model.py

"""
model.py — Liquid Chess Model (LCM) architecture.

Hybrid transformer with 6 GQA attention blocks and 10 LIV convolution blocks,
distributed evenly via Bresenham algorithm. Trained with dual NTP + TOP objectives.

Architecture highlights:
  - GQA (Grouped Query Attention) with RoPE positional embeddings
  - LIV (Local Input-dependent Value) causal convolution blocks
  - LRM (Learnable Rate Multipliers) on every block
  - Weight tying between embedding and NTP head
  - PyTorch SDPA for efficient attention
"""

import math
import torch
import torch.nn as nn
import torch.nn.functional as F

from config import ChessModelConfig


# ══════════════════════════════════════════════════════════════════════════════
# SHARED COMPONENTS
# ══════════════════════════════════════════════════════════════════════════════

class RMSNorm(nn.Module):
    """Root Mean Square Layer Normalization."""

    def __init__(self, d_model: int, eps: float = 1e-6):
        super().__init__()
        self.weight = nn.Parameter(torch.ones(d_model))
        self.eps    = eps

    def forward(self, x: torch.Tensor) -> torch.Tensor:
        rms = x.pow(2).mean(dim=-1, keepdim=True).add(self.eps).sqrt()
        return (x / rms) * self.weight


# ══════════════════════════════════════════════════════════════════════════════
# LIV CONVOLUTION BLOCK
# ══════════════════════════════════════════════════════════════════════════════

class LIVBlock(nn.Module):
    """
    Local Input-dependent Value convolution block.

    Each token attends to itself and its nearest neighbors (kernel_size=4)
    using double gating. Efficient for capturing local sequential patterns.

    Structure:
        input → RMSNorm → project to 3× → split (B, C, x)
        → B gates x → causal conv → C gates result → project back
        → LRM scale → residual add
    """

    def __init__(self, config: ChessModelConfig):
        super().__init__()
        d = config.d_model
        k = config.conv_kernel_size

        self.norm        = RMSNorm(d)
        self.input_proj  = nn.Linear(d, 3 * d, bias=False)
        self.conv        = nn.Conv1d(
            in_channels=d, out_channels=d, kernel_size=k,
            padding=k - 1, groups=d, bias=False,
        )
        self.output_proj = nn.Linear(d, d, bias=False)
        self.dropout     = nn.Dropout(config.dropout)
        self.lrm         = nn.Parameter(torch.ones(d)) if config.use_lrm else None

    def forward(self, x: torch.Tensor) -> torch.Tensor:
        residual = x
        x        = self.norm(x)
        B, C, x  = self.input_proj(x).chunk(3, dim=-1)
        x        = B * x
        x        = self.conv(x.transpose(1, 2))
        x        = x[:, :, :residual.shape[1]]   # trim for causality
        x        = C * x.transpose(1, 2)
        x        = self.dropout(self.output_proj(x))
        if self.lrm is not None:
            x = x * self.lrm
        return residual + x


# ══════════════════════════════════════════════════════════════════════════════
# GQA ATTENTION BLOCK
# ══════════════════════════════════════════════════════════════════════════════

def build_rope_cache(
    seq_len: int, head_dim: int, device: torch.device
) -> tuple[torch.Tensor, torch.Tensor]:
    """Precompute RoPE cosine and sine tables."""
    theta     = 1.0 / (10000 ** (torch.arange(0, head_dim, 2, device=device).float() / head_dim))
    positions = torch.arange(seq_len, device=device).float()
    freqs     = torch.outer(positions, theta)
    return torch.cos(freqs), torch.sin(freqs)


def apply_rope(
    x: torch.Tensor, cos: torch.Tensor, sin: torch.Tensor
) -> torch.Tensor:
    """Apply rotary position embeddings to a query or key tensor."""
    x1, x2 = x[..., ::2], x[..., 1::2]
    cos     = cos.unsqueeze(0).unsqueeze(0)
    sin     = sin.unsqueeze(0).unsqueeze(0)
    return torch.stack([x1 * cos - x2 * sin, x1 * sin + x2 * cos], dim=-1).flatten(-2)


class SwiGLU(nn.Module):
    """SwiGLU feed-forward network."""

    def __init__(self, config: ChessModelConfig):
        super().__init__()
        d, h = config.d_model, config.ffn_hidden_size
        self.gate_proj  = nn.Linear(d, h, bias=False)
        self.up_proj    = nn.Linear(d, h, bias=False)
        self.down_proj  = nn.Linear(h, d, bias=False)
        self.dropout    = nn.Dropout(config.dropout)

    def forward(self, x: torch.Tensor) -> torch.Tensor:
        return self.dropout(self.down_proj(F.silu(self.gate_proj(x)) * self.up_proj(x)))


class GQABlock(nn.Module):
    """
    Grouped Query Attention block with SwiGLU FFN and RoPE.
    Uses PyTorch's scaled_dot_product_attention for efficiency.
    """

    def __init__(self, config: ChessModelConfig):
        super().__init__()
        d = config.d_model
        self.n_heads    = config.n_heads
        self.n_kv_heads = config.n_kv_heads
        self.head_dim   = config.head_dim
        self.repeats    = config.n_heads // config.n_kv_heads

        self.attn_norm = RMSNorm(d)
        self.ffn_norm  = RMSNorm(d)

        self.q_proj = nn.Linear(d, self.n_heads    * self.head_dim, bias=False)
        self.k_proj = nn.Linear(d, self.n_kv_heads * self.head_dim, bias=False)
        self.v_proj = nn.Linear(d, self.n_kv_heads * self.head_dim, bias=False)
        self.o_proj = nn.Linear(d, d, bias=False)

        self.ffn     = SwiGLU(config)
        self.dropout = nn.Dropout(config.dropout)

        self.attn_lrm = nn.Parameter(torch.ones(d)) if config.use_lrm else None
        self.ffn_lrm  = nn.Parameter(torch.ones(d)) if config.use_lrm else None

    def forward(
        self, x: torch.Tensor, freqs_cos: torch.Tensor, freqs_sin: torch.Tensor
    ) -> torch.Tensor:
        B, T, _ = x.shape

        # ── Attention ─────────────────────────────────────────────────────────
        residual = x
        x_norm   = self.attn_norm(x)

        q = self.q_proj(x_norm).view(B, T, self.n_heads,    self.head_dim)
        k = self.k_proj(x_norm).view(B, T, self.n_kv_heads, self.head_dim)
        v = self.v_proj(x_norm).view(B, T, self.n_kv_heads, self.head_dim)

        q = apply_rope(q.transpose(1, 2), freqs_cos, freqs_sin).transpose(1, 2)
        k = apply_rope(k.transpose(1, 2), freqs_cos, freqs_sin).transpose(1, 2)

        # Expand KV heads to match query heads
        k = k.repeat_interleave(self.repeats, dim=2).transpose(1, 2)
        v = v.repeat_interleave(self.repeats, dim=2).transpose(1, 2)
        q = q.transpose(1, 2)

        attn_out = F.scaled_dot_product_attention(
            q, k, v,
            dropout_p=self.dropout.p if self.training else 0.0,
            is_causal=True,
        ).transpose(1, 2).reshape(B, T, -1)

        attn_out = self.o_proj(attn_out)
        if self.attn_lrm is not None:
            attn_out = attn_out * self.attn_lrm
        x = residual + attn_out

        # ── FFN ───────────────────────────────────────────────────────────────
        residual = x
        ffn_out  = self.ffn(self.ffn_norm(x))
        if self.ffn_lrm is not None:
            ffn_out = ffn_out * self.ffn_lrm
        return residual + ffn_out


# ══════════════════════════════════════════════════════════════════════════════
# LAYER DISTRIBUTION
# ══════════════════════════════════════════════════════════════════════════════

def get_layer_types(n_layers: int, n_gqa: int) -> list[str]:
    """
    Distribute GQA layers evenly through the network using a Bresenham-style
    integer accumulator. Avoids floating-point rounding collisions.
    Always places a GQA block first.

    Example (16 layers, 6 GQA):
        GQA LIV LIV GQA LIV LIV GQA LIV LIV GQA LIV LIV GQA LIV LIV GQA
    """
    if n_gqa == 0:
        return ["liv"] * n_layers
    if n_gqa >= n_layers:
        return ["gqa"] * n_layers

    layer_types    = ["liv"] * n_layers
    layer_types[0] = "gqa"
    gqa_placed     = 1
    remaining      = n_gqa - 1
    slots          = n_layers - 1
    accumulator    = 0

    for i in range(1, n_layers):
        accumulator += remaining
        if accumulator >= slots:
            layer_types[i] = "gqa"
            accumulator   -= slots
            gqa_placed    += 1
        if gqa_placed == n_gqa:
            break

    return layer_types


# ══════════════════════════════════════════════════════════════════════════════
# FULL MODEL
# ══════════════════════════════════════════════════════════════════════════════

class ChessModel(nn.Module):
    """
    Liquid Chess Model (LCM).

    Input:  token IDs  (batch_size, seq_len)
    Output: ntp_logits (batch_size, seq_len, vocab_size)  — move generation
            top_logits (batch_size, seq_len, vocab_size)  — auxiliary training only
    """

    def __init__(self, config: ChessModelConfig):
        super().__init__()
        self.config = config

        self.embedding = nn.Embedding(
            config.vocab_size, config.d_model, padding_idx=config.pad_id
        )

        layer_types  = get_layer_types(config.n_layers, config.n_gqa_layers)
        self.blocks  = nn.ModuleList([
            GQABlock(config) if lt == "gqa" else LIVBlock(config)
            for lt in layer_types
        ])
        self.layer_types = layer_types

        self.norm     = RMSNorm(config.d_model)
        self.ntp_head = nn.Linear(config.d_model, config.vocab_size, bias=False)
        self.top_head = nn.Linear(config.d_model, config.vocab_size, bias=False)

        # Weight tying: embedding and NTP head are inverse operations
        self.ntp_head.weight = self.embedding.weight

        freqs_cos, freqs_sin = build_rope_cache(
            config.max_seq_len, config.head_dim, device=torch.device("cpu")
        )
        self.register_buffer("freqs_cos", freqs_cos)
        self.register_buffer("freqs_sin", freqs_sin)

        self._init_weights()

    def _init_weights(self):
        for module in self.modules():
            if isinstance(module, nn.Linear):
                nn.init.normal_(module.weight, mean=0.0, std=0.02)
                if module.bias is not None:
                    nn.init.zeros_(module.bias)
            elif isinstance(module, nn.Embedding):
                nn.init.normal_(module.weight, mean=0.0, std=0.02)
                if module.padding_idx is not None:
                    module.weight.data[module.padding_idx].zero_()

        # Scale down output projections to stabilize residual stream
        for name, param in self.named_parameters():
            if "o_proj" in name or "down_proj" in name:
                nn.init.normal_(param, mean=0.0,
                                std=0.02 / math.sqrt(2 * self.config.n_layers))

    def forward(
        self, token_ids: torch.Tensor
    ) -> tuple[torch.Tensor, torch.Tensor]:
        B, T = token_ids.shape
        assert T <= self.config.max_seq_len, \
            f"Sequence length {T} exceeds maximum {self.config.max_seq_len}"

        x         = self.embedding(token_ids)
        freqs_cos = self.freqs_cos[:T]
        freqs_sin = self.freqs_sin[:T]

        for block, lt in zip(self.blocks, self.layer_types):
            x = block(x, freqs_cos, freqs_sin) if lt == "gqa" else block(x)

        x = self.norm(x)
        return self.ntp_head(x), self.top_head(x)

    def count_parameters(self) -> int:
        return sum(p.numel() for p in self.parameters() if p.requires_grad)


if __name__ == "__main__":
    from model.config import ChessModelConfig

    config = ChessModelConfig()
    model  = ChessModel(config)
    params = model.count_parameters()
    print(f"Parameters: {params:,} ({params/1e6:.1f}M)")

    x              = torch.randint(0, config.vocab_size, (2, 255))
    ntp, top       = model(x)
    assert ntp.shape == (2, 255, config.vocab_size)
    assert top.shape == (2, 255, config.vocab_size)
    print(f"Forward pass: {ntp.shape} ✓")