Added unit tests for SlugBuilder.

Had to add separator field, so that can keep track of how long the separator is
and fix bug that occurs with long separator Strings.

src/main/java/com/eddarmitage/slugger/SlugBuilder.java

@@ -23,23 +23,27 @@ class SlugBuilder {
 
     private final OptionalInt targetLength;
     private final boolean enforceHardLimit;
+    private final CharSequence separator;
     private final StringJoiner joiner;
 
     SlugBuilder(CharSequence separator) {
         this.targetLength = OptionalInt.empty();
         this.enforceHardLimit = false;
+        this.separator = separator;
         this.joiner = new StringJoiner(separator);
     }
 
     SlugBuilder(OptionalInt targetLength, boolean enforceHardLimit, CharSequence separator) {
         this.targetLength = targetLength;
         this.enforceHardLimit = enforceHardLimit;
+        this.separator = separator;
         joiner = new StringJoiner(separator);
     }
 
-    private SlugBuilder(OptionalInt targetLength, boolean enforceHardLimit, StringJoiner joiner) {
+    private SlugBuilder(OptionalInt targetLength, boolean enforceHardLimit, CharSequence separator, StringJoiner joiner) {
         this.targetLength = targetLength;
         this.enforceHardLimit = enforceHardLimit;
+        this.separator = separator;
         this.joiner = joiner;
     }
 
@@ -56,7 +60,7 @@ public void addWord(String word) {
     }
 
     public SlugBuilder merge(SlugBuilder other) {
-        return new SlugBuilder(targetLength, enforceHardLimit, joiner.merge(other.joiner));
+        return new SlugBuilder(targetLength, enforceHardLimit, separator, joiner.merge(other.joiner));
     }
 
     public String build() {
@@ -72,7 +76,7 @@ private void addFirstWord(String word) {
     }
 
     private boolean wordWillFit(String word) {
-        return (targetLength.isPresent() && joiner.length() + word.length() <= targetLength.getAsInt()) || !targetLength.isPresent();
+        return !targetLength.isPresent() || joiner.length() + word.length() + separator.length() <= targetLength.getAsInt();
     }
 
     public static Collector<String, SlugBuilder, String> collector(OptionalInt targetLength, boolean enforceHardLimit, CharSequence separator) {

src/test/java/com/eddarmitage/slugger/SlugBuilderTest.java

@@ -0,0 +1,74 @@
+package com.eddarmitage.slugger;
+
+import org.assertj.core.api.SoftAssertions;
+import org.junit.Before;
+import org.junit.Test;
+
+import java.util.Arrays;
+import java.util.OptionalInt;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class SlugBuilderTest {
+
+    @Before
+    public void setup() {
+
+    }
+
+    @Test
+    public void testSlugBuilder_withDefaultConfiguration() {
+        SlugBuilder slugBuilder = new SlugBuilder("-");
+        Arrays.asList("one", "two", "three").forEach(slugBuilder::addWord);
+
+        assertThat(slugBuilder.build()).isEqualTo("one-two-three");
+    }
+
+    @Test
+    public void testSlugBuilder_withLengthRestriction() {
+        SlugBuilder slugBuilder = new SlugBuilder(OptionalInt.of(10), false, "-");
+        Arrays.asList("one", "two", "three").forEach(slugBuilder::addWord);
+
+        assertThat(slugBuilder.build()).isEqualTo("one-two");
+    }
+
+    @Test
+    public void testSlugBuilder_withLengthRestrictionAndLongSeparator() {
+        SlugBuilder slugBuilder = new SlugBuilder(OptionalInt.of(10), false, "-----");
+        Arrays.asList("one", "two", "three").forEach(slugBuilder::addWord);
+
+        assertThat(slugBuilder.build()).isEqualTo("one");
+    }
+
+    @Test
+    public void testSlugBuilder_withLongFirstWord() {
+        SlugBuilder slugBuilder = new SlugBuilder(OptionalInt.of(10), false, "-");
+        slugBuilder.addWord("refrigerator");
+
+        assertThat(slugBuilder.build()).isEqualTo("refrigerator");
+    }
+
+    @Test
+    public void testSlugBuilder_withLongFirstWordAndHardLimitsEnforced() {
+        SlugBuilder slugBuilder = new SlugBuilder(OptionalInt.of(10), true, "-");
+        slugBuilder.addWord("refrigerator");
+
+        assertThat(slugBuilder.build()).isEqualTo("refrigerat");
+    }
+
+    @Test
+    public void testSlugBuilder_isMergedWithOtherSlugBuilder() {
+        SlugBuilder firstBuilder = new SlugBuilder("-");
+        Arrays.asList("one", "two").forEach(firstBuilder::addWord);
+
+        SlugBuilder secondBuilder = new SlugBuilder("-");
+        Arrays.asList("three", "four").forEach(secondBuilder::addWord);
+
+        String mergedResult = firstBuilder.merge(secondBuilder).build();
+
+        SoftAssertions softly = new SoftAssertions();
+        softly.assertThat(mergedResult).hasSize(18);
+        softly.assertThat(mergedResult).isEqualTo("one-two-three-four");
+        softly.assertAll();
+    }
+}