import logging
import torch
from torch import nn
import triton
import triton.language as tl

from flash_attn.flash_attn_interface import flash_attn_varlen_func, flash_attn_with_kvcache
from nanovllm.utils.context import get_context

logger = logging.getLogger(__name__)


@triton.jit
def store_kvcache_kernel(
    key_ptr,
    key_stride,
    value_ptr,
    value_stride,
    k_cache_ptr,
    v_cache_ptr,
    slot_mapping_ptr,
    D: tl.constexpr,
):
    idx = tl.program_id(0)
    slot = tl.load(slot_mapping_ptr + idx)
    if slot == -1: return
    key_offsets = idx * key_stride + tl.arange(0, D)
    value_offsets = idx * value_stride + tl.arange(0, D)
    key = tl.load(key_ptr + key_offsets)
    value = tl.load(value_ptr + value_offsets)
    cache_offsets = slot * D + tl.arange(0, D)
    tl.store(k_cache_ptr + cache_offsets, key)
    tl.store(v_cache_ptr + cache_offsets, value)


def store_kvcache(key: torch.Tensor, value: torch.Tensor, k_cache: torch.Tensor, v_cache: torch.Tensor, slot_mapping: torch.Tensor):
    N, num_heads, head_dim = key.shape
    D = num_heads * head_dim
    assert key.stride(-1) == 1 and value.stride(-1) == 1
    assert key.stride(1) == head_dim and value.stride(1) == head_dim
    assert k_cache.stride(1) == D and v_cache.stride(1) == D
    assert slot_mapping.numel() == N
    store_kvcache_kernel[(N,)](key, key.stride(0), value, value.stride(0), k_cache, v_cache, slot_mapping, D)


class Attention(nn.Module):

    def __init__(
        self,
        num_heads,
        head_dim,
        scale,
        num_kv_heads,
    ):
        super().__init__()
        self.num_heads = num_heads
        self.head_dim = head_dim
        self.scale = scale
        self.num_kv_heads = num_kv_heads
        self.k_cache = self.v_cache = torch.tensor([])
        # Layer ID set by model_runner after model creation
        self.layer_id: int = -1

    def forward(self, q: torch.Tensor, k: torch.Tensor, v: torch.Tensor):
        context = get_context()
        k_cache, v_cache = self.k_cache, self.v_cache
        if k_cache.numel() and v_cache.numel():
            store_kvcache(k, v, k_cache, v_cache, context.slot_mapping)

        if context.is_prefill:
            if context.is_chunked_prefill:
                # Chunked prefill: merge attention from previous KV
                o = self._chunked_prefill_attention(q, k, v, context)
            elif context.block_tables is not None:    # prefix cache
                k, v = k_cache, v_cache
                o = flash_attn_varlen_func(q, k, v,
                                           max_seqlen_q=context.max_seqlen_q, cu_seqlens_q=context.cu_seqlens_q,
                                           max_seqlen_k=context.max_seqlen_k, cu_seqlens_k=context.cu_seqlens_k,
                                           softmax_scale=self.scale, causal=True, block_table=context.block_tables)
            else:
                o = flash_attn_varlen_func(q, k, v,
                                           max_seqlen_q=context.max_seqlen_q, cu_seqlens_q=context.cu_seqlens_q,
                                           max_seqlen_k=context.max_seqlen_k, cu_seqlens_k=context.cu_seqlens_k,
                                           softmax_scale=self.scale, causal=True, block_table=context.block_tables)
        else:    # decode
            if context.is_chunked_prefill:
                # Chunked decode: need to load all KV from CPU+GPU
                o = self._chunked_decode_attention(q, k, v, context)
            else:
                o = flash_attn_with_kvcache(q.unsqueeze(1), k_cache, v_cache,
                                            cache_seqlens=context.context_lens, block_table=context.block_tables,
                                            softmax_scale=self.scale, causal=True)
        return o

    def _chunked_prefill_attention(
        self,
        q: torch.Tensor,
        k: torch.Tensor,
        v: torch.Tensor,
        context,
    ) -> torch.Tensor:
        """
        Compute attention with unified ring buffer for chunked prefill.

        Ring buffer design:
        - Current chunk's KV is written to ring_slot[chunk_idx % N]
        - Previous chunks' KV are loaded from CPU using N-1 available slots
        - Pipeline: pre-fill slots, then process with overlapped load/compute

        For each layer:
        1. Current chunk's KV is in k_batched, v_batched (just written by model)
        2. Load previous chunks from CPU using available slots (pipeline)
        3. Compute attention against previous KV (no causal mask)
        4. Compute attention against current KV (causal)
        5. Merge all results using online softmax
        """
        from nanovllm.kvcache.chunked_attention import flash_attn_with_lse, merge_attention_outputs

        # q, k, v shape: [total_tokens, num_heads, head_dim]
        # Reshape for flash attention: [batch, seq, heads, dim]
        q_batched = q.unsqueeze(0)  # [1, total_tokens, heads, dim]
        k_batched = k.unsqueeze(0)
        v_batched = v.unsqueeze(0)

        o_acc = None
        lse_acc = None

        kvcache_manager = context.kvcache_manager
        seq = context.chunked_seq if hasattr(context, 'chunked_seq') else None
        current_chunk_idx = context.current_chunk_idx

        if kvcache_manager is not None and seq is not None and self.layer_id >= 0:
            # Get prefilled CPU blocks (blocks from previous chunks)
            cpu_block_table = kvcache_manager.get_prefilled_cpu_blocks(seq)

            if cpu_block_table:
                offload_engine = kvcache_manager.offload_engine

                # Get write slot for current chunk and available load slots
                write_slot = offload_engine.get_write_slot_for_prefill(current_chunk_idx)
                load_slots = offload_engine.get_load_slots_for_prefill(write_slot)
                pipeline_depth = len(load_slots)

                if pipeline_depth == 0:
                    # Only 1 slot total, cannot pipeline - use sync loading
                    o_acc, lse_acc = self._sync_load_previous_chunks(
                        q_batched, cpu_block_table, offload_engine
                    )
                else:
                    # Use ring buffer pipeline
                    o_acc, lse_acc = self._ring_buffer_pipeline_load(
                        q_batched, cpu_block_table, load_slots, offload_engine
                    )

        # Compute attention against current chunk's KV (with causal mask)
        current_o, current_lse = flash_attn_with_lse(
            q_batched,
            k_batched,
            v_batched,
            softmax_scale=self.scale,
            causal=True,
        )

        # Merge with accumulated
        if o_acc is None:
            final_o = current_o
        else:
            final_o, _ = merge_attention_outputs(o_acc, lse_acc, current_o, current_lse)

        # Remove batch dimension: [1, total_tokens, heads, dim] -> [total_tokens, heads, dim]
        return final_o.squeeze(0)

    def _sync_load_previous_chunks(
        self,
        q_batched: torch.Tensor,
        cpu_block_table: list,
        offload_engine,
    ):
        """Synchronous loading fallback when pipeline_depth=0."""
        from nanovllm.kvcache.chunked_attention import flash_attn_with_lse, merge_attention_outputs

        o_acc, lse_acc = None, None

        for block_idx, cpu_block_id in enumerate(cpu_block_table):
            # Load to slot 0 (single slot)
            offload_engine.load_to_slot_layer(0, self.layer_id, cpu_block_id)
            offload_engine.wait_slot_layer(0, self.layer_id)

            prev_k, prev_v = offload_engine.get_kv_for_slot(0, self.layer_id)

            prev_o, prev_lse = flash_attn_with_lse(
                q_batched, prev_k, prev_v,
                softmax_scale=self.scale,
                causal=False,
            )

            if o_acc is None:
                o_acc, lse_acc = prev_o, prev_lse
            else:
                o_acc, lse_acc = merge_attention_outputs(o_acc, lse_acc, prev_o, prev_lse)

        return o_acc, lse_acc

    def _ring_buffer_pipeline_load(
        self,
        q_batched: torch.Tensor,
        cpu_block_table: list,
        load_slots: list,
        offload_engine,
    ):
        """
        Ring buffer async pipeline loading with double buffering.

        Uses compute_done events to ensure safe buffer reuse:
        - Before loading to slot X, wait for previous compute on slot X to finish
        - Before computing on slot X, wait for load to slot X to finish

        Timeline with 2 slots (A, B):
        ┌──────────────┐
        │ Load B0→A    │
        └──────────────┘
                       ┌──────────────┐ ┌──────────────┐
                       │ Load B1→B    │ │ Load B2→A    │ ...
                       └──────────────┘ └──────────────┘
                                      ↘               ↘
                        ┌──────────────┐ ┌──────────────┐
                        │ Compute(A)   │ │ Compute(B)   │ ...
                        └──────────────┘ └──────────────┘

        The load_to_slot_layer internally waits for compute_done[slot] before
        starting the transfer, ensuring no data race.
        """
        from nanovllm.kvcache.chunked_attention import flash_attn_with_lse, merge_attention_outputs

        num_blocks = len(cpu_block_table)
        if num_blocks == 0:
            return None, None

        pipeline_depth = len(load_slots)
        if pipeline_depth == 0:
            return None, None

        o_acc, lse_acc = None, None

        if pipeline_depth == 1:
            # Only 1 slot available, cannot pipeline - use synchronous mode
            slot = load_slots[0]
            for block_idx in range(num_blocks):
                offload_engine.load_to_slot_layer(slot, self.layer_id, cpu_block_table[block_idx])
                offload_engine.wait_slot_layer(slot, self.layer_id)
                prev_k, prev_v = offload_engine.get_kv_for_slot(slot, self.layer_id)
                prev_o, prev_lse = flash_attn_with_lse(
                    q_batched, prev_k, prev_v,
                    softmax_scale=self.scale,
                    causal=False,
                )
                # Record compute done so next load can safely reuse this slot
                offload_engine.record_slot_compute_done(slot, self.layer_id)
                if o_acc is None:
                    o_acc, lse_acc = prev_o, prev_lse
                else:
                    o_acc, lse_acc = merge_attention_outputs(o_acc, lse_acc, prev_o, prev_lse)
            return o_acc, lse_acc

        # Double buffering with 2 slots
        slot_A = load_slots[0]
        slot_B = load_slots[1]

        # Pre-load first block to slot_A (async)
        offload_engine.load_to_slot_layer(slot_A, self.layer_id, cpu_block_table[0])

        for block_idx in range(num_blocks):
            # Alternate between slot_A and slot_B
            current_slot = slot_A if block_idx % 2 == 0 else slot_B
            next_slot = slot_B if block_idx % 2 == 0 else slot_A

            # Wait for current slot's transfer to complete
            offload_engine.wait_slot_layer(current_slot, self.layer_id)

            # Start async load of next block to the OTHER slot
            # load_to_slot_layer internally waits for next_slot's compute_done
            if block_idx + 1 < num_blocks:
                offload_engine.load_to_slot_layer(next_slot, self.layer_id, cpu_block_table[block_idx + 1])

            # Compute attention on current slot's data
            prev_k, prev_v = offload_engine.get_kv_for_slot(current_slot, self.layer_id)
            prev_o, prev_lse = flash_attn_with_lse(
                q_batched, prev_k, prev_v,
                softmax_scale=self.scale,
                causal=False,
            )

            # Record compute done - this allows the next round to safely load into this slot
            offload_engine.record_slot_compute_done(current_slot, self.layer_id)

            # Merge with accumulated
            if o_acc is None:
                o_acc, lse_acc = prev_o, prev_lse
            else:
                o_acc, lse_acc = merge_attention_outputs(o_acc, lse_acc, prev_o, prev_lse)

        return o_acc, lse_acc

    def _chunked_decode_attention(
        self,
        q: torch.Tensor,
        k: torch.Tensor,
        v: torch.Tensor,
        context,
    ) -> torch.Tensor:
        """
        Compute decode attention with double-buffering using decode_load_slots.

        Decode uses:
        - decode_slot (slot[0]): writes new token's KV
        - decode_load_slots (slots[1:]): load previous chunks from CPU

        Pipeline design:
        - First half of decode_load_slots: 'compute' buffer
        - Second half: 'prefetch' buffer
        - Double-buffer between them for async overlap

        Timeline:
        ┌─────────────┐ ┌─────────────┐ ┌─────────────┐
        │Load C0→buf0 │ │Load C1→buf1 │ │Load C2→buf0 │ ...
        └─────────────┘ └─────────────┘ └─────────────┘
                      ↘               ↘               ↘
                       ┌─────────────┐ ┌─────────────┐ ┌─────────────┐
                       │ Attn(C0)    │ │ Attn(C1)    │ │ Attn(C2)    │
                       └─────────────┘ └─────────────┘ └─────────────┘
        """
        from nanovllm.kvcache.chunked_attention import flash_attn_with_lse, merge_attention_outputs

        # q shape: [batch_size, num_heads, head_dim] (single decode token per sequence)
        q_batched = q.unsqueeze(1)  # [batch, 1, heads, dim]

        kvcache_manager = context.kvcache_manager
        seq = context.chunked_seq

        # Get all CPU blocks for this sequence
        cpu_block_table, _ = kvcache_manager.get_all_cpu_blocks(seq)
        if self.layer_id == 0:
            logger.debug(f"Decode attention: cpu_block_table={cpu_block_table}, seq.block_table={list(seq.block_table)}")
        if not cpu_block_table:
            raise RuntimeError("Chunked decode attention failed: no CPU blocks available")

        offload_engine = kvcache_manager.offload_engine

        # Use prefetch_size as chunk size for double buffering
        # This ensures both Compute and Prefetch regions can hold a full chunk
        chunk_size = offload_engine.num_prefetch_blocks
        num_chunks = (len(cpu_block_table) + chunk_size - 1) // chunk_size

        o_acc = None
        lse_acc = None

        # Double buffering state: True = use Compute region, False = use Prefetch region
        use_compute = True

        # Pre-load first chunk to Compute region (async)
        first_chunk_ids = cpu_block_table[:min(chunk_size, len(cpu_block_table))]
        offload_engine.load_to_compute_layer(self.layer_id, first_chunk_ids)

        for chunk_idx in range(num_chunks):
            start = chunk_idx * chunk_size
            end = min(start + chunk_size, len(cpu_block_table))
            num_blocks_in_chunk = end - start

            # Wait for current buffer to be ready
            if use_compute:
                offload_engine.wait_compute_layer(self.layer_id)
            else:
                offload_engine.wait_prefetch_layer(self.layer_id)

            # Trigger async prefetch of next chunk to the OTHER buffer
            # This overlaps transfer with current chunk's computation
            if chunk_idx + 1 < num_chunks:
                next_start = end
                next_end = min(next_start + chunk_size, len(cpu_block_table))
                next_chunk_ids = cpu_block_table[next_start:next_end]
                if use_compute:
                    # Current in Compute, prefetch next to Prefetch region
                    offload_engine.load_to_prefetch_layer(self.layer_id, next_chunk_ids)
                else:
                    # Current in Prefetch, prefetch next to Compute region
                    offload_engine.load_to_compute_layer(self.layer_id, next_chunk_ids)

            # Get KV from current buffer
            if use_compute:
                k_chunk, v_chunk = offload_engine.get_kv_for_compute(
                    self.layer_id, num_blocks_in_chunk
                )
            else:
                k_chunk, v_chunk = offload_engine.get_kv_for_prefetch(
                    self.layer_id, num_blocks_in_chunk
                )

            # Compute attention for this chunk
            o_chunk, lse_chunk = flash_attn_with_lse(
                q_batched, k_chunk, v_chunk,
                softmax_scale=self.scale,
                causal=False,
            )

            # Merge with accumulated
            if o_acc is None:
                o_acc, lse_acc = o_chunk, lse_chunk
            else:
                o_acc, lse_acc = merge_attention_outputs(o_acc, lse_acc, o_chunk, lse_chunk)

            # Swap buffers for next iteration
            use_compute = not use_compute

        # Now attend to Decode region (contains accumulated decode tokens)
        pos_in_block = context.decode_pos_in_block
        start_pos = context.decode_start_pos_in_block
        num_accumulated = pos_in_block - start_pos + 1

        if num_accumulated > 0:
            decode_k = offload_engine.k_cache_gpu[self.layer_id, offload_engine.decode_slot, start_pos:pos_in_block+1]
            decode_v = offload_engine.v_cache_gpu[self.layer_id, offload_engine.decode_slot, start_pos:pos_in_block+1]
            decode_k = decode_k.unsqueeze(0)
            decode_v = decode_v.unsqueeze(0)

            decode_o, decode_lse = flash_attn_with_lse(
                q_batched, decode_k, decode_v,
                softmax_scale=self.scale,
                causal=False,
            )

            if o_acc is None:
                o_acc = decode_o
            else:
                o_acc, _ = merge_attention_outputs(o_acc, lse_acc, decode_o, decode_lse)

        if o_acc is None:
            raise RuntimeError("Chunked decode attention failed: no KV available")

        return o_acc