"""
This file is copied from https://github.com/deepseek-ai/EPLB/blob/main/eplb.py
"""
"""Expert Parallelism Load Balancer (EPLB)"""

from typing import Tuple

import numpy as np


def balanced_packing(weight: np.ndarray,
                     num_packs: int) -> Tuple[np.ndarray, np.ndarray]:
    """
    Pack n weighted objects to m packs, such that each bin contains exactly n/m objects and the weights of all packs
    are as balanced as possible.

    Parameters:
        weight: [X, n], the weight of each item
        num_packs: number of packs

    Returns:
        pack_index: [X, n], the pack index of each item
        rank_in_pack: [X, n], the rank of the item in the pack
    """
    num_layers, num_groups = weight.shape
    assert num_groups % num_packs == 0
    groups_per_pack = num_groups // num_packs

    if groups_per_pack == 1:
        pack_index = np.arange(weight.shape[-1],
                               dtype=np.int32).reshape(1,
                                                       -1).repeat(num_layers,
                                                                  axis=0)
        rank_in_pack = np.zeros_like(weight, dtype=np.int32)
        return pack_index, rank_in_pack

    indices = np.argsort(-weight.astype(np.float32), axis=-1)
    pack_index = np.full_like(weight, fill_value=-1, dtype=np.int32)
    rank_in_pack = np.full_like(pack_index, fill_value=-1)
    for i in range(num_layers):
        pack_weights = [0] * num_packs
        pack_items = [0] * num_packs
        for group in indices[i]:
            pack = min(
                (i
                 for i in range(num_packs) if pack_items[i] < groups_per_pack),
                key=pack_weights.__getitem__)
            assert pack_items[pack] < groups_per_pack
            pack_index[i, group] = pack
            rank_in_pack[i, group] = pack_items[pack]
            pack_weights[pack] += weight[i, group]
            pack_items[pack] += 1
    return pack_index, rank_in_pack


def replicate_experts(
        weight: np.ndarray,
        num_phy: int) -> Tuple[np.ndarray, np.ndarray, np.ndarray]:
    """
    Replicate `num_log` experts to `num_phy` replicas, such that the maximum load of all replicas is minimized.

    Parameters:
        weight: [X, num_log]
        num_phy: total number of experts after replication

    Returns:
        phy2log: [X, num_phy], logical expert id of each physical expert
        rank: [X, num_phy], the replica rank
        logcnt: [X, num_log], number of replicas for each logical expert
    """
    n, num_log = weight.shape
    num_redundant = num_phy - num_log
    assert num_redundant >= 0
    phy2log = np.arange(num_phy, dtype=np.int32).reshape(1, -1).repeat(n,
                                                                       axis=0)
    rank = np.zeros((n, num_phy), dtype=np.int32)
    logcnt = np.ones((n, num_log), dtype=np.int32)
    arangen = np.arange(n, dtype=np.int32)
    for i in range(num_log, num_phy):
        redundant_indices = np.argmax(weight / logcnt, axis=-1)
        phy2log[:, i] = redundant_indices
        rank[:, i] = logcnt[arangen, redundant_indices]
        logcnt[arangen, redundant_indices] += 1
    return phy2log, rank, logcnt


def rebalance_experts_hierarchical(
        weight: np.ndarray, num_physical_experts: int, num_groups: int,
        num_nodes: int,
        num_gpus: int) -> Tuple[np.ndarray, np.ndarray, np.ndarray]:
    """
    Parameters:
        weight: [num_moe_layers, num_logical_experts]
        num_physical_experts: number of physical experts after replication
        num_groups: number of expert groups
        num_nodes: number of server nodes, where the intra-node network (e.g, NVLink) is faster
        num_gpus: number of GPUs, must be a multiple of `num_nodes`

    Returns:
        physical_to_logical_map: [num_moe_layers, num_physical_experts]
        logical_to_physical_map: [num_moe_layers, num_logical_experts, X]
        logical_count: [num_moe_layers, num_logical_experts]
    """
    num_layers, num_logical_experts = weight.shape
    assert num_logical_experts % num_groups == 0
    group_size = num_logical_experts // num_groups
    assert num_groups % num_nodes == 0
    groups_per_node = num_groups // num_nodes
    assert num_gpus % num_nodes == 0
    assert num_physical_experts % num_gpus == 0
    phy_experts_per_gpu = num_physical_experts // num_gpus

    def inverse(perm: np.ndarray) -> np.ndarray:
        inv = np.empty_like(perm)
        inv[np.arange(perm.shape[0])[:, None],
            perm] = np.arange(perm.shape[1], dtype=np.int32).reshape(1, -1)
        return inv

    # Step 1: pack groups to nodes
    tokens_per_group = weight.reshape(num_layers, num_groups,
                                      group_size).sum(axis=-1)
    group_pack_index, group_rank_in_pack = balanced_packing(
        tokens_per_group, num_nodes)
    log2mlog = (((group_pack_index * groups_per_node + group_rank_in_pack) *
                 group_size)[:, :, None] +
                np.arange(group_size, dtype=np.int32)).reshape(num_layers, -1)
    mlog2log = inverse(log2mlog)

    # Step 2: construct redundant experts within nodes
    tokens_per_mlog = np.take_along_axis(weight, mlog2log, axis=-1).reshape(
        -1, num_logical_experts // num_nodes)
    phy2mlog, phyrank, mlogcnt = replicate_experts(
        tokens_per_mlog, num_physical_experts // num_nodes)

    # Step 3: pack physical_experts to GPUs
    tokens_per_phy = np.take_along_axis(tokens_per_mlog / mlogcnt,
                                        phy2mlog,
                                        axis=-1)
    pack_index, rank_in_pack = balanced_packing(tokens_per_phy,
                                                num_gpus // num_nodes)
    phy2pphy = pack_index * phy_experts_per_gpu + rank_in_pack
    pphy2phy = inverse(phy2pphy)

    pphy2mlog = np.take_along_axis(
        phy2mlog, pphy2phy,
        axis=-1)  # [num_layers * num_nodes, num_log_per_nodes]
    pphy2mlog = (pphy2mlog.reshape(num_layers, num_nodes, -1) +
                 np.arange(0,
                           num_logical_experts,
                           num_logical_experts // num_nodes,
                           dtype=np.int32).reshape(1, -1, 1)).reshape(
                               num_layers, -1)
    pphy2log = np.take_along_axis(mlog2log, pphy2mlog, axis=-1)
    pphyrank = np.take_along_axis(phyrank, pphy2phy,
                                  axis=-1).reshape(num_layers, -1)
    logcnt = np.take_along_axis(mlogcnt.reshape(num_layers, -1),
                                log2mlog,
                                axis=-1)
    return pphy2log, pphyrank, logcnt


def rebalance_experts(
        weight: np.ndarray, num_replicas: int, num_groups: int, num_nodes: int,
        num_gpus: int) -> Tuple[np.ndarray, np.ndarray, np.ndarray]:
    """
    Entry point for expert-parallelism load balancer.

    Parameters:
        weight: [layers, num_logical_experts], the load statistics for all logical experts
        num_replicas: number of physical experts, must be a multiple of `num_gpus`
        num_groups: number of expert groups
        num_nodes: number of server nodes, where the intra-node network (e.g, NVLink) is faster
        num_gpus: number of GPUs, must be a multiple of `num_nodes`

    Returns:
        physical_to_logical_map: [layers, num_replicas], the expert index of each replica
        logical_to_physical_map: [layers, num_logical_experts, X], the replica indices for each expert
        expert_count: [layers, num_logical_experts], number of physical replicas for each logical expert
    """
    num_layers, num_logical_experts = weight.shape
    weight = weight.astype(np.float32)
    if num_groups % num_nodes == 0:
        # use hierarchical load-balance policy
        phy2log, phyrank, logcnt = rebalance_experts_hierarchical(
            weight, num_replicas, num_groups, num_nodes, num_gpus)
    else:
        # use global load-balance policy
        phy2log, phyrank, logcnt = replicate_experts(weight, num_replicas)
    maxlogcnt = logcnt.max()
    log2phy = np.full((num_layers, num_logical_experts, maxlogcnt),
                      -1,
                      dtype=np.int32)
    np.put_along_axis(log2phy.reshape(num_layers, -1)[:, :, None],
                      (phy2log * maxlogcnt + phyrank)[:, :, None],
                      np.arange(num_replicas, dtype=np.int32).reshape(
                          1, -1).repeat(num_layers, axis=0)[:, :, None],
                      axis=1)
    return phy2log, log2phy, logcnt


__all__ = ['rebalance_experts']


def main():
    """ """
    num_hidden_layers = 3
    num_expert = 64
    num_groups = 8

    num_replicas = 64
    num_nodes = 4
    num_gpus = 4 * 8

    model_tokens_per_expert_stats_list = np.random.randint(
        low=1, high=10, size=(num_hidden_layers, num_expert))

    phy2log, phyrank, logcnt = rebalance_experts(
        model_tokens_per_expert_stats_list, num_replicas, num_groups,
        num_nodes, num_gpus)

    print(phy2log)
    print(phyrank)
    print(logcnt)


if __name__ == '__main__':
    main()
