mvbg

VM_INIT_RNG

VM_INIT_RNG is a GBVM instruction used to initialize the seed of the Game Boy’s pseudo-random number generator (PRNG) with a specific value from a variable.

Purpose: Randomness is crucial for many game elements, such as enemy behavior, item drops, critical hits, or procedural generation. However, computers are deterministic, meaning they follow instructions precisely. To simulate randomness, they use PRNGs, which generate sequences of numbers that appear random but are actually determined by an initial “seed” value.

VM_INIT_RNG allows you to:

• Control randomness: By setting a specific seed, you can reproduce the exact same sequence of “random” numbers. This is invaluable for debugging, testing, and ensuring consistent gameplay experiences.
• Create unique game sessions: By seeding the PRNG with a truly random or time-based value (e.g., from the Real-Time Clock if available), you can ensure that each new game playthrough feels fresh and unpredictable.

Syntax:

VM_INIT_RNG IDX

• IDX: The variable whose value will be used as the seed for the random number generator.

Usage Example: Seeding RNG for Reproducible Level Generation

Imagine you have a game with procedurally generated levels. For testing purposes, you want to be able to reproduce a specific level layout that you found interesting. You can store a seed value in a variable and use VM_INIT_RNG to ensure the same level is generated every time.

; In your game initialization script:

; Variable to hold the RNG seed
VAR_RNG_SEED:
  .R_INT16 0 ; Initialize to 0

; For testing, set a specific seed value
VM_SET_INT16 VAR_RNG_SEED, 12345 ; A fixed seed for reproducible results

; Initialize the RNG with the chosen seed
VM_INIT_RNG VAR_RNG_SEED

; Now, any subsequent calls to VM_RAND will produce a predictable sequence
VM_CALL GENERATE_LEVEL_LAYOUT ; This routine will use VM_RAND

; --- Later, for a new game, you might use a dynamic seed ---
; VM_GET_RTC_TIME VAR_RNG_SEED ; Get current time for a more random seed
; VM_INIT_RNG VAR_RNG_SEED

; --- Example of a routine that uses random numbers ---
GENERATE_LEVEL_LAYOUT:
  ; Generate a random number for room type
  VM_RAND VAR_ROOM_TYPE, 0, 3 ; Generate a number between 0 and 3
  ; ... use VAR_ROOM_TYPE to determine room layout ...

  ; Generate a random number for enemy count
  VM_RAND VAR_ENEMY_COUNT, 1, 5 ; Generate a number between 1 and 5
  ; ... spawn enemies ...

  VM_RET

In this example, by setting VAR_RNG_SEED to 12345 and then calling VM_INIT_RNG, the sequence of numbers generated by VM_RAND within GENERATE_LEVEL_LAYOUT will always be the same. This is incredibly useful for debugging and ensuring that specific game scenarios can be re-tested reliably.

Analogy to other programming languages: This is analogous to calling srand() in C/C++ or random.seed() in Python. You provide an initial value to the random number generator, which then determines the sequence of numbers it produces. If you provide the same seed, you get the same sequence.