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Fix documentation about Magikarp length calculation and bugfix about "Magikarp lengths can be miscalculated" #1100

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Fix documentation about Magikarp length calculation and bugfix about "Magikarp lengths can be miscalculated" #1100

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6508421
Fix doc about Magikarp length calculation and bugfix doc
Idain Dec 24, 2023
08358d8
Fix comment about maximum possible value
Idain Dec 24, 2023
497eb87
Update bugfix diff and comments in the code
Idain Dec 24, 2023
12e5c90
Move bug's explanation to bugs_and_glitches.md
Idain Dec 25, 2023
6f430d4
Shorten bug explanation
Idain Jan 2, 2024
8a4d9ab
Delete comment with possible X, Y and Z values.
Idain Jan 2, 2024
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6 changes: 2 additions & 4 deletions data/events/magikarp_lengths.asm
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Original file line number Diff line number Diff line change
Expand Up @@ -2,9 +2,7 @@ MagikarpLengths:
; [wMagikarpLength] = z * 100 + (bc - x) / y
; First argument is the bc threshold as well as x.
; Second argument is y.
; In reality, due to the bug at .BCLessThanDE,
; the threshold is determined by only register b.
dwb 110, 1 ; not used unless the bug is fixed
dwb 110, 1
dwb 310, 2
dwb 710, 4
dwb 2710, 20
Expand All @@ -17,4 +15,4 @@ MagikarpLengths:
dwb 64710, 20
dwb 65210, 5
dwb 65410, 2
dwb 65510, 1 ; not used
dwb 65510, 1
7 changes: 6 additions & 1 deletion docs/bugs_and_glitches.md
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Expand Up @@ -2407,15 +2407,20 @@ CopyPokemonName_Buffer1_Buffer3:

### Magikarp lengths can be miscalculated

The function `CalcMagikarpLength` is supposed to determine a Magikarp's length by using the Trainer's ID and the Magikarp's DVs, `xor`ing the values, store the result in `bc`, use it as an index to select an entry from the `MagikarpLengths` table and calculate the Magikarp's length, but it doesn't work as intended. The local function `.BCLessThanDE` is supposed to compare the index (stored in `bc`) with a threshold (stored in `de`) from the table to select the correct entry, but it only compares the high bytes of both inputs, which means that the wrong entry might be selected.

This bug also causes the first entry of the `MagikarpLengths` table to go unused and `bc` values between 65280 and 65509 to use the wrong formula. As a side effect, the highest possible length is capped at 1625mm (before being converted to feet and inches).
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**Fix:** Edit `CalcMagikarpLength.BCLessThanDE` in [engine/events/magikarp.asm](https://github.com/pret/pokecrystal/blob/master/engine/events/magikarp.asm):

```diff
.BCLessThanDE:
-; BUG: Magikarp lengths can be miscalculated (see docs/bugs_and_glitches.md)
ld a, b
cp d
ret c
- ret c
- ret nc
+ ret nz
ld a, c
cp e
ret
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47 changes: 23 additions & 24 deletions engine/events/magikarp.asm
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Expand Up @@ -108,35 +108,34 @@ CalcMagikarpLength:
; de: wEnemyMonDVs
; bc: wPlayerID
Comment on lines 108 to 109
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Suggested change
; de: wEnemyMonDVs
; bc: wPlayerID
; de: Magikarp's DVs (MON_DVS / wEnemyMonDVs )
; bc: Trainer ID (MON_ID / wPlayerID)

Those are only the inputs when called from LoadEnemyMon in engine/battle/core.asm. When called from CheckMagikarpLength, the inputs are MON_DVS and MON_ID. (This distinction is important, as MON_ID is the Original Trainer's ID, not the player's.)

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I see. I'll address your suggestions when I get my laptop back.

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This would be MON_OT_ID now.


; This function is poorly commented.

; In short, it generates a value between 190 and 1786 using
; a Magikarp's DVs and its trainer ID. This value is further
; filtered in LoadEnemyMon to make longer Magikarp even rarer.
; It generates a value between 190 and 1755 using a Magikarp's DVs
; and its trainer ID. This value is further filtered in LoadEnemyMon
; to make longer Magikarp even rarer.
Comment on lines +111 to +113
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Suggested change
; It generates a value between 190 and 1755 using a Magikarp's DVs
; and its trainer ID. This value is further filtered in LoadEnemyMon
; to make longer Magikarp even rarer.
; It generates a length (in mm) between 190 and 1755 using a Magikarp's DVs
; and its Trainer ID.

The comment about being further filtered in LoadEnemyMon assumes this function is only called from LoadEnemyMon (ignoring the case when it is called from CheckMagikarpLength). IMO, what a calling function does with the return value is out of the scope of what belongs in comments on this function.


; The value is generated from a lookup table.
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Suggested change
; The value is generated from a lookup table.
; The length is generated from a lookup table.

; The index is determined by the dv xored with the player's trainer id.

; bc = rrc(dv[0]) ++ rrc(dv[1]) ^ rrc(id)
; The index is determined by the DV xored with the player's trainer ID
; and then stored in bc.
Comment on lines +116 to +117
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Suggested change
; The index is determined by the DV xored with the player's trainer ID
; and then stored in bc.
; The index is determined by the DV xored with the Magikarp's Trainer ID
; and then stored in bc.

The player's Trainer ID is only used when called from LoadEnemyMon. When this function is called from CheckMagikarpLength, it uses Magikarp's Original Trainer's Trainer ID, not the player's. Even though the Magikarp technically doesn't have a Trainer ID when it is generated in the wild, I think it's clear enough what is being referred to if this is just generalized as "the Magikarp's Trainer ID".

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In this case it can feel misleading if we don't also clarify it uses the Player's ID when fighting against a wild Magikarp.

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That's completely reasonable. Perhaps something like this would be better

Suggested change
; The index is determined by the DV xored with the player's trainer ID
; and then stored in bc.
; The index is determined by the DV xored with a Trainer ID
; (the player's for wild Magikarp; the OT's when checking owned Magikarp)
; then stored in bc.

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It sounds better.


; if bc < 10: [wMagikarpLength] = c + 190
; if bc ≥ $ff00: [wMagikarpLength] = c + 1370
; if bc < 10: [wMagikarpLength] = bc + 190
; if bc ≥ 65510: [wMagikarpLength] = bc - 65510 + 1600
; else: [wMagikarpLength] = z * 100 + (bc - x) / y

; X, Y, and Z depend on the value of b as follows:

; if b = 0: x = 310, y = 2, z = 3
; if b = 1: x = 710, y = 4, z = 4
; if b = 2-9: x = 2710, y = 20, z = 5
; if b = 10-29: x = 7710, y = 50, z = 6
; if b = 30-68: x = 17710, y = 100, z = 7
; if b = 69-126: x = 32710, y = 150, z = 8
; if b = 127-185: x = 47710, y = 150, z = 9
; if b = 186-224: x = 57710, y = 100, z = 10
; if b = 225-243: x = 62710, y = 50, z = 11
; if b = 244-251: x = 64710, y = 20, z = 12
; if b = 252-253: x = 65210, y = 5, z = 13
; if b = 254: x = 65410, y = 2, z = 14
; X, Y, and Z depend on the value of bc as follows:

; if bc = 10-109: x = 110, y = 1, z = 2
; if bc = 110-309: x = 310, y = 2, z = 3
; if bc = 310-709: x = 710, y = 4, z = 4
; if bc = 710-2709: x = 2710, y = 20, z = 5
; if bc = 2710-7709: x = 7710, y = 50, z = 6
; if bc = 7710-17709: x = 17710, y = 100, z = 7
; if bc = 17710-32709: x = 32710, y = 150, z = 8
; if bc = 32710-47709: x = 47710, y = 150, z = 9
; if bc = 47710-57709: x = 57710, y = 100, z = 10
; if bc = 57710-62709: x = 62710, y = 50, z = 11
; if bc = 62710-64709: x = 64710, y = 20, z = 12
; if bc = 64710-65209: x = 65210, y = 5, z = 13
; if bc = 65210-65409: x = 65410, y = 2, z = 14
; if bc = 65410-65509: x = 65510, y = 1, z = 15
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; bc = rrc(dv[0]) ++ rrc(dv[1]) ^ rrc(id)

Expand Down