import torch


def pitch_transform_custom(pitch, pitch_lens):
    """Apply a custom pitch transformation to predicted pitch values.

    This sample modification linearly increases the pitch throughout
    the utterance from 0.5 of predicted pitch to 1.5 of predicted pitch.
    In other words, it starts low and ends high.

    PARAMS
    ------
    pitch: torch.Tensor (bs, max_len)
        Predicted pitch values for each lexical unit, padded to max_len (in Hz).
    pitch_lens: torch.Tensor (bs, max_len)
        Number of lexical units in each utterance.

    RETURNS
    -------
    pitch: torch.Tensor
        Modified pitch (in Hz).
    """

    weights = torch.arange(pitch.size(1), dtype=torch.float32, device=pitch.device)

    # The weights increase linearly from 0.0 to 1.0 in every i-th row
    # in the range (0, pitch_lens[i])
    weights = weights.unsqueeze(0) / pitch_lens.unsqueeze(1)

    # Shift the range from (0.0, 1.0) to (0.5, 1.5)
    weights += 0.5

    return pitch * weights