2019-8-31-C#-对-byte-数组进行模式搜索

title	author	date	CreateTime	categories
C# 对 byte 数组进行模式搜索	lindexi	2019-08-31 16:55:58 +0800	2018-07-19 11:16:46 +0800	C#

本文告诉大家几个方法从 byte 数组找到对应的相同序列的数组

最简单的方法是进行数值判断，但是代码最少是使用Linq ，效率比较高是使用 Boyer-Moore 算法，下面就告诉大家几个算法的代码

判断数值

    class ByteArrayRocks
    {
        public static IEnumerable<int> IndexOf(byte[] source, int start, byte[] pattern)
        {
            if (IsEmptyLocate(source, start, pattern))
            {
                yield break;
            }

            for (int i = start; i < source.Length; i++)
            {
                if (!IsMatch(source, i, pattern))
                {
                    continue;
                }

                yield return i;
            }
        }

        private static readonly int[] Empty = new int[0];

        private static bool IsMatch(byte[] array, int position, byte[] candidate)
        {
            if (candidate.Length > (array.Length - position))
            {
                return false;
            }

            for (int i = 0; i < candidate.Length; i++)
            {
                if (array[position + i] != candidate[i])
                {
                    return false;
                }
            }

            return true;
        }

        private static bool IsEmptyLocate(byte[] array, int start, byte[] candidate)
        {
            return array == null
                   || candidate == null
                   || array.Length == 0
                   || array.Length < start
                   || candidate.Length == 0
                   || candidate.Length + start > array.Length;
        }
    }

这是最简单的方法，参见 https://stackoverflow.com/a/283648/6116637

linq

这个方法的代码最少

    class LinqArraySearch
    {
        public static IEnumerable<int> IndexOf(byte[] source, int start, byte[] pattern)
        {
            for (int i = start; i < source.Length; i++)
            {
                if (source.Skip(i).Take(pattern.Length).SequenceEqual(pattern))
                {
                    yield return i;
                }
            }
        }
    }

Boyer-Moore-Horspool 搜索

这是最快的方法

    class BoyerMooreHorspool
    {
        public static IEnumerable<long> IndexesOf(byte[] source, int start, byte[] pattern)
        {
            if (source == null)
            {
                throw new ArgumentNullException(nameof(source));
            }

            if (pattern == null)
            {
                throw new ArgumentNullException(nameof(pattern));
            }

            long valueLength = source.LongLength;
            long patternLength = pattern.LongLength;

            if ((valueLength == 0) || (patternLength == 0) || (patternLength > valueLength))
            {
                yield break;
            }

            var badCharacters = new long[256];

            for (var i = 0; i < 256; i++)
            {
                badCharacters[i] = patternLength;
            }

            var lastPatternByte = patternLength - 1;

            for (long i = 0; i < lastPatternByte; i++)
            {
                badCharacters[pattern[i]] = lastPatternByte - i;
            }

            long index = start;

            while (index <= valueLength - patternLength)
            {
                for (var i = lastPatternByte; source[index + i] == pattern[i]; i--)
                {
                    if (i == 0)
                    {
                        yield return index;
                        break;
                    }
                }

                index += badCharacters[source[index + lastPatternByte]];
            }
        }
    }

参见：https://stackoverflow.com/q/16252518/6116637

测试了三个方法的性能，请看下面

BenchmarkDotNet=v0.10.14, OS=Windows 10.0.17134
Intel Core i7-6700 CPU 3.40GHz (Skylake), 1 CPU, 8 logical and 4 physical cores
.NET Core SDK=2.1.202
  [Host]     : .NET Core 2.0.9 (CoreCLR 4.6.26614.01, CoreFX 4.6.26614.01), 64bit RyuJIT
  DefaultJob : .NET Core 2.0.9 (CoreCLR 4.6.26614.01, CoreFX 4.6.26614.01), 64bit RyuJIT

Method	Mean	Error	StdDev
Linq	13,332.8 us	251.782 us	466.694 us
ByteArrayRocks	371.3 us	7.327 us	14.462 us
BoyerMooreHorspool	108.3 us	1.153 us	1.079 us

其他方法请看下面

使用不安全代码的 Boyer Moore 算法 C# High Performance Boyer Moore Byte Array Search Algorithm