batch file calling to function fail - windows

Question

been trying to figure how to make the random string generation as a function, then call to function to return new random for each for loop.
but unable to make it work...
blank of idea, batch programming seems lot more complicated than web..
#echo off
GOTO :MAIN
:TestFunc
set orig=%1
set %~2=%random%
goto :eof
:MAIN
for %%a in (C:\folder\*.png) do (
set /a count+=1
set "fname=%%~a"
setlocal enabledelayedexpansion
set param_to_function=LetItBeA
call :TestFunc %param_to_function% return_value
set random=%return_value%
echo !random!
echo !fname!
ren "!fname!" img_%date:~10,4%-%date:~4,2%-%date:~7,2%_%HR%%time:~3,2%-!random!.png
endlocal
)
goto :eof

Answer 1

Original question answered (before edit). Explanation:
Use for /F against dir /b as unlike for /F, for starts parsing files immediately so it could get a renamed file again and again...
File renaming treated in a subroutine with test on file existence before ren.
Note endlocal&set "%1=%_RndAlphaNum%"&goto :eof tricky part how-to return a value to a variable (parameter) passed by reference.
ren command echoed merely for debugging purposes.
Resources (required reading):
An A-Z Index of the Windows CMD command line
Windows CMD Shell Command Line Syntax
The script:
#ECHO OFF
SETLOCAL enableextensions disabledelayedexpansion
Set /A "_RNDLength=6"
Set "_Alphanumeric=ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789"
set "_folder=C:\folder"
for /F "delims=" %%a in ('dir /B "%_folder%\*.png"') do (
set /a count+=1
set "fname=%%~nxa"
call :renameFile
)
ENDLOCAL
goto :eof
:renameFile
call :getRandomString _RndANum
set "newFileName=img_%date:~10,4%-%date:~4,2%-%date:~7,2%_%HR%%time:~3,2%-%_RndANum%.png"
if exist "%_folder%\%newFileName%" goto :renameFile
echo ren "%_folder%\%fname%" "%newFileName%"
goto :eof
:getRandomString
rem usage
rem call :getRandomString varname
rem %1 = a variable name (pass by reference)
Setlocal EnableDelayedExpansion
Set "_Str=%_Alphanumeric%987654321"
:_LenLoop
IF NOT "%_Str:~18%"=="" (
SET _Str=%_Str:~9%
SET /A "_Len+=9"
GOTO :_LenLoop
)
SET _tmp=%_Str:~9,1%
SET /A _Len=_Len+_tmp
Set _count=0
SET _RndAlphaNum=
:_loop
Set /a _count+=1
SET _RND=%Random%
Set /A _RND=_RND%%%_Len%
SET _RndAlphaNum=!_RndAlphaNum!!_Alphanumeric:~%_RND%,1!
If !_count! lss %_RNDLength% goto _loop
rem do not split next line
endlocal&set "%1=%_RndAlphaNum%"&goto :eof