feat: Add SpellBackwardDataset with word reversal and length filtering

tests/test_word_reversal.py

@@ -54,6 +54,60 @@ def test_word_reversal_dataset_items():
         assert answer_words == list(reversed(question_words))
 
 
+def test_spell_backward_config_validation():
+    """Test that invalid configs raise appropriate errors"""
+    with pytest.raises(AssertionError):
+        config = SpellBackwardConfig(min_word_len=0)
+        config.validate()
+
+
+def test_spell_backward_dataset_deterministic():
+    """Test that dataset generates same items with same seed"""
+    config = SpellBackwardConfig(seed=42, size=10)
+    dataset1 = SpellBackwardDataset(config)
+    dataset2 = SpellBackwardDataset(config)
+
+    for i in range(len(dataset1)):
+        assert dataset1[i] == dataset2[i]
+
+
+def test_spell_backward_dataset_items():
+    """Test basic properties of generated items"""
+    config = SpellBackwardConfig(min_word_len=3, size=10, seed=42)
+    dataset = SpellBackwardDataset(config)
+
+    for i in range(len(dataset)):
+        item = dataset[i]
+        # Check item structure
+        assert isinstance(item, dict)
+        assert "question" in item
+        assert "answer" in item
+        assert "metadata" in item
+
+        # Check metadata
+        assert "word" in item["metadata"]
+        assert "word_len" in item["metadata"]
+
+        # Verify word length constraint
+        word = item["metadata"]["word"]
+        assert len(word) >= config.min_word_len
+
+        # Verify answer is correct
+        assert item["answer"] == word[::-1]
+
+
+def test_spell_backward_dataset_iteration():
+    """Test that iteration respects dataset size"""
+    config = SpellBackwardConfig(size=5, seed=42)
+    dataset = SpellBackwardDataset(config)
+
+    items = list(dataset)
+    assert len(items) == config.size
+
+    # Test multiple iterations yield same items
+    assert items == list(dataset)
+
+
 def test_word_reversal_dataset_iteration():
     """Test that iteration respects dataset size"""
     config = WordReversalConfig(size=5, seed=42)