Fast open source checksum for small strings

Question

I need a quick checksum (as fast as possilbe) for small strings (20-500 chars).

I need the source code and that must be small! (about 100 LOC max)

If it could generate strings in Base32/64. (or something similar) it would be perfect. Basically the checksums cannot use any "bad" chars.. you know.. the usual (){}[].,;:/+-\| etc

Clarifications

It could be strong/weak, that really doesn't matter since it is only for behind-the-scenes purposes.

It need not contain all the data of the original string since I will be only doing comparison with generated checksums, I don't expect any sort of "decryption".

Answer 1

schnaader's implementation is indeed very fast. Here it is in Javascript:

function checksum(s) {   var chk = 0x12345678;   var len = s.length;   for (var i = 0; i < len; i++) {       chk += (s.charCodeAt(i) * (i + 1));   }    return (chk & 0xffffffff).toString(16); } 

Using Google Chrome, this function takes just 5ms to run for 1-megabyte strings, versus 330ms using a crc32 function.

Answer 2

Quick implementation in C, no copyrights from my side, so use it as you wish. But please note that this is a very weak "checksum", so don't use it for serious things :) - but that's what you wanted, isn't it?

This returns an 32-bit integer checksum encoded as an string containing its hex value. If the checksum function doesn't satisfy your needs, you can change the chk += ((int)(str[i]) * (i + 1)); line to something better (f.e. multiplication, addition and bitwise rotating would be much better).

EDIT: Following hughdbrown's advice and one of the answers he linked, I changed the for loop so it doesn't call strlen with every iteration.

#include <stdio.h> #include <stdlib.h> #include <string>  char* hextab = "0123456789ABCDEF";  char* encode_int(int i) {   char* c = (char*)malloc(sizeof(char) * 9);    for (int j = 0; j < 4; j++) {     c[(j << 1)] = hextab[((i % 256) >> 4)];     c[(j << 1) + 1] = hextab[((i % 256) % 16)];      i = (i >> 8);   }   c[8] = 0;    return c; }  int checksum(char* str) {   int i;   int chk = 0x12345678;    for (i = 0; str[i] != '\0'; i++) {     chk += ((int)(str[i]) * (i + 1));   }    return chk; }  int main() {   char* str1 = "Teststring";   char* str2 = "Teststring2";    printf("string: %s, checksum string: %s\n", str1, encode_int(checksum(str1)));   printf("string: %s, checksum string: %s\n", str2, encode_int(checksum(str2)));    return 0; }