From 1752f6efc1ce24508e65ef89d63a4432a8d9a47e Mon Sep 17 00:00:00 2001
From: Tatsuru <44866610+Ikkarin@users.noreply.github.com>
Date: Mon, 4 Jan 2021 18:10:41 -0300
Subject: [PATCH] Allow water running in reverse gravity

---
 src/p_mobj.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/src/p_mobj.c b/src/p_mobj.c
index 58124fb9b..5cde1639c 100644
--- a/src/p_mobj.c
+++ b/src/p_mobj.c
@@ -3188,13 +3188,16 @@ boolean P_SceneryZMovement(mobj_t *mo)
 //
 boolean P_CanRunOnWater(player_t *player, ffloor_t *rover)
 {
-	fixed_t topheight = P_GetFFloorTopZAt(rover, player->mo->x, player->mo->y);
+	boolean flip = player->mo->eflags & MFE_VERTICALFLIP;
+	fixed_t surfaceheight = flip ? P_GetFFloorBottomZAt(rover, player->mo->x, player->mo->y) : P_GetFFloorTopZAt(rover, player->mo->x, player->mo->y);
+	fixed_t playerbottom = flip ? (player->mo->z + player->mo->height) : player->mo->z;
+	boolean doifit = flip ? (surfaceheight - player->mo->floorz >= player->mo->height) : (player->mo->ceilingz - surfaceheight >= player->mo->height);
 
 	if (!player->powers[pw_carry] && !player->homing
-		&& ((player->powers[pw_super] || player->charflags & SF_RUNONWATER || player->dashmode >= DASHMODE_THRESHOLD) && player->mo->ceilingz-topheight >= player->mo->height)
+		&& ((player->powers[pw_super] || player->charflags & SF_RUNONWATER || player->dashmode >= DASHMODE_THRESHOLD) && doifit)
 		&& (rover->flags & FF_SWIMMABLE) && !(player->pflags & PF_SPINNING) && player->speed > FixedMul(player->runspeed, player->mo->scale)
 		&& !(player->pflags & PF_SLIDING)
-		&& abs(player->mo->z - topheight) < FixedMul(30*FRACUNIT, player->mo->scale))
+		&& abs(playerbottom - surfaceheight) < FixedMul(30*FRACUNIT, player->mo->scale))
 		return true;
 
 	return false;