Add Accumulators

[redwizard42]

Drawing from comments in RetroAchievements/rcheevos#339 ...

Building more complex math and being able to reuse these values would be of a benefit. This could be done by using something like this, drawn from Jamiras's example in the linked PR:

Get Offset to boss pointer in Object Table of Metroid PRime:

SetAccumulator Mem      16-Bit BE BossUID     & Value 0x3FF // accumulator = (BossUID & 0x3ff)
SetAccumulator $accumulator                   * Value 0x08  // accumulator = (BossUID & 0x3FF) * 8
SetAccumulator $accumulator                   + Value 0x04  // accumulator = (BossUID & 0x3FF) * 8 + 4

And extending this chain to calculate the direct address of the boss pointer and use it to get other data:

SetAccumulator $accumulator + Mem      32-Bit BE  ObjTablePtr // accumulator = ObjTablePtr + (BossUID & 0x3FF) * 8 + 4

Add Address  $accumulator
SetAccumulator Mem      16-Bit BE  OfstToM1UID & Value 0x3FF // accumulator = (MirrorUID & 0x3FF)
SetAccumulator $accumulator                    * Value 0x08  // accumulator = (MirrorUID & 0x3FF) * 8
SetAccumulator $accumulator                    + Value 0x04  // accumulator = (MirrorUID & 0x3FF) * 8 + 4
SetAccumulator $accumulator + Mem     32-Bit BE  ObjTablePtr // accumulator = ObjTablePtr + (MirrorUID & 0x3FF) * 8 + 4

Add Address $accumulator
            Mem 16-BitBE OfstToData = Value Target // compare 16-bit value at ($accumulator + OfstToData) to Target

This can be further chained to calculate other pointer locations from that pointer.

[redwizard42]

Other considerations:

1. Is an accumulator in-scope for an entire trigger/value? Per group?
2. How should they interact with pause?
   • Done before pauses in their own pass?
   • Intermixed, such that a pause will prevent later SetAccumulator conditions from being processed?
   • Done after pause/as part of the non-pause processing?
3. How many accumulators should there be?
   • Fixed amount?
   • Definable?
4. Prefix for accumulator/variables? $ is probably bad due to it being already used in values when there are multiple.

[redwizard42]

Today I realized that if Set Accumulator is combining (like add source and sub source), it'll work in chains properly and I wouldn't need multiple for my use case.

Not sure that behavior would be obvious, but it can work.