import os
import json
import torch
import matplotlib.pyplot as plt
import torchvision.models as models
from torch.utils.data import DataLoader
from tqdm import tqdm

from verl.cifar10_experiments.sampling_based_rl_objective_experiments import HFImageNet, calculate_loss

from datasets import load_dataset
from torchvision import transforms

def gradient_snr(gradients: torch.Tensor, eps: float = 1e-8) -> torch.Tensor:
    """
    Compute the gradient SNR given a set of per-sample gradients.

    Args:
        gradients: (N, D) tensor of N gradient vectors, each of dimension D.
        eps: small constant for numerical stability.

    Returns:
        snr: scalar tensor, ||mean||^2 / ||var||.
    """
    # gradients: (N, D)
    mu  = gradients.mean(dim=0)                # (D,)
    var = gradients.var(dim=0, unbiased=False)  # (D,)
    mean_sq_norm = mu.pow(2).sum()             # ||mu||^2
    var_sum      = var.sum()                   # ||var||
    snr = mean_sq_norm / (var_sum + eps)
    return snr, mean_sq_norm, var_sum


CHECKPOINT_PATH = "/data/imagenet256_checkpoints/cross_entropy_1024/checkpoint_latest_final.pth"
BATCH_SIZE = 1024
BATCH_CACHE_PATH = f"batch_{BATCH_SIZE}.pt"
S = 64    # number of gradient samples used to estimate SNR

ADVANTAGE_TYPES  = ["grpo", "reinforce_with_baseline", "reinforce_with_p_normalization",]
# ADVANTAGE_TYPES  = ["grpo"]
# ROLLOUT_COUNTS   = [4, 8, 16, 32, 64]
# ROLLOUT_COUNTS   = [64, 128, 256]
# ROLLOUT_COUNTS = [4, 16, 64, 256, 1024, 4096]
ROLLOUT_COUNTS = [2**i for i in range(2, 22, 2)]
print(ROLLOUT_COUNTS)
# ROLLOUT_COUNTS = [2**14, 2**16, 2**18, 2**20]

# ROLLOUT_COUNTS = [2**22, 2**24, 2**26]

SNR_STATS_PATH   = "snr_stats_large.json"
MINI_BATCH_SIZE  = 512  # number of images per minibatch to avoid OOM

device = torch.device("cuda" if torch.cuda.is_available() else "cpu")

# Load model
model = models.resnet50(num_classes=1000)
checkpoint = torch.load(CHECKPOINT_PATH, map_location="cpu")
model.load_state_dict(checkpoint["model_state"])
model = model.to(device)
model.eval()

# Load one batch of BATCH_SIZE images from imagenet (cached)
if os.path.exists(BATCH_CACHE_PATH):
    print(f"Loading cached batch from {BATCH_CACHE_PATH}")
    batch = torch.load(BATCH_CACHE_PATH)
else:
    print("Cache not found, loading dataset...")
    mean = (0.485, 0.456, 0.406)
    std  = (0.229, 0.224, 0.225)

    test_transform = transforms.Compose([
        transforms.Resize(256),
        transforms.CenterCrop(224),
        transforms.ToTensor(),
        transforms.Normalize(mean=mean, std=std),
    ])

    hf_ds = load_dataset("benjamin-paine/imagenet-1k-256x256")
    dataset = HFImageNet(split="validation", transform=test_transform, hf_ds=hf_ds)
    loader = DataLoader(dataset, batch_size=BATCH_SIZE, shuffle=False, num_workers=4, pin_memory=True)

    batch = next(iter(loader))
    torch.save(batch, BATCH_CACHE_PATH)
    print(f"Batch cached to {BATCH_CACHE_PATH}")

inputs, targets = batch
inputs, targets = inputs.to(device), targets.to(device)

snr_stats = {}

for advantage_type in ADVANTAGE_TYPES:
    snr_stats[advantage_type] = {}
    for N in ROLLOUT_COUNTS:
        print(f"\n[advantage={advantage_type}, N={N}] collecting {S} gradient samples...")
        try:
            all_grads = []

            num_minibatches = BATCH_SIZE // MINI_BATCH_SIZE

            for s in tqdm(range(S)):
                model.zero_grad()

                for mb_start in range(0, BATCH_SIZE, MINI_BATCH_SIZE):
                    mb_inputs  = inputs [mb_start : mb_start + MINI_BATCH_SIZE]
                    mb_targets = targets[mb_start : mb_start + MINI_BATCH_SIZE]

                    logits = model(mb_inputs)

                    loss = calculate_loss(
                        logits=logits,
                        targets=mb_targets,
                        num_train_rollouts_per_example=N,
                        advantage_type=advantage_type,
                        max_k=None,
                    )

                    # scale so accumulated gradient == average over full batch
                    (loss / num_minibatches).backward()

                flat_grad = torch.cat([
                    p.grad.detach().view(-1)
                    for p in model.parameters()
                    if p.grad is not None
                ])
                all_grads.append(flat_grad)

            gradients = torch.stack(all_grads)  # (S, D)
            snr, mean_sq_norm, var_sum = gradient_snr(gradients)
            snr, mean_sq_norm, var_sum = snr.item(), mean_sq_norm.item(), var_sum.item()
            print(f"  SNR = {snr:.6f}, mean = {mean_sq_norm:.6f}, var = {var_sum:.6f}")
            snr_stats[advantage_type][N] = {
                "snr":  snr,
                "mean": mean_sq_norm,
                "var":  var_sum,
            }

        except Exception as e:
            print(f"  ERROR: {e}")
            snr_stats[advantage_type][N] = None

with open(SNR_STATS_PATH, "w") as f:
    json.dump(snr_stats, f, indent=2)

print(f"\nSaved SNR stats to {SNR_STATS_PATH}")

# --- Plot ---
LABEL_MAP = {
    "reinforce_with_baseline":       "RLOO",
    "grpo":                          "GRPO",
    "reinforce_with_p_normalization": "MaxRL",
}

with open(SNR_STATS_PATH) as f:
    plot_data = json.load(f)

fig, ax = plt.subplots(figsize=(8, 5))

for advantage_type, rollout_snrs in plot_data.items():
    xs = [int(k) for k in rollout_snrs]
    ys = [v["snr"] if isinstance(v, dict) else v for v in rollout_snrs.values()]
    label = LABEL_MAP.get(advantage_type, advantage_type)
    ax.plot(xs, ys, marker="o", label=label)

ax.set_xscale("log", base=2)
ax.set_xlabel("Rollouts (N)")
ax.set_ylabel("Gradient SNR")
ax.set_title(f"Gradient SNR vs Rollouts  (S={S} gradient samples)")
ax.legend()
ax.grid(True, which="both", linestyle="--", alpha=0.5)

plt.tight_layout()
plt.savefig("snr.png", dpi=150)
print("Saved plot to snr.png")