REXX the Halls

Festive Coding with Mainframe Languages
Holiday spirit & Mainframe Coding fun 👨🏻‍💻
Henri Kuiper, CTO @ Mainframe Society
Part of IBM ZDay 2025

Who Am I?

CTO and co-founder at Mainframe Society
Security Architect at Dutch Tax Administration
Founder of zdevops
7 Year IBM Champion
Mainframe Nerd

What's Advent of Code?

A global holiday coding tradition made by Eric Wastl

25 days of logic puzzles released every December
Solve using any programming language
Developers worldwide compete, learn, and share
Fun, festive, and surprisingly addictive!
All info: adventofcode.com

First ever challenge
2015 day one part one

Santa is trying to deliver presents in a large
apartment building. He starts on the ground
floor (0) and then follows the instructions one
character at a time.

( ==> go up one floor
) ==> go down one floor

The apartment building is very tall,
Santa will never find the top or bottom floors.

For example:  ((( results in floor 3

To what floor do the instructions take Santa?

Python Solution

instruction_set = "((((()(..."
floor = 0
for instruction in instruction_set:
    if instruction == "(":
        floor = floor +1
    elif instruction == ")":
        floor = floor -1
print("Santa is at floor: {}".format(floor))

The Mainframe Twist

What if we solved them… on z/OS?

Introducing Advent of Code for Mainframers (MFAOC)
Solve the challenges in z/OS languages like REXX, COBOL, PL/I, or JCL
Learn a new language, or polish up your skills
Friendly competition, fame and swag
This will be our 6th year

REXX Solution

/* REXX */
INST = "((((()(..."
floor = 0
do i = 1 to length(INST)
  inst = substr(INST,i,1)
  if inst = "(" then floor = floor + 1
  if inst = ")" then floor = floor - 1
end
say "Santa is at floor" floor

PL/1 Solution

SANTAFLOOR: PROC OPTIONS(MAIN);
  DCL instructions CHAR(100) INIT('((((()(...');
  DCL i FIXED BIN(31);
  DCL floor FIXED BIN(31) INIT(0);
  DCL c CHAR(1);
  DO i = 1 TO LENGTH(instructions);
      c = SUBSTR(instructions, i, 1);
      IF c = '(' THEN
        floor = floor + 1;
      ELSE IF c = ')' THEN
        floor = floor - 1;
  END;
  PUT SKIP LIST('Santa is at floor:', floor);
 END SANTAFLOOR;

COBOL Solution

IDENTIFICATION DIVISION.
PROGRAM-ID. SANTAFLOOR.

DATA DIVISION.
WORKING-STORAGE SECTION.
01 INSTRUCTION-SET      PIC X(100) VALUE "((((()(...".
01 I                    PIC 9(3) VALUE 1.
01 LEN                  PIC 9(3).
01 CHAR                 PIC X.
01 FLOOR                PIC S9(5) VALUE 0.

PROCEDURE DIVISION.
    COMPUTE LEN = FUNCTION LENGTH(INSTRUCTION-SET)

    PERFORM VARYING I FROM 1 BY 1 UNTIL I > LEN
        MOVE INSTRUCTION-SET(I:1) TO CHAR
        EVALUATE CHAR
            WHEN "("
                ADD 1 TO FLOOR
            WHEN ")"
                SUBTRACT 1 FROM FLOOR
        END-EVALUATE
    END-PERFORM

    DISPLAY "Santa is at floor: " FLOOR
    STOP RUN.

HLASM Solution

SANTAFLR CSECT
    USING  SANTAFLR,R15         Establish base register
    STM    R14,R12,12(R13)      Save caller registers
    LR     R12,R15              Set up base
    LA     R11,SAVEAREA         Get save area
    ST     R13,4(R11)           Chain back
    ST     R11,8(R13)
    LR     R13,R11

***********************************************************************
*  WORKING STORAGE
***********************************************************************
INSTRSET DC    C'((((()(...'         Instruction string
INSTRLEN DC    F'11'                 Length of the string
FLOOR    DC    F'0'                  Current floor counter
I        DC    F'1'                  Loop index
CHAR     DS    CL1                   Current character
MSG      DC    C'Santa is at floor: ' 

***********************************************************************
*  MAIN LOGIC
***********************************************************************
MAINLOOP DS    0H
    L      R1,I                 Load loop index
    C      R1,INSTRLEN          Compare to length
    BH     DONE                 If higher, we’re done

    LA     R2,INSTRSET          Load address of string
    AR     R2,R1                Add index
    SR     R2,=F'1'             Adjust for 0-based
    MVC    CHAR(1),0(R2)        Get character

    CLI    CHAR,'('
    BE     UP
    CLI    CHAR,')'
    BE     DW
    B      NX

UP  L      R3,FLOOR
    LA     R3,1(R3)
    ST     R3,FLOOR
    B      NEXT

DW  L      R3,FLOOR
    LA     R3,-1(R3)
    ST     R3,FLOOR

NX  LA     R1,1(R1)
    ST     R1,I
    B      MAINLOOP

***********************************************************************
*  PRINT RESULT
***********************************************************************
DONE     WTO    'Santa is at floor:'         Print message
         MVC    BUF(4),FLOOR                 Move floor value to buffer


***********************************************************************
*  CLEANUP AND RETURN
***********************************************************************
    L      R13,4(R13)
    LM     R14,R12,12(R13)
    BR     R14

SAVEAREA DS    18F
BUF      DS    CL10
         END   SANTAFLR