【安全加密】私聊消息的敏感词过滤：DFA 匹配实现

---DFA 自动机的构建
function DFA.build(wordList)
    local root = {}
    for _, word in ipairs(wordList) do
        local node = root
        for i = 1, #word do
            local ch = string.sub(word, i, i)
            node.children = node.children or {}
            node.children[ch] = node.children[ch] or {}
            node = node.children[ch]
        end
        node.isEnd = true
    end
    return root
end

---DFA 匹配：检测文本中的敏感词
function DFA.match(dfa, text)
    local node = dfa
    local start = 1
    for i = 1, #text do
        local ch = string.sub(text, i, i)
        if node.children and node.children[ch] then
            node = node.children[ch]
            if node.isEnd then
                return true, start, i
            end
        else
            node = dfa
            start = i + 1
        end
    end
    return false
end