liferules.cpp

// This file is part of Golly.
// See docs/License.html for the copyright notice.

#include "liferules.h"
#include "util.h"
#include <cstdlib>
#include <string.h>
#include <stdlib.h>
#include <ctype.h>

#if defined(WIN32) || defined(WIN64)
#define strncasecmp _strnicmp
#endif

liferules::liferules() {
   int i ;

   // base64 encoding characters
   base64_characters = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/" ;

   // all valid rule letters
   valid_rule_letters = "012345678ceaiknjqrytwz-" ;

   // rule letters per neighbor count
   rule_letters[0] = "ce" ;
   rule_letters[1] = "ceaikn" ;
   rule_letters[2] = "ceaiknjqry" ;
   rule_letters[3] = "ceaiknjqrytwz" ;

   // isotropic neighborhoods per neighbor count
   static int entry0[2] = { 1, 2 } ;
   static int entry1[6] = { 5, 10, 3, 40, 33, 68 } ;
   static int entry2[10] = { 69, 42, 11, 7, 98, 13, 14, 70, 41, 97 } ;
   static int entry3[13] = { 325, 170, 15, 45, 99, 71, 106, 102, 43, 101, 105, 78, 108 } ;
   rule_neighborhoods[0] = entry0 ;
   rule_neighborhoods[1] = entry1 ;
   rule_neighborhoods[2] = entry2 ;
   rule_neighborhoods[3] = entry3 ;

   // bit offset for suvival part of rule
   survival_offset = 9 ;

   // bit in letter bit mask indicating negative
   negative_bit = 13 ; 

   // maximum number of letters per neighbor count
   max_letters[0] = 0 ;
   max_letters[1] = (int) strlen(rule_letters[0]) ;
   max_letters[2] = (int) strlen(rule_letters[1]) ;
   max_letters[3] = (int) strlen(rule_letters[2]) ;
   max_letters[4] = (int) strlen(rule_letters[3]) ;
   max_letters[5] = max_letters[3] ;
   max_letters[6] = max_letters[2] ;
   max_letters[7] = max_letters[1] ;
   max_letters[8] = max_letters[0] ;
   for (i = 0 ; i < survival_offset ; i++) {
      max_letters[i + survival_offset] = max_letters[i] ;
   }

   // canonical letter order per neighbor count
   static int order0[1] = { 0 } ;
   static int order1[2] = { 0, 1 } ;
   static int order2[6] = { 2, 0, 1, 3, 4, 5 } ;
   static int order3[10] = { 2, 0, 1, 3, 6, 4, 5, 7, 8, 9 } ;
   static int order4[13] = { 2, 0, 1, 3, 6, 4, 5, 7, 8, 10, 11, 9, 12 } ;
   order_letters[0] = order0 ;
   order_letters[1] = order1 ;
   order_letters[2] = order2 ;
   order_letters[3] = order3 ;
   order_letters[4] = order4 ;
   order_letters[5] = order3 ;
   order_letters[6] = order2 ;
   order_letters[7] = order1 ;
   order_letters[8] = order0 ;
   for (i = 0 ; i < survival_offset ; i++) {
      order_letters[i + survival_offset] = order_letters[i] ;
   }

   // initialize
   initRule() ;
}

liferules::~liferules() {
}

// returns a count of the number of bits set in given int
static int bitcount(int v) {
   int r = 0 ;
   while (v) {
      r++ ;
      v &= v - 1 ;
   }
   return r ;
}

// initialize
void liferules::initRule() {
   // default to Moore neighbourhood totalistic rule
   neighbormask = MOORE ; 
   neighbors = 8 ;
   wolfram = -1 ;
   totalistic = true ;
   using_map = false ;

   // one bit for each neighbor count
   // s = survival, b = birth
   // bit:     17 16 15 14 13 12 11 10  9  8  7  6  5  4  3  2  1  0
   // meaning: s8 s7 s6 s5 s4 s3 s2 s1 s0 b8 b7 b6 b5 b4 b3 b2 b1 b0
   rulebits = 0 ;

   // one bit for each letter per neighbor count
   // N = negative bit
   // bit:     13 12 11 10  9  8  7  6  5  4  3  2  1  0
   // meaning:  N  z  w  t  y  r  q  j  n  k  i  a  e  c
   memset(letter_bits, 0, sizeof(letter_bits)) ;

   // two 4x4 rule maps (second used for B0-not-Smax rule emulation)
   memset(rule0, 0, sizeof(rule0)) ;
   memset(rule1, 0, sizeof(rule1)) ;

   // 3x3 rule map
   memset(rule3x3, 0, sizeof(rule3x3)) ;

   // canonical rule string
   memset(canonrule, 0, sizeof(canonrule)) ;
}

// set 3x3 grid based on totalistic value
void liferules::setTotalistic(int value, bool survival) {
   int mask = 0 ;
   int nbrs = 0 ;
   int nhood = 0 ;
   int i = 0 ;
   int j = 0 ;
   int offset = 0 ;

   // check if this value has already been processed
   if (survival) {
      offset = survival_offset ;
   }
   if ((rulebits & (1 << (value + offset))) == 0) {
       // update the rulebits
       rulebits |= 1 << (value + offset) ;

       // update the mask if survival
       if (survival) {
          mask = 0x10 ;
       }

       // fill the array based on totalistic value
       for (i = 0 ; i < ALL3X3 ; i += 32) {
          for (j = 0 ; j < 16 ; j++) {
             nbrs = 0 ;
             nhood = (i+j) & neighbormask ;
             while (nhood > 0) {
                nbrs += (nhood & 1) ;
                nhood >>= 1 ;
             }
             if (value == nbrs) {
                rule3x3[i+j+mask] = 1 ;
             }
          }
      }
   }
}

// flip bits
int liferules::flipBits(int x) {
   return ((x & 0x07) << 6) | ((x & 0x1c0) >> 6) | (x & 0x38) ;
}

// rotate 90
int liferules::rotateBits90Clockwise(int x) {
   return ((x & 0x4) << 6) | ((x & 0x20) << 2) | ((x & 0x100) >> 2)
                        | ((x & 0x2) << 4) | (x & 0x10) | ((x & 0x80) >> 4)
                        | ((x & 0x1) << 2) | ((x & 0x8) >> 2) | ((x & 0x40) >> 6) ;
}

// set symmetrical neighborhood into 3x3 map
void liferules::setSymmetrical512(int x, int b) {
   int y = x ;
   int i = 0 ;

   // process each of the 4 rotations
   for (i = 0 ; i < 4 ; i++) {
      rule3x3[y] = (char) b ;
      y = rotateBits90Clockwise(y) ;
   }

   // flip
   y = flipBits(y) ;

   // process each of the 4 rotations
   for (i = 0 ; i < 4 ; i++) {
      rule3x3[y] = (char) b ;
      y = rotateBits90Clockwise(y) ;
   }
}

// set symmetrical neighborhood
void liferules::setSymmetrical(int value, bool survival, int lindex, int normal) {
   int xorbit = 0 ;
   int nindex = value - 1 ;
   int x = 0 ;
   int offset = 0 ;

   // check for homogeneous bits
   if (value == 0 || value == 8) {
      setTotalistic(value, survival) ;
   }
   else {
      // update the rulebits
      if (survival) {
         offset = survival_offset ;
      }
      rulebits |= 1 << (value + offset) ;

      // reflect the index if in second half
      if (nindex > 3) {
         nindex = 6 - nindex ;
         xorbit = 0x1ef ;
      }

      // update the letterbits
      letter_bits[value + offset] |= 1 << lindex ;

      if (!normal) {
         // set the negative bit
         letter_bits[value + offset] |= 1 << negative_bit ;
      }

      // lookup the neighborhood
      x = rule_neighborhoods[nindex][lindex] ^ xorbit ;
      if (survival) {
         x |= 0x10 ;
      }
      setSymmetrical512(x, normal) ;
   }
}

// set totalistic birth or survival rule from a string
void liferules::setTotalisticRuleFromString(const char *rule, bool survival) {
   char current ;

   // process each character in the rule string
   while ( *rule ) {
      current = *rule ;
      rule++ ;

      // convert the digit to an integer
      current -= '0' ;

      // set totalistic
      setTotalistic(current, survival) ;
   }
}

// set rule from birth or survival string
void liferules::setRuleFromString(const char *rule, bool survival) {
   // current and next character
   char current ;
   char next ;

   // whether character normal or inverted
   int normal = 1 ;

   // letter index
   char *letterindex = 0 ;
   int lindex = 0 ;
   int nindex = 0 ;

   // process each character
   while ( *rule ) {
      current = *rule ;
      rule++ ;

      // find the index in the valid character list
      letterindex = strchr((char*) valid_rule_letters, current) ;
      lindex = letterindex ? int(letterindex - valid_rule_letters) : -1 ;

      // check if it is a digit
      if (lindex >= 0 && lindex <= 8) {
         // determine what follows the digit
         next = *rule ;
         nindex = -1 ;
         if (next) {
            letterindex = strchr((char*) rule_letters[3], next) ;
            if (letterindex) {
               nindex = int(letterindex - rule_letters[3]) ;
            }
         }

         // is the next character a digit or minus?
         if (nindex == -1) {
            setTotalistic(lindex, survival) ;
         }

         // check for inversion
         normal = 1 ;
         if (next == '-') {
            rule++ ;
            next = *rule ;

            // invert following character meanings
            normal = 0 ;
         }

         // process non-totalistic characters
         if (next) {
            letterindex = strchr((char*) rule_letters[3], next) ;
            nindex = -1 ;
            if (letterindex) {
               nindex = int(letterindex - rule_letters[3]) ;
            }
            while (nindex >= 0) {
               // set symmetrical
               setSymmetrical(lindex, survival, nindex, normal) ;

               // get next character
               rule++ ;
               next = *rule ;
               nindex = -1 ;
               if (next) {
                  letterindex = strchr((char*) rule_letters[3], next) ;
                  if (letterindex) {
                     nindex = int(letterindex - rule_letters[3]) ;
                  }
               }
            }
         }
      }
   }
}

// create the rule map from Wolfram number
void liferules::createWolframMap() {
   int i = 0 ;

   // clear the rule array
   memset(rule3x3, 0, ALL3X3) ;

   // set in the 3x3 map
   for (i = 0 ; i < ALL3X3 ; i++) {
      if ((wolfram & (1 << (i & 7))) || (i & 16)) {
         rule3x3[i] = 1 ;
      }
   }
}

// create the rule map from the base64 encoded map
void liferules::createRuleMapFromMAP(const char *base64) {
   // set the number of characters to read
   int power2 = 1 << (neighbors + 1) ;
   int fullchars = power2 / 6 ;
   int remainbits = power2 % 6 ;

   // create an array to read the MAP bits
   char bits[ALL3X3] ;

   // decode the base64 string
   int i = 0 ;
   char c = 0 ;
   int j = 0 ;
   const char *index = 0 ;

   for (i = 0 ; i < fullchars ; i++) {
      // convert character to base64 index
      index = strchr(base64_characters, *base64) ;
      base64++ ;
      c = index ? (char)(index - base64_characters) : 0 ;
 
      // decode the character
      bits[j] = c >> 5 ;
      j++ ;
      bits[j] = (c >> 4) & 1 ;
      j++ ;
      bits[j] = (c >> 3) & 1 ;
      j++ ;
      bits[j] = (c >> 2) & 1 ;
      j++ ;
      bits[j] = (c >> 1) & 1 ;
      j++ ;
      bits[j] = c & 1 ;
      j++ ;
   }

   // decode remaining bits from final character
   if (remainbits > 0) {
      index = strchr(base64_characters, *base64) ;
      c = index ? (char)(index - base64_characters) : 0 ;
      int b = 5 ;

      while (remainbits > 0) {
          bits[j] = (c >> b) & 1 ;
          b-- ;
          j++ ;
          remainbits-- ;
      }
   }
 
   // copy into rule array using the neighborhood mask
   int k, m ;
   for (i = 0 ; i < ALL3X3 ; i++) {
      k = 0 ;
      m = neighbors ;
      for (j = 8 ; j >= 0 ; j--) {
         if (neighbormask & (1 << j)) {
             if (i & (1 << j)) {
                k |= (1 << m) ;
             }
             m-- ;
         }
      }
      rule3x3[i] = bits[k] ;
   }
}
   
// create the rule map from birth and survival strings
void liferules::createRuleMap(const char *birth, const char *survival) {
   // clear the rule array
   memset(rule3x3, 0, ALL3X3) ;

   // check for totalistic rule
   if (totalistic) {
      // set the totalistic birth rule
      setTotalisticRuleFromString(birth, false) ;

      // set the totalistic surivival rule
      setTotalisticRuleFromString(survival, true) ;
   }
   else {
      // set the non-totalistic birth rule
      setRuleFromString(birth, false) ;

      // set the non-totalistic survival rule
      setRuleFromString(survival, true) ;
   }
}

// add canonical letter representation
int liferules::addLetters(int count, int p) {
   int bits ;            // bitmask of letters defined at this count
   int negative = 0 ;    // whether negative
   int setbits ;         // how many bits are defined
   int maxbits ;         // maximum number of letters at this count
   int letter = 0 ;
   int j ;

   // check if letters are defined for this neighbor count
   if (letter_bits[count]) {
      // get the bit mask
      bits = letter_bits[count] ;

      // check for negative
      if (bits & (1 << negative_bit)) {
         // letters are negative
         negative = 1 ;
         bits &= ~(1 << negative_bit) ;
      }

      // compute the number of bits set
      setbits = bitcount(bits) ;

      // get the maximum number of allowed letters at this neighbor count
      maxbits = max_letters[count] ;

      // do not invert if not negative and seven letters
      if (!(!negative && setbits == 7 && maxbits == 13)) {
         // if maximum letters minus number used is greater than number used then invert
         if (setbits + negative > (maxbits >> 1)) {
            // invert maximum letters for this count
            bits = ~bits & ((1 << maxbits) - 1) ;
            if (bits) {
               negative = !negative ;
            }
         }
      }

      // if negative and no letters then remove neighborhood count
      if (negative && !bits) {
         canonrule[p] = 0 ;
         p-- ;
      }
      else {
         // check whether to output minus
         if (negative) {
            canonrule[p++] = '-' ;
         }

         // add defined letters
         for (j = 0 ; j < maxbits ; j++) {
            // lookup the letter in order
            letter = order_letters[count][j] ;
            if (bits & (1 << letter)) {
               canonrule[p++] = rule_letters[3][letter] ;
            }
         }
      }
   }

   return p ;
}

// convert the 3x3 map to the 4x4 map
void liferules::convertTo4x4Map(char *which) {
   int i = 0 ;
   int v = 0 ;

   // create every possible cell combination for 4x4
   for (i = 0 ; i < ALL4X4 ; i ++) {
      // perform 4 lookups in the 3x3 map to create the 4x4 entry
      // 15 14 13  x       7  6  5
      // 11 10  9  x  ->  11 10  9  ->  10' x 0 0 x x
      //  7  6  5  x      15 14 13
      //  x  x  x  x
      v = rule3x3[((i & 57344) >> 13) | ((i & 3584) >> 6) | ((i & 224) << 1)] << 5 ;
     
      //  x 14 13 12       6  5  4
      //  x 10  9  8  ->  10  9  8  ->  x 9' 0 0 x x
      //  x  6  5  4      14 13 12
      //  x  x  x  x
      v |= rule3x3[((i & 28672) >> 12) | ((i & 1792) >> 5) | ((i & 112) << 2)] << 4 ;

      //  x  x  x  x
      // 11 10  9  x  ->   3  2  1  ->  x x 0 0 6' x
      //  7  6  5  x       7  6  5
      //  3  2  1  x      11 10  9
      v |= rule3x3[((i & 3584) >> 9) | ((i & 224) >> 2) | ((i & 14) << 5)] << 1 ;

      //  x  x  x  x
      //  x 10  9  8  ->   2  1  0  ->  x x 0 0 x 5' 
      //  x  6  5  4       6  5  4
      //  x  2  1  0      10  9  8
      v |= rule3x3[((i & 1792) >> 8) | ((i & 112) >> 1) | ((i & 7) << 6)] ;

      // save the entry
      which[i] = (char) v ;
   }
}

// save the rule (and handle B0)
void liferules::saveRule() {
   int i = 0 ;
   char tmp ;

   if (wolfram == -1) {
      // check for B0
      if (rule3x3[0]) {
         // check for Smax
         if (rule3x3[ALL3X3 - 1]) {
            // B0 with Smax: rule -> NOT(reverse(bits))
            for (i = 0 ; i < ALL3X3 / 2 ; i++) {
               tmp = rule3x3[i] ;
               rule3x3[i] = 1 - rule3x3[ALL3X3 - i - 1] ;
               rule3x3[ALL3X3 - i - 1] = 1 - tmp ;
            }
         }
         else {
            // B0 without Smax needs two rules: one for odd and one for even generations
            alternate_rules = true ;

            // odd rule -> reverse(bits)
            for (i = 0 ; i < ALL3X3 / 2 ; i++) {
               tmp = rule3x3[i] ;
               rule3x3[i] = rule3x3[ALL3X3 - i - 1] ;
               rule3x3[ALL3X3 - i - 1] = tmp ;
            }
            convertTo4x4Map(rule1) ;

            // even rule -> NOT(bits)
            for (i = 0 ; i < ALL3X3 / 2 ; i++) {
               tmp = rule3x3[i] ;
               // need to reverse then invert due to even rule above
               rule3x3[i] = 1 - rule3x3[ALL3X3 - i - 1] ;
               rule3x3[ALL3X3 - i - 1] = 1 - tmp ;
            }
         }
      }
   }

   // convert to 4x4 map
   convertTo4x4Map(rule0) ;
}

// remove character from a string in place
void liferules::removeChar(char *string, char skip) {
   int src = 0 ;
   int dst = 0 ;
   char c = string[src++] ;

   // copy characters other than skip
   while ( c ) {
      if (c != skip) {
         string[dst++] = c ;
      }
      c = string[src++] ;
   }

   // ensure null terminated
   string[dst] = 0 ;
}

// check whether non-totalistic letters are valid for defined neighbor counts
bool liferules::lettersValid(const char *part) {
   char c ;
   int nindex = 0 ;
   int currentCount = -1 ;

   // get next character
   while ( *part ) {
      c = *part ;
      if (c >= '0' && c <= '8') {
         currentCount = c - '0' ;
         nindex = currentCount - 1 ;
         if (nindex > 3) {
            nindex = 6 - nindex ;
         }
      }
      else {
         // ignore minus
         if (c != '-') {
            // not valid if 0 or 8
            if (currentCount == 0 || currentCount == 8) {
               return false ;
            }

            // check against valid rule letters for this neighbor count
            if (strchr((char*) rule_letters[nindex], c) == 0) {
               return false ;
            }
         }
      }
      part++ ;
   }

   return true ;
}

// set rule
const char *liferules::setrule(const char *rulestring, lifealgo *algo) {
   char *r = (char *)rulestring ;
   char tidystring[MAXRULESIZE] ;  // tidy version of rule string
   char *t = (char *)tidystring ;
   char *end = r + strlen(r) ;     // end of rule string
   char c ;
   char *charpos = 0 ;
   int digit ;
   int maxdigit = 0 ;              // maximum digit value found
   char *colonpos = 0 ;            // position of colon
   char *slashpos = 0 ;            // position of slash
   char *underscorepos = 0 ;       // position of underscore
   char *bpos = 0 ;                // position of b
   char *spos = 0 ;                // position of s

   // initialize rule type
   initRule() ;

   // we might need to emulate B0 rule by using two different rules for odd/even gens
   alternate_rules = false ;

   // check if rule is too long
   if (strlen(rulestring) > (size_t) MAXRULESIZE) {
      return "Rule name is too long." ;
   }

   // check for colon
   colonpos = strchr(r, ':') ;
   if (colonpos) {
      // only process up to the colon
      end = colonpos ;
   }

   // skip any whitespace
   while (*r == ' ') {
      r++ ;
   }

   // check for map
   if (strncasecmp(r, "map", 3) == 0) {
      // attempt to decode map
      r += 3 ;
      bpos = r ;

      // terminate at the colon if one is present
      if (colonpos) *colonpos = 0 ;

      // check the length of the map
      int maplen = (int) strlen(r) ;

      // replace the colon if one was present
      if (colonpos) *colonpos = ':' ;
      
      // check if there is base64 padding
      if (maplen > 2 && !strncmp(r + maplen - 2, "==", 2)) {
         // remove padding
         maplen -= 2 ;
      }

      // check if the map length is valid for Moore, Hexagonal or von Neumann neighborhoods
      if (!(maplen == MAP512LENGTH || maplen == MAP128LENGTH || maplen == MAP32LENGTH)) {
          return "MAP rule needs 6, 22 or 86 base64 characters." ;
      }

      // validate the characters
      spos = r + maplen ;
      while (r < spos) {
         if (!strchr(base64_characters, *r)) {
             return "MAP contains illegal base64 character." ;
         }
         r++ ;
      }

      // set the neighborhood based on the map length
      if (maplen == MAP128LENGTH) {
         neighbormask = HEXAGONAL ;
         neighbors = 6 ;
      } else {
         if (maplen == MAP32LENGTH) {
            neighbormask = VON_NEUMANN ;
            neighbors = 4 ;
         }
      }

      // map looks valid
      using_map = true ;
   }
   else {
      // create lower case version of rule name without spaces
      while (r < end) {
         // get the next character and convert to lowercase
         c = (char) tolower(*r) ;
   
         // process the character
         switch (c) {
         // birth
         case 'b':
            if (bpos) {
               // multiple b found
               return "Only one B allowed." ;
            }
            bpos = t ;
            *t = c ;
            t++ ;
            break ;
   
         // survival
         case 's':
            if (spos) {
               // multiple s found
               return "Only one S allowed." ;
            }
            spos = t ;
            *t = c ;
            t++ ;
            break ;
   
         // slash
         case '/':
            if (slashpos) {
               // multiple slashes found
               return "Only one slash allowed." ;
            }
            slashpos = t ;
            *t = c ;
            t++ ;
            break ;
           
         // underscore
         case '_':
            if (underscorepos) {
               // multiple underscores found
               return "Only one underscore allowed." ;
            }
            underscorepos = t ;
            *t = c ;
            t++ ;
            break ;
   
         // hex
         case 'h':
            if (neighbormask != MOORE || wolfram != -1) {
               // multiple neighborhoods specified
               return "Only one neighborhood allowed." ;
            }
            neighbormask = HEXAGONAL ;
            neighbors = 6 ;
            *t = c ;
            t++ ;
            break ;
   
         // von neumann
         case 'v':
            if (neighbormask != MOORE || wolfram != -1) {
               // multiple neighborhoods specified
               return "Only one neighborhood allowed." ;
            }
            neighbormask = VON_NEUMANN ;
            neighbors = 4 ;
            *t = c ;
            t++ ;
            break ;
   
         // wolfram
         case 'w':
            // check if at beginning of string
            if (t == tidystring) {
               if (neighbormask != MOORE || wolfram != -1) {
                  // multiple neighborhoods specified
                  return "Only one neighborhood allowed." ;
               }
               wolfram = 0 ;
            }
            else {
               // copy character
               *t = c ;
               t++ ;
               totalistic = false ;
            }
            break ;
   
         // minus
         case '-':
            // check if previous character is a digit
            if (t == tidystring || *(t-1) < '0' || *(t-1) > '8') {
               // minus can only follow a digit
               return "Minus can only follow a digit." ;
            }
            *t = c ;
            t++ ;
            totalistic = false ;
            break ;
   
         // other characters
         default:
            // ignore space
            if (c != ' ') {
               // check character is valid
               charpos = strchr((char*) valid_rule_letters, c) ;
               if (charpos) {
                  // copy character
                  *t = c ; 
                  t++ ;
   
                  // check if totalistic (i.e. found a valid non-digit)
                  digit = int(charpos - valid_rule_letters) ;
                  if (digit > 8) {
                     totalistic = false ;
                  }
                  else {
                     // update maximum digit found
                     if (digit > maxdigit) {
                        maxdigit = digit ;
                     }
                  }
               }
               else if (wolfram == 0 && c == '9') {
                  *t = c ; 
                  t++ ;
               }
               else {
                  return "Bad character found.";
               }
            }
            break ;
         }
   
         // next character
         r++ ;
      }
   
      // ensure null terminated
      *t = 0 ;
   
      // don't allow empty rule string
      t = tidystring ;
      if (*t == 0) {
         return "Rule cannot be empty string." ;
      }
   
      // can't have slash and underscore
      if (underscorepos && slashpos) {
         return "Can't have slash and underscore." ;
      }
   
      // underscore only valid for non-totalistic rules
      if (underscorepos && totalistic) {
         return "Underscore not valid for totalistic rules, use slash." ;
      }
   
      // if underscore defined then set the slash position
      if (underscorepos) {
         slashpos = underscorepos ;
      }
   
      // check for Wolfram
      if (wolfram == 0) {
         // parse Wolfram 1D rule
         while (*t >= '0' && *t <= '9') {
            wolfram = 10 * wolfram + *t - '0' ;
            t++ ;
         }
         if (wolfram < 0 || wolfram > 254 || wolfram & 1) {
            return "Wolfram rule must be an even number from 0 to 254." ;
         }
         if (*t) {
            return "Bad character in Wolfram rule." ;
         }
      }
      else {
         // if neighborhood specified then must be last character
         if (neighbormask != MOORE) {
            size_t len = strlen(t) ;
            if (len) {
               c = t[len - 1] ;
               if (!((c == 'h') || (c == 'v'))) {
                  return "Neighborhood must be at end of rule." ;
               }
               // remove character
               t[len - 1] = 0 ;
            }
         }
   
         // at least one of slash, b or s must be present
         if (!(slashpos || bpos || spos)) {
            return "Rule must contain a slash or B or S." ;
         }
      
         // digits can not be greater than the number of neighbors for the defined neighborhood
         if (maxdigit > neighbors) {
            return "Digit greater than neighborhood allows." ;
         }
   
         // if slash present and both b and s then one must be each side of the slash
         if (slashpos && bpos && spos) {
            if ((bpos < slashpos && spos < slashpos) || (bpos > slashpos && spos > slashpos)) {
               return "B and S must be either side of slash." ;
            }
         }
   
         // check if there was a slash to divide birth from survival
         if (!slashpos) {
            // check if both b and s exist
            if (bpos && spos) {
               // determine whether b or s is first
               if (bpos < spos) {
                  // skip b and cut the string using s
                  bpos++ ;
                  *spos = 0 ;
                  spos++ ;
               }
               else {
                  // skip s and cut the string using b
                  spos++ ;
                  *bpos = 0 ;
                  bpos++ ;
               }
            }
            else {
               // just bpos
               if (bpos) {
                  bpos = t ;
                  removeChar(bpos, 'b') ;
                  spos = bpos + strlen(bpos) ;
               }
               else {
                  // just spos
                  spos = t ;
                  removeChar(spos, 's') ;
                  bpos = spos + strlen(spos) ;
               }
            }
         }
         else {
            // slash exists so set determine which part is b and which is s
            *slashpos = 0 ;
   
            // check if b or s are defined
            if (bpos || spos) {
               // check for birth first
               if ((bpos && bpos < slashpos) || (spos && spos > slashpos)) {
                  // birth then survival
                  bpos = t ;
                  spos = slashpos + 1 ;
               }
               else {
                  // survival then birth
                  bpos = slashpos + 1 ;
                  spos = t ;
               }
   
               // remove b or s from rule parts
               removeChar(bpos, 'b') ;
               removeChar(spos, 's') ;
            }
            else {
               // no b or s so survival first
               spos = t ;
               bpos = slashpos + 1 ;
            }
         }
   
         // if not totalistic and a part exists it must start with a digit
         if (!totalistic) {
            // check birth
            c = *bpos ;
            if (c && (c < '0' || c > '8')) {
               return "Non-totalistic birth must start with a digit." ;
            }
            // check survival
            c = *spos ;
            if (c && (c < '0' || c > '8')) {
               return "Non-totalistic survival must start with a digit." ;
            }
         }
      
         // if not totalistic then neighborhood must be Moore
         if (!totalistic && neighbormask != MOORE) {
            return "Non-totalistic only supported with Moore neighborhood." ;
         }
   
         // validate letters used against each specified neighbor count
         if (!lettersValid(bpos)) {
            return "Letter not valid for birth neighbor count." ;
         }
         if (!lettersValid(spos)) {
            return "Letter not valid for survival neighbor count." ;
         }
      }
   }

   // create the rule map
   if (wolfram >= 0) {
      // create the 3x3 map from the wolfram code
      createWolframMap() ;
   }
   else {
      // check for map
      if (using_map) {
         // generate the 3x3 map from the supplied MAP string
         createRuleMapFromMAP(bpos) ;
      }
      else {
         // generate the 3x3 map from the birth and survival rules
         createRuleMap(bpos, spos) ;
      }
   }

   // save the rule
   saveRule() ;

   // exit with success
   return 0 ;
}

const char* liferules::getrule() {
   return canonrule ;
}

// B3/S23 -> (1 << 3) + (1 << (9 + 2)) + (1 << (9 + 3)) = 0x1808
bool liferules::isRegularLife() {
   return (neighbormask == MOORE && totalistic && rulebits == 0x1808 && wolfram < 0) ;
}