Large integer multiplication -- dramatically improved

[Geoff Canyon]

Somewhere along the way I lost the "if either argument is 0, return 0"
statement at the beginning. Correction below:

function bTimes X,Y
   if X = 0 or Y = 0 then return 0
   if char 1 of X is "-" then
      put "-" into leadChar
      delete char 1 of X
   end if
   if char 1 of Y is "-"  then
      if leadChar is "-" then put empty into leadChar else put "-" into
leadChar
      delete char 1 of Y
   end if
   put X & Y into R
   put "0000000000000000" into char 1 to 10 of R
   put (6 + length(X)) div 7 * 7 into XL
   put char 1 to XL - length(X) of "000000" before X
   put (6 + length(Y)) div 7 * 7 into YL
   put char 1 to YL - length(Y) of "000000" before Y
   put length(R) - 6 into rStart
   repeat with N = XL + YL down to 15 step -7
      put min(XL,N - 7) into finalM
      put 0 into C
      repeat with startM = max(7,N - YL) to finalM - 7 step 630
         repeat with M = startM to min(startM + 623,finalM) step 7
            add (char M - 6 to M of X) * (char N - M - 6 to N - M of Y) to S
         end repeat
         add char 1 to -8 of S to C
         delete char 1 to -8 of S
      end repeat
      put char -7 to -1 of ("000000" & S) into char rStart to rStart + 6 of
R
      subtract 7 from rStart
      put C into S
   end repeat
   if S > 0 then put S into char max(1,rStart) to rStart + 6 of R
   put 0 into zR
   repeat until zR > 0
      put 0 + char 1 to 15 of R into zR
      put zR into char 1 to 15 of R
   end repeat
   return leadChar & R
end bTimes