(i, c, b)
    end
  else
    for b, c in s:gmatch('()(' .. pattern .. ')') do
      i = i + 1
      f(i, c, b)
    end
  end
end

-- THE REST

function utf8.chars(s, no_subs)
  return coroutine.wrap(function()
    return utf8.map(s, coroutine.yield, no_subs)
  end)
end

function utf8.len(s)
  return select(2, s:gsub('[^\128-\193]', ''))
end

function utf8.replace(s, map)
  return s:gsub(pattern, map)
end

function utf8.reverse(s)
  s = s:gsub(pattern, function(c)
    return #c > 1 and c:reverse()
  end)
  return s:reverse()
end

function utf8.strip(s)
  return s:gsub(pattern, function(c)
    return #c > 1 and ''
  end)
end

function utf8.sub(s, i, j)
  local l = utf8.len(s)

  i = posrelat(i, l)
  j = j and posrelat(j, l) or l

  if i < 1 then i = 1 end
  if j > l then j = l end
  if i > j then return '' end

  local diff = j - i
  local iter = utf8.chars(s, true)

  for _ = 1, i - 1 do iter() end
  local c, b = select(2, iter())

  if diff == 0
  then return s:sub(b, b + c - 1) end

  i = b
  for _ = 1, diff - 1 do iter() end
  local c, b = select(2, iter())

  return s:sub(i, b + c - 1)
end

return utf8