# coding=utf-8

# Copyright 2025 The HuggingFace Inc. team.

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# Licensed under the Apache License, Version 2.0 (the "License");

# you may not use this file except in compliance with the License.

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# http://www.apache.org/licenses/LICENSE-2.0

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"""PyTorch optimization for diffusion models."""

import math

from enum import Enum

from torch.optim import Optimizer

from torch.optim.lr_scheduler import LambdaLR

from .utils import logging

logger = logging.get_logger(__name__)

class SchedulerType(Enum):

LINEAR = "linear"

COSINE = "cosine"

COSINE_WITH_RESTARTS = "cosine_with_restarts"

POLYNOMIAL = "polynomial"

CONSTANT = "constant"

CONSTANT_WITH_WARMUP = "constant_with_warmup"

PIECEWISE_CONSTANT = "piecewise_constant"

def get_constant_schedule(optimizer: Optimizer, last_epoch: int = -1) -> LambdaLR:

"""

Create a schedule with a constant learning rate, using the learning rate set in optimizer.

Args:

optimizer ([`~torch.optim.Optimizer`]):

The optimizer for which to schedule the learning rate.

last_epoch (`int`, *optional*, defaults to -1):

The index of the last epoch when resuming training.

Return:

`torch.optim.lr_scheduler.LambdaLR` with the appropriate schedule.

"""

return LambdaLR(optimizer, lambda _: 1, last_epoch=last_epoch)

def get_constant_schedule_with_warmup(optimizer: Optimizer, num_warmup_steps: int, last_epoch: int = -1) -> LambdaLR:

"""

Create a schedule with a constant learning rate preceded by a warmup period during which the learning rate

increases linearly between 0 and the initial lr set in the optimizer.

Args:

optimizer ([`~torch.optim.Optimizer`]):

The optimizer for which to schedule the learning rate.

num_warmup_steps (`int`):

The number of steps for the warmup phase.

last_epoch (`int`, *optional*, defaults to -1):

The index of the last epoch when resuming training.

Return:

`torch.optim.lr_scheduler.LambdaLR` with the appropriate schedule.

"""

def lr_lambda(current_step: int):

if current_step < num_warmup_steps:

return float(current_step) / float(max(1.0, num_warmup_steps))

return 1.0

return LambdaLR(optimizer, lr_lambda, last_epoch=last_epoch)

def get_piecewise_constant_schedule(optimizer: Optimizer, step_rules: str, last_epoch: int = -1) -> LambdaLR:

"""

Create a schedule with a constant learning rate, using the learning rate set in optimizer.

Args:

optimizer ([`~torch.optim.Optimizer`]):

The optimizer for which to schedule the learning rate.

step_rules (`string`):

The rules for the learning rate. ex: rule_steps="1:10,0.1:20,0.01:30,0.005" it means that the learning rate

if multiple 1 for the first 10 steps, multiple 0.1 for the next 20 steps, multiple 0.01 for the next 30

steps and multiple 0.005 for the other steps.

last_epoch (`int`, *optional*, defaults to -1):

The index of the last epoch when resuming training.

Return:

`torch.optim.lr_scheduler.LambdaLR` with the appropriate schedule.

"""

rules_dict = {}

rule_list = step_rules.split(",")

for rule_str in rule_list[:-1]:

value_str, steps_str = rule_str.split(":")

steps = int(steps_str)

value = float(value_str)

rules_dict[steps] = value

last_lr_multiple = float(rule_list[-1])

def create_rules_function(rules_dict, last_lr_multiple):

def rule_func(steps: int) -> float:

sorted_steps = sorted(rules_dict.keys())

for i, sorted_step in enumerate(sorted_steps):

if steps < sorted_step:

return rules_dict[sorted_steps[i]]

return last_lr_multiple

return rule_func

rules_func = create_rules_function(rules_dict, last_lr_multiple)

return LambdaLR(optimizer, rules_func, last_epoch=last_epoch)

def get_linear_schedule_with_warmup(

optimizer: Optimizer,

num_warmup_steps: int,

num_training_steps: int,

last_epoch: int = -1,

f_min: float = 0.0,

f_max: float = 1.0,

) -> LambdaLR:

"""

Create a schedule with a learning rate that decreases linearly from the initial lr set in the optimizer to 0, after

a warmup period during which it increases linearly from 0 to the initial lr set in the optimizer.

Args:

optimizer ([`~torch.optim.Optimizer`]):

The optimizer for which to schedule the learning rate.

num_warmup_steps (`int`):

The number of steps for the warmup phase.

num_training_steps (`int`):

The total number of training steps.

last_epoch (`int`, *optional*, defaults to -1):

The index of the last epoch when resuming training.

f_min (`float`, *optional*, defaults to 0.0):

Minimum lr multiplier (floor of the linear decay). The lr will not fall below `f_min * initial_lr`.

f_max (`float`, *optional*, defaults to 1.0):

Maximum lr multiplier (peak reached after warmup). The lr peaks at `f_max * initial_lr`.

Return:

`torch.optim.lr_scheduler.LambdaLR` with the appropriate schedule.

"""

def lr_lambda(current_step: int):

if current_step < num_warmup_steps:

return f_max * float(current_step) / float(max(1, num_warmup_steps))

progress = float(num_training_steps - current_step) / float(max(1, num_training_steps - num_warmup_steps))

return f_min + (f_max - f_min) * max(0.0, progress)

return LambdaLR(optimizer, lr_lambda, last_epoch)

def get_cosine_schedule_with_warmup(

optimizer: Optimizer, num_warmup_steps: int, num_training_steps: int, num_cycles: float = 0.5, last_epoch: int = -1

) -> LambdaLR:

"""

Create a schedule with a learning rate that decreases following the values of the cosine function between the

initial lr set in the optimizer to 0, after a warmup period during which it increases linearly between 0 and the

initial lr set in the optimizer.

Args:

optimizer ([`~torch.optim.Optimizer`]):

The optimizer for which to schedule the learning rate.

num_warmup_steps (`int`):

The number of steps for the warmup phase.

num_training_steps (`int`):

The total number of training steps.

num_periods (`float`, *optional*, defaults to 0.5):

The number of periods of the cosine function in a schedule (the default is to just decrease from the max

value to 0 following a half-cosine).

last_epoch (`int`, *optional*, defaults to -1):

The index of the last epoch when resuming training.

Return:

`torch.optim.lr_scheduler.LambdaLR` with the appropriate schedule.

"""

def lr_lambda(current_step):

if current_step < num_warmup_steps:

return float(current_step) / float(max(1, num_warmup_steps))

progress = float(current_step - num_warmup_steps) / float(max(1, num_training_steps - num_warmup_steps))

return max(0.0, 0.5 * (1.0 + math.cos(math.pi * float(num_cycles) * 2.0 * progress)))

return LambdaLR(optimizer, lr_lambda, last_epoch)

def get_cosine_with_hard_restarts_schedule_with_warmup(

optimizer: Optimizer, num_warmup_steps: int, num_training_steps: int, num_cycles: int = 1, last_epoch: int = -1

) -> LambdaLR:

"""

Create a schedule with a learning rate that decreases following the values of the cosine function between the

initial lr set in the optimizer to 0, with several hard restarts, after a warmup period during which it increases

linearly between 0 and the initial lr set in the optimizer.

Args:

optimizer ([`~torch.optim.Optimizer`]):

The optimizer for which to schedule the learning rate.

num_warmup_steps (`int`):

The number of steps for the warmup phase.

num_training_steps (`int`):

The total number of training steps.

num_cycles (`int`, *optional*, defaults to 1):

The number of hard restarts to use.

last_epoch (`int`, *optional*, defaults to -1):

The index of the last epoch when resuming training.

Return:

`torch.optim.lr_scheduler.LambdaLR` with the appropriate schedule.

"""

def lr_lambda(current_step):

if current_step < num_warmup_steps:

return float(current_step) / float(max(1, num_warmup_steps))

progress = float(current_step - num_warmup_steps) / float(max(1, num_training_steps - num_warmup_steps))

if progress >= 1.0:

return 0.0

return max(0.0, 0.5 * (1.0 + math.cos(math.pi * ((float(num_cycles) * progress) % 1.0))))

return LambdaLR(optimizer, lr_lambda, last_epoch)

def get_polynomial_decay_schedule_with_warmup(

optimizer: Optimizer,

num_warmup_steps: int,

num_training_steps: int,

lr_end: float = 1e-7,

power: float = 1.0,

last_epoch: int = -1,

) -> LambdaLR:

"""

Create a schedule with a learning rate that decreases as a polynomial decay from the initial lr set in the

optimizer to end lr defined by *lr_end*, after a warmup period during which it increases linearly from 0 to the

initial lr set in the optimizer.

Args:

optimizer ([`~torch.optim.Optimizer`]):

The optimizer for which to schedule the learning rate.

num_warmup_steps (`int`):

The number of steps for the warmup phase.

num_training_steps (`int`):

The total number of training steps.

lr_end (`float`, *optional*, defaults to 1e-7):

The end LR.

power (`float`, *optional*, defaults to 1.0):

Power factor.

last_epoch (`int`, *optional*, defaults to -1):

The index of the last epoch when resuming training.

Note: *power* defaults to 1.0 as in the fairseq implementation, which in turn is based on the original BERT

implementation at

https://github.com/google-research/bert/blob/f39e881b169b9d53bea03d2d341b31707a6c052b/optimization.py#L37

Return:

`torch.optim.lr_scheduler.LambdaLR` with the appropriate schedule.

"""

lr_init = optimizer.defaults["lr"]

if not (lr_init > lr_end):

raise ValueError(f"lr_end ({lr_end}) must be smaller than initial lr ({lr_init})")

def lr_lambda(current_step: int):

if current_step < num_warmup_steps:

return float(current_step) / float(max(1, num_warmup_steps))

elif current_step > num_training_steps:

return lr_end / lr_init # as LambdaLR multiplies by lr_init

else:

lr_range = lr_init - lr_end

decay_steps = num_training_steps - num_warmup_steps

pct_remaining = 1 - (current_step - num_warmup_steps) / decay_steps

decay = lr_range * pct_remaining**power + lr_end

return decay / lr_init # as LambdaLR multiplies by lr_init

return LambdaLR(optimizer, lr_lambda, last_epoch)

TYPE_TO_SCHEDULER_FUNCTION = {

SchedulerType.LINEAR: get_linear_schedule_with_warmup,

SchedulerType.COSINE: get_cosine_schedule_with_warmup,

SchedulerType.COSINE_WITH_RESTARTS: get_cosine_with_hard_restarts_schedule_with_warmup,

SchedulerType.POLYNOMIAL: get_polynomial_decay_schedule_with_warmup,

SchedulerType.CONSTANT: get_constant_schedule,

SchedulerType.CONSTANT_WITH_WARMUP: get_constant_schedule_with_warmup,

SchedulerType.PIECEWISE_CONSTANT: get_piecewise_constant_schedule,

}

def get_scheduler(

name: str | SchedulerType,

optimizer: Optimizer,

step_rules: str | None = None,

num_warmup_steps: int | None = None,

num_training_steps: int | None = None,

num_cycles: int = 1,

power: float = 1.0,

last_epoch: int = -1,

) -> LambdaLR:

"""

Unified API to get any scheduler from its name.

Args:

name (`str` or `SchedulerType`):

The name of the scheduler to use.

optimizer (`torch.optim.Optimizer`):

The optimizer that will be used during training.

step_rules (`str`, *optional*):

A string representing the step rules to use. This is only used by the `PIECEWISE_CONSTANT` scheduler.

num_warmup_steps (`int`, *optional*):

The number of warmup steps to do. This is not required by all schedulers (hence the argument being

optional), the function will raise an error if it's unset and the scheduler type requires it.

num_training_steps (`int``, *optional*):

The number of training steps to do. This is not required by all schedulers (hence the argument being

optional), the function will raise an error if it's unset and the scheduler type requires it.

num_cycles (`int`, *optional*):

The number of hard restarts used in `COSINE_WITH_RESTARTS` scheduler.

power (`float`, *optional*, defaults to 1.0):

Power factor. See `POLYNOMIAL` scheduler

last_epoch (`int`, *optional*, defaults to -1):

The index of the last epoch when resuming training.

"""

name = SchedulerType(name)

schedule_func = TYPE_TO_SCHEDULER_FUNCTION[name]

if name == SchedulerType.CONSTANT:

return schedule_func(optimizer, last_epoch=last_epoch)

if name == SchedulerType.PIECEWISE_CONSTANT:

return schedule_func(optimizer, step_rules=step_rules, last_epoch=last_epoch)

# All other schedulers require `num_warmup_steps`

if num_warmup_steps is None:

raise ValueError(f"{name} requires `num_warmup_steps`, please provide that argument.")

if name == SchedulerType.CONSTANT_WITH_WARMUP:

return schedule_func(optimizer, num_warmup_steps=num_warmup_steps, last_epoch=last_epoch)

# All other schedulers require `num_training_steps`

if num_training_steps is None:

raise ValueError(f"{name} requires `num_training_steps`, please provide that argument.")

if name == SchedulerType.COSINE_WITH_RESTARTS:

return schedule_func(

optimizer,

num_warmup_steps=num_warmup_steps,

num_training_steps=num_training_steps,

num_cycles=num_cycles,

last_epoch=last_epoch,

)

if name == SchedulerType.POLYNOMIAL:

return schedule_func(

optimizer,

num_warmup_steps=num_warmup_steps,

num_training_steps=num_training_steps,

power=power,

last_epoch=last_epoch,

)

return schedule_func(

optimizer, num_warmup_steps=num_warmup_steps, num_training_steps=num_training_steps, last_epoch=last_epoch

)