Implement Waterfall technique (#34)

engine/overworld/events.asm

@@ -769,6 +769,22 @@ CheckFacingTileEvent:
 	ret ; c
 
 .next_event_1
+;; Waterfall
+	ld a, [wFacingTileID]
+	call CheckWaterfallTile
+	jr nz, .next_event_2
+
+; Must be facing up and facing a waterfall tile to trigger the waterfall up sequence.
+; Otherwise HI_NYBBLE_CURRENT collision (forced walking in walking direction) applies.
+	farcall CheckMapCanWaterfall
+	jr c, .next_event_2
+
+	ld a, BANK(Script_WaterfallAuto)
+	ld hl, Script_WaterfallAuto
+	call CallScript
+	ret ; c
+
+.next_event_2
 .no_event
 	xor a
 	ret ; nc

engine/overworld/player_movement.asm

@@ -340,7 +340,7 @@ DoPlayerMovement::
 	add hl, bc
 	ld a, [hl]
 	ld [wWalkingDirection], a
-	jr .continue_walk
+	jr .continue_fast_slide
 
 .water_table
 	db RIGHT ; COLL_WATERFALL_RIGHT
@@ -414,8 +414,15 @@ DoPlayerMovement::
 	xor a
 	ret
 
+.continue_fast_slide
+	ld a, STEP_ICE
+	jr .continue_step
+
 .continue_walk
 	ld a, STEP_WALK
+	; fallthrough
+
+.continue_step
 	call .DoStep
 	ld a, PLAYERMOVEMENT_CONTINUE
 	scf