local mt = {}
mt.__index = mt

local function find_char(self, current, is_head, is_tail)
    if not current then
        current = 0
    end
    for i = current + 1, #self.char_list do
        local char = self.char_list[i]
        if (is_head or is_tail) and char == '_' then
            goto CONTINUE
        end
        if is_head and not char:find '%a' then
            goto CONTINUE
        end
        do return i, char end
        :: CONTINUE ::
    end
    return nil
end

local function find_new_name(self)
    for i = 1, #self.confuse_bytes + 1 do
        local byte, char = find_char(self, self.confuse_bytes[i], i >= #self.confuse_bytes, i == 1)
        if byte then
            self.confuse_bytes[i] = byte
            self.confuse_chars[i] = char
            break
        else
            self.confuse_bytes[i], self.confuse_chars[i] = find_char(self, 0, i >= #self.confuse_bytes, i == 1)
        end
    end
    return string.reverse(table.concat(self.confuse_chars))
end

local function find_usable_name(self)
    while true do
        local new_name = find_new_name(self)
        if not self.on_find then
            return new_name
        end
        local new_name = self:on_find(new_name)
        if new_name then
            return new_name
        end
    end
end

function mt:__call(name)
    if not self.name_list[name] then
        self.name_list[name] = find_usable_name(self)
    end
    return self.name_list[name]
end

return function (confusion)
    local self = setmetatable({}, mt)
    self.char_list = {}
    self.name_list = {}
    self.head_list = {}
    self.confuse_bytes = {}
    self.confuse_chars = {}
    for char in confusion:gmatch '.' do
        self.char_list[#self.char_list+1] = char
    end
    return self
end