Optimize multiplies and divides in factor.

Several of the multiplies and divides are now implemented via shifts and additions, which should be significantly faster. (These also take advantage of the limited range of operands to use only 16-bit operations where possible.)

time.asm

@@ -257,7 +257,7 @@ factor   private
          using TimeCommon
          
 ;
-;  sign-extend time components to 4 bytes
+;  sign-extend time components (except month) to 4 bytes
 ;
 
          stz  second+2
@@ -269,8 +269,7 @@ lb0      stz  year+2
          lda  year
          bpl  lb0a
          dec  year+2
-lb0a     stz  month+2
-         stz  day+2
+lb0a     stz  day+2
          lda  day
          bpl  lb0b
          dec  day+2
@@ -303,7 +302,16 @@ lb0f     sec
 ;
 lb0x     mul4  year,#365,count          count := 365*year + day + 31*month
          add4  count,day
-         mul4  month,#31,t1
+         lda   month
+         asl   a
+         asl   a
+         asl   a
+         asl   a
+         asl   a
+         sec
+         sbc   month
+         sta   t1
+         stz   t1+2
          add4  count,t1
          add4  year,#32800,t2           t2 := year + 32800 (so it is positive)
          lda   month                    if January or February then
@@ -311,17 +319,42 @@ lb0x     mul4  year,#365,count          count := 365*year + day + 31*month
          bge   lb1
          dec4  t2                         year := year-1
          bra   lb2                      else
-lb1      mul4  month,#4,t1                count := count - (month*4+27) div 10
-         add4  t1,#27
-         div4  t1,#10
+lb1      asl   a                          count := count - (month*4+27) div 10
+         asl   a
+         adc   #27+1                      ( x div 10 ~= (x+1)*$33 div 512 )
+         sta   t1
+         asl   a
+         adc   t1
+         asl   a
+         asl   a
+         asl   a
+         adc   t1
+         asl   a
+         adc   t1
+         lsr   a
+         xba
+         and   #$00ff
+         sta   t1
+         stz   t1+2
          sub4  count,t1
-lb2      div4  t2,#4,t1                 count := count + (year+32800) div 4
+lb2      lda   t2+2                     count := count + (year+32800) div 4
+         lsr   a
+         lda   t2
+         ror   a
+         lsr   a
+         sta   t1
+         stz   t1+2
          add4  count,t1
          add4  t2,#300                  count := count -
          div4  t2,#100                    ((300+year+32800) div 100+1)*3 div 4
-         inc4  t2
-         mul4  t2,#3
-         div4  t2,#4
+         inc   t2
+         lda   t2
+         asl   a
+         adc   t2
+         lsr   a
+         lsr   a
+         sta   t2
+         stz   t2+2
          sub4  count,t2
          sub4  count,#25518-2+7954      subtract off days between 1 Jan 1900
 !                                        and 13 Nov 1969, minus 2 to adjust for