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"""Multibox location transform for object detection."""

from . import _ffi_api


def multibox_transform_loc(
    cls_pred,
    loc_pred,
    anchor,
    clip=False,
    threshold=0.0,
    variances=(1.0, 1.0, 1.0, 1.0),
    keep_background=True,
):
    """SSD / TFLite-style decode: priors + offsets → boxes; logits → softmax scores.

    Box decode follows TFLite ``DecodeCenterSizeBoxes``; expected tensor layout matches
    ``tflite_frontend.convert_detection_postprocess`` (loc reorder yxhw→xywh, anchor ltrb).

    Parameters
    ----------
    cls_pred : relax.Expr
        ``[B, C, N]`` class logits (pre-softmax).
    loc_pred : relax.Expr
        ``[B, 4*N]`` per-anchor encodings as ``(x,y,w,h)`` after reorder (see above).
    anchor : relax.Expr
        ``[1, N, 4]`` priors: ``(left, top, right, bottom)``.
    clip : bool
        If True, clip ``ymin,xmin,ymax,xmax`` to ``[0, 1]``.
    threshold : float
        After softmax, multiply scores by mask ``(score >= threshold)``.
    variances : tuple of 4 floats
        ``(x,y,w,h)`` = TFLite ``1/x_scale, 1/y_scale, 1/w_scale, 1/h_scale``.
        Use magnitudes consistent with the model: very large ``w``/``h`` entries scale the
        encoded height/width terms inside ``exp(...)`` and can overflow in float32/float16.
    keep_background : bool
        If False, set output scores at class index 0 to zero.

    Returns
    -------
    result : relax.Expr
        Tuple ``(boxes, scores)``: ``boxes`` is ``[B, N, 4]`` as ``(ymin,xmin,ymax,xmax)``;
        ``scores`` is ``[B, C, N]`` softmax, post-processed like the implementation.

    Notes
    -----
    **Shape/dtype (checked in ``FInferStructInfo`` when static):**

    - ``cls_pred``: 3-D; ``loc_pred``: 2-D; ``anchor``: 3-D.
    - ``cls_pred``, ``loc_pred``, ``anchor`` dtypes must match.
    - ``N = cls_pred.shape[2]``; ``loc_pred.shape[1] == 4*N``; ``anchor.shape == [1,N,4]``.
    - ``loc_pred.shape[1]`` must be divisible by 4.
    - ``cls_pred.shape[0]`` must equal ``loc_pred.shape[0]`` (batch).

    If ``cls_pred`` has **unknown** shape, inference only returns generic rank-3 tensor
    struct info for the two outputs; it does **not** verify ``4*N`` vs ``loc_pred`` or
    ``anchor.shape[1]`` vs ``N``, because ``N`` is not available statically. Other checks
    (ranks, dtypes, ``loc_pred.shape[1] % 4 == 0`` when known, batch match when both batch
    axes are known, etc.) still run where applicable.
    """
    return _ffi_api.multibox_transform_loc(
        cls_pred,
        loc_pred,
        anchor,
        clip,
        threshold,
        variances,
        keep_background,
    )
