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# coding=utf-8
# Copyright 2023 Authors of "A Watermark for Large Language Models"
# available at https://arxiv.org/abs/2301.10226
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
import os
import torch
from transformers import LogitsProcessor
from typing import List, Union
GAMMA = os.getenv("WATERMARK_GAMMA", 0.5)
DELTA = os.getenv("WATERMARK_DELTA", 2.0)
class WatermarkLogitsProcessor(LogitsProcessor):
def __init__(
self,
gamma: float = GAMMA,
delta: float = DELTA,
hash_key: int = 15485863, # just a large prime number to create a rng seed with sufficient bit width
device: str = "cpu",
):
# watermarking parameters
self.gamma = gamma
self.delta = delta
self.rng = torch.Generator(device=device)
self.hash_key = hash_key
def _seed_rng(self, input_ids: Union[List[int], torch.LongTensor]):
if isinstance(input_ids, list):
assert (
len(input_ids) >= 1
), "requires at least a 1 token prefix sequence to seed rng"
prev_token = input_ids[-1]
else:
assert len(input_ids) == 1
input_ids = input_ids[0]
assert (
input_ids.shape[-1] >= 1
), "requires at least a 1 token prefix sequence to seed rng"
prev_token = input_ids[-1].item()
self.rng.manual_seed(self.hash_key * prev_token)
def _get_greenlist_ids(
self,
input_ids: Union[List[int], torch.LongTensor],
max_value: int,
device: torch.device,
) -> List[int]:
# seed the rng using the previous tokens/prefix
self._seed_rng(input_ids)
greenlist_size = int(max_value * self.gamma)
vocab_permutation = torch.randperm(max_value, device=device, generator=self.rng)
greenlist_ids = vocab_permutation[:greenlist_size]
return greenlist_ids
@staticmethod
def _calc_greenlist_mask(
scores: torch.FloatTensor, greenlist_token_ids
) -> torch.BoolTensor:
green_tokens_mask = torch.zeros_like(scores)
green_tokens_mask[-1, greenlist_token_ids] = 1
final_mask = green_tokens_mask.bool()
return final_mask
@staticmethod
def _bias_greenlist_logits(
scores: torch.Tensor, greenlist_mask: torch.Tensor, greenlist_bias: float
) -> torch.Tensor:
scores[greenlist_mask] = scores[greenlist_mask] + greenlist_bias
return scores
def __call__(
self, input_ids: Union[List[int], torch.LongTensor], scores: torch.FloatTensor
) -> torch.FloatTensor:
greenlist_ids = self._get_greenlist_ids(
input_ids, scores.shape[-1], scores.device
)
green_tokens_mask = self._calc_greenlist_mask(
scores=scores, greenlist_token_ids=greenlist_ids
)
scores = self._bias_greenlist_logits(
scores=scores, greenlist_mask=green_tokens_mask, greenlist_bias=self.delta
)
return scores
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