"""
GLADIUS v2.0 — Shared Embeddings

Token embeddings and output projection head.
Optionally weight-tied (saves ~8M params at full scale).
Tool embeddings live in the same space — see tools.py.
"""

import torch
import torch.nn as nn
import math

from .config import KernelConfig


class SharedEmbeddings(nn.Module):
    """
    Shared vocabulary embedding layer.

    Tokens, tools, and specialist routing all project into the same
    hidden_dim space. This is what makes tool activation = generation:
    they live in the same manifold.
    """

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

        # Token embeddings
        self.token_embed = nn.Embedding(
            config.vocab_size, config.hidden_dim,
            padding_idx=config.pad_token_id
        )

        # Output projection (vocab logits)
        self.output_head = nn.Linear(config.hidden_dim, config.vocab_size, bias=False)

        # Weight tying: output_head shares weights with token_embed
        self.output_head.weight = self.token_embed.weight

        # Scaling factor (Vaswani et al.)
        self.scale = math.sqrt(config.hidden_dim)

        self._init_weights()

    def _init_weights(self):
        nn.init.normal_(self.token_embed.weight, mean=0.0, std=0.02)
        # Pad token should be zero
        with torch.no_grad():
            self.token_embed.weight[self.config.pad_token_id].zero_()

    def embed(self, input_ids: torch.Tensor) -> torch.Tensor:
        """Token IDs → hidden representations."""
        return self.token_embed(input_ids) * self.scale

    def project(self, hidden: torch.Tensor) -> torch.Tensor:
        """Hidden representations → vocabulary logits."""
        return self.output_head(hidden)

    def forward(self, input_ids: torch.Tensor) -> torch.Tensor:
        """Convenience: embed."""
        return self.embed(input_ids)