Java 一个指针变量。这通过消除索引指令来优化代码。非常感谢。我在C中使用了这个。时间从约5400减少到约5350。当然，有一些改进。非常感谢。我在C中使用了这个。时间从约5400减少到约5350。当然，有一些改进。你能解释一下你在做什么以及这些指令来自哪里（编译

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一个指针变量。这通过消除索引指令来优化代码。非常感谢。我在C中使用了这个。时间从约5400减少到约5350。当然，有一些改进。非常感谢。我在C中使用了这个。时间从约5400减少到约5350。当然，有一些改进。你能解释一下你在做什么以及这些指令来自哪里（编译器、库等等）？@PaulOgilvie用AVX2 SIMD重新实现了OP的代码。这些指令是在2013后的CPU中内置的，并且由所有的现代C和C++编译器支持。正如您所看到的，性能差异是巨大的。非常感谢这段代码。我想我需要一个向导来理解这一点。你似乎对C语言的编码非常精通。请注意，你只能一次循环调用

rand（）

，这也会有很大帮助：-@AmarjitSingh添加了更多的注释，希望现在更容易理解。你能解释一下你在做什么以及这些指令来自哪里（编译器、库等）？@PaulOgilvie用AVX2 SIMD重新实现了OP的代码。这些指令是在2013后的CPU中内置的，并且由所有的现代C和C++编译器支持。正如您所看到的，性能差异是巨大的。非常感谢这段代码。我想我需要一个向导来理解这一点。你似乎非常精通C语言的编码。请注意，你只能循环调用

rand（）

，这也会有很大帮助：-）@AmarjitSingh添加了更多注释，希望现在更容易理解。

public static void main(String[] args) {
    Long startTime = System.currentTimeMillis();

    String name = "abcdefghijklmnopqrstuvwxyz";
    char[] stringCharArray = name.toCharArray();
    Random random = new Random();
    for (Integer i = 0; i <100000000; i++) {
        {
            for (int j = 0; j < random.nextInt(26) + 1; j++) {
                if (stringCharArray[j] == 'z') {
                    stringCharArray[j] = 'a';
                } else {
                    stringCharArray[j] = (char) (((int) (stringCharArray[j])) + 1);
                }
            }
        }
    }
    Long endtime = System.currentTimeMillis();
    System.out.println(endtime-startTime+" ms");

}

public static void main(String[] args) {
    HashMap hashMap = new HashMap();
    hashMap.put('a', 'b');
    hashMap.put('b', 'c');
    hashMap.put('c', 'd');
    hashMap.put('d', 'e');
    hashMap.put('e', 'f');
    hashMap.put('f', 'g');
    hashMap.put('g', 'h');
    hashMap.put('h', 'i');
    hashMap.put('i', 'j');
    hashMap.put('j', 'k');
    hashMap.put('k', 'l');
    hashMap.put('l', 'm');
    hashMap.put('m', 'n');
    hashMap.put('n', 'o');
    hashMap.put('o', 'p');
    hashMap.put('p', 'q');
    hashMap.put('q', 'r');
    hashMap.put('r', 's');
    hashMap.put('s', 't');
    hashMap.put('t', 'u');
    hashMap.put('u', 'v');
    hashMap.put('v', 'w');
    hashMap.put('w', 'x');
    hashMap.put('x', 'y');
    hashMap.put('y', 'z');
    hashMap.put('z', 'a');
    Long startTime = System.currentTimeMillis();
    String name = "abcdefghijklmnopqrstuvwxyz";
    char[] stringCharArray = name.toCharArray();
    Random random = new Random();
    for (Integer i = 0; i <100000000; i++) {
        {
            for (Integer j = 0; j < random.nextInt(26) + 1; j++) {
                    stringCharArray[j] = (char) hashMap.get(stringCharArray[j]);
            }
        }
    }
    Long endtime = System.currentTimeMillis();
    System.out.println(endtime-startTime+" ms");
}

#include <stdio.h>
#include <time.h>
#include <zconf.h>
#include <stdlib.h>

int main() {
    long start = clock();
    char name[] = "abcdefghijklmnopqrstuvwxyz";
    for (int i = 0; i <100000000; i++) {
        {
            for (int j = 0; j < rand() % 25; j++) {
                if (name[j] == 'z') {
                    name[j] = 'a';
                } else {
                    name[j] = (char) (((int) (name[j])) + 1);
                }
            }
        }
    }
    long stop = clock();
    printf("time taken = %ld sec \n",( stop-start)/1000);
}

char[] nextChar = {'b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z','a'};

                stringCharArray[j] = nextChar[j];//by index
                stringCharArray[j] = nextChar[stringCharArray[j]-'a'];// using ascii

    Long startTime = System.currentTimeMillis();

    char[] nextChar = {'b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z','a'};
    String name = "abcdefghijklmnopqrstuvwxyz";
    char[] stringCharArray = name.toCharArray();
    Random random = new Random();
    for (int i = 0; i <100000000; i++) {
        {
            for (int j = 0; j < random.nextInt(27); j++) {
                stringCharArray[j] = nextChar[j];//by index
                //stringCharArray[j] = nextChar[stringCharArray[j]-'a'];// using ascii
            }
        }
    }
    Long endtime = System.currentTimeMillis();
    System.out.println(endtime-startTime+" ms");

#include <immintrin.h>

int main()
{
    const __m256i compareConstant = _mm256_setr_epi8( 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12,
        13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31 );

    long start = clock();
    char name[ 32 ] = "abcdefghijklmnopqrstuvwxyz";
    // __m256i is a C name for AVX register.
    // AVX registers are 32 bytes in size, so your 26 bytes string fits in just a single one.
    // The following line loads your string from memory to that register.
    __m256i n = _mm256_loadu_si256( ( const __m256i* )name );
    for( int i = 0; i < 100000000; i++ )
    {
        // Increment the letters, all 32 of them.
        // `_mm256_set1_epi8` creates a value with all 32 bytes set to the same value.
        // `_mm256_add_epi8` adds one set of 32 signed bytes to another set of 32 signed bytes.
        // It's not a loop i.e. it's very fast, CPUs can actually run 2-3 such instructions per cycle. 
        __m256i n2 = _mm256_add_epi8( n, _mm256_set1_epi8( 1 ) );

        // Wrap any > 'z' letters back to 'a'
        // _mm256_cmpgt_epi8 compares one set of bytes to another set, for `>`.
        // When it's `>` the result byte is set to 0xFF, when it's `<=` the result byte is 0.
        // _mm256_blendv_epi8 combines bytes from 2 registers based on the bytes from the third one.
        // In this case, the third one is the result of the comparison.
        n2 = _mm256_blendv_epi8( n2, _mm256_set1_epi8( 'a' ), _mm256_cmpgt_epi8( n2, _mm256_set1_epi8( 'z' ) ) );

        // Combine incremented ones with old, using random number of first characters
        const int r = rand() % 25;
        // This sets all 32 bytes in rv to the random number r
        __m256i rv = _mm256_broadcastb_epi8( _mm_cvtsi32_si128( r ) );
        // Compares all bytes in rv with the constant value [0, 1, 2, 3, ...]
        // For bytes where r>cc, blendv_epi8 will select a byte from n2.
        // For bytes where r<=cc, n will not change because blendv_epi8 will select an old value.
        n = _mm256_blendv_epi8( n, n2, _mm256_cmpgt_epi8( rv, compareConstant ) );
    }
    long stop = clock();
    printf( "time taken = %ld sec \n", ( stop - start ) / 1000 );
}