blockgame/mods/bg_player/run.lua

local walk_speed = 1
local walk_time = 2
local speedup_time = 6
local run_factor = 3
local slowdown_factor = 8
local jump_height = 1
local jump_factor = 1.5

local speed_data = {}

local function handle_speed (player, delta_time)
	speed_data[player] = speed_data[player] or {}
	local data = speed_data[player]

	-- data.start = data.start or minetest.get_gametime()
	data.duration = (data.duration or 0) + delta_time

	local control = player:get_player_control()

	local moving = control.up or control.down or control.left or control.right
	if not moving then
		data.duration = data.duration - delta_time * (slowdown_factor + 1)
	end
	if data.duration < 0 then data.duration = 0 end
	if data.duration > walk_time + speedup_time then data.duration = walk_time + speedup_time end

	local duration = data.duration

	local physics = player:get_physics_override()

	if duration < walk_time then
		physics.speed = walk_speed
		physics.jump = jump_height
	else
		local run_time = duration - walk_time
		local t = run_time / speedup_time

		local factor = (run_factor - 1) * t + 1
		if factor > run_factor then factor = run_factor end

		physics.speed = walk_speed * factor
		physics.jump = jump_height + (jump_height * jump_factor - 1) * t
	end

	player:set_physics_override(physics)
end

minetest.register_globalstep(function(delta_time)
	for _, player in pairs(minetest.get_connected_players()) do
		handle_speed(player, delta_time)
	end
end)