September 2017 Update

Task/Long-multiplication/Batch-File/long-multiplication.bat

@@ -1,37 +1,58 @@
+::Long Multiplication Task from Rosetta Code
+::Batch File Implementation
+
 @echo off
-setlocal enabledelayedexpansion
-
-set num1=18446744073709551616
-set num2=18446744073709551616
-
-set limit_a=-1&set limit_b=-1&set length=0
-for %%A in (1,2) do for /l %%B in (0,1,9) do set num%%A=!num%%A:%%B=%%B !
-for %%. in (!num1!) do set/a limit_a+=1&set a1=%%.!a1!
-for %%. in (!num2!) do set/a limit_b+=1&set a2=%%.!a2!
-
-for /l %%a in (0,1,!limit_a!) do (
-for /l %%b in (0,1,!limit_b!) do (
-	set/a pos=%%a+%%b
-	set/a next=!pos!+1
-	set/a temp0=result!pos!
-	set/a result!pos!=!a1:~%%a,1!*!a2:~%%b,1!
-	if !temp0! equ 0 set/a length+=1
-	if !pos! lss !length! set/a result!pos!+=!temp0!
-
-	set/a temp0=result!pos!
-	set/a temp1=result!next!
-
-	if !temp0! gtr 9 (
-		set/a result!next!=!temp0!/10
-		set temp2=!length!
-		if !temp1! equ 0 set/a length+=1
-		if !next! lss !temp2! set/a result!next!+=!temp1!
-		set/a result!pos!=!temp0!%%10
-	)
-)
-)
-for /l %%. in (0,1,!length!) do set product=!result%%.!!product!
-echo.!product!
-echo.
-pause>nul
+call :longmul 18446744073709551616 18446744073709551616 answer
+echo(%answer%
 exit /b 0
+
+rem The Hellish Procedure
+rem Syntax: call :longmul <n1> <n2> <variable to store product>
+:longmul
+    setlocal enabledelayedexpansion
+
+    rem Define variables
+    set "num1=%1"
+    set "num2=%2"
+    set "limit1=-1"
+    set "limit2=-1"
+    set "length=0"
+    set "prod="
+
+    rem Reverse the digits of each factor
+    for %%A in (1,2) do (
+        for /l %%B in (0,1,9) do set "num%%A=!num%%A:%%B=%%B !"
+        for %%C in (!num%%A!) do ( set /a limit%%A+=1 & set "rev%%A=%%C!rev%%A!" )
+    )
+
+    rem Do the multiplication
+    for /l %%A in (0,1,%limit1%) do (
+        for /l %%B in (0,1,%limit2%) do (
+            set /a iter=%%A+%%B
+            set /a iternext=iter+1
+            set /a iternext2=iter+2
+
+            set /a prev=digit!iter!
+            set /a digit!iter!=!rev1:~%%A,1!*!rev2:~%%B,1!
+
+            rem The next line updates the length of "digits"
+            if !iternext! gtr !length! set length=!iternext!
+            if !iter! lss !length! set /a digit!iter!+=prev
+
+            set /a currdigit=digit!iter!
+            if !currDigit! gtr 9 (
+                set /a prev=digit!iternext!
+                set /a digit!iternext!=currdigit/10
+                set /a digit!iter!=currdigit%%10
+
+                rem The next line updates the length of "digits"
+                if !iternext2! gtr !length! set length=!iternext2!
+                if !iternext! lss !length! set /a digit!iternext!+=prev
+            )
+        )
+    )
+
+    rem Finalize product reversing the digits
+    for /l %%F in (0,1,%length%) do set "prod=!digit%%F!!prod!"
+    endlocal & set "%3=%prod%"
+goto :eof