From 5d0b18a0d0bd02a9b77b8948d6887bb661a385da Mon Sep 17 00:00:00 2001
From: pecksin <78765996+pecksin@users.noreply.github.com>
Date: Wed, 16 Feb 2022 17:06:00 -0500
Subject: [PATCH] Use absolute value for bouncy in collision (#11969)

* use abs(bouncy) in collision
* test case for negative bouncy
* send abs(bouncy) to old clients
---
 games/devtest/mods/testnodes/properties.lua | 4 ++--
 src/collision.cpp                           | 3 ++-
 src/nodedef.cpp                             | 7 ++++++-
 3 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/games/devtest/mods/testnodes/properties.lua b/games/devtest/mods/testnodes/properties.lua
index 51f703d7c..89facf71c 100644
--- a/games/devtest/mods/testnodes/properties.lua
+++ b/games/devtest/mods/testnodes/properties.lua
@@ -252,9 +252,9 @@ for i=-100, 100, 25 do
 end
 
 -- Bouncy nodes (various bounce levels)
-for i=20, 180, 20 do
+for i=-140, 180, 20 do
 	local val = math.floor(((i-20)/200)*255)
-	minetest.register_node("testnodes:bouncy"..i, {
+	minetest.register_node(("testnodes:bouncy"..i):gsub("-","NEG"), {
 		description = S("Bouncy Node (@1%)", i),
 		groups = {bouncy=i, dig_immediate=3},
 
diff --git a/src/collision.cpp b/src/collision.cpp
index d85a56884..ccc3a058d 100644
--- a/src/collision.cpp
+++ b/src/collision.cpp
@@ -303,7 +303,8 @@ collisionMoveResult collisionMoveSimple(Environment *env, IGameDef *gamedef,
 			if (!f.walkable)
 				continue;
 
-			int n_bouncy_value = itemgroup_get(f.groups, "bouncy");
+			// Negative bouncy may have a meaning, but we need +value here.
+			int n_bouncy_value = abs(itemgroup_get(f.groups, "bouncy"));
 
 			int neighbors = 0;
 			if (f.drawtype == NDT_NODEBOX &&
diff --git a/src/nodedef.cpp b/src/nodedef.cpp
index 8a5542837..fe0cc4bb0 100644
--- a/src/nodedef.cpp
+++ b/src/nodedef.cpp
@@ -452,7 +452,12 @@ void ContentFeatures::serialize(std::ostream &os, u16 protocol_version) const
 	writeU16(os, groups.size());
 	for (const auto &group : groups) {
 		os << serializeString16(group.first);
-		writeS16(os, group.second);
+		if (protocol_version < 41 && group.first.compare("bouncy") == 0) {
+			// Old clients may choke on negative bouncy value
+			writeS16(os, abs(group.second));
+		} else {
+			writeS16(os, group.second);
+		}
 	}
 	writeU8(os, param_type);
 	writeU8(os, param_type_2);