"""
DomainTransformer Configuration.

HF-compatible config following Nubank nuFormer architecture choices:
  - GPT-style causal decoder
  - NoPE (no positional encoding) by default — Kazemnejad et al. 2023
  - Pre-norm (LayerNorm before attention and FFN)
  - Weight-tied embedding ↔ LM head
  - Two reference sizes: 24M (d=512, L=6) and 330M (d=1024, L=24)
"""

from transformers import PretrainedConfig


class DomainTransformerConfig(PretrainedConfig):
    """Configuration for DomainTransformer causal language model.

    This config produces a GPT-style decoder-only Transformer with:
      - No positional encoding (NoPE) by default
      - Pre-norm architecture (LayerNorm before attn/FFN)
      - GELU activation in FFN
      - Weight tying between token embeddings and LM head

    Predefined sizes following Nubank nuFormer (arXiv:2507.23267):
      - "24m":  6 layers, d_model=512,  8 heads, FFN=2048   (~24M params)
      - "85m":  12 layers, d_model=768,  12 heads, FFN=3072  (~85M params)
      - "330m": 24 layers, d_model=1024, 16 heads, FFN=4096  (~330M params)

    Args:
        vocab_size: Size of the token vocabulary.
        hidden_size: Dimension of hidden representations (d_model).
        num_hidden_layers: Number of transformer blocks.
        num_attention_heads: Number of attention heads.
        intermediate_size: FFN intermediate dimension (default: 4 * hidden_size).
        hidden_act: Activation function in FFN.
        hidden_dropout_prob: Dropout rate for embeddings and residual connections.
        attention_probs_dropout_prob: Dropout rate for attention weights.
        max_position_embeddings: Maximum sequence length (for buffer sizing, not PE).
        initializer_range: Std for weight initialization (normal distribution).
        layer_norm_eps: Epsilon for LayerNorm.
        use_cache: Whether to return past key values for generation.
        tie_word_embeddings: Whether to tie input/output embeddings.
    """

    model_type = "domain_transformer"

    def __init__(
        self,
        vocab_size: int = 32000,
        hidden_size: int = 512,
        num_hidden_layers: int = 6,
        num_attention_heads: int = 8,
        intermediate_size: int = None,
        hidden_act: str = "gelu",
        hidden_dropout_prob: float = 0.0,
        attention_probs_dropout_prob: float = 0.0,
        max_position_embeddings: int = 2048,
        initializer_range: float = 0.02,
        layer_norm_eps: float = 1e-5,
        use_cache: bool = True,
        tie_word_embeddings: bool = True,
        **kwargs,
    ):
        self.vocab_size = vocab_size
        self.hidden_size = hidden_size
        self.num_hidden_layers = num_hidden_layers
        self.num_attention_heads = num_attention_heads
        self.intermediate_size = intermediate_size if intermediate_size is not None else 4 * hidden_size
        self.hidden_act = hidden_act
        self.hidden_dropout_prob = hidden_dropout_prob
        self.attention_probs_dropout_prob = attention_probs_dropout_prob
        self.max_position_embeddings = max_position_embeddings
        self.initializer_range = initializer_range
        self.layer_norm_eps = layer_norm_eps
        self.use_cache = use_cache

        assert hidden_size % num_attention_heads == 0, (
            f"hidden_size ({hidden_size}) must be divisible by "
            f"num_attention_heads ({num_attention_heads})"
        )

        super().__init__(tie_word_embeddings=tie_word_embeddings, **kwargs)

    @classmethod
    def from_preset(cls, name: str, vocab_size: int = 32000, **overrides) -> "DomainTransformerConfig":
        """Create config from a named preset.

        Presets:
            "24m":  ~24M params  (6 layers, d=512, 8 heads)
            "85m":  ~85M params  (12 layers, d=768, 12 heads)
            "330m": ~330M params (24 layers, d=1024, 16 heads)
        """
        presets = {
            "24m": dict(hidden_size=512, num_hidden_layers=6, num_attention_heads=8, intermediate_size=2048),
            "85m": dict(hidden_size=768, num_hidden_layers=12, num_attention_heads=12, intermediate_size=3072),
            "330m": dict(hidden_size=1024, num_hidden_layers=24, num_attention_heads=16, intermediate_size=4096),
        }
        if name not in presets:
            raise ValueError(f"Unknown preset '{name}'. Available: {list(presets.keys())}")
        params = {**presets[name], "vocab_size": vocab_size, **overrides}
        return cls(**params)