"""Causal LM stem: embed -> DecoderBlock* -> norm -> lm_head.

KDA-only and K3-like both use this class. Config.layer_specs() chooses
attn/ffn per layer: ("kda"|"mla", "swiglu"|"moe").

``config.attnres`` selects the depth mixer:
    off   — standard residual inside each DecoderBlock (default)
    full  — Full AttnRes over attn|ffn sublayers
    block — Block AttnRes (K3); block size from ``attnres_block_size``
"""

from __future__ import annotations

import torch
import torch.nn.functional as F
from torch import nn
from torch.utils.checkpoint import checkpoint as activation_checkpoint

from ..layers.attn_res import (
    BlockAttnResStack,
    BorrowedSubLayer,
    FullAttnResStack,
    atomic_block_size,
)
from ..layers.block import DecoderBlock
from ..layers.rmsnorm import RMSNorm


def _build_mixer(config, blocks: nn.ModuleList):
    mode = getattr(config, "attnres", "off")
    if mode == "off":
        return None
    atomics = []
    for block in blocks:
        atomics.append(BorrowedSubLayer(block.attn_norm, block.attn))
        atomics.append(BorrowedSubLayer(block.ffn_norm, block.ffn))
    kwargs = dict(
        eps=config.norm_eps,
        zero_init_queries=getattr(config, "attnres_zero_init_queries", True),
        is_final_aggregate=getattr(config, "attnres_final_aggregate", True),
    )
    if mode == "full":
        return FullAttnResStack(config.hidden_size, atomics, **kwargs)
    if mode == "block":
        return BlockAttnResStack(
            config.hidden_size,
            atomics,
            block_size=atomic_block_size(
                config.num_hidden_layers, getattr(config, "attnres_block_size", None)
            ),
            **kwargs,
        )
    raise ValueError(f"unknown attnres mode: {mode!r}")


class CausalLM(nn.Module):
    def __init__(self, config):
        super().__init__()
        self.config = config
        self.attnres = getattr(config, "attnres", "off")
        self.embedding = nn.Embedding(config.vocab_size, config.hidden_size)
        self.blocks = nn.ModuleList(
            [
                DecoderBlock.from_spec(config, attn, ffn)
                for attn, ffn in config.layer_specs()
            ]
        )
        self.mixer = _build_mixer(config, self.blocks)
        self.gradient_checkpointing = bool(
            getattr(config, "gradient_checkpointing", False)
        )
        self.norm = RMSNorm(config.hidden_size, config.norm_eps)
        self.lm_head = nn.Linear(config.hidden_size, config.vocab_size, bias=False)
        nn.init.normal_(self.embedding.weight, std=config.initializer_range)
        nn.init.normal_(self.lm_head.weight, std=config.initializer_range)
        if config.tie_word_embeddings:
            self.lm_head.weight = self.embedding.weight

    def forward(
        self,
        input_ids: torch.Tensor,
        labels: torch.Tensor | None = None,
        ignore_index: int = -100,
    ):
        x = self.embedding(input_ids)
        if self.mixer is None:
            for block in self.blocks:
                if self.gradient_checkpointing and self.training:
                    x = activation_checkpoint(block, x, use_reentrant=False)
                else:
                    x = block(x)
        elif self.gradient_checkpointing and self.training:
            x = activation_checkpoint(self.mixer, x, use_reentrant=False)
        else:
            x = self.mixer(x)
        logits = self.lm_head(self.norm(x))
        if labels is None:
            return logits
        return F.cross_entropy(
            logits[:, :-1].reshape(-1, logits.size(-1)),
            labels[:, 1:].reshape(-1),
            ignore_index=ignore_index,
        )

    @torch.inference_mode()
    def generate(
        self,
        input_ids: torch.Tensor,
        max_new_tokens: int,
        temperature: float = 0.0,
        eos_token_id: int | None = None,
    ):
        for _ in range(max_new_tokens):
            logits = self(input_ids)[:, -1]
            if temperature > 0:
                probs = F.softmax(logits / temperature, dim=-1)
                next_token = torch.multinomial(probs, 1)
            else:
                next_token = logits.argmax(-1, keepdim=True)
            input_ids = torch.cat((input_ids, next_token), dim=1)
            if eos_token_id is not None and (next_token.squeeze(-1) == eos_token_id).all():
                break
        return input_ids