Implementing ANLS
Average Normalized Levenshtein Similarity, abbreviated as ANLS, is a metric used to compute the similarity between two strings.
In natural language processing (NLP), it's often necessary to compare the similarity of two strings.
Levenshtein Similarity is a common measure that assesses the "edit distance" between two strings, which is the minimum number of single-character edits (insertions, deletions, or substitutions) required to change one string into the other. However, Levenshtein Similarity itself isn't intuitive as it depends on the lengths of the strings.
To address this issue, we can normalize Levenshtein Similarity to the [0, 1] range, making it easier to understand and compare the similarity between different strings, known as Normalized Levenshtein Similarity (NLS).
As NLS refers to the similarity between sets of strings, we can further extend it to ANLS, which computes the average similarity among multiple sets of strings, thereby quantifying the performance of a model.
And then...
I always struggle to find implementations that I like, so I decided to write one myself.
Import Necessary Libraries
First, we need to import some necessary libraries, especially the EditDistance implemented by torchmetrics:
from typing import Any, Literal, Optional, Sequence, Union
import torch
from torch import Tensor
from torchmetrics.metric import Metric
from torchmetrics.text import EditDistance
from torchmetrics.utilities.data import dim_zero_cat
Since EditDistance can already compute the Levenshtein distance, we can directly use it to calculate the edit distance between two strings. However, EditDistance doesn't provide normalization functionality, so we need to implement this part ourselves.
Implement Normalization Functionality
Here, we inherit the interface of torchmetrics.metric.Metric, so we need to implement the update and compute methods:
class NormalizedLevenshteinSimilarity(Metric):
def __init__(
self,
substitution_cost: int = 1,
reduction: Optional[Literal["mean", "sum", "none"]] = "mean",
**kwargs: Any
) -> None:
super().__init__(**kwargs)
self.edit_distance = EditDistance(
substitution_cost=substitution_cost,
reduction=None # Set to None to get distances for all string pairs
)
# ...
Here are a few key points:
- Ensure that the input
predsandtargetare lists of strings, otherwise the function will calculate on a character level. - Calculate the maximum length of each string, so that we can perform normalization.
def update(self, preds: Union[str, Sequence[str]], target: Union[str, Sequence[str]]) -> None:
"""Update state with predictions and targets."""
if isinstance(preds, str):
preds = [preds]
if isinstance(target, str):
target = [target]
distances = self.edit_distance(preds, target)
max_lengths = torch.tensor([
max(len(p), len(t))
for p, t in zip(preds, target)
], dtype=torch.float)
ratio = torch.where(
max_lengths == 0,
torch.zeros_like(distances).float(),
distances.float() / max_lengths
)
nls_values = 1 - ratio
# ...
Implement the reduction Parameter
We also need to accommodate the reduction parameter, where if we specify mean, it will be the common ANLS score.
In addition to the usual mean, we can also use sum or none to fulfill different needs.
def _compute(
self,
nls_score: Tensor,
num_elements: Union[Tensor, int],
) -> Tensor:
"""Compute the ANLS over state."""
if nls_score.numel() == 0:
return torch.tensor(0, dtype=torch.int32)
if self.reduction == "mean":
return nls_score.sum() / num_elements
if self.reduction == "sum":
return nls_score.sum()
if self.reduction is None or self.reduction == "none":
return nls_score
def compute(self) -> torch.Tensor:
"""Compute the NLS over state."""
if self.reduction == "none" or self.reduction is None:
return self._compute(dim_zero_cat(self.nls_values_list), 1)
return self._compute(self.nls_score, self.num_elements)
Here, it's noteworthy that when we specify reduction as none, we need to return all NLS values instead of computing the average. In this case, I referenced the implementation of torchmetrics.text.EditDistance, using dim_zero_cat to concatenate values in the list together, ensuring that the return value is a Tensor.
Complete Implementation
We have synchronized the code to DocsaidKit/.../normalized_levenshtein_similarity.py
The complete implementation is as follows:
from typing import Any, Literal, Optional, Sequence, Union
import torch
from torch import Tensor
from torchmetrics.metric import Metric
from torchmetrics.text import EditDistance
from torchmetrics.utilities.data import dim_zero_cat
class NormalizedLevenshteinSimilarity(Metric):
"""
Normalized Levenshtein Similarity (NLS) is a metric that computes the
normalized Levenshtein similarity between two sequences.
This metric is calculated as 1 - (levenshtein_distance / max_length),
where `levenshtein_distance` is the Levenshtein distance between the two
sequences and `max_length` is the maximum length of the two sequences.
NLS aims to provide a similarity measure for character sequences
(such as text), making it useful in areas like text similarity analysis,
Optical Character Recognition (OCR), and Natural Language Processing (NLP).
This class inherits from `Metric` and uses the `EditDistance` class to
compute the Levenshtein distance.
Inputs to the ``update`` and ``compute`` methods are as follows:
- ``preds`` (:class:`~Union[str, Sequence[str]]`):
Predicted text sequences or a collection of sequences.
- ``target`` (:class:`~Union[str, Sequence[str]]`):
Target text sequences or a collection of sequences.
Output from the ``compute`` method is as follows:
- ``nls`` (:class:`~torch.Tensor`): A tensor containing the NLS value.
Returns 0.0 when there are no samples; otherwise, it returns the NLS.
Args:
substitution_cost:
The cost of substituting one character for another. Default is 1.
reduction:
Method to aggregate metric scores.
Default is 'mean', options are 'sum' or None.
- ``'mean'``: takes the mean over samples, which is ANLS.
- ``'sum'``: takes the sum over samples
- ``None`` or ``'none'``: returns the score per sample
kwargs: Additional keyword arguments.
Example::
Multiple strings example:
>>> metric = NormalizedLevenshteinSimilarity(reduction=None)
>>> preds = ["rain", "lnaguaeg"]
>>> target = ["shine", "language"]
>>> metric(preds, target)
tensor([0.4000, 0.5000])
>>> metric = NormalizedLevenshteinSimilarity(reduction="mean")
>>> metric(preds, target)
tensor(0.4500)
"""
def __init__(
self,
substitution_cost: int = 1,
reduction: Optional[Literal["mean", "sum", "none"]] = "mean",
**kwargs: Any
) -> None:
super().__init__(**kwargs)
self.edit_distance = EditDistance(
substitution_cost=substitution_cost,
reduction=None # Set to None to get distances for all string pairs
)
allowed_reduction = (None, "mean", "sum", "none")
if reduction not in allowed_reduction:
raise ValueError(
f"Expected argument `reduction` to be one of {allowed_reduction}, but got {reduction}")
self.reduction = reduction
if self.reduction == "none" or self.reduction is None:
self.add_state(
"nls_values_list",
default=[],
dist_reduce_fx="cat"
)
else:
self.add_state(
"nls_score",
default=torch.tensor(0.0),
dist_reduce_fx="sum"
)
self.add_state(
"num_elements",
default=torch.tensor(0),
dist_reduce_fx="sum"
)
def update(self, preds: Union[str, Sequence[str]], target: Union[str, Sequence[str]]) -> None:
"""Update state with predictions and targets."""
if isinstance(preds, str):
preds = [preds]
if isinstance(target, str):
target = [target]
distances = self.edit_distance(preds, target)
max_lengths = torch.tensor([
max(len(p), len(t))
for p, t in zip(preds, target)
], dtype=torch.float)
ratio = torch.where(
max_lengths == 0,
torch.zeros_like(distances).float(),
distances.float() / max_lengths
)
nls_values = 1 - ratio
if self.reduction == "none" or self.reduction is None:
self.nls_values_list.append(nls_values)
else:
self.nls_score += nls_values.sum()
self.num_elements += nls_values.shape[0]
def _compute(
self,
nls_score: Tensor,
num_elements: Union[Tensor, int],
) -> Tensor:
"""Compute the ANLS over state."""
if nls_score.numel() == 0:
return torch.tensor(0, dtype=torch.int32)
if self.reduction == "mean":
return nls_score.sum() / num_elements
if self.reduction == "sum":
return nls_score.sum()
if self.reduction is None or self.reduction == "none":
return nls_score
def compute(self) -> torch.Tensor:
"""Compute the NLS over state."""
if self.reduction == "none" or self.reduction is None:
return self._compute(dim_zero_cat(self.nls_values_list), 1)
return self._compute(self.nls_score, self.num_elements)
if __name__ == "__main__":
anls = NormalizedLevenshteinSimilarity(reduction='mean')
preds = ["rain", "lnaguaeg"]
target = ["shine", "language"]
print(anls(preds, target))