Kejardon: I am failing all the maths

2012-11-19T20:39:16Z

I am failing all the maths

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Semi-commented drop-chance code.

Summary of it: Drop chance is a ratio of probabilities for each type of object:

0 = Small energy, 1 = Large, 2 = missiles, 3 = nothing, 4 = super, 5 = power bomb
Example:
0x10, 0x20, 0x08, 0x20, 0x08, 0x08
Total drop chance is 0x68. 2/13 chance of small energy, 4/13 large energy, 1/13 missiles, 4/13 nothing, 1/13 super missiles, 1/13 power bombs.
Note: Default Super Metroid drop chances all(?) add up to 0xFF if I remember right.
Furthermore, these drop chances are lumped into two groups: 0-3 and 4-5. The value of these groups should add up to less than 256 or overflow errors will happen during item drop chance calculations.

When a category is full (current == max, includes not having collected it yet), its probability is set to 0 and the other probabilities (in group one only, options 0-3) are multiplied to make up the difference. Group two, supers and power bombs, do NOT get their drop chance multiplied in this way - overflow probability defaults to dropping nothing.
Example (using the drop chances from last):
A person has full energy and full super missiles. Group 1 drops are multiplied by 13/6 to account for this. Final drop chances:
1/6 missiles, 4/6 nothing, 1/13 power bombs.
Total drop chance is ~91%. There is a ~9% chance of the drop defaulting to nothing (in addition to the given 4/6 drop chance).

Note: The final chances are actually calculated as x/255 chance. The values are multiplied up to these odds and then rounded down to be (integer)/255 chance. This can cause particularly high rolls from the RNG to drop nothing even if nothing is not a drop option.

<pre>
$37106-$3725D
{
PHX
PHY
PHB
PEA #$A000
PLB
PLB
LDA $7EF410, X
AND #$7FFF
STA $0E2A
LDA $7EF3C8, X
TAX
STA $0E28
LDA $A0003A, X ; Get drop selection
TAX

BNE BRANCH_ALPHA

JMP $F24D ; Drop Nothing if no drop selection is given.

BRANCH_ALPHA

JSL $808111 ; Produce a random number
AND #$00FF

BEQ BRANCH_ALPHA ; If number is 0, get a different number

STA $1A ;I'll call this 'drop chance quota'. It'll make sense later.
LDA #$00FF
STA $14
STZ $18
LDY #$0001 ; Default for $0E1A
LDA $09C2 ; Current Health
CLC
ADC $09D6 ; Reserve Tank health
CMP #$001E ; 30

BCC BRANCH_BETA

CMP #$0032 ; 50

BCC BRANCH_GAMMA

DEY

BRANCH_BETA

STY $0E1A ; Basically, if total health is <30, set 0E1A. If >49, clear 0E1A. Else leave it alone.

BRANCH_GAMMA

SEP #$20
LDA $0E1A

BEQ BRANCH_DELTA ; If cleared, go to normal drops

; This is for energy only drops.
LDA $B40000, X
CLC
ADC $B40001, X
STA $12 ;Set $12 to small and large enemy maximum
LDA #$03
STA $16 ;Set small and large energy drop opportunities
JMP $F1DF ; Go to choose drop code

BRANCH_DELTA

LDA $B40003, X
STA $12 ;Set $12 to 'nothing' drop chance
LDA #$08
STA $16 ;Set nothing drop chance
LDY $09C2
CPY $09C4 ;Check health and max

BNE BRANCH_EPSILON

LDY $09D6
CPY $09D4 ;Also reserves and max

BEQ BRANCH_ZETA

BRANCH_EPSILON ;Enable energy drop

LDA $12
CLC
ADC $B40000, X ;Small drop
ADC $B40001, X ;Large drop
STA $12 ;Added chance for small and large energy
LDA $16
ORA #$03
STA $16 ;Set small and large energy drop opportunities

BRANCH_ZETA

LDY $09C6
CPY $09C8 ;Check missiles and max

BEQ BRANCH_THETA

LDA $12
CLC
ADC $B40002, X
STA $12 ;Add missile drop chance
LDA $16
ORA #$04
STA $16 ;Add missile drop opportunity

BRANCH_THETA

LDY $09CA
CPY $09CC ;Check supers and max

BEQ BRANCH_IOTA

LDA $14
SEC
SBC $B40004, X
STA $14 ;'Add' supers drop chance. Odd code, I know.
LDA $16
ORA #$10
STA $16 ;Add super drop opportunity.

BRANCH_IOTA

LDY $09CE
CPY $09D0 ;Check power bombs and max

BEQ BRANCH_KAPPA

LDA $14
SEC
SBC $B40005, X
STA $14 ;'Add' power bomb drop chance
LDA $16
ORA #$20
STA $16 ;Add power bomb drop opportunity

$371DF ALTERNATE ENTRY POINT
BRANCH_KAPPA ;Choose item drop

LDY #$0000

BRANCH_XI

SEP #$20
LDA $12

BEQ BRANCH_LAMBDA ;If no chance to drop small energy, large energy, missiles, or nothing, go here.

LSR $16 ;Check the next drop opportunity bit

BCC BRANCH_MU ;If clear, skip that item.

LDA $14 ;Acumulative 'negative' drop chance of supers and power bombs. This acts basically as a X/256 multiplier for now
STA $4202
LDA $B40000, X ;Drop chance
STA $4203
PHY
NOP
NOP
LDY $4216 ;Multiplier * drop chance
STY $4204
LDA $12 ;Cumulative drop chance of first four items (including no drop)
STA $4206
PLY
NOP
REP #$20
LDA $18 ; Acumulated drop chance
CLC
ADC $4214 ;This is basically current drop chance * (acumulative first 4 drop chance / acumulative super and power drop chance)
CMP $1A ; Compared to the drop chance quota.

BCS BRANCH_NU ;When the drop chance quota is met or exceeded, go here to set the drop

STA $18 ;Else update the cumulative drop chance

BRANCH_MU

INX ;Go to next drop chance
INY ;Go to next drop type
CPY #$0004

BNE BRANCH_XI ;Loop back to choose item loop, unless

BRA BRANCH_OMICRON ;The first four items haven't triggered the drop, go on to supers and power bombs.

BRANCH_LAMBDA

LDA $16
LSR A
LSR A
LSR A
LSR A
STA $16 ;Skip first 4 drop opportunity bits
INX
INX
INX
INX ;Skip 4 drop chances
LDY #$0004 ;Go straight to drop type 4 (supers)

BRANCH_OMICRON ;Super and power bomb drop choice loop

SEP #$20
LSR $16 ;Check the next drop opportunity bit

BCC BRANCH_PI ;If clear, skip that item.

REP #$20
LDA $B40000, X ;Get drop chance
AND #$00FF
CLC
ADC $18 ;Add to accumulated drop chance
CMP $1A ;Compare to drop quota

BCS BRANCH_NU

STA $18 ;Update drop chance

BRANCH_PI

INX ;Go to next drop chance
INY ;Go to next drop type
CPY #$0006

BNE BRANCH_OMICRON ;Loop back

$3724D ALTERNATE ENTRY POINT

LDY #$0003 ;Drop nothing

BRANCH_NU ;At this point, Y determines the drop. 0 = Small energy, 1 = Large, 2 = missiles, 3 = nothing, 4 = super, 5 = power bomb.

REP #$20
TYX
LDA $86F25E, X ;Translate chosen drop to some other index value.
AND #$00FF ;1 byte
PLB
PLY
PLX
RTS
}
</pre>

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Kejardon: I am failing all the maths