feat: Add SpellBackwardDataset with word reversal and length filtering

reasoning_gym/algorithmic/word_reversal.py

@@ -9,6 +9,19 @@ from ..data import read_data_file
 from ..factory import ProceduralDataset, register_dataset
 
 
+@dataclass 
+class SpellBackwardConfig:
+    """Configuration for spelling words backward task generation"""
+    
+    min_word_len: int = 3  # Minimum word length
+    seed: Optional[int] = None 
+    size: int = 500  # Virtual dataset size
+
+    def validate(self) -> None:
+        """Validate configuration parameters"""
+        assert self.min_word_len > 0, "min_word_len must be positive"
+
+
 @dataclass
 class WordReversalConfig:
     """Configuration for word reversal task generation"""
@@ -55,4 +68,32 @@ class WordReversalDataset(ProceduralDataset):
         }
 
 
+class SpellBackwardDataset(ProceduralDataset):
+    """Generates tasks to spell words backward"""
+
+    def __init__(self, config: SpellBackwardConfig):
+        super().__init__(config=config, seed=config.seed, size=config.size)
+
+        # Load and preprocess text
+        text = read_data_file("in_the_year_2889.txt")
+        # Extract words and clean them to contain only alphanumeric characters
+        self.words = [word for word in re.findall(r"\b\w+\b", text) 
+                     if word.isalnum() and len(word) >= config.min_word_len]
+
+    def __getitem__(self, idx: int) -> dict:
+        """Generate a single spell backward task"""
+        rng = Random(self.seed + idx)
+
+        # Select random word
+        word = rng.choice(self.words)
+        answer = word[::-1]
+
+        return {
+            "question": f"Spell this word backward (example: sun -> nus): {word}",
+            "answer": answer,
+            "metadata": {"word": word, "word_len": len(word)},
+        }
+
+
+register_dataset("spell_backward", SpellBackwardDataset, SpellBackwardConfig)
 register_dataset("word_reversal", WordReversalDataset, WordReversalConfig)