physix/training/sft.py

"""SFT warm-start before GRPO training.

Trains Qwen2.5-1.5B-Instruct for 2 epochs on supervised (prompt, completion)
pairs where the completion is the ground-truth equation in the action JSON
format the env expects. This is the essential bootstrap step: without it a
cold 1.5B model outputs LaTeX / incoherent text on ~80% of turns, yielding
near-zero GRPO advantages and a flat loss curve that wastes GPU credits.

After SFT the model:
- Emits valid JSON with ``equation``, ``params``, ``rationale`` on >90% turns.
- Writes equations in the ASCII grammar (``d2y/dt2 = ...``), not LaTeX.
- Knows the per-system equation family (gravity, drag, pendulum, spring).

Then GRPO refines physics accuracy via the verifiable R² reward.

Run::

    python -m physix.training.sft \
        --model Qwen/Qwen2.5-1.5B-Instruct \
        --output-dir runs/physix-1.5b-sft \
        --epochs 2 \
        --instances-per-system 32

Typical runtime: 5-8 min on an A10G, 3-4 min on an A100.
"""

from __future__ import annotations

import argparse
import json
import logging
import os
from pathlib import Path

import numpy as np
from datasets import Dataset

from physix.systems import (
    SUPPORTED_SYSTEMS,
    SYSTEM_REGISTRY,
    get_system,
)
from physix.systems.base import PhysicalSystem, TrajectoryData
from physix.training.prompt import build_prompt
from physix.models import DEFAULT_MAX_TURNS, PhysiXObservation


_log = logging.getLogger(__name__)


# ─── Dataset ──────────────────────────────────────────────────────────────────

def _gt_completion(system: PhysicalSystem) -> str:
    """Build the ground-truth completion JSON for one system.

    We include the system's sampled parameters so the model learns that the
    ``params`` field must contain the symbols it references in the equation.
    The SFT target is the *exact* JSON string the env's verifier accepts;
    GRPO will later teach the model to refine parameter values per trajectory.
    """
    import re as _re
    eq = system.ground_truth_equation()
    # Extract all identifier tokens that appear in the equation, then keep
    # only those that are declared as system parameters. We use a proper
    # identifier regex (not split-on-whitespace) so symbols inside function
    # calls like sin(theta) and fractions like -(g/L) are caught.
    reserved = set(system.state_variables) | {"dt", "d", "t", "sin", "cos",
                                               "tan", "exp", "log", "sqrt", "abs"}
    eq_tokens = set(_re.findall(r'\b([A-Za-z_][A-Za-z0-9_]*)\b', eq))
    relevant_keys = eq_tokens & set(system.parameters) - reserved
    relevant = {k: round(system.parameters[k], 4) for k in sorted(relevant_keys)}
    return json.dumps({
        "equation": eq,
        "params": relevant,
        "rationale": (
            f"Ground-truth equation for {system.system_id.replace('_', ' ')}."
        ),
    })


def build_sft_dataset(
    system_ids: tuple[str, ...] = SUPPORTED_SYSTEMS,
    instances_per_system: int = 32,
    seed: int = 0,
) -> Dataset:
    if not system_ids:
        raise ValueError("system_ids must be non-empty.")
    unknown = [sid for sid in system_ids if sid not in SYSTEM_REGISTRY]
    if unknown:
        raise ValueError(
            f"Unknown system_ids in build_sft_dataset: {unknown!r}. "
            f"Registered: {sorted(SYSTEM_REGISTRY)!r}."
        )

    rng = np.random.default_rng(seed)
    rows: list[dict] = []

    for system_id in system_ids:
        system = get_system(system_id)
        for _ in range(instances_per_system):
            trajectory = system.simulate(rng)
            obs = _build_obs(system, trajectory)
            prompt_messages = build_prompt(obs)
            completion = _gt_completion(system)
            rows.append({"prompt": prompt_messages, "completion": completion})

    _log.info(
        "Built SFT dataset: %d rows across %d systems (%s)",
        len(rows),
        len(system_ids),
        ", ".join(system_ids),
    )
    return Dataset.from_list(rows)


def _build_obs(system: PhysicalSystem, trajectory: TrajectoryData) -> PhysiXObservation:
    return PhysiXObservation(
        done=False,
        reward=None,
        trajectory=trajectory.to_observation_samples(),
        state_variables=list(system.state_variables),
        hint=system.hint(system.parameters),
        history=[],
        mismatch_summary="",
        turn=0,
        turn_remaining=DEFAULT_MAX_TURNS,
        system_id=system.system_id,
        stats=trajectory.stats(),
        reward_breakdown={},
    )


# ─── Training ─────────────────────────────────────────────────────────────────

def train_sft(
    model_name: str = "Qwen/Qwen2.5-1.5B-Instruct",
    output_dir: str = "runs/physix-1.5b-sft",
    epochs: int = 2,
    max_seq_length: int = 2048,
    lora_r: int = 16,
    lora_alpha: int = 32,
    per_device_batch_size: int = 2,
    gradient_accumulation_steps: int = 4,
    learning_rate: float = 2e-5,
    instances_per_system: int = 32,
    system_ids: tuple[str, ...] = SUPPORTED_SYSTEMS,
    seed: int = 0,
    wandb_run_name: str | None = None,
    hub_checkpoint_repo_id: str | None = None,
    hub_token: str | None = None,
) -> None:
    _configure_logging()

    # Heavy imports: only available in [train] env.
    import wandb
    from unsloth import FastLanguageModel
    from trl import SFTTrainer, SFTConfig

    # Force a fresh W&B run for SFT regardless of any inherited WANDB_RUN_ID
    # / WANDB_RESUME env vars (those are intended for the GRPO stage). If we
    # let wandb.init() try to resume a foreign run id it will block for ~90s
    # fetching that run's history before giving up.
    for stale in ("WANDB_RUN_ID", "WANDB_RESUME"):
        os.environ.pop(stale, None)

    wandb.init(
        project=os.environ.get("WANDB_PROJECT", "physix-live"),
        name=wandb_run_name or f"physix-sft-{epochs}ep",
        config={
            "stage": "sft",
            "model_name": model_name,
            "epochs": epochs,
            "lora_r": lora_r,
            "lora_alpha": lora_alpha,
            "learning_rate": learning_rate,
            "per_device_batch_size": per_device_batch_size,
            "gradient_accumulation_steps": gradient_accumulation_steps,
            "instances_per_system": instances_per_system,
            "seed": seed,
        },
        tags=["sft", "physix", model_name.split("/")[-1]],
    )

    _log.info("Loading model %s (4-bit, LoRA-%d)", model_name, lora_r)
    model, tokenizer = FastLanguageModel.from_pretrained(
        model_name=model_name,
        max_seq_length=max_seq_length,
        load_in_4bit=True,
        dtype=None,
    )
    model = FastLanguageModel.get_peft_model(
        model,
        r=lora_r,
        lora_alpha=lora_alpha,
        target_modules=["q_proj", "k_proj", "v_proj", "o_proj",
                        "gate_proj", "up_proj", "down_proj"],
        bias="none",
        use_gradient_checkpointing="unsloth",
        random_state=seed,
    )

    dataset = build_sft_dataset(system_ids=system_ids, instances_per_system=instances_per_system, seed=seed)

    def _format_row(row: dict) -> dict:
        """Combine prompt + completion into a single training string."""
        messages = row["prompt"] + [{"role": "assistant", "content": row["completion"]}]
        text = tokenizer.apply_chat_template(
            messages, tokenize=False, add_generation_prompt=False
        )
        return {"text": text}

    formatted = dataset.map(_format_row, remove_columns=["prompt", "completion"])
    _log.info("SFT dataset ready: %d rows", len(formatted))

    import torch
    sft_config = SFTConfig(
        output_dir=output_dir,
        num_train_epochs=epochs,
        per_device_train_batch_size=per_device_batch_size,
        gradient_accumulation_steps=gradient_accumulation_steps,
        learning_rate=learning_rate,
        max_seq_length=max_seq_length,
        dataset_text_field="text",
        packing=True,
        logging_steps=1,
        save_strategy="epoch",
        report_to=["wandb"],
        seed=seed,
        bf16=torch.cuda.is_bf16_supported() if torch.cuda.is_available() else False,
        fp16=not torch.cuda.is_bf16_supported() if torch.cuda.is_available() else False,
    )

    trainer = SFTTrainer(
        model=model,
        tokenizer=tokenizer,
        args=sft_config,
        train_dataset=formatted,
    )

    _log.info("Starting SFT for %d epochs on %d examples", epochs, len(formatted))
    trainer.train()

    # We save as merged_16bit (full model + config + tokenizer) rather than
    # "lora" (adapter weights only). GRPO's downstream
    # ``FastLanguageModel.from_pretrained(sft_checkpoint)`` needs a complete
    # model directory — config.json + tokenizer + weights — to load. A bare
    # adapter shard makes Unsloth raise "No config file found". The merged
    # checkpoint is ~3 GB (1.5B params × 2 bytes) which is fine on /tmp.
    out_path = Path(output_dir) / "merged"
    out_path.mkdir(parents=True, exist_ok=True)
    model.save_pretrained_merged(
        save_directory=str(out_path),
        tokenizer=tokenizer,
        save_method="merged_16bit",
    )
    _log.info("SFT model (merged 16-bit) saved → %s", out_path)

    if hub_checkpoint_repo_id:
        # Push the merged SFT model to the same checkpoint repo GRPO uses,
        # under a fixed `sft/` subfolder. Re-runs overwrite the subfolder
        # but produce a new commit, so the revision SHA still uniquely
        # identifies *this* SFT result.
        from physix.training.checkpoints import (
            SFT_SUBFOLDER,
            log_link_artifact_to_wandb,
            push_checkpoint_to_hub,
        )

        try:
            handle = push_checkpoint_to_hub(
                local_dir=out_path,
                repo_id=hub_checkpoint_repo_id,
                subfolder=SFT_SUBFOLDER,
                commit_message=(
                    f"SFT merged_16bit: {model_name} | "
                    f"epochs={epochs} lora_r={lora_r}"
                ),
                token=hub_token,
            )
            _log.info("SFT checkpoint pushed to Hub: %s", handle.hub_url)
            wandb.run.summary["sft/hub_repo"] = handle.repo_id
            wandb.run.summary["sft/hub_url"] = handle.hub_url
            wandb.run.summary["sft/hub_revision"] = handle.revision
            log_link_artifact_to_wandb(
                handle,
                artifact_name="physix-sft-checkpoint",
                extra={"model_name": model_name, "epochs": epochs, "lora_r": lora_r},
            )
        except Exception as exc:  # noqa: BLE001
            # Don't kill SFT just because the hub push failed; the GRPO step
            # downstream can fall back to the local /tmp checkpoint.
            _log.error("SFT hub push failed (non-fatal): %s", exc)

    wandb.finish()


# ─── CLI ──────────────────────────────────────────────────────────────────────

def _configure_logging() -> None:
    logging.basicConfig(
        level=os.environ.get("PHYSIX_LOG_LEVEL", "INFO"),
        format="[%(asctime)s] %(levelname)s %(name)s | %(message)s",
    )


def main() -> None:
    parser = argparse.ArgumentParser(description="SFT warm-start for PhysiX RLVR.")
    parser.add_argument("--model", default="Qwen/Qwen2.5-1.5B-Instruct")
    parser.add_argument("--output-dir", default="runs/physix-1.5b-sft")
    parser.add_argument("--epochs", type=int, default=2)
    parser.add_argument("--instances-per-system", type=int, default=32)
    parser.add_argument(
        "--system-ids",
        default=None,
        help=(
            "Comma-separated list of system IDs to include in the SFT dataset "
            "(e.g. 'damped_spring'). Defaults to all SUPPORTED_SYSTEMS."
        ),
    )
    parser.add_argument("--lora-r", type=int, default=32)
    parser.add_argument("--learning-rate", type=float, default=2e-5)
    parser.add_argument("--seed", type=int, default=0)
    parser.add_argument("--wandb-run-name", default=None,
                        help="Override W&B run name. Defaults to physix-sft-{epochs}ep.")
    parser.add_argument(
        "--hub-checkpoint-repo-id",
        default=None,
        help=(
            "If set, push the merged SFT model to <repo>/sft on the Hub "
            "and log a pointer-only artifact to W&B."
        ),
    )
    args = parser.parse_args()

    system_ids = (
        tuple(s.strip() for s in args.system_ids.split(",") if s.strip())
        if args.system_ids
        else SUPPORTED_SYSTEMS
    )

    os.environ.setdefault("WANDB_PROJECT", "physix-live")
    train_sft(
        model_name=args.model,
        output_dir=args.output_dir,
        epochs=args.epochs,
        lora_r=args.lora_r,
        learning_rate=args.learning_rate,
        instances_per_system=args.instances_per_system,
        system_ids=system_ids,
        seed=args.seed,
        wandb_run_name=args.wandb_run_name,
        hub_checkpoint_repo_id=args.hub_checkpoint_repo_id,
        hub_token=os.environ.get("HF_TOKEN"),
    )


if __name__ == "__main__":
    main()