_ALGORITHM ALLEY_
by Gene Callahan
edited by Bruce Schneier

Listing One

/* given an ASCII character, what is its value in the base? */
#define MAX_BASE     256
typedef struct baserep
{
     int  Radix;
     int  NumValues[MAX_BASE];
     char CharValues[MAX_BASE + 1];
} BaseRep;

BaseRep Base36DigitsLower = {
     { 36 },
/* 0 - 47 */
     {-1, -1, -1, /* ...and so on, until 48 negative ones in all
*/
/* 48 ('0') - 57 ('9') */
     0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 
/* 58 - 96: another run of -1 */
/* 97 ('a') - 122 ('z') */
     10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22,         
23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 
/* 123 - 255: whole bunch o' negative ones go here */},
     {"0123456789abcdefghijklmnopqrstuvwxyz"}
};


Listing Two

long BaseXToLong(char* number, BaseRep* br)
{
     long PowerOfRadix;
     long ret = 0;
     int  i;
     int  digits = strlen(number);

     for(i = digits - 1, base_power = 1; 
          i >= 0; 
          i--, PowerOfRadix *= br->Radix)
          ret += (br->NumValues[(int)number[i]] * PowerOfRadix);

     return ret;
}


Listing Three

char* LongToBaseX(long number, char* buf, 
                  int buf_len, BaseRep* br)
{
     int i;

     for(i = buf_len - 1; i >= 0; i--)
     {
          buf[i] = br->CharValues[number % br->Radix];
          number /= br->Radix;
     }
     if(number) return 0;  /* null: error return */
     return buf;  /* so function will generate rvalue */
}



Example 1:

char* BaseXIncr(char* number, BaseRep* br)
{
  return LongToBaseX(BaseXToLong(number, br) + 1, number, strlen(number), br);
}


Example 2:

int LessThan(char* num1, BaseRep* br1, char* num2, BaseRep* br2)
{
     return BaseXToLong(num1, br1) 
          < BaseXToLong(num2, br2);
}