import paddle
import paddle.fluid as fluid
import paddle.fluid.layers.ops as ops
from paddle.fluid.initializer import init_on_cpu
from paddle.fluid.layers.learning_rate_scheduler import _decay_step_counter
import math


def cosine_decay(learning_rate, step_each_epoch, epochs=120):
    """Applies cosine decay to the learning rate.
    lr = 0.05 * (math.cos(epoch * (math.pi / 120)) + 1)
    """
    global_step = _decay_step_counter()

    with init_on_cpu():
        epoch = ops.floor(global_step / step_each_epoch)
        decayed_lr = learning_rate * \
                     (ops.cos(epoch * (math.pi / epochs)) + 1)/2
    return decayed_lr
