rohitg00/ai-engineering-from-scratch · error · ValueError

bands ({bands}) must divide num_hashes ({num_hashes})

Error message

bands ({bands}) must divide num_hashes ({num_hashes})

What it means

Error "bands ({bands}) must divide num_hashes ({num_hashes})" thrown in rohitg00/ai-engineering-from-scratch.

Source

Thrown at phases/19-capstone-projects/42-large-corpus-downloader/code/main.py:177

                if candidate < best:
                    best = candidate
            sig.append(best)
        return sig


class LSHIndex:
    """Locality-sensitive hashing index over MinHash signatures.

    Splits each signature into `bands` bands of `rows = num_hashes / bands` rows.
    Two signatures collide if they agree on at least one band. The collision
    probability is 1 - (1 - s^r)^b where s is Jaccard similarity, which gives
    a sharp threshold near s = (1/b)^(1/r). For (b=32, r=4) the threshold is
    near s = 0.42; for (b=20, r=5) it is near s = 0.55.
    """

    def __init__(self, num_hashes: int, bands: int = DEFAULT_BANDS) -> None:
        if bands <= 0 or num_hashes % bands != 0:
            raise ValueError(f"bands ({bands}) must divide num_hashes ({num_hashes})")
        self.num_hashes = num_hashes
        self.bands = bands
        self.rows = num_hashes // bands
        self._buckets: list[dict[bytes, list[str]]] = [{} for _ in range(bands)]
        self._signatures: dict[str, list[int]] = {}

    @staticmethod
    def _band_key(band: list[int]) -> bytes:
        return hashlib.blake2b(b"".join(struct.pack("<Q", v) for v in band), digest_size=16).digest()

    def query(self, signature: list[int]) -> str | None:
        """Return the doc id of a near-duplicate keeper or None."""

        for i in range(self.bands):
            band = signature[i * self.rows : (i + 1) * self.rows]
            key = self._band_key(band)
            bucket = self._buckets[i].get(key)
            if bucket:

View on GitHub (pinned to 39ea8a1c6d)

When it happens

Trigger: Thrown at phases/19-capstone-projects/42-large-corpus-downloader/code/main.py:177 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of rohitg00/ai-engineering-from-scratch@39ea8a1c6d (2026-08-26). Data as JSON: /api/errors/cbd5891bd68b6488. Report an issue: GitHub.