import math
from typing import Optional

import torch

from colossalai.kernel.kernel_loader import CPUAdamLoader

from .nvme_optimizer import NVMeOptimizer


class CPUAdam(NVMeOptimizer):
    """
    Implements Adam algorithm.

    Supports parameters updating on both GPU and CPU, depending on the device of parameters.
    But the parameters and gradients should on the same device:
      * Parameters on CPU and gradients on CPU is allowed.
      * Parameters on GPU and gradients on GPU is allowed.
      * Parameters on GPU and gradients on CPU is **not** allowed.

    `CPUAdam` requires CUDA extensions which can be built during installation or runtime.

    This version of CPU Adam accelerates parameters updating on CPU with SIMD.
    Support of AVX2 or AVX512 is required.

    The GPU part is implemented in an naive way.

    CPU Adam also supports the hybrid precision calculation, eg. fp32 parameters and fp16 gradients.

    :class:`colossalai.nn.optimizer.CPUAdam` may be used as a drop-in replacement for ``torch.optim.AdamW``,
    or ``torch.optim.Adam`` with ``adamw_mode=False``

    Adam was been proposed in `Adam: A Method for Stochastic Optimization`_.

    Arguments:
        model_params (iterable): iterable of parameters of dicts defining
            parameter groups.
        lr (float, optional): learning rate. (default: 1e-3)
        betas (Tuple[float, float], optional): coefficients used for computing
            running averages of gradient and its square. (default: (0.9, 0.999))
        eps (float, optional): term added to the denominator to improve
            numerical stability. (default: 1e-8)
        weight_decay (float, optional): weight decay (L2 penalty) (default: 0)
        amsgrad (boolean, optional): whether to use the AMSGrad variant of this
            algorithm from the paper `On the Convergence of Adam and Beyond`_
            (default: False) NOT SUPPORTED yet in CPUAdam!
        adamw_mode (boolean, optional): Apply L2 regularization or weight decay
            True for decoupled weight decay(also known as AdamW) (default: True)
        simd_log (boolean, optional): whether to show if you are using SIMD to
            accelerate. (default: False)
        nvme_offload_fraction (float, optional): Fraction of optimizer states to be offloaded to NVMe. Defaults to 0.0.
        nvme_offload_dir (Optional[str], optional): Directory to save NVMe offload files.
            If it's ``None``, a random temporary directory will be used. Defaults to None.

    .. _Adam\: A Method for Stochastic Optimization:
        https://arxiv.org/abs/1412.6980
    .. _On the Convergence of Adam and Beyond:
        https://openreview.net/forum?id=ryQu7f-RZ
    """

    # Number of fp32 shards for per parameter
    # Param weight, grad, momentum and variance
    num_fp32_shards_per_param = 4

    def __init__(
        self,
        model_params,
        lr=1e-3,
        bias_correction=True,
        betas=(0.9, 0.999),
        eps=1e-8,
        weight_decay=0,
        adamw_mode=True,
        nvme_offload_fraction: float = 0.0,
        nvme_offload_dir: Optional[str] = None,
    ):
        default_args = dict(lr=lr, betas=betas, eps=eps, weight_decay=weight_decay, bias_correction=bias_correction)
        super(CPUAdam, self).__init__(model_params, default_args, nvme_offload_fraction, nvme_offload_dir)
        self.adamw_mode = adamw_mode
        cpu_adam = CPUAdamLoader().load()
        # if you find yourself stuck here, make sure that you install colossalai with BUILD_EXT=1 specification
        self.cpu_adam_op = cpu_adam.CPUAdamOptimizer(lr, betas[0], betas[1], eps, weight_decay, adamw_mode)

    def torch_adam_update(
        self,
        data,
        grad,
        exp_avg,
        exp_avg_sq,
        lr,
        beta1,
        beta2,
        eps,
        weight_decay,
        bias_correction1,
        bias_correction2,
        use_adamw=False,
    ):
        grad = grad.to(data.dtype)

        if weight_decay != 0:
            if use_adamw:
                data.mul_(1 - lr * weight_decay)
            else:
                grad = grad.add(data, alpha=weight_decay)

        # Decay the first and second moment running average coefficient
        exp_avg.mul_(beta1).add_(grad, alpha=1 - beta1)
        exp_avg_sq.mul_(beta2).addcmul_(grad, grad, value=1 - beta2)

        # TODO(jiaruifang) dose not support amsgrad
        denom = (exp_avg_sq.sqrt() / math.sqrt(bias_correction2)).add_(eps)

        step_size = lr / bias_correction1

        data.addcdiv_(exp_avg, denom, value=-step_size)

    @torch.no_grad()
    def step(self, closure=None, div_scale: float = -1):
        loss = None
        if closure is not None:
            with torch.enable_grad():
                loss = closure()

        self._pre_step("exp_avg", "exp_avg_sq")
        for _, group in enumerate(self.param_groups):
            for _, p in enumerate(group["params"]):
                if p.grad is None:
                    continue

                state = self.state[p]

                target_device = p.device
                if len(state) == 0:
                    state["step"] = 0
                    # gradient momentums
                    state["exp_avg"] = torch.zeros_like(p, device=target_device)
                    # gradient variances
                    state["exp_avg_sq"] = torch.zeros_like(p, device=target_device)
                    self._post_state_init(p)

                state["step"] += 1
                beta1, beta2 = group["betas"]

                if target_device.type == "cpu":
                    assert p.data.numel() == p.grad.data.numel(), "parameter and gradient should have the same size"
                    assert state["exp_avg"].device.type == "cpu", "exp_avg should stay on cpu"
                    assert state["exp_avg_sq"].device.type == "cpu", "exp_avg should stay on cpu"
                    self._pre_update(p, "exp_avg", "exp_avg_sq")
                    if p.grad.dtype is torch.bfloat16:
                        # cpu adam kernel does not support bf16 now
                        bias_correction1 = 1 - beta1 ** state["step"]
                        bias_correction2 = 1 - beta2 ** state["step"]
                        self.torch_adam_update(
                            p.data,
                            p.grad.data,
                            state["exp_avg"],
                            state["exp_avg_sq"],
                            group["lr"],
                            beta1,
                            beta2,
                            group["eps"],
                            group["weight_decay"],
                            bias_correction1,
                            bias_correction2,
                            self.adamw_mode,
                        )
                    else:
                        self.cpu_adam_op.step(
                            state["step"],
                            group["lr"],
                            beta1,
                            beta2,
                            group["eps"],
                            group["weight_decay"],
                            group["bias_correction"],
                            p.data,
                            p.grad.data,
                            state["exp_avg"],
                            state["exp_avg_sq"],
                            div_scale,
                        )
                    self._post_update(p, "exp_avg", "exp_avg_sq")
                elif target_device.type == "cuda":
                    assert div_scale == -1, "div_scale should remain default"
                    assert state["exp_avg"].device.type == "cuda", "exp_avg should stay on cuda"
                    assert state["exp_avg_sq"].device.type == "cuda", "exp_avg should stay on cuda"

                    bias_correction1 = 1 - beta1 ** state["step"]
                    bias_correction2 = 1 - beta2 ** state["step"]

                    # adam on cuda
                    self.torch_adam_update(
                        p.data,
                        p.grad.data,
                        state["exp_avg"],
                        state["exp_avg_sq"],
                        group["lr"],
                        beta1,
                        beta2,
                        group["eps"],
                        group["weight_decay"],
                        bias_correction1,
                        bias_correction2,
                        self.adamw_mode,
                    )
                else:
                    raise RuntimeError
        self._post_step()
        return loss
