enable reverse driving

defold_game/main/player.script

@@ -1,7 +1,7 @@
-go.property("acceleration", 100)
-go.property("deceleration", 200)
-go.property("max_speed", 400)
-go.property("wheel_base", 40)  -- pixels between left and right wheels
+local ACCELERATION = 100
+local DECELERATION = 200
+local MAX_SPEED = 400
+local WHEEL_BASE = 80
 
 local UP = vmath.vector3(0, 1, 0)
 
@@ -10,27 +10,41 @@ function init(self)
 
     self.left_speed = 0
     self.right_speed = 0
-    self.left_input = false
-    self.right_input = false
+    self.left_fwd = false
+    self.left_rev = false
+    self.right_fwd = false
+    self.right_rev = false
+end
+
+local function update_speed(speed, drive, dt)
+    if drive ~= 0 then
+        local target = drive * MAX_SPEED
+        local step = ACCELERATION * dt
+        if speed < target then
+            return math.min(speed + step, target)
+        else
+            return math.max(speed - step, target)
+        end
+    else
+        local step = DECELERATION * dt
+        if speed > 0 then
+            return math.max(speed - step, 0)
+        else
+            return math.min(speed + step, 0)
+        end
+    end
 end
 
 function update(self, dt)
-    -- each wheel accelerates when its key is held, decelerates otherwise
-    if self.left_input then
-        self.left_speed = math.min(self.left_speed + self.acceleration * dt, self.max_speed)
-    else
-        self.left_speed = math.max(self.left_speed - self.deceleration * dt, 0)
-    end
+    local left_drive  = (self.left_fwd  and 1 or 0) - (self.left_rev  and 1 or 0)
+    local right_drive = (self.right_fwd and 1 or 0) - (self.right_rev and 1 or 0)
 
-    if self.right_input then
-        self.right_speed = math.min(self.right_speed + self.acceleration * dt, self.max_speed)
-    else
-        self.right_speed = math.max(self.right_speed - self.deceleration * dt, 0)
-    end
+    self.left_speed  = update_speed(self.left_speed,  left_drive,  dt)
+    self.right_speed = update_speed(self.right_speed, right_drive, dt)
 
     -- differential drive: speed difference creates rotation, average creates forward motion
     local linear_speed = (self.left_speed + self.right_speed) / 2
-    local angular_vel = (self.right_speed - self.left_speed) / self.wheel_base
+    local angular_vel = (self.right_speed - self.left_speed) / WHEEL_BASE
 
     local rot = go.get_rotation()
     rot = rot * vmath.quat_rotation_z(angular_vel * dt)
@@ -40,15 +54,13 @@ function update(self, dt)
     p = p + vmath.rotate(rot, UP * linear_speed * dt)
     go.set_position(p)
 
-    -- reset each frame (on_input re-sets if key is still held)
-    self.left_input = false
-    self.right_input = false
 end
 
 function on_input(self, action_id, action)
-    if action_id == hash("a_key") then
-        self.left_input = true
-    elseif action_id == hash("d_key") then
-        self.right_input = true
+    local held = action.pressed or not action.released
+    if     action_id == hash("a_key") then self.left_fwd  = held
+    elseif action_id == hash("q_key") then self.left_rev  = held
+    elseif action_id == hash("d_key") then self.right_fwd = held
+    elseif action_id == hash("e_key") then self.right_rev = held
     end
 end