code/src/generators/sparse_encoder.py

"""
BM25-based sparse vector encoder for Qdrant hybrid search.

Workflow
--------
1. fit(corpus)    — build vocab + IDF from a list of texts, persist to disk.
2. encode(text)   — per-document sparse vector (BM25 term weights).
3. encode_query(text) — per-query sparse vector (IDF weights only, standard pattern).
4. load(path)     — restore a previously fitted encoder.

Sparse vector format matches Qdrant's SparseVector:
    {"indices": [int, ...], "values": [float, ...]}
"""

from __future__ import annotations

import json
import math
import re
from collections import Counter
from pathlib import Path
from typing import Any, Dict, List, Tuple

from src.generators.rag_config import BM25_K1, BM25_B, BM25_MIN_DF, BM25_MAX_VOCAB, BM25_VOCAB_PATH


def _tokenize(text: str) -> List[str]:
    """Lowercase, split on non-alphanumeric, keep tokens ≥ 2 chars."""
    return [t for t in re.split(r"[^a-z0-9]+", text.lower()) if len(t) >= 2]


class BM25SparseEncoder:
    """
    Corpus-level BM25 sparse encoder.

    Attributes
    ----------
    vocab   : token → integer token_id
    idf     : token → IDF weight (Robertson–Spärck Jones)
    avgdl   : average document length in tokens
    """

    def __init__(self) -> None:
        self.vocab:  Dict[str, int]   = {}
        self.idf:    Dict[str, float] = {}
        self.avgdl:  float             = 0.0

    # ── fitting ──────────────────────────────────────────────────────────────

    def fit(self, corpus: List[str]) -> "BM25SparseEncoder":
        """
        Build vocab and IDF from a list of raw texts.
        Persists the fitted state to BM25_VOCAB_PATH automatically.
        """
        tokenized = [_tokenize(text) for text in corpus]
        N  = len(tokenized)
        df: Counter = Counter()

        for tokens in tokenized:
            for t in set(tokens):
                df[t] += 1

        self.avgdl = sum(len(t) for t in tokenized) / max(1, N)

        # Build vocab: filter low-df, keep top MAX_VOCAB by df desc
        filtered = [(t, f) for t, f in df.items() if f >= BM25_MIN_DF]
        filtered.sort(key=lambda x: -x[1])
        filtered = filtered[:BM25_MAX_VOCAB]

        self.vocab = {t: idx for idx, (t, _) in enumerate(filtered)}
        self.idf   = {
            t: math.log((N - f + 0.5) / (f + 0.5) + 1.0)
            for t, f in filtered
        }

        self.save()
        return self

    # ── encoding ─────────────────────────────────────────────────────────────

    def encode(self, text: str) -> Dict[str, Any]:
        """BM25 document vector — term-frequency weighted."""
        tokens = _tokenize(text)
        dl     = len(tokens)
        tf     = Counter(tokens)

        indices: List[int]   = []
        values:  List[float] = []

        for token, count in tf.items():
            tid = self.vocab.get(token)
            if tid is None:
                continue
            idf = self.idf[token]
            # BM25 TF normalisation
            tf_norm = count * (BM25_K1 + 1) / (
                count + BM25_K1 * (1 - BM25_B + BM25_B * dl / max(1, self.avgdl))
            )
            w = idf * tf_norm
            if w > 0:
                indices.append(tid)
                values.append(round(w, 6))

        return {"indices": indices, "values": values}

    def encode_query(self, text: str) -> Dict[str, Any]:
        """Query vector — IDF weights only (asymmetric BM25 pattern)."""
        tokens = set(_tokenize(text))
        indices: List[int]   = []
        values:  List[float] = []

        for token in tokens:
            tid = self.vocab.get(token)
            if tid is None:
                continue
            indices.append(tid)
            values.append(round(self.idf[token], 6))

        return {"indices": indices, "values": values}

    # ── persistence ───────────────────────────────────────────────────────────

    def save(self, path: str | Path | None = None) -> None:
        path = Path(path or BM25_VOCAB_PATH)
        path.parent.mkdir(parents=True, exist_ok=True)
        state = {
            "vocab":  self.vocab,
            "idf":    self.idf,
            "avgdl":  self.avgdl,
        }
        with open(path, "w", encoding="utf-8") as f:
            json.dump(state, f)

    @classmethod
    def load(cls, path: str | Path | None = None) -> "BM25SparseEncoder":
        path = Path(path or BM25_VOCAB_PATH)
        with open(path, encoding="utf-8") as f:
            state = json.load(f)
        enc = cls()
        enc.vocab  = state["vocab"]
        enc.idf    = state["idf"]
        enc.avgdl  = state["avgdl"]
        return enc

    @classmethod
    def load_or_none(cls, path: str | Path | None = None) -> "BM25SparseEncoder | None":
        import logging
        _logger = logging.getLogger(__name__)

        p = Path(path or BM25_VOCAB_PATH)
        if not p.exists():
            _logger.warning(
                "BM25 vocabulary not found at %s. Hybrid search will fall back to "
                "dense-only (sparse vectors empty). Run --build to create the index.",
                p,
            )
            return None

        try:
            enc = cls.load(p)
            if len(enc.vocab) < 10:
                _logger.warning(
                    "BM25 vocabulary at %s contains only %d tokens — sparse search "
                    "will be effectively disabled. Run --build to rebuild with full corpus.",
                    p, len(enc.vocab),
                )
            return enc
        except Exception as e:
            _logger.warning("Failed to load BM25 vocabulary from %s: %s", p, e)
            return None