sampling.py

"""
RND1 sampling module for masked diffusion generation.

This module implements entropy-based token selection for iterative denoising
in diffusion language models. Supports both greedy and stochastic sampling
with optional prefix/suffix constraints and infilling.
"""

import torch
import torch.nn as nn
import torch.nn.functional as F
from typing import Optional, Tuple, Union


def apply_top_k_filtering(logits: torch.Tensor, k: int) -> torch.Tensor:
    """
    Apply top-k filtering to logits: with non-top-k values set to -inf
    """
    top_k_values, top_k_indices = torch.topk(logits, min(k, logits.size(-1)), dim=-1)
    filtered_logits = torch.full_like(logits, float('-inf'))
    filtered_logits.scatter_(-1, top_k_indices, top_k_values)
    return filtered_logits


def apply_top_p_filtering(logits: torch.Tensor, p: float) -> torch.Tensor:
    """
    Apply top-p (nucleus) filtering to logits: with tokens beyond threshold set to -inf
    """
    sorted_logits, sorted_indices = torch.sort(logits, descending=True, dim=-1)
    cumulative_probs = torch.cumsum(torch.softmax(sorted_logits, dim=-1), dim=-1)

    # Remove tokens with cumulative probability above threshold
    sorted_indices_to_remove = cumulative_probs > p
    sorted_indices_to_remove[..., 0] = False  # Keep at least one token
    sorted_indices_to_remove[..., 1:] = sorted_indices_to_remove[..., :-1].clone()
    indices_to_remove = sorted_indices_to_remove.scatter(-1, sorted_indices, sorted_indices_to_remove)
    return logits.masked_fill(indices_to_remove, float('-inf'))


@torch.no_grad()
def diffusion_sample(
    model: nn.Module,
    seq_len: int = 256,
    num_steps: int = 256,
    top_k: Optional[int] = None,
    top_p: Optional[float] = None,
    temperature: float = 1.0,
    greedy: bool = True,
    mask_token_id: int = 151669,
    prefix_ids: Optional[torch.LongTensor] = None,
    suffix_ids: Optional[torch.LongTensor] = None,
    infill_length: Optional[int] = None,
    eos_token_id: int = 151645,
    pad_token_id: Optional[int] = None,
    bos_token_id: Optional[int] = None,
    device: Optional[Union[str, torch.device]] = None,
    generator: Optional[torch.Generator] = None,
    visualizer: Optional['TerminalVisualizer'] = None,
) -> torch.LongTensor:
    """
    Perform masked diffusion sampling with entropy-based token selection.

    Args:
        model: The RND1 language model
        seq_len: Target sequence length
        num_steps: Number of denoising steps
        top_k: Optional top-k filtering for sampling (None = no filtering)
        top_p: Optional nucleus (top-p) filtering for sampling (None = no filtering)
               When both top_k and top_p are set, top_k is applied first, then top_p
        temperature: Temperature for sampling (higher = more random, lower = more deterministic)
                    Values close to 0 are clamped to 1e-8 to avoid division by zero
        greedy: Whether to use greedy sampling (True) or stochastic (False)
        mask_token_id: Token ID for masked positions (default: 151669)
        prefix_ids: Optional prefix token IDs to preserve
        suffix_ids: Optional suffix token IDs to preserve
        infill_length: Length of infill region between prefix/suffix
        eos_token_id: End of sequence token ID (default: 151645)
        pad_token_id: Padding token ID (default: None, uses 0 if needed)
        bos_token_id: Beginning of sequence token ID (default: None)
        device: Device for computation (None = infer from model)
        generator: Optional torch generator for reproducible sampling
        visualizer: Optional TerminalVisualizer for live visualization

    Returns:
        Generated token IDs as LongTensor
    """
    model.eval()

    if device is None:
        device = next(model.parameters()).device
    else:
        device = torch.device(device)
    dtype = next(model.parameters()).dtype

    if pad_token_id is None:
        pad_token_id = 0

    # Build initial masked sequence
    # When prefix_ids is provided, we create a sequence of length seq_len where:
    # - The prefix occupies the first pre_len positions
    # - The remaining (seq_len - pre_len) positions are filled with mask tokens to be generated
    if prefix_ids is not None or suffix_ids is not None:
        if prefix_ids is not None:
            prefix_ids = prefix_ids.to(device) if isinstance(prefix_ids, torch.Tensor) else torch.tensor(prefix_ids, device=device)
            pre_len = prefix_ids.shape[-1] if prefix_ids.dim() > 0 else 0
        else:
            pre_len = 0

        if suffix_ids is not None:
            suffix_ids = suffix_ids.to(device) if isinstance(suffix_ids, torch.Tensor) else torch.tensor(suffix_ids, device=device)
            suf_len = suffix_ids.shape[-1] if suffix_ids.dim() > 0 else 0
        else:
            suf_len = 0

        reserved = (1 if bos_token_id is not None else 0) + (1 if eos_token_id is not None else 0)
        used = pre_len + suf_len + reserved

        if used > seq_len:
            raise ValueError(
                f"Combined length of prefix ({pre_len}), suffix ({suf_len}), "
                f"and special tokens ({reserved}) = {used} exceeds seq_len ({seq_len}). "
                f"Please increase seq_len or reduce input lengths."
            )
        elif used == seq_len:
            raise ValueError(
                f"No space for generation: prefix ({pre_len}) + suffix ({suf_len}) "
                f"+ special tokens ({reserved}) = seq_len ({seq_len}). "
                f"Need at least 1 position for generation."
            )

        infill_length = min(infill_length or (seq_len - used), seq_len - used)

        x = torch.full((1, seq_len), pad_token_id, dtype=torch.long, device=device)
        pos = 0
        if bos_token_id is not None:
            x[0, pos] = bos_token_id; pos += 1
        if pre_len > 0:
            x[0, pos:pos+pre_len] = prefix_ids.flatten()[:pre_len]; pos += pre_len
        fill_start, fill_end = pos, pos + infill_length
        x[0, fill_start:fill_end] = mask_token_id
        pos = fill_end
        if suf_len > 0:
            x[0, pos:pos+suf_len] = suffix_ids.flatten()[:suf_len]; pos += suf_len

        if eos_token_id is not None and pos < seq_len:
            if isinstance(eos_token_id, (list, tuple)):
                x[0, pos] = eos_token_id[0]
            else:
                x[0, pos] = eos_token_id

        init_maskable = torch.zeros_like(x, dtype=torch.bool)
        init_maskable[0, fill_start:fill_end] = True
    else:
        x = torch.full((1, seq_len), mask_token_id, dtype=torch.long, device=device)
        if bos_token_id is not None:
            x[0, 0] = bos_token_id
        if eos_token_id is not None:
            x[0, -1] = eos_token_id
        init_maskable = x.eq(mask_token_id)

    if bos_token_id is not None:
        init_maskable[:, 0] = False
    if eos_token_id is not None:
        init_maskable &= x.ne(eos_token_id)
    init_maskable &= x.ne(pad_token_id)

    maskable = init_maskable.clone()
    xt = x.clone()

    if visualizer:
        visualizer.start_visualization(xt, maskable, num_steps)

    def forward_scores(tokens):
        """Compute predictions and entropy scores for next tokens."""
        # Try with input_ids parameter first (standard HF models)
        try:
            model_output = model(input_ids=tokens)
        except TypeError:
            # Fall back to positional argument
            model_output = model(tokens)

        # Apply temperature scaling (with safety for near-zero temperature)
        safe_temperature = max(temperature, 1e-8)  # Prevent division by zero
        logits = model_output.logits / safe_temperature


        # Apply filtering strategies
        # Note: When both top_k and top_p are provided, they are applied sequentially:
        # First top_k filters to k tokens, then top_p filters from those k tokens
        if top_k is not None and top_k > 0:
            logits = apply_top_k_filtering(logits, top_k)

        if top_p is not None and 0 < top_p < 1.0:
            logits = apply_top_p_filtering(logits, top_p)

        # Convert to log probabilities
        logp = torch.log_softmax(logits, dim=-1)

        # Greedy or stochastic sampling
        if greedy:
            pred_next = logp.argmax(-1)
        else:
            pred_next = torch.distributions.Categorical(logits=logp).sample(generator=generator)

        conf_next = torch.gather(logp, -1, pred_next.unsqueeze(-1)).squeeze(-1)

        p = logp.exp()
        ent_next = -(p * logp).sum(-1)

        # Shift predictions: pos i predicts token i+1
        pred_i = tokens.clone()
        conf_i = torch.full_like(conf_next, torch.finfo(conf_next.dtype).min)
        ent_i = torch.zeros_like(ent_next)

        pred_i[:, 1:] = pred_next[:, :-1]
        conf_i[:, 1:] = conf_next[:, :-1]
        ent_i[:, 1:] = ent_next[:, :-1]

        return pred_i, conf_i, ent_i

    pred_i, conf_i, ent_i = forward_scores(xt)
    total_masked = init_maskable.sum(1, keepdim=True)
    finf = torch.finfo(conf_i.dtype)

    for step in range(num_steps - 1, 0, -1):
        rate = step / num_steps
        cutoff_len = (total_masked * rate).long().clamp(min=0)

        # Choose HIGH-entropy tokens to keep masked
        sel_scores = ent_i.masked_fill(~maskable, -finf.max)
        B, L = sel_scores.shape
        k_max = cutoff_len.max().item()
        if k_max > 0:
            sss, idx = torch.topk(sel_scores, k_max, dim=-1, largest=True)
            keep_mask = torch.zeros_like(sel_scores, dtype=torch.bool)
            for b in range(B):
                k_b = int(cutoff_len[b].item())
                if k_b > 0:
                    keep_mask[b, idx[b, :k_b]] = True
        else:
            keep_mask = torch.zeros_like(sel_scores, dtype=torch.bool)

        to_unmask = maskable & ~keep_mask
        if to_unmask.any():
            xt[to_unmask] = pred_i[to_unmask]
            maskable[to_unmask] = False

        if visualizer:
            visualizer.update_step(xt, maskable, num_steps - step, ent_i, conf_i)

        if maskable.any():
            pred_i, conf_i, ent_i = forward_scores(xt)

    if maskable.any():
        xt[maskable] = pred_i[maskable]

    if visualizer:
        visualizer.stop_visualization()

    return xt