Exponent can't be first in number

+ tests for NumberInputFilter

app/src/main/java/org/solovyev/android/text/method/NumberInputFilter.java

@@ -61,7 +61,7 @@ public class NumberInputFilter implements InputFilter {
         findChars(dest, dend, dest.length(), end - start, CHARS);
 
         SpannableStringBuilder filtered = null;
-        for (int i = end - 1; i >= start; i--) {
+        for (int i = start; i < end; i++) {
             final char c = source.charAt(i);
 
             boolean filter = false;
@@ -93,6 +93,9 @@ public class NumberInputFilter implements InputFilter {
                 } else if (CHARS[CHAR_POINT] >= 0 && CHARS[CHAR_POINT] > i + dstart) {
                     // no exponent before decimal point
                     filter = true;
+                } else if (i + dstart == 0) {
+                    // exponent can't be first
+                    filter = true;
                 } else {
                     CHARS[CHAR_EXP] = i + dstart;
                 }

app/src/test/java/org/solovyev/android/text/method/NumberInputFilterTest.java

@@ -0,0 +1,112 @@
+package org.solovyev.android.text.method;
+
+import android.os.Build;
+import android.text.Editable;
+import android.text.InputFilter;
+import android.text.SpannableStringBuilder;
+import org.junit.Before;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.robolectric.RobolectricGradleTestRunner;
+import org.robolectric.annotation.Config;
+import org.solovyev.android.calculator.BuildConfig;
+
+import static org.junit.Assert.assertEquals;
+
+@Config(constants = BuildConfig.class, sdk = Build.VERSION_CODES.LOLLIPOP)
+@RunWith(RobolectricGradleTestRunner.class)
+public class NumberInputFilterTest {
+
+    private Editable editable;
+
+    @Before
+    public void setUp() throws Exception {
+        editable = new SpannableStringBuilder();
+        editable.setFilters(new InputFilter[]{new NumberInputFilter()});
+    }
+
+    @Test
+    public void testShouldNotInsertExponentInTheBeginning() throws Exception {
+        editable.insert(0, "E");
+        assertEquals("", editable.toString());
+    }
+
+    @Test
+    public void testShouldInsertExponentAtTheEnd() throws Exception {
+        editable.insert(0, "1");
+        editable.insert(1, "E");
+        assertEquals("1E", editable.toString());
+    }
+
+    @Test
+    public void testShouldNotInsertSecondMinusSign() throws Exception {
+        editable.insert(0, "-");
+        editable.insert(1, "-");
+        assertEquals("-", editable.toString());
+    }
+
+    @Test
+    public void testShouldNotInsertTwoMinusSigns() throws Exception {
+        editable.insert(0, "--");
+        assertEquals("-", editable.toString());
+    }
+
+    @Test
+    public void testShouldInsertSecondMinusSignAfterExponent() throws Exception {
+        editable.insert(0, "-");
+        editable.insert(1, "E");
+        editable.insert(2, "-");
+        assertEquals("-E-", editable.toString());
+    }
+
+    @Test
+    public void testShouldInsertSecondMinusSignAlongWithExponent() throws Exception {
+        editable.insert(0, "-");
+        editable.insert(1, "E-");
+        assertEquals("-E-", editable.toString());
+    }
+
+    @Test
+    public void testShouldNotInsertMinusSignBeforeExistingMinusSIgn() throws Exception {
+        editable.insert(0, "-");
+        editable.insert(0, "-");
+        assertEquals("-", editable.toString());
+    }
+
+    @Test
+    public void testShouldNotInsertSecondDecimalPoint() throws Exception {
+        editable.insert(0, "0.2");
+        editable.insert(3, ".");
+        assertEquals("0.2", editable.toString());
+    }
+
+    @Test
+    public void testShouldNotInsertTwoDecimalPoints() throws Exception {
+        editable.insert(0, "..");
+        assertEquals(".", editable.toString());
+    }
+
+    @Test
+    public void testShouldNotInsertDecimalPointAfterExponent() throws Exception {
+        editable.insert(0, "2E");
+        editable.insert(2, ".");
+        assertEquals("2E", editable.toString());
+
+        editable.clear();
+        editable.insert(0, "2E.");
+        assertEquals("2E", editable.toString());
+    }
+
+    @Test
+    public void testShouldNotInsertTwoExcponents() throws Exception {
+        editable.insert(0, "2EE");
+        assertEquals("2E", editable.toString());
+    }
+
+    @Test
+    public void testShouldNotInsertExponentBeforeDecimalPoint() throws Exception {
+        editable.insert(0, "0.2");
+        editable.insert(0, "E");
+        assertEquals("0.2", editable.toString());
+    }
+}