Revert "Remove unused EmailHelper"

package com.fsck.k9.helper;

import org.junit.Test;

import static org.junit.Assert.assertEquals;

import static org.junit.Assert.assertNull;

public class EmailHelperTest {

    @Test

    public void getDomainFromEmailAddress_withRegularEmail_shouldReturnsDomain() {

        String result = EmailHelper.getDomainFromEmailAddress("user@domain.com");

        assertEquals("domain.com", result);

    }

    @Test

    public void getDomainFromEmailAddress_withInvalidEmail_shouldReturnNull() {

        String result = EmailHelper.getDomainFromEmailAddress("user");

        assertNull(result);

    }

    @Test

    public void getDomainFromEmailAddress_withTLD_shouldReturnDomain() {

        String result = EmailHelper.getDomainFromEmailAddress("user@domain");

        assertEquals("domain", result);

    }

    @Test

    public void getDomainFromEmailAddress_withEmptyDomain_shouldReturnNull() {

        String result = EmailHelper.getDomainFromEmailAddress("user@");

        assertNull(result);

    }

    @Test

    public void getLocalPartFromEmailAddress_withRegularEmail_shouldReturnLocalPart() {

        String result = EmailHelper.getLocalPartFromEmailAddress("user@domain.com");

        assertEquals("user", result);

    }

    @Test

    public void getLocalPartFromEmailAddress_withAtInLocalPart_shouldReturnLocalPart() {

        String result = EmailHelper.getLocalPartFromEmailAddress("\"user@work\"@domain");

        assertEquals("\"user@work\"", result);

    }

    @Test

    public void getLocalPartFromEmailAddress_withInvalidEmail_shouldReturnNull() {

        String result = EmailHelper.getLocalPartFromEmailAddress("user");

        assertNull(result);

    }

    @Test

    public void getLocalPartFromEmailAddress_withEmptyDomain_shouldReturnNull() {

        String result = EmailHelper.getLocalPartFromEmailAddress("user@");

        assertNull(result);

    }

}

package com.fsck.k9.helper

object EmailHelper {

    @JvmStatic

    fun getLocalPartFromEmailAddress(email: String): String? {

        val index = email.lastIndexOf('@')

        return if (index == -1 || index == email.lastIndex) null else email.substring(0, index)

    }

    @JvmStatic

    fun getDomainFromEmailAddress(email: String): String? {

        val index = email.lastIndexOf('@')

        return if (index == -1 || index == email.lastIndex) null else email.substring(index + 1)

    }

}