stepprobe/segment.py

"""
StepProbe: CoT Step Segmentation

Parses a reasoning model's chain-of-thought output into discrete steps.
Handles both explicit markers (numbered steps, reflection cues) and 
implicit boundaries via an LLM-based fallback segmenter.
"""

import re
import json
from dataclasses import dataclass, field, asdict
from typing import List, Optional


@dataclass
class ReasoningStep:
    """A single step in a chain-of-thought trace."""
    index: int
    text: str
    step_type: str = "reasoning"  # reasoning | reflection | verification | conclusion
    is_correct: Optional[bool] = None  # filled in by diagnosis
    error_type: Optional[str] = None   # conceptual | methodological | executional | logical


@dataclass
class SegmentedCoT:
    """A full CoT trace parsed into steps."""
    problem_id: str
    model: str
    quantization: str  # "fp16" | "awq_w4" | "gptq_w4" | etc.
    raw_output: str
    final_answer: str
    steps: List[ReasoningStep] = field(default_factory=list)
    
    def to_dict(self):
        d = asdict(self)
        return d
    
    @classmethod
    def from_dict(cls, d):
        steps = [ReasoningStep(**s) for s in d.pop("steps", [])]
        return cls(**d, steps=steps)


# ============================================================
# Rule-based segmentation patterns for reasoning models
# ============================================================

# DeepSeek-R1 patterns
DEEPSEEK_PATTERNS = [
    r"(?:^|\n)\s*(?:Step\s+\d+[:.)])",                 # "Step 1:"
    r"(?:^|\n)\s*(?:\d+[.)]\s)",                         # "1. " or "1) "
    r"(?:^|\n)\s*(?:First|Second|Third|Next|Then|Finally|Now)[,:]",
    r"(?:^|\n)\s*(?:Let me|Let's|I need to|I should|I'll)",
    r"(?:^|\n)\s*(?:Wait|Hmm|Actually|Oh|But wait)",     # Reflection cues
    r"(?:^|\n)\s*(?:So |Therefore |Thus |Hence )",        # Conclusion cues  
    r"(?:^|\n)\s*(?:To verify|Let me check|Double.?check)", # Verification
]

# Classify step type based on content
STEP_TYPE_PATTERNS = {
    "reflection": [
        r"(?:Wait|Hmm|Actually|Oh|But wait|I made|mistake|error|reconsider|wrong)",
    ],
    "verification": [
        r"(?:verify|check|double.?check|confirm|validate|makes sense|correct\?)",
    ],
    "conclusion": [
        r"(?:therefore|thus|hence|so the answer|final answer|in conclusion|the result)",
        r"(?:boxed\{|\\boxed|answer is|= \d+$)",
    ],
}


def classify_step_type(text: str) -> str:
    """Classify a step as reasoning, reflection, verification, or conclusion."""
    text_lower = text.lower().strip()
    for stype, patterns in STEP_TYPE_PATTERNS.items():
        for pat in patterns:
            if re.search(pat, text_lower, re.IGNORECASE):
                return stype
    return "reasoning"


def segment_cot_rule_based(raw_output: str) -> List[str]:
    """
    Segment a CoT trace into steps using rule-based patterns.
    Returns a list of step strings.
    """
    # Combine all patterns
    combined = "|".join(f"({p})" for p in DEEPSEEK_PATTERNS)
    
    # Find all split points
    splits = []
    for match in re.finditer(combined, raw_output):
        splits.append(match.start())
    
    if not splits:
        # No explicit markers found; split by double newline
        parts = re.split(r"\n\s*\n", raw_output)
        return [p.strip() for p in parts if p.strip()]
    
    # Build segments
    segments = []
    for i, start in enumerate(splits):
        end = splits[i + 1] if i + 1 < len(splits) else len(raw_output)
        segment = raw_output[start:end].strip()
        if segment:
            segments.append(segment)
    
    # Prepend any text before the first marker
    if splits[0] > 0:
        preamble = raw_output[:splits[0]].strip()
        if preamble:
            segments.insert(0, preamble)
    
    return segments


def extract_final_answer(raw_output: str) -> str:
    """Extract the final answer from a CoT trace."""
    # Try LaTeX boxed format first
    boxed_match = re.search(r"\\boxed\{([^}]+)\}", raw_output)
    if boxed_match:
        return boxed_match.group(1).strip()
    
    # Try "The answer is X" pattern
    answer_match = re.search(
        r"(?:the\s+)?(?:final\s+)?answer\s+is[:\s]+(.+?)(?:\.|$)",
        raw_output, re.IGNORECASE
    )
    if answer_match:
        return answer_match.group(1).strip()
    
    # Last number in the output as fallback
    numbers = re.findall(r"-?\d+\.?\d*", raw_output)
    if numbers:
        return numbers[-1]
    
    return ""


def segment_cot(
    problem_id: str,
    raw_output: str,
    model: str = "",
    quantization: str = "fp16",
) -> SegmentedCoT:
    """
    Main segmentation function.
    
    Args:
        problem_id: Unique identifier for the problem
        raw_output: Raw CoT text from the model
        model: Model name
        quantization: Quantization method string
        
    Returns:
        SegmentedCoT with parsed steps
    """
    # Segment
    step_texts = segment_cot_rule_based(raw_output)
    
    # Build step objects
    steps = []
    for i, text in enumerate(step_texts):
        step = ReasoningStep(
            index=i,
            text=text,
            step_type=classify_step_type(text),
        )
        steps.append(step)
    
    # Extract answer
    final_answer = extract_final_answer(raw_output)
    
    return SegmentedCoT(
        problem_id=problem_id,
        model=model,
        quantization=quantization,
        raw_output=raw_output,
        final_answer=final_answer,
        steps=steps,
    )


# ============================================================
# CLI
# ============================================================

if __name__ == "__main__":
    import argparse
    import glob
    
    parser = argparse.ArgumentParser(description="Segment CoT traces into steps")
    parser.add_argument("--input", required=True, help="Directory with inference outputs (jsonl)")
    parser.add_argument("--output", required=True, help="Output directory for segmented steps")
    parser.add_argument("--model", default="", help="Model name tag")
    parser.add_argument("--quant", default="fp16", help="Quantization tag")
    args = parser.parse_args()
    
    import os
    os.makedirs(args.output, exist_ok=True)
    
    # Process all jsonl files
    for fpath in glob.glob(os.path.join(args.input, "*.jsonl")):
        basename = os.path.basename(fpath)
        out_path = os.path.join(args.output, basename)
        
        results = []
        with open(fpath) as f:
            for line in f:
                record = json.loads(line)
                seg = segment_cot(
                    problem_id=record.get("problem_id", record.get("id", "")),
                    raw_output=record.get("output", record.get("response", "")),
                    model=args.model,
                    quantization=args.quant,
                )
                results.append(seg.to_dict())
        
        with open(out_path, "w") as f:
            for r in results:
                f.write(json.dumps(r, ensure_ascii=False) + "\n")
        
        print(f"Segmented {len(results)} traces -> {out_path}")