DeviceListener: Rejig calculateGestureDelta algorithm

Get rid of the silly precomputed tables, and do More Maths(TM) instead!
Thanks to @zwim for the magic sauce ;).

Minor simplification of the API while I'm in there, and unify the warmth
computations, do everything in the native scale (much like what
effectively happens for intensity) to workaround the silly public API
being an unhelpful PITA, ensuring consistent & effective changes.

frontend/device/devicelistener.lua

@@ -55,68 +55,54 @@ if Device:hasFrontlight() then
     local function calculateGestureDelta(ges, direction, min, max)
         local delta_int
         if type(ges) == "table" then
-            -- here we are using just two scales
-            -- big scale is for high dynamic ranges (e.g. brightness from 1..100)
-            --           original scale maybe tuned by hand
-            -- small scale is for lower dynamic ranges (e.g. warmth from 1..10)
-            --           scale entries are calculated by math.round(1*sqrt(2)^n)
-            --- @fixme: An intermediary scale is probably necessary for Kindle, which goes from 0 to 24...
-            local steps_fl_big_scale = { 0.1, 0.1, 0.2, 0.4, 0.7, 1.1, 1.6, 2.2, 2.9, 3.7, 4.6, 5.6, 6.7, 7.9, 9.2, 10.6, }
-            local steps_fl_small_scale = { 1.0, 1.0, 2.0, 3.0, 4.0, 6.0, 8.1, 11.3 }
-            local steps_fl = steps_fl_big_scale
-            if (max - min) < 50  then
-                steps_fl = steps_fl_small_scale
-            end
-            local gestureScale
-            local scale_multiplier
+            local gesture_multiplier
             if ges.ges == "two_finger_swipe" or ges.ges == "swipe" then
-                scale_multiplier = 0.8
+                gesture_multiplier = 0.8
             else
-                scale_multiplier = 1
+                gesture_multiplier = 1
             end
 
+            local gestureScale
             if ges.direction == "south" or ges.direction == "north" then
-                gestureScale = Screen:getHeight() * scale_multiplier
+                gestureScale = Screen:getHeight() * gesture_multiplier
             elseif ges.direction == "west" or ges.direction == "east" then
-                gestureScale = Screen:getWidth() * scale_multiplier
+                gestureScale = Screen:getWidth() * gesture_multiplier
             else
                 local width = Screen:getWidth()
                 local height = Screen:getHeight()
                 -- diagonal
-                gestureScale = math.sqrt(width * width + height * height) * scale_multiplier
-            end
-
-            local steps_tbl = {}
-            local scale = (max - min) / steps_fl[#steps_fl] / 2 -- full swipe gives half scale
-            for i = 1, #steps_fl, 1 do
-                steps_tbl[i] = math.ceil(steps_fl[i] * scale)
+                gestureScale = math.sqrt(width^2 + height^2) * gesture_multiplier
             end
 
+            -- In case we're passed a gesture that doesn't imply movement (e.g., tap or hold)
             if ges.distance == nil then
                 ges.distance = 1
             end
 
-            local step = math.ceil(#steps_tbl * ges.distance / gestureScale)
-            delta_int = steps_tbl[step] or steps_tbl[#steps_tbl]
+            -- delta_int is calculated by a function f(x) = coeff * x^2
+            -- *) f(x) has the boundary condition: f(1) = max/2;
+            -- *) x is roughly the swipe distance as a fraction of the screen geometry,
+            --    clamped between 0 and 1
+            local x = math.min(1, ges.distance / gestureScale)
+            delta_int = math.ceil(1/2 * max * x^2)
         else
-            -- received amount to change
+            -- The ges arg passed by our caller wasn't a gesture, but an absolute integer increment
             delta_int = ges
         end
         if direction ~= -1 and direction ~= 1 then
-            -- set default value (increase frontlight)
+            -- If the caller didn't specify, opt to *increase* by default
             direction = 1
         end
-        return direction, delta_int
+        return direction * delta_int
     end
 
     -- direction +1 - increase frontlight
     -- direction -1 - decrease frontlight
     function DeviceListener:onChangeFlIntensity(ges, direction)
         local powerd = Device:getPowerDevice()
-        local delta_int
-        direction, delta_int = calculateGestureDelta(ges, direction, powerd.fl_min, powerd.fl_max)
+        local delta = calculateGestureDelta(ges, direction, powerd.fl_min, powerd.fl_max)
 
-        local new_intensity = powerd:frontlightIntensity() + direction * delta_int
+        local new_intensity = powerd:frontlightIntensity() + delta
         -- when new_intensity <= 0, toggle light off
         self:onSetFlIntensity(new_intensity)
         self:onShowIntensity()
@@ -149,17 +135,13 @@ if Device:hasFrontlight() then
         if not Device:hasNaturalLight() then return true end
 
         local powerd = Device:getPowerDevice()
-        local delta_int
-        direction, delta_int = calculateGestureDelta(ges, direction, powerd.fl_warmth_min, powerd.fl_warmth_max)
+        local delta = calculateGestureDelta(ges, direction, powerd.fl_warmth_min, powerd.fl_warmth_max)
 
-        local warmth
-        if type(ges) == "table" then
-            -- received a gesture, scale the gesture delta to the API range
-            warmth = powerd:frontlightWarmth() + powerd:fromNativeWarmth(direction * delta_int)
-        else
-            -- received an absolute increment, use it as-is in the native scale
-            warmth = powerd:fromNativeWarmth(powerd:toNativeWarmth(powerd:frontlightWarmth()) + ges)
-        end
+        -- Given that the native warmth ranges are usually pretty restrictive (e.g., [0, 10] or [0, 24]),
+        -- do the computations in the native scale, to ensure we always actually *change* something,
+        -- in case both the old and new value would round to the same native step,
+        -- despite being different in the API scale, which is stupidly fixed at [0, 100]...
+        local warmth = powerd:fromNativeWarmth(powerd:toNativeWarmth(powerd:frontlightWarmth()) + delta)
 
         self:onSetFlWarmth(warmth)
         self:onShowWarmth()