Alternative code for slipping (#6256)

* Alternative code for slipping

- does not depend on frame rate
- controllable via environment variables for now

* Adjust slipping speed for item entities too.

* Final version of framerate-independent slippery code

* Remove dead code and fix formatting

* getStandingNodePos should only look 0.05 nodes downwards

This ensures that, even if the player is standing on a partially
filled node, this node is used as the standing node and not the
node below it.

Specific use: enables slippery slabs

* Exchange global getStandingPosNode change for local inline change

Reverts previous commit

* Revert the item movement changes

* Slippery nodes now slip over cliffs and edges

Players no longer suddenly stop before falling off.
Also refactored slippery code into getSlipFactor method.

* Slipping over an edge gated by player's is_slipping state

A new flag for just this case, to reduce costly node lookups in
the normal case of leaning over a non-slippery edge.
Public access for consistency and potential future uses.

* Minor code tweaks / cosmetics

* Add temp variable to improve readability and fix indentation issues

2 files changed, 55 insertions, 16 deletions

diff --git a/src/localplayer.cpp b/src/localplayer.cpp
index 2a098ba0a..81e4009df 100644
--- a/src/localplayer.cpp
+++ b/src/localplayer.cpp
@@ -656,13 +656,10 @@ void LocalPlayer::applyControl(float dtime, Environment *env)
 	else
 		incH = incV = movement_acceleration_default * BS * dtime;
 
-	const INodeDefManager *nodemgr = env->getGameDef()->ndef();
-	Map *map = &env->getMap();
-	const ContentFeatures &f = nodemgr->get(map->getNodeNoEx(getStandingNodePos()));
-	bool slippery = (itemgroup_get(f.groups, "slippery") != 0);
+	float slip_factor = getSlipFactor(env, speedH);
 	// Accelerate to target speed with maximum increment
 	accelerateHorizontal(speedH * physics_override_speed,
-			incH * physics_override_speed, slippery);
+			incH * physics_override_speed * slip_factor);
 	accelerateVertical(speedV * physics_override_speed,
 			incV * physics_override_speed);
 }
@@ -702,19 +699,12 @@ v3f LocalPlayer::getEyeOffset() const
 
 // Horizontal acceleration (X and Z), Y direction is ignored
 void LocalPlayer::accelerateHorizontal(const v3f &target_speed,
-	const f32 max_increase, bool slippery)
+	const f32 max_increase)
 {
         if (max_increase == 0)
                 return;
 
 	v3f d_wanted = target_speed - m_speed;
-	if (slippery) {
-		if (target_speed == v3f())
-			d_wanted = -m_speed * 0.05f;
-		else
-			d_wanted *= 0.1f;
-	}
-
 	d_wanted.Y = 0.0f;
 	f32 dl = d_wanted.getLength();
 	if (dl > max_increase)
@@ -1038,3 +1028,52 @@ void LocalPlayer::old_move(f32 dtime, Environment *env, f32 pos_max_d,
 		m_can_jump = false;
 	}
 }
+
+float LocalPlayer::getSlipFactor(Environment *env, const v3f &speedH)
+{
+
+	if (!touching_ground)
+		return 1.0f;
+
+	float slip_factor = 1.0f;
+	// Slip on slippery nodes
+	const INodeDefManager *nodemgr = env->getGameDef()->ndef();
+	Map *map = &env->getMap();
+	const ContentFeatures &f = nodemgr->get(map->getNodeNoEx(
+			floatToInt(getPosition() - v3f(0, 0.05f * BS, 0), BS)));
+	int slippery = 0;
+	if (f.walkable) {
+		slippery = itemgroup_get(f.groups, "slippery");
+	} else if (is_slipping) {
+		// slipping over an edge? Check surroundings for slippery nodes
+		slippery = 2 << 16; // guard value, bigger than all realistic ones
+		for (int z = 0; z <= 1; z++) {
+			for (int x = 0; x <= 1; x++) {
+				// this should cover all nodes surrounding player position
+				v3f offset((x - 0.5f) * BS, 0.05f * BS, (z - 0.5f) * BS);
+				const ContentFeatures &f2 = nodemgr->get(map->getNodeNoEx(
+						floatToInt(getPosition() - offset, BS)));
+				if (f2.walkable) {
+					// find least slippery node we might be standing on
+					int s = itemgroup_get(f2.groups, "slippery");
+					if (s < slippery)
+						slippery = s;
+				}
+			}
+		}
+		// without any hits, ignore slippery
+		if (slippery >= (2 << 16))
+			slippery = 0;
+	}
+	if (slippery >= 1) {
+		if (speedH == v3f(0.0f)) {
+			slippery = slippery * 2;
+		}
+		slip_factor = core::clamp(1.0f / (slippery + 1), 0.001f, 1.0f);
+		is_slipping = true;
+	} else {
+		// remember this to avoid checking the edge case above too often
+		is_slipping = false;
+	}
+	return slip_factor;
+}
diff --git a/src/localplayer.h b/src/localplayer.h
index c64e0042a..3521512af 100644
--- a/src/localplayer.h
+++ b/src/localplayer.h
@@ -60,6 +60,7 @@ public:
 	u8 liquid_viscosity = 0;
 	bool is_climbing = false;
 	bool swimming_vertical = false;
+	bool is_slipping = false;
 
 	float physics_override_speed = 1.0f;
 	float physics_override_jump = 1.0f;
@@ -143,11 +144,10 @@ public:
 	void setCollisionbox(const aabb3f &box) { m_collisionbox = box; }
 
 private:
-	// clang-format off
-	void accelerateHorizontal(const v3f &target_speed, f32 max_increase, bool slippery);
-	// clang-format on
+	void accelerateHorizontal(const v3f &target_speed, const f32 max_increase);
 	void accelerateVertical(const v3f &target_speed, const f32 max_increase);
 	bool updateSneakNode(Map *map, const v3f &position, const v3f &sneak_max);
+	float getSlipFactor(Environment *env, const v3f &speedH);
 
 	v3f m_position;