# Source code for lhotse.dataset.cut_transforms.concatenate

```
from typing import Optional, Sequence
from lhotse import CutSet
from lhotse.cut import Cut
from lhotse.utils import Seconds
[docs]
class CutConcatenate:
"""
A transform on batch of cuts (``CutSet``) that concatenates the cuts to minimize the total amount of padding;
e.g. instead of creating a batch with 40 examples, we will merge some of the examples together
adding some silence between them to avoid a large number of padding frames that waste the computation.
"""
[docs]
def __init__(
self,
gap: Seconds = 1.0,
duration_factor: float = 1.0,
max_duration: Optional[Seconds] = None,
) -> None:
"""
CutConcatenate's constructor.
:param gap: The duration of silence in seconds that is inserted between the cuts;
it's goal is to let the model "know" that there are separate utterances in a single example.
:param duration_factor: Determines the maximum duration of the concatenated cuts;
by default it's 1, setting the limit at the duration of the longest cut in the batch.
:param max_duration: If a value is given (in seconds), the maximum duration of concatenated cuts
is fixed to the value while duration_factor is ignored.
"""
self.gap = gap
self.duration_factor = duration_factor
self.max_duration = max_duration
def __call__(self, cuts: CutSet) -> CutSet:
cuts = cuts.sort_by_duration(ascending=False)
return concat_cuts(
cuts,
gap=self.gap,
max_duration=self.max_duration
if self.max_duration
else cuts[0].duration * self.duration_factor,
)
def concat_cuts(
cuts: Sequence[Cut], gap: Seconds = 1.0, max_duration: Optional[Seconds] = None
) -> CutSet:
"""
We're going to concatenate the cuts to minimize the amount of total padding frames used.
This means that some samples in the batch will be merged together into one sample,
separated by an interval of silence.
This is actually solving a knapsack problem.
In this initial implementation we're using a greedy approach:
going from the back (i.e. the shortest cuts) we'll try to concat them to the longest cut
that still has some "space" at the end.
:param cuts: a list of cuts to pack.
:param gap: the duration of silence inserted between concatenated cuts.
:param max_duration: the maximum duration for the concatenated cuts
(by default set to the duration of the first cut).
:return a list of packed cuts.
"""
if len(cuts) <= 1:
# Nothing to do.
return CutSet.from_cuts(cuts)
cuts = sorted(cuts, key=lambda c: c.duration, reverse=True)
max_duration = cuts[0].duration if max_duration is None else max_duration
current_idx = 0
while True:
can_fit = False
shortest = cuts[-1]
for idx in range(current_idx, len(cuts) - 1):
cut = cuts[current_idx]
can_fit = cut.duration + gap + shortest.duration <= max_duration
if can_fit:
cuts[current_idx] = cut.pad(cut.duration + gap).append(shortest)
cuts = cuts[:-1]
break
current_idx += 1
if not can_fit:
break
return CutSet.from_cuts(cuts)
```