ref: 5207615cdcc9ce8fadd4cb76cf0040cc5a407275
parent: 3650232398d6d75a5bcdfc99c3c3bb73ddd54751
author: Rangi <remy.oukaour+rangi42@gmail.com>
date: Sat Jan 5 15:29:45 EST 2019
Document a bugfix: 1/256 failure rate for 100% effect chance
--- a/docs/bugs_and_glitches.md
+++ b/docs/bugs_and_glitches.md
@@ -16,6 +16,7 @@
- [Thick Club and Light Ball can make (Special) Attack wrap around above 1024](#thick-club-and-light-ball-can-make-special-attack-wrap-around-above-1024)
- [Metal Powder can increase damage taken with boosted (Special) Defense](#metal-powder-can-increase-damage-taken-with-boosted-special-defense)
- [Reflect and Light Screen can make (Special) Defense wrap around above 1024](#reflect-and-light-screen-can-make-special-defense-wrap-around-above-1024)
+- [Moves with a 100% secondary effect chance will not trigger it in 1/256 uses](#moves-with-a-100-secondary-effect-chance-will-not-trigger-it-in-1256-uses)
- [Belly Drum sharply boosts Attack even with under 50% HP](#belly-drum-sharply-boosts-attack-even-with-under-50-hp)
- [Confusion damage is affected by type-boosting items and Explosion/Self-Destruct doubling](#confusion-damage-is-affected-by-type-boosting-items-and-explosionself-destruct-doubling)
- [Moves that lower Defense can do so after breaking a Substitute](#moves-that-lower-defense-can-do-so-after-breaking-a-substitute)
@@ -160,6 +161,35 @@
(This fix also affects Thick Club, Light Ball, and Metal Powder, as described above, but their specific fixes in the above bugs allow more accurate damage calculations.)
+
+## Moves with a 100% secondary effect chance will not trigger it in 1/256 uses
+
+*Fixing this bug will break compatibility with standard Pokémon Crystal for link battles.*
+
+([Video](https://www.youtube.com/watch?v=mHkyO5T5wZU&t=206))
+
+**Fix:** Edit `BattleCommand_EffectChance` in [engine/battle/effect_commands.asm](/engine/battle/effect_commands.asm):
+
+```diff
+- ; BUG: 1/256 chance to fail even for a 100% effect chance,
+- ; since carry is not set if BattleRandom == [hl] == 255
++ ld a, [hl]
++ cp 100 percent
++ jr z, .ok
+ call BattleRandom
+ cp [hl]
+- pop hl
+- ret c
++ jr c, .ok
+
+ .failed
+ ld a, 1
+ ld [wEffectFailed], a
+ and a
++.ok
++ pop hl
+ ret
+```
## Belly Drum sharply boosts Attack even with under 50% HP
--- a/engine/battle/effect_commands.asm
+++ b/engine/battle/effect_commands.asm
@@ -1877,6 +1877,8 @@
ld hl, wEnemyMoveStruct + MOVE_CHANCE
.got_move_chance
+ ; BUG: 1/256 chance to fail even for a 100% effect chance,
+ ; since carry is not set if BattleRandom == [hl] == 255
call BattleRandom
cp [hl]
pop hl