def compute_mask_indices(
    shape: Tuple[int, int],
    padding_mask: Optional[torch.Tensor],
    mask_prob: float,
    mask_length: int,
    mask_type: str = "static",
    mask_other: float = 0.0,
    min_masks: int = 0,
    no_overlap: bool = False,
    min_space: int = 0,
    require_same_masks: bool = True,
    mask_dropout: float = 0.0,
    add_masks: bool = False,
    seed: Optional[int] = None,
    epoch: Optional[int] = None,
    indices: Optional[torch.Tensor] = None,
    idc_select_ver: int = 1,  # 2 to reproduce mask_tokens_dataset
    num_mask_ver: int = 2,  # 2 to reproduce mask_tokens_dataset
) -> np.ndarray:
    """
    Computes random mask spans for a given shape

    Args:
        shape: the the shape for which to compute masks.
            should be of size 2 where first element is batch size and 2nd is timesteps
        padding_mask: optional padding mask of the same size as shape, which will prevent masking padded elements
        mask_prob: probability for each token to be chosen as start of the span to be masked. this will be multiplied by
            number of timesteps divided by length of mask span to mask approximately this percentage of all elements.
            however due to overlaps, the actual number will be smaller (unless no_overlap is True)
        mask_type: how to compute mask lengths
            static = fixed size
            uniform = sample from uniform distribution [mask_other, mask_length*2]
            normal = sample from normal distribution with mean mask_length and stdev mask_other. mask is min 1 element
            poisson = sample from possion distribution with lambda = mask length
        min_masks: minimum number of masked spans
        no_overlap: if false, will switch to an alternative recursive algorithm that prevents spans from overlapping
        min_space: only used if no_overlap is True, this is how many elements to keep unmasked between spans
        require_same_masks: if true, will randomly drop out masks until same amount of masks remains in each sample
        mask_dropout: randomly dropout this percentage of masks in each example
    """

    bsz, all_sz = shape
    mask = np.full((bsz, all_sz), False)

    if num_mask_ver == 1:
        all_num_mask = int(
            # add a random number for probabilistic rounding
            mask_prob * all_sz / float(mask_length)
            + np.random.rand()
        )
        all_num_mask = max(min_masks, all_num_mask)

    mask_idcs = []
    for i in range(bsz):
        if seed is not None and epoch is not None and indices is not None:
            seed_i = int(hash((seed, epoch, indices[i].item())) % 1e6)
        else:
            seed_i = None

        rng = np.random.default_rng(seed_i)

        if padding_mask is not None:
            sz = all_sz - padding_mask[i].long().sum().item()
            assert sz >= 0, sz
        else:
            sz = all_sz

        if num_mask_ver == 1:
            if padding_mask is not None:
                num_mask = int(
                    # add a random number for probabilistic rounding
                    mask_prob * sz / float(mask_length)
                    + np.random.rand()
                )
                num_mask = max(min_masks, num_mask)
            else:
                num_mask = all_num_mask
        elif num_mask_ver == 2:
            num_mask = int(
                # add a random number for probabilistic rounding
                mask_prob * sz / float(mask_length)
                + rng.random()
            )
            num_mask = max(min_masks, num_mask)
        else:
            raise ValueError()

        if mask_type == "static":
            lengths = np.full(num_mask, mask_length)
        elif mask_type == "uniform":
            lengths = rng.randint(mask_other, mask_length * 2 + 1, size=num_mask)
        elif mask_type == "normal":
            lengths = rng.normal(mask_length, mask_other, size=num_mask)
            lengths = [max(1, int(round(x))) for x in lengths]
        elif mask_type == "poisson":
            lengths = rng.poisson(mask_length, size=num_mask)
            lengths = [int(round(x)) for x in lengths]
        else:
            raise Exception("unknown mask selection " + mask_type)

        if sum(lengths) == 0:
            if mask_type == "static":
                raise ValueError(f"this should never happens")
            else:
                lengths = [min(mask_length, sz - 1)]

        if no_overlap:
            mask_idc = []

            def arrange(s, e, length, keep_length):
                span_start = rng.randint(s, e - length)
                mask_idc.extend(span_start + i for i in range(length))

                new_parts = []
                if span_start - s - min_space >= keep_length:
                    new_parts.append((s, span_start - min_space + 1))
                if e - span_start - length - min_space > keep_length:
                    new_parts.append((span_start + length + min_space, e))
                return new_parts

            parts = [(0, sz)]
            min_length = min(lengths)
            for length in sorted(lengths, reverse=True):
                lens = np.fromiter(
                    (e - s if e - s >= length + min_space else 0 for s, e in parts),
                    np.int,
                )
                l_sum = np.sum(lens)
                if l_sum == 0:
                    break
                probs = lens / np.sum(lens)
                c = rng.choice(len(parts), p=probs)
                s, e = parts.pop(c)
                parts.extend(arrange(s, e, length, min_length))
            mask_idc = np.asarray(mask_idc)
        else:
            if idc_select_ver == 1:
                min_len = min(lengths)
                if sz - min_len <= num_mask:
                    min_len = sz - num_mask - 1
                mask_idc = rng.choice(sz - min_len, num_mask, replace=False)
            elif idc_select_ver == 2:
                mask_idc = rng.choice(sz, num_mask, replace=False)
            else:
                raise ValueError()

            mask_idc = np.asarray(
                [
                    mask_idc[j] + offset
                    for j in range(len(mask_idc))
                    for offset in range(lengths[j])
                ]
            )

        mask_idc = np.unique(mask_idc[mask_idc < sz])
        if len(mask_idc) >= sz:
            raise ValueError(
                (
                    f"the entire sequence is masked. "
                    f"sz={sz}; mask_idc[mask_idc]; "
                    f"index={indices[i] if indices is not None else None}"
                )
            )
        mask_idcs.append(mask_idc)

    target_len = None
    if require_same_masks:
        if add_masks:
            target_len = max([len(m) for m in mask_idcs])
        else:
            target_len = min([len(m) for m in mask_idcs])

    for i, mask_idc in enumerate(mask_idcs):
        if target_len is not None and len(mask_idc) > target_len:
            mask_idc = rng.choice(mask_idc, target_len, replace=False)

        mask[i, mask_idc] = True

        if target_len is not None and len(mask_idc) < target_len:
            unmasked = np.flatnonzero(~mask[i])
            to_mask = rng.choice(unmasked, target_len - len(mask_idc), replace=False)
            mask[i, to_mask] = True

        if mask_dropout > 0:
            masked = np.flatnonzero(mask[i])
            num_holes = np.rint(len(masked) * mask_dropout).astype(int)
            to_drop = rng.choice(masked, num_holes, replace=False)
            mask[i, to_drop] = False

    return mask
Computes random mask spans for a given shape

Args:
shape: the the shape for which to compute masks.
should be of size 2 where first element is batch size and 2nd is timesteps
padding_mask: optional padding mask of the same size as shape, which will prevent masking padded elements
mask_prob: probability for each token to be chosen as start of the span to be masked. this will be multiplied by
number of timesteps divided by length of mask span to mask approximately this percentage of all elements.
however due to overlaps, the actual number will be smaller (unless no_overlap is True)
mask_type: how to compute mask lengths
static = fixed size
uniform = sample from uniform distribution [mask_other, mask_length*2]
normal = sample from normal distribution with mean mask_length and stdev mask_other. mask is min 1 element
poisson = sample from possion distribution with lambda = mask length
min_masks: minimum number of masked spans
no_overlap: if false, will switch to an alternative recursive algorithm that prevents spans from overlapping
min_space: only used if no_overlap is True, this is how many elements to keep unmasked between spans
require_same_masks: if true, will randomly drop out masks until same amount of masks remains in each sample
mask_dropout: randomly dropout this percentage of masks in each example
fairseq/data/data_utils.py