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Document fix for game freezing while switching mon (#869)
Fixes #868 Co-authored-by: Rangi <35663410+Rangi42@users.noreply.github.com>
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@ -35,9 +35,10 @@ Fixes in the [multi-player battle engine](#multi-player-battle-engine) category
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- [Beat Up may trigger King's Rock even if it failed](#beat-up-may-trigger-kings-rock-even-if-it-failed)
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- [Present damage is incorrect in link battles](#present-damage-is-incorrect-in-link-battles)
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- [Dragon Scale, not Dragon Fang, boosts Dragon-type moves](#dragon-scale-not-dragon-fang-boosts-dragon-type-moves)
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- [Switching out or switching against a Pokémon with max HP below 4 freezes the game](#switching-out-or-switching-against-a-pokémon-with-max-HP-below-4-freezes-the-game)
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- [Moves that do damage and increase your stats do not increase stats after a KO](#moves-that-do-damage-and-increase-your-stats-do-not-increase-stats-after-a-ko)
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- [HP bar animation is slow for high HP](#hp-bar-animation-is-slow-for-high-hp)
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- [HP bar animation off-by-one error for low HP](#hp-bar-animation-off-by-one-error-for-low-hp)
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- [Moves that do damage and increase your stats do not increase stats after a KO](#moves-that-do-damage-and-increase-your-stats-do-not-increase-stats-after-a-ko)
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- [Single-player battle engine](#single-player-battle-engine)
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- [A Transformed Pokémon can use Sketch and learn otherwise unobtainable moves](#a-transformed-pokémon-can-use-sketch-and-learn-otherwise-unobtainable-moves)
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- [Catching a Transformed Pokémon always catches a Ditto](#catching-a-transformed-pokémon-always-catches-a-ditto)
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@ -714,6 +715,143 @@ This bug existed for all battles in Gold and Silver, and was only fixed for sing
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```
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### Switching out or switching against a Pokémon with max HP below 4 freezes the game
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This happens because switching involves calculating a percentage of maximum enemy HP. Directly calculating *HP* × 100 / *max HP* would require a two-byte denominator, so instead the game calculates *HP* × 25 / (*max HP* / 4), since even a maximum HP of 999 divided by 4 is 249, which fits in one byte. However, if the maximum HP is below 4 this will divide by 0, which enters an infinite loop in `_Divide`.
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**Fix:** First, edit `SendOutMonText` in [engine/battle/core.asm](https://github.com/pret/pokecrystal/blob/master/engine/battle/core.asm):
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```diff
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; compute enemy health remaining as a percentage
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xor a
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ldh [hMultiplicand + 0], a
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ld hl, wEnemyMonHP
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ld a, [hli]
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ld [wEnemyHPAtTimeOfPlayerSwitch], a
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ldh [hMultiplicand + 1], a
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ld a, [hl]
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ld [wEnemyHPAtTimeOfPlayerSwitch + 1], a
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ldh [hMultiplicand + 2], a
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- ld a, 25
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- ldh [hMultiplier], a
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- call Multiply
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ld hl, wEnemyMonMaxHP
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ld a, [hli]
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ld b, [hl]
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- srl a
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- rr b
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- srl a
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- rr b
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+ ld c, 100
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+ and a
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+ jr z, .shift_done
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+.shift
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+ rra
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+ rr b
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+ srl c
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+ and a
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+ jr nz, .shift
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+.shift_done
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+ ld a, c
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+ ldh [hMultiplier], a
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+ call Multiply
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ld a, b
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ld b, 4
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ldh [hDivisor], a
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call Divide
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```
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Then edit `WithdrawMonText` in the same file:
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```diff
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; compute enemy health lost as a percentage
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ld hl, wEnemyMonHP + 1
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ld de, wEnemyHPAtTimeOfPlayerSwitch + 1
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ld b, [hl]
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dec hl
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ld a, [de]
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sub b
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ldh [hMultiplicand + 2], a
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dec de
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ld b, [hl]
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ld a, [de]
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sbc b
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ldh [hMultiplicand + 1], a
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- ld a, 25
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- ldh [hMultiplier], a
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- call Multiply
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ld hl, wEnemyMonMaxHP
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ld a, [hli]
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ld b, [hl]
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- srl a
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- rr b
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- srl a
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- rr b
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+ ld c, 100
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+ and a
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+ jr z, .shift_done
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+.shift
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+ rra
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+ rr b
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+ srl c
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+ and a
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+ jr nz, .shift
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+.shift_done
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+ ld a, c
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+ ldh [hMultiplier], a
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+ call Multiply
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ld a, b
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ld b, 4
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ldh [hDivisor], a
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call Divide
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```
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This changes both calculations to *HP* × (100 / *N*) / (*max HP* / *N*) for the smallest necessary *N*, which will be at least 1, so it avoids dividing by zero and is also more accurate.
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### Moves that do damage and increase your stats do not increase stats after a KO
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`BattleCommand_CheckFaint` "ends the move effect if the opponent faints", and these moves attempt to raise the user's stats *after* `checkfaint`. Note that fixing this can lead to stats being increased at the end of battle, but will not have any negative effects.
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**Fix:** Edit [data/moves/effects.asm](https://github.com/pret/pokecrystal/blob/master/data/moves/effects.asm):
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```diff
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DefenseUpHit:
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...
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criticaltext
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supereffectivetext
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+ defenseup
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+ statupmessage
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checkfaint
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buildopponentrage
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- defenseup
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- statupmessage
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endmove
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AttackUpHit:
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...
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criticaltext
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supereffectivetext
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+ attackup
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+ statupmessage
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checkfaint
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buildopponentrage
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- attackup
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- statupmessage
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endmove
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AllUpHit:
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...
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criticaltext
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supereffectivetext
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+ allstatsup
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checkfaint
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buildopponentrage
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- allstatsup
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endmove
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```
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### HP bar animation is slow for high HP
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*Fixing this cosmetic bug will* not *break link battle compatibility.*
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@ -770,48 +908,6 @@ This bug existed for all battles in Gold and Silver, and was only fixed for sing
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jr .loop
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```
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### Moves that do damage and increase your stats do not increase stats after a KO
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`BattleCommand_CheckFaint` "ends the move effect if the opponent faints", and these moves attempt to raise the user's stats *after* `checkfaint`. Note that fixing this can lead to stats being increased at the end of battle, but will not have any negative effects.
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**Fix:** Edit [data/moves/effects.asm](https://github.com/pret/pokecrystal/blob/master/data/moves/effects.asm):
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```diff
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DefenseUpHit:
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...
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criticaltext
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supereffectivetext
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+ defenseup
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+ statupmessage
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checkfaint
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buildopponentrage
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- defenseup
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- statupmessage
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endmove
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AttackUpHit:
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...
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criticaltext
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supereffectivetext
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+ attackup
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+ statupmessage
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checkfaint
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buildopponentrage
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- attackup
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- statupmessage
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endmove
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AllUpHit:
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...
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criticaltext
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supereffectivetext
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+ allstatsup
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checkfaint
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buildopponentrage
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- allstatsup
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endmove
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```
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## Single-player battle engine
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