Add player knockback on punch to builtin

Author: sfan5

builtin/game/init.lua

@@ -33,5 +33,6 @@ dofile(gamepath .. "features.lua")
 dofile(gamepath .. "voxelarea.lua")
 dofile(gamepath .. "forceloading.lua")
 dofile(gamepath .. "statbars.lua")
+dofile(gamepath .. "knockback.lua")
 
 profiler = nil

builtin/game/knockback.lua

@@ -0,0 +1,46 @@
+-- can be overriden by mods
+function core.calculate_knockback(player, hitter, time_from_last_punch, tool_capabilities, dir, distance, damage)
+	if damage == 0 or player:get_armor_groups().immortal then
+		return 0.0
+	end
+
+	local m = 8
+	-- solve m - m*e^(k*4) = 4 for k
+	local k = -0.17328
+	local res = m - m * math.exp(k * damage)
+
+	if distance < 2.0 then
+		res = res * 1.1 -- more knockback when closer
+	elseif distance > 4.0 then
+		res = res * 0.9 -- less when far away
+	end
+	return res
+end
+
+local function vector_absmax(v)
+	local max, abs = math.max, math.abs
+	return max(max(abs(v.x), abs(v.y)), abs(v.z))
+end
+
+core.register_on_punchplayer(function(player, hitter, time_from_last_punch, tool_capabilities, unused_dir, damage)
+	if player:get_hp() == 0 then
+		return -- RIP
+	end
+
+	-- Server::handleCommand_Interact() adds eye offset to one but not the other
+	-- so the direction is slightly off, calculate it ourselves
+	local dir = vector.subtract(player:get_pos(), hitter:get_pos())
+	local d = vector.length(dir)
+	if d ~= 0.0 then
+		dir = vector.divide(dir, d)
+	end
+
+	local k = core.calculate_knockback(player, hitter, time_from_last_punch, tool_capabilities, dir, d, damage)
+
+	local kdir = vector.multiply(dir, k)
+	if vector_absmax(kdir) < 1.0 then
+		return -- barely noticeable, so don't even send
+	end
+
+	player:add_player_velocity(kdir)
+end)

doc/lua_api.txt

@@ -5019,6 +5019,15 @@ Misc.
       of the creative mode setting, checks for "sneak" to set the `invert_wall`
       parameter and `prevent_after_place` set to `true`.
 
+* `minetest.calculate_knockback(player, hitter, time_from_last_punch,
+  tool_capabilities, dir, distance, damage)`
+    * Returns the amount of knockback applied on the punched player.
+    * Arguments are equivalent to `register_on_punchplayer`, except the following:
+        * `distance`: distance between puncher and punched player
+    * This function can be overriden by mods that wish to modify this behaviour.
+    * You may want to cache and call the old function to allow multiple mods to
+      change knockback behaviour.
+
 * `minetest.forceload_block(pos[, transient])`
     * forceloads the position `pos`.
     * returns `true` if area could be forceloaded