IOS 长链接与短链接之间的转换

#import <commoncrypto/commondigest.h>
 
@interface nsstring (md5)
 
-(nsstring *) md5hexdigest;
 
@end

#import "nsstring+md5.h"
 
@implementation nsstring (md5)
 
- (nsstring *) md5hexdigest
{
  const char *original_str = [self utf8string];
  unsignedchar result[cc_md5_digest_length];
  cc_md5(original_str, strlen(original_str), result);
  nsmutablestring *hash = [nsmutablestringstring];
  for (int i = 0; i < 16; i++)
    [hash appendformat:@"%02x", result[i]];
  nslog(@"lowercasestring = %@",[hash lowercasestring]);
  return [hash lowercasestring];
}
 
@end

- (nsstring *)shorturl:(nsstring *)url
{
  nsarray *chars = [[nsarray alloc] initwithobjects:@"a" , @"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" , @"0" , @"1" , @"2" , @"3" , @"4" , @"5" ,
           @"6" , @"7" , @"8" , @"9" , @"a" , @"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", nil];
  nslog(@"chars = %d", [chars count]);
  nsstring *key = @"xxxxxx";
  nsstring *hex = [nsstringstringwithformat:@"%@",[[key stringbyappendingformat:@"%@",url] md5hexdigest]];
 
  nslog(@"hex = %@", hex);
  nsmutablearray *resurl = [[nsmutablearrayalloc] initwithcapacity:4];
  for (int i=0; i<4; i++) {
    // 把加密字符按照 8 位一组 16 进制与 0x3fffffff 进行位与运算
    nsstring *stempsubstring = [hex substringwithrange:nsmakerange(i*8, 8)];
    
    // 这里需要使用 long 型来转换，因为 inteper只能处理 31 位 , 首位为符号位 , 如果不用 long ，则会越界
    long longoftemp;
    sscanf([stempsubstring cstringusingencoding:nsasciistringencoding], "%lx", &longoftemp);
    long lhexlong = 0x3fffffff & longoftemp;
    nsstring *outchars = @"";
    for (int j=0; j<6; j++) {
      // 把得到的值与 0x0000003d 进行位与运算，取得字符数组 chars 索引
      long index = 0x0000003d & lhexlong;
      // 把取得的字符相加
      outchars = [outchars stringbyappendingformat:@"%@",[chars objectatindex:(int)index]];
      // 每次循环按位右移 5 位
      lhexlong = lhexlong >> 5;
    }
    // 把字符串存入对应索引的输出数组
    [resurl insertobject:outchars atindex:i];
  }
  return [resurl objectatindex:0];//这里可以返回任意一个元素作为短链接（0，1，2，3）
}