Use resistance measurement to calculate body composition data.

Coëfficients were obtained by reverse-engineering the discontinued Trisa Vital
Bluetooth app. The resulting figures do not seem to be very accurate.

android_app/app/src/main/java/com/health/openscale/core/bluetooth/BluetoothTrisaBodyAnalyze.java

@@ -23,7 +23,9 @@ import android.preference.PreferenceManager;
 import android.support.annotation.Nullable;
 
 import com.health.openscale.R;
+import com.health.openscale.core.OpenScale;
 import com.health.openscale.core.datatypes.ScaleMeasurement;
+import com.health.openscale.core.datatypes.ScaleUser;
 
 import java.util.UUID;
 
@@ -252,7 +254,8 @@ public class BluetoothTrisaBodyAnalyze extends BluetoothCommunication {
     }
 
     private void onScaleMeasurumentReceived(byte[] data) {
-        ScaleMeasurement scaleMeasurement = parseScaleMeasurementData(data);
+        ScaleUser user = OpenScale.getInstance().getSelectedScaleUser();
+        ScaleMeasurement scaleMeasurement = parseScaleMeasurementData(data, user);
         if (scaleMeasurement == null) {
             Timber.e("Failed to parse scale measure measurement data: %s", byteInHex(data));
             return;

android_app/app/src/main/java/com/health/openscale/core/bluetooth/lib/TrisaBodyAnalyzeLib.java

@@ -18,6 +18,8 @@ package com.health.openscale.core.bluetooth.lib;
 import android.support.annotation.Nullable;
 
 import com.health.openscale.core.datatypes.ScaleMeasurement;
+import com.health.openscale.core.datatypes.ScaleUser;
+import com.health.openscale.core.utils.Converters;
 
 import java.util.Date;
 
@@ -64,25 +66,75 @@ public class TrisaBodyAnalyzeLib {
     }
 
     @Nullable
-    public static ScaleMeasurement parseScaleMeasurementData(byte[] data) {
+    public static ScaleMeasurement parseScaleMeasurementData(byte[] data, @Nullable ScaleUser user) {
-        // Byte 0 contains info.
+        // data contains:
-        // Byte 1-4 contains weight.
+        //
-        // Byte 5-8 contains timestamp, if bit 0 in info byte is set.
+        //   1 byte: info about presence of other fields:
+        //           bit 0: timestamp
+        //           bit 1: resistance1
+        //           bit 2: resistance2
+        //           (other bits aren't used here)
+        //   4 bytes: weight
+        //   4 bytes: timestamp (if info bit 0 is set)
+        //   4 bytes: resistance1 (if info bit 1 is set)
+        //   4 bytes: resistance2 (if info bit 2 is set)
+        //   (following fields aren't used here)
+
         // Check that we have at least weight & timestamp, which is the minimum information that
         // ScaleMeasurement needs.
-        if (data.length < 9 || (data[0] & 1) == 0) {
+        if (data.length < 9) {
+            return null;  // data is too short
+        }
+        byte infoByte = data[0];
+        boolean hasTimestamp = (infoByte & 1) == 1;
+        boolean hasResistance1 = (infoByte & 2) == 2;
+        boolean hasResistance2 = (infoByte & 4) == 4;
+        if (!hasTimestamp) {
             return null;
         }
-
+        double weightKg = getBase10Float(data, 1);
-        double weight = getBase10Float(data, 1);
         int deviceTimestamp = getInt32(data, 5);
 
         ScaleMeasurement measurement = new ScaleMeasurement();
         measurement.setDateTime(new Date(convertDeviceTimestampToJava(deviceTimestamp)));
-        measurement.setWeight((float)weight);
+        measurement.setWeight((float) weightKg);
-        // TODO: calculate body composition (if possible) and set those fields too
+
+        // Only resistance 2 is used; resistance 1 is 0, even if it is present.
+        int resistance2Offset = 9 + (hasResistance1 ? 4 : 0);
+        if (hasResistance2 && resistance2Offset + 4 <= data.length && isValidUser(user)) {
+            // Calculate body composition statistics from measured weight & resistance, combined
+            // with age, height and sex from the user profile. The accuracy of the resulting figures
+            // is questionable, but it's better than nothing. Even if the absolute numbers aren't
+            // very meaningful, it might still be useful to track changes over time.
+            double resistance2 = getBase10Float(data, resistance2Offset);
+            int ageYears = user.getAge();
+            double heightCm = Converters.toCentimeter(user.getBodyHeight(), user.getMeasureUnit());
+            boolean isMale = user.getGender().isMale();
+            double impedance = resistance2 < 410 ? 3.0 : 0.3 * (resistance2 - 400);
+            double bmi = weightKg * 1e4 / (heightCm * heightCm);
+            double fat = isMale
+                    ? bmi * (1.479 + 4.4e-4 * impedance) + 0.1 * ageYears - 21.764
+                    : bmi * (1.506 + 3.908e-4 * impedance) + 0.1 * ageYears - 12.834;
+            double water = isMale
+                    ? 87.51 + (-1.162 * bmi - 0.00813 * impedance + 0.07594 * ageYears)
+                    : 77.721 + (-1.148 * bmi - 0.00573 * impedance + 0.06448 * ageYears);
+            double muscle = isMale
+                    ? 74.627 + (-0.811 * bmi - 0.00565 * impedance - 0.367 * ageYears)
+                    : 57.0 + (-0.694 * bmi - 0.00344 * impedance - 0.255 * ageYears);
+            double bone = isMale
+                    ? 7.829 + (-0.0855 * bmi - 5.92e-4 * impedance - 0.0389 * ageYears)
+                    : 7.98 + (-0.0973 * bmi - 4.84e-4 * impedance - 0.036 * ageYears);
+            measurement.setFat((float) fat);
+            measurement.setWater((float) water);
+            measurement.setMuscle((float) muscle);
+            measurement.setBone((float) bone);
+        }
         return measurement;
     }
 
+    private static boolean isValidUser(@Nullable ScaleUser user) {
+        return user != null && user.getAge() > 0 && user.getBodyHeight() > 0;
+    }
+
     private TrisaBodyAnalyzeLib() {}
 }

android_app/app/src/test/java/com/health/openscale/TrisaBodyAnalyzeLibTest.java

@@ -1,12 +1,16 @@
 package com.health.openscale;
 
 import com.health.openscale.core.datatypes.ScaleMeasurement;
+import com.health.openscale.core.datatypes.ScaleUser;
+import com.health.openscale.core.utils.Converters;
 
 import junit.framework.Assert;
 
 import org.junit.Test;
 
+import java.util.Calendar;
 import java.util.Date;
+import java.util.GregorianCalendar;
 
 import static com.health.openscale.core.bluetooth.lib.TrisaBodyAnalyzeLib.convertDeviceTimestampToJava;
 import static com.health.openscale.core.bluetooth.lib.TrisaBodyAnalyzeLib.convertJavaTimestampToDevice;
@@ -68,14 +72,49 @@ public class TrisaBodyAnalyzeLibTest {
     }
 
     @Test
-    public void parseScaleMeasurementDataTests() {
+    public void parseScaleMeasurementData_validUserData() {
+        long expected_timestamp_seconds = 1539205852L;  // Wed Oct 10 21:10:52 UTC 2018
+        byte[] bytes = hexToBytes("9f:b0:1d:00:fe:dc:2f:81:10:00:00:00:ff:0a:15:00:ff:00:09:00");
+
+        ScaleUser user = new ScaleUser();
+        user.setGender(Converters.Gender.MALE);
+        user.setBirthday(ageToBirthday(36));
+        user.setBodyHeight(186);
+        user.setMeasureUnit(Converters.MeasureUnit.CM);
+
+        ScaleMeasurement measurement = parseScaleMeasurementData(bytes, user);
+
+        float eps = 1e-3f;
+        assertEquals(76.0f, measurement.getWeight(), eps);
+        assertEquals(new Date(expected_timestamp_seconds * 1000), measurement.getDateTime());
+        assertEquals(14.728368f, measurement.getFat(), eps);
+        assertEquals(64.37914f, measurement.getWater(), eps);
+        assertEquals(43.36414f, measurement.getMuscle(), eps);
+        assertEquals(4.525733f, measurement.getBone());
+    }
+
+    @Test
+    public void parseScaleMeasurementData_missingUserData() {
         long expected_timestamp_seconds = 1538156082L;  // Fri Sep 28 17:34:42 UTC 2018
         byte[] bytes = hexToBytes("9f:ba:1d:00:fe:32:2b:71:10:00:00:00:ff:8d:14:00:ff:00:09:00");
 
-        ScaleMeasurement measurement = parseScaleMeasurementData(bytes);
+        ScaleMeasurement measurement = parseScaleMeasurementData(bytes, null);
 
-        assertEquals(measurement.getWeight(), 76.1f, 1e-6f);
+        assertEquals(76.1f, measurement.getWeight(), 1e-3f);
         assertEquals(new Date(expected_timestamp_seconds * 1000), measurement.getDateTime());
+        assertEquals(0f, measurement.getFat());
+    }
+
+    @Test
+    public void parseScaleMeasurementData_invalidUserData() {
+        long expected_timestamp_seconds = 1538156082L;  // Fri Sep 28 17:34:42 UTC 2018
+        byte[] bytes = hexToBytes("9f:ba:1d:00:fe:32:2b:71:10:00:00:00:ff:8d:14:00:ff:00:09:00");
+
+        ScaleMeasurement measurement = parseScaleMeasurementData(bytes, new ScaleUser());
+
+        assertEquals(76.1f, measurement.getWeight(), 1e-3f);
+        assertEquals(new Date(expected_timestamp_seconds * 1000), measurement.getDateTime());
+        assertEquals(0f, measurement.getFat());
     }
 
     /**
@@ -124,7 +163,7 @@ public class TrisaBodyAnalyzeLibTest {
     }
 
     /** Parses a colon-separated hex-encoded string like "aa:bb:cc:dd" into an array of bytes. */
-    private byte[] hexToBytes(String s) {
+    private static byte[] hexToBytes(String s) {
         String[] parts = s.split(":");
         byte[] bytes = new byte[parts.length];
         for (int i = 0; i < bytes.length; ++i) {
@@ -135,4 +174,9 @@ public class TrisaBodyAnalyzeLibTest {
         }
         return bytes;
     }
+
+    private static Date ageToBirthday(int years) {
+        int currentYear = GregorianCalendar.getInstance().get(Calendar.YEAR);
+        return new GregorianCalendar(currentYear - years, Calendar.JANUARY, 1).getTime();
+    }
 }