在 C# 中的字节数组中搜索最长模式 [英] Search longest pattern in byte array in C#

问题描述

我需要编写有效且快速的方法来搜索给定模式的字节数组.我是这样写的，你怎么看，如何改进?它有一个错误，它不能返回长度为 1 的匹配项.

I need to write effective and quick method to search byte array for given pattern. I write it this way, what do you think , how to improve? And it has one bug, it cannot return match with length 1.

public static bool SearchByteByByte(byte[] bytes, byte[] pattern)
    {
        bool found = false;
        int matchedBytes = 0;
        for (int i = 0; i < bytes.Length; i++)
        {
            if (pattern[0] == bytes[i] && bytes.Length - i >= pattern.Length)
            {
                for (int j = 1; j < pattern.Length; j++)
                {
                    if (bytes[i + j] == pattern[j])
                    {
                        matchedBytes++;
                        if (matchedBytes == pattern.Length - 1)
                        {
                            return true;
                        }
                        continue;
                    }
                    else
                    {
                        matchedBytes = 0;
                        break;
                    }
                }
            }
        }
        return found;
    }

有什么建议吗?

推荐答案

grep 中使用的 Boyer-Moore 算法非常有效，并且对于更长的模式尺寸更有效.我很确定你可以让它在没有太多困难的情况下为字节数组工作，它的 维基百科页面 有一个 Java 实现，应该很容易移植到 C#.

The Boyer-Moore algorithm that is used in grep is pretty efficient, and gets more efficient for longer pattern sizes. I'm pretty sure you could make it work for a byte array without too much difficulty, and its wikipedia page has an implementation in Java that should be fairly easy to port to C#.

更新:

这里是 C# 中字节数组的 Boyer-Moore 算法的简化版本的实现.它只使用完整算法的第二个跳转表.根据您所说的数组大小(haystack:2000000 字节，needle:10 字节)，它比简单的逐字节算法快约 5-8 倍.

Here's an implementation of a simplified version of the Boyer-Moore algorithm for byte arrays in C#. It only uses the second jump table of the full algorithm. Based on the array sizes that you said (haystack: 2000000 bytes, needle: 10 bytes), it's about 5-8 times faster than a simple byte by byte algorithm.

    static int SimpleBoyerMooreSearch(byte[] haystack, byte[] needle)
    {
        int[] lookup = new int[256];
        for (int i = 0; i < lookup.Length; i++) { lookup[i] = needle.Length; }

        for (int i = 0; i < needle.Length; i++)
        {
            lookup[needle[i]] = needle.Length - i - 1;
        }

        int index = needle.Length - 1;
        var lastByte = needle.Last();
        while (index < haystack.Length)
        {
            var checkByte = haystack[index];
            if (haystack[index] == lastByte)
            {
                bool found = true;
                for (int j = needle.Length - 2; j >= 0; j--)
                {
                    if (haystack[index - needle.Length + j + 1] != needle[j])
                    {
                        found = false;
                        break;
                    }
                }

                if (found)
                    return index - needle.Length + 1;
                else
                    index++;
            }
            else
            {
                index += lookup[checkByte];
            }
        }
        return -1;
    }

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