"""Tests for the SFT dataset builder.

Why a dedicated file: :func:`physix.training.sft.build_sft_dataset` is
imported through a module that *also* imports heavy ML packages
(``unsloth``, ``trl``, ``wandb``) lazily inside :func:`train_sft`. Keeping
the dataset-only tests here documents the boundary — these tests
exercise the pure-Python data path and must not require GPU deps.

The whole point of changes captured by these tests: SFT must warm the
model on *the same systems GRPO will refine*. Any divergence between
SFT and GRPO curricula wastes the warm-start budget on shapes that
never see a reinforcement signal.
"""

from __future__ import annotations

import json

import pytest

from physix.systems import SUPPORTED_SYSTEMS
from physix.training.sft import build_sft_dataset


def test_build_sft_dataset_default_curriculum_size() -> None:
    ds = build_sft_dataset(instances_per_system=1)
    assert len(ds) == len(SUPPORTED_SYSTEMS)


def test_build_sft_dataset_default_scales_with_instances_per_system() -> None:
    instances = 4
    ds = build_sft_dataset(instances_per_system=instances)
    assert len(ds) == len(SUPPORTED_SYSTEMS) * instances


def test_build_sft_dataset_completions_are_valid_json_action() -> None:
    ds = build_sft_dataset(instances_per_system=1)
    for row in ds:
        action = json.loads(row["completion"])
        assert "equation" in action
        assert "params" in action
        assert "rationale" in action


def test_build_sft_dataset_explicit_system_ids_override_default() -> None:
    ds = build_sft_dataset(
        system_ids=("free_fall",),
        instances_per_system=3,
    )
    assert len(ds) == 3


def test_build_sft_dataset_rejects_unknown_system_ids() -> None:
    with pytest.raises(ValueError, match="Unknown system_ids"):
        build_sft_dataset(system_ids=("definitely_not_a_system",))


def test_build_sft_dataset_rejects_empty_system_ids() -> None:
    with pytest.raises(ValueError, match="non-empty"):
        build_sft_dataset(system_ids=())