Proper battery percentage

src/components/battery/BatteryController.cpp

@@ -1,4 +1,5 @@
 #include "components/battery/BatteryController.h"
+#include "components/utility/LinearApproximation.h"
 #include "drivers/PinMap.h"
 #include <hal/nrf_gpio.h>
 #include <nrfx_saadc.h>
@@ -60,8 +61,14 @@ void Battery::SaadcInit() {
 }
 
 void Battery::SaadcEventHandler(nrfx_saadc_evt_t const* p_event) {
-  const uint16_t battery_max = 4180; // maximum voltage of battery ( max charging voltage is 4.21 )
-  const uint16_t battery_min = 3200; // minimum voltage of battery before shutdown ( depends on the battery )
+  static const Utility::LinearApproximation<uint16_t, uint8_t, 6> aprox {{{
+    {3500, 0},  // Minimum voltage before shutdown (depends on the battery)
+    {3600, 10}, // Keen point that corresponds to 10%
+    {3700, 25},
+    {3750, 50},
+    {3900, 75},
+    {4180, 100} // Maximum voltage during charging is 4.21V
+  }}};
 
   if (p_event->type == NRFX_SAADC_EVT_DONE) {
 
@@ -77,10 +84,8 @@ void Battery::SaadcEventHandler(nrfx_saadc_evt_t const* p_event) {
     uint8_t newPercent;
     if (isFull) {
       newPercent = 100;
-    } else if (voltage < battery_min) {
-      newPercent = 0;
     } else {
-      newPercent = std::min((voltage - battery_min) * 100 / (battery_max - battery_min), isCharging ? 99 : 100);
+      newPercent = std::min(aprox.GetValue(voltage), isCharging ? uint8_t {99} : uint8_t {100});
     }
 
     if ((isPowerPresent && newPercent > percentRemaining) || (!isPowerPresent && newPercent < percentRemaining) || firstMeasurement) {

src/components/utility/LinearApproximation.h

@@ -0,0 +1,41 @@
+#pragma once
+
+#include <cstddef>
+#include <array>
+
+namespace Pinetime {
+  namespace Utility {
+
+    // based on: https://github.com/SHristov92/LinearApproximation/blob/main/Linear.h
+    template <typename Key, typename Value, std::size_t Size> class LinearApproximation {
+      using Point = struct {
+        Key key;
+        Value value;
+      };
+
+    public:
+      LinearApproximation(const std::array<Point, Size>&& sorted_points) : points {sorted_points} {
+      }
+
+      Value GetValue(Key key) const {
+        if (key <= points[0].key) {
+          return points[0].value;
+        }
+
+        for (std::size_t i = 1; i < Size; i++) {
+          const auto& p = points[i];
+          const auto& p_prev = points[i - 1];
+
+          if (key < p.key) {
+            return p_prev.value + (key - p_prev.key) * (p.value - p_prev.value) / (p.key - p_prev.key);
+          }
+        }
+
+        return points[Size - 1].value;
+      }
+
+    private:
+      std::array<Point, Size> points;
+    };
+  }
+}