"""
Tests the performance of torch.nn.MultiheadAttention's fast path (BetterTransformer)
vs the slow path (torch.nn.functional.multi_head_attention)

To run this script install these dependencies:

pip install tqdm
pip install prettytable
"""

import torch
import random
import numpy as np
from pprint import pprint
import itertools
import json
import argparse
from pathlib import Path
from typing import Optional

from prettytable import PrettyTable
from collections import defaultdict, OrderedDict
from tqdm import tqdm


import warnings

warnings.filterwarnings("ignore")

error_dict = defaultdict(int)


def benchmark_torch_function(iters, f, *args, **kwargs):
    f(*args, **kwargs)
    f(*args, **kwargs)
    torch.cuda.synchronize()
    start_event = torch.cuda.Event(enable_timing=True)
    end_event = torch.cuda.Event(enable_timing=True)
    start_event.record()
    for _ in range(iters):
        f(*args, **kwargs)
    end_event.record()
    torch.cuda.synchronize()
    # elapsed_time has a resolution of 0.5 microseconds:
    # but returns milliseconds, so we need to multiply it to increase resolution
    return start_event.elapsed_time(end_event) * 1000 / iters, *f(*args, **kwargs)


def run(a: int, b: int, iters: int, batch_size: int, sequence_length: int,
        embed_dim: int, num_heads: int, device: str, dtype: str, block_size: int, seed):
    random.seed(seed)
    torch.manual_seed(seed)
    np.random.seed(seed)

    from scipy.stats import beta
    lengths = beta.rvs(a, b, size=batch_size) * (sequence_length + block_size - 1) // block_size
    lengths = list(map(int, list(lengths)))
    lengths = [l * block_size for l in lengths]
    lengths = [max(l, block_size) for l in lengths]

    # Used to enforce no padding
    # lengths = [sequence_length] * batch_size

    # Ensure one row in the batch of ele has the max_sequence_length
    lengths[random.randint(0, batch_size - 1)] = sequence_length

    q = [torch.randn(l, embed_dim, device=device, dtype=dtype)
         for l in lengths]
    q = torch.nested.nested_tensor(q, device=device, dtype=dtype)
    k, v = q, q

    qkv = torch.nn.Linear(embed_dim, 3 * embed_dim, device=device, dtype=dtype)
    proj = torch.nn.Linear(embed_dim, embed_dim, device=device, dtype=dtype)

    native_mha = torch.nn.MultiheadAttention(
        embed_dim, num_heads, batch_first=True, device=device, dtype=dtype
    ).eval()
    native_mha.in_proj_weight = qkv.weight
    native_mha.in_proj_bias = qkv.bias
    native_mha.out_proj.weight = proj.weight
    native_mha.out_proj.bias = proj.bias

    # Create query mask
    q_mask = torch.nested.to_padded_tensor(
        torch.nested.nested_tensor([
            torch.tensor([True] * length, dtype=torch.bool)
            for length in lengths
        ]), 0)
    q_mask = q_mask.cuda()

    if q_mask.size(1) == 0:
        return None

    # Benchmark the native MHA in core
    with torch.backends.cuda.sdp_kernel(enable_math=False, enable_flash=True):
        with torch.inference_mode():
            time_native_mha_fast, y_native_mha_fast, _ = benchmark_torch_function(
                iters, native_mha, q, k, v, need_weights=False)
    q = q.to_padded_tensor(0)
    k = q
    v = q
    # Internal Flash Attention
    time_native_mha_slow, y_native_mha_slow, _ = benchmark_torch_function(
        iters, native_mha, q, k, v, key_padding_mask=~q_mask, need_weights=False)

    # Convert to padded for comparison
    if y_native_mha_fast.is_nested:
        y_native_mha_fast = torch.nested.to_padded_tensor(y_native_mha_fast, 0)
    y_native_mha_fast = y_native_mha_fast * q_mask.unsqueeze(-1)

    if y_native_mha_slow.is_nested:
        y_native_mha_slow = torch.nested.to_padded_tensor(y_native_mha_slow, 0)
    y_native_mha_slow = y_native_mha_slow * q_mask.unsqueeze(-1)

    # Correctness check
    entry_name = f"batch:{batch_size}_seq_len:{sequence_length}_n_heads:{num_heads}_embed_dim:{embed_dim}"
    try:
        torch.testing.assert_close(y_native_mha_fast, y_native_mha_slow, atol=1e-3, rtol=1e-3)
    except AssertionError as e:
        error_dict[entry_name] += 1
        pprint(error_dict)

    # Calculate amount of padding
    padding = 1 - q_mask.float().mean().item()

    # Calculate the speedup for flash attention
    speedup_fast_internal = time_native_mha_slow / time_native_mha_fast

    result_entry = OrderedDict()
    result_entry['dtype'] = dtype
    result_entry["batch_size"] = batch_size
    result_entry["sequence_length"] = sequence_length
    result_entry["n_heads"] = num_heads
    result_entry["embed_dim"] = embed_dim
    result_entry["time_native_mha_slow(μs)"] = f"{time_native_mha_slow:.3f}"
    result_entry["time_native_mha_fast (μs)"] = f"{time_native_mha_fast:.3f}"
    result_entry["speedup flash_mha v native_mha"] = f"{speedup_fast_internal:.3f}"
    result_entry["padding"] = f"{padding:.3f}"
    return result_entry


def main(save_path: Optional[Path], error_path: Optional[Path]):
    table = PrettyTable()
    entries = defaultdict(list)

    print("CUDA device: ", torch.cuda.get_device_name(0))
    iters = 100
    header = None
    batch_sizes = [16, 32, 64, 128, 256]
    sequence_lengths = [64, 128, 256, 512]
    embed_dims = [512, 1024]
    num_heads_list = [8, 16]
    betas = range(1, 64, 4)

    for (batch_size, sequence_length, embed_dim, num_heads, block_size, b) in tqdm(
            list(itertools.product(batch_sizes, sequence_lengths, embed_dims, num_heads_list, [2], betas))):
        seed = 26214  # Magic number that works well for higher b values
        entry = run(1, b * 0.05, iters, batch_size, sequence_length,
                    embed_dim, num_heads, "cuda", torch.float16, block_size, seed)
        if entry is None:
            continue
        if header is None:
            table.field_names = list(entry.keys())
            header = list(entry.keys())
        row = []
        for k, v in entry.items():
            row.append(v)
            entries[k].append(v)
        table.add_row(row)

    # Print the full table to console
    print(table)
    pprint(error_dict)

    csv_string = table.get_csv_string()
    if save_path is not None:
        with open(save_path, 'w') as csvfile:
            csvfile.write(csv_string)

    print(f"Total errors: {sum(error_dict.values())}")
    if error_path is not None:
        with open(error_path, 'w') as file:
            file.write(json.dumps(error_dict))


if __name__ == "__main__":
    parser = argparse.ArgumentParser()
    parser.add_argument("--save-path", "--save_path", type=str, help="Path to save the results")
    parser.add_argument("--error-save-path", "--error_save_path", type=str, help="Path to save the errors")

    args = parser.parse_args()
    save_path = Path(args.save_path) if args.save_path else None
    error_path = Path(args.error_save_path) if args.error_save_path else None

    main(save_path, error_path)
