Input: Simplify input slot storage alloc (#11296)

* Input: Harden setCurrentMtSlotChecked
  The current implementation was assuming that the only case where we
  might be missing slot storage was for the *first* contact point,
  given that ABS_MT_SLOT is (if all goes well) guaranteed to be present
  and come first for every subsequent additional contact points.
  While this works just fine in practice, we can simplify and generalize
  the check by just checking if we've actually recorded the requested
  slot, even if it's not the first contact point.
  The hit check is possibly ever so slightly faster than the length
  computation, to boot.
* Input: Handle snow_protocol devices with newer hardware revisions that do *NOT* need the snow quirks.
  If a sane input frame is detected, the snow quirks will be disabled at runtime, ensuring sane behavior.
  Given the extremely non-standard behavior of the snow quirks, this is fairly easy to detect,
  as a snow device will *never* emit EV_ABS:ABS_MT_TRACKING_ID:-1, so if we catch one, it's not snow ;).
  (We've had reports of this on a Clara HD, FWIW)

frontend/device/input.lua

@@ -532,15 +532,17 @@ function Input:resetState()
 end
 
 function Input:handleKeyBoardEv(ev)
-    -- Detect loss of contact for the "snow" protocol...
-    -- NOTE: Some ST devices may also behave similarly, but we handle those via ABS_PRESSURE
+    -- Detect loss of contact for the "snow" protocol, as we *never* get EV_ABS:ABS_MT_TRACKING_ID:-1 on those...
+    -- NOTE: The same logic *could* be used on *some* ST devices to detect contact states,
+    --       but we instead prefer using EV_ABS:ABS_PRESSURE on those,
+    --       as it appears to be more common than EV_KEY:BTN_TOUCH on the devices we care about...
     if self.snow_protocol then
         if ev.code == C.BTN_TOUCH then
             if ev.value == 0 then
                 -- Kernel sends it after loss of contact for *all* slots,
                 -- only once the final contact point has been lifted.
                 if #self.MTSlots == 0 then
-                    -- Likely, since this is usually in its own event stream,
+                    -- Likely, since this is usually in its own input frame,
                     -- meaning self.MTSlots has *just* been cleared by our last EV_SYN:SYN_REPORT handler...
                     -- So, poke at the actual data to find the slots that are currently active (i.e., in the down state),
                     -- and re-populate a minimal self.MTSlots array that simply switches them to the up state ;).
@@ -553,7 +555,7 @@ function Input:handleKeyBoardEv(ev)
                 else
                     -- Unlikely, given what we mentioned above...
                     -- Note that, funnily enough, its EV_KEY:BTN_TOUCH:1 counterpart
-                    -- *can* be in the same initial event stream as the EV_ABS batch...
+                    -- *can* be in the same initial input frame as the EV_ABS batch...
                     for _, MTSlot in ipairs(self.MTSlots) do
                         self:setMtSlot(MTSlot.slot, "id", -1)
                     end
@@ -789,17 +791,25 @@ function Input:handleTouchEv(ev)
         elseif ev.code == C.ABS_MT_TRACKING_ID then
             if self.snow_protocol then
                 -- NOTE: We'll never get an ABS_MT_SLOT event, instead we have a slot-like ABS_MT_TRACKING_ID value...
-                --       This also means this may never be set to -1 on contact lift,
-                --       which is why we instead rely on EV_KEY:BTN_TOUCH:0 for that (c.f., handleKeyBoardEv).
-                self:setupSlotData(ev.value)
-            else
-                -- The Elan driver needlessly repeats unchanged ABS_MT_TRACKING_ID values,
-                -- which allows us to do this here instead of relying more aggressively on setCurrentMtSlotChecked.
-                if #self.MTSlots == 0 then
-                    self:addSlot(self.cur_slot)
+                --       This also means that, unlike on sane devices, this will *never* be set to -1 on contact lift,
+                --       which is why we instead have to rely on EV_KEY:BTN_TOUCH:0 for that (c.f., handleKeyBoardEv).
+                if ev.value == -1 then
+                    -- NOTE: While *actual* snow_protocol devices will *never* emit an EV_ABS:ABS_MT_TRACKING_ID:-1 event,
+                    --       we've seen brand new revisions of snow_protocol devices shipping with sane panels instead,
+                    --       so we'll need to disable the quirks at runtime to handle these properly...
+                    --       (c.f., https://www.mobileread.com/forums/showpost.php?p=4383629&postcount=997).
+                    -- NOTE: Simply skipping the slot storage setup for -1 would not be enough, as it would only fix ST handling.
+                    --       MT would be broken, because buddy contact detection in GestureDetector looks at slot +/- 1,
+                    --       whereas we'd be having the main contact point at a stupidly large slot number
+                    --       (because it would match ABS_MT_TRACKING_ID, given the lack of ABS_MT_SLOT, at least for the first input frame),
+                    --       while the second contact would be at slot 1, because it would immediately have required emitting a proper ABS_MT_SLOT event...
+                    logger.warn("Input: Disabled snow_protocol quirks because your device's hardware revision doesn't appear to need them!")
+                    self.snow_protocol = false
+                else
+                    self:setupSlotData(ev.value)
                 end
             end
-            self:setCurrentMtSlot("id", ev.value)
+            self:setCurrentMtSlotChecked("id", ev.value)
         elseif ev.code == C.ABS_MT_TOOL_TYPE then
             -- NOTE: On the Elipsa: Finger == 0; Pen == 1
             self:setCurrentMtSlot("tool", ev.value)
@@ -934,14 +944,15 @@ end
 
 function Input:handleTouchEvLegacy(ev)
     -- Single Touch Protocol.
-    -- Some devices emit both singletouch and multitouch events,
-    -- on those devices, the 'handleTouchEv' function may not behave as expected. Use this one instead.
+    -- Some devices emit both singletouch and multitouch events.
+    -- On those devices, `handleTouchEv` may not behave as expected. Use this one instead.
     if ev.type == C.EV_ABS then
         if ev.code == C.ABS_X then
             self:setCurrentMtSlotChecked("x", ev.value)
         elseif ev.code == C.ABS_Y then
             self:setCurrentMtSlotChecked("y", ev.value)
         elseif ev.code == C.ABS_PRESSURE then
+            -- This is the least common denominator we can use to detect contact down & lift...
             if ev.value ~= 0 then
                 self:setCurrentMtSlotChecked("id", 1)
             else
@@ -1051,29 +1062,29 @@ function Input:initMtSlot(slot)
     end
 end
 
+function Input:getMtSlot(slot)
+    return self.ev_slots[slot]
+end
+
+function Input:getCurrentMtSlot()
+    return self.ev_slots[self.cur_slot]
+end
+
 function Input:setMtSlot(slot, key, val)
     self.ev_slots[slot][key] = val
 end
 
 function Input:setCurrentMtSlot(key, val)
-    self:setMtSlot(self.cur_slot, key, val)
+    self.ev_slots[self.cur_slot][key] = val
 end
 
 -- Same as above, but ensures the current slot actually has a live ref first
 function Input:setCurrentMtSlotChecked(key, val)
-    if #self.MTSlots == 0 then
+    if not self.active_slots[self.cur_slot] then
         self:addSlot(self.cur_slot)
     end
 
-    self:setMtSlot(self.cur_slot, key, val)
-end
-
-function Input:getMtSlot(slot)
-    return self.ev_slots[slot]
-end
-
-function Input:getCurrentMtSlot()
-    return self:getMtSlot(self.cur_slot)
+    self.ev_slots[self.cur_slot][key] = val
 end
 
 function Input:getCurrentMtSlotData(key)
@@ -1100,22 +1111,13 @@ function Input:addSlot(value)
     self.cur_slot = value
 end
 
-function Input:addSlotIfChanged(value)
-    if self.cur_slot ~= value then
-        -- We've already seen that slot in this frame, don't insert a duplicate reference!
-        if self.active_slots[value] then
-            self.cur_slot = value
-        else
-            self:addSlot(value)
-        end
-    end
-end
-
 function Input:setupSlotData(value)
-    if #self.MTSlots == 0 then
+    if not self.active_slots[value] then
         self:addSlot(value)
     else
-        self:addSlotIfChanged(value)
+        -- We've already seen that slot in this frame, don't insert a duplicate reference!
+        -- NOTE: May already be set to the correct value if the driver repeats ABS_MT_SLOT (e.g., our android/PB translation layers; or ABS_MT_TRACKING_ID for snow_protocol).
+        self.cur_slot = value
     end
 end