フォールド別およびHAC指標による取引予測の評価

# metrics.py


```py
"""Metric computation for predictions and backtests."""

from __future__ import annotations

import logging
import math
from typing import Any

logger = logging.getLogger(__name__)


def compute_prediction_fold_metrics(
    predictions,
    *,
    y_true_col: str = "y_true",
    y_score_col: str = "y_score",
    fold_col: str = "fold_id",
    date_col: str = "timestamp",
    entity_col: str = "symbol",
    task_type: str = "regression",
    class_values: list | None = None,
    eval_col: str | None = None,
    label: str | None = None,
    label_buffer: str | None = None,
    direction_labels=None,
    direction_col: str | None = None,
) -> tuple[dict[str, float], dict[int, dict[str, float]]]:
    """Compute standardized metrics from a predictions DataFrame.

    Uses the provided ``task_type`` to decide which metrics to compute.

    ``label_buffer`` is the holding period the case study declares in ``setup.yaml``.
    Divided by the step of the prediction timestamps, it sets the HAC bandwidth for a
    sub-daily label, whose overlap the label name alone cannot express. Without it the
    horizon falls back to parsing the name, which reads a minute label as a monthly one.

    For ``task_type="classification"``, ``eval_col`` must be provided and must
    name a column holding the continuous return that the binary/categorical
    label was derived from. IC is computed against that continuous return;
    AUC, log_loss, accuracy, etc. are computed against the binary ``y_true_col``.
    Computing IC against a binary label collapses to ``2·(AUC − 0.5)`` and is
    not a valid Spearman rank correlation against returns.

    Returns (headline_metrics, fold_metrics) where:
    - headline_metrics: aggregated across all folds
    - fold_metrics: per-fold breakdown keyed by fold_id

    Folds whose scores are constant produce no cross-sectional IC. Headline
    ``ic_mean`` / ``ic_std`` / ``ic_t`` / ``pct_positive`` are computed over the
    folds that did produce one, and ``n_folds_ic`` reports how many that was
    against ``n_folds``. ``ic_t`` is None when fewer than two folds have a defined
    IC or the folds show no dispersion. The fold-based ``ic_t`` is a diagnostic:
    the inferential statistic is ``ic_t_hac``, computed below on the daily IC
    series with its confidence interval.

    Regression metrics: ic, ic_std, rmse, mae, n_entities
    Classification metrics: ic, ic_std, auc_roc, log_loss, brier_score,
        accuracy, balanced_accuracy, auc_pr, n_entities
    """
    import numpy as np
    import polars as pl
    from ml4t.diagnostic.metrics import (
        compute_auc_uncertainty,
        compute_ic_uncertainty,
        cross_sectional_auc_series,
        cross_sectional_ic,
        cross_sectional_ic_series,
    )

    from case_studies.utils.notebook_contracts import defined_ic
    from utils.modeling import compute_classification_metrics

    if not isinstance(predictions, pl.DataFrame):
        predictions = pl.from_pandas(predictions)

    if class_values is None:
        class_values = []

    is_classification = task_type == "classification"
    if is_classification:
        if not eval_col:
            raise ValueError(
                "compute_prediction_fold_metrics(task_type='classification') requires "
                "eval_col — the continuous return column to compute IC against. "
                "Computing IC vs the binary label is 2·(AUC − 0.5) in disguise."
            )
        if eval_col not in predictions.columns:
            raise KeyError(
                f"eval_col {eval_col!r} not present in predictions DataFrame "
                f"(columns: {predictions.columns}). The caller must materialize the "
                f"continuous-return column on every prediction row before registering."
            )
        ic_target_col = eval_col
    else:
        ic_target_col = y_true_col

    folds = sorted(predictions[fold_col].unique().drop_nulls().to_list())
    fold_results = {}

    for fold_id in folds:
        fold_preds = predictions.filter(pl.col(fold_col) == fold_id)

        # Per-date cross-sectional IC — pass polars frame directly (no numpy round-trip).
        _entity = entity_col if entity_col and entity_col in fold_preds.columns else None
        ic_result = cross_sectional_ic(
            fold_preds,
            fold_preds,
            pred_col=y_score_col,
            ret_col=ic_target_col,
            date_col=date_col,
            entity_col=_entity,
            method="spearman",
            min_obs=5,
        )

        yt_fold = fold_preds[y_true_col].to_numpy().astype(float)
        yp_fold = fold_preds[y_score_col].to_numpy().astype(float)
        valid_all = ~(np.isnan(yt_fold) | np.isnan(yp_fold))

        fold_m: dict[str, float] = {
            "ic": ic_result["ic_mean"],
            "ic_std": ic_result["ic_std"],
            "n_entities": int(fold_preds[entity_col].n_unique())
            if entity_col in fold_preds.columns
            else 0,
        }

        if not is_classification:
            fold_m["rmse"] = (
                float(np.sqrt(np.mean((yt_fold[valid_all] - yp_fold[valid_all]) ** 2)))
                if valid_all.any()
                else 0.0
            )
            fold_m["mae"] = (
                float(np.mean(np.abs(yt_fold[valid_all] - yp_fold[valid_all])))
                if valid_all.any()
                else 0.0
            )
        else:
            # Classification metrics: AUC/log_loss/accuracy on the binary y_true.
            cls_m = compute_classification_metrics(yt_fold, yp_fold, class_values)
            # Multiclass ordinal labels (e.g. {-1, 0, 1}) don't get a single
            # auc_roc from compute_classification_metrics. Derive one by
            # collapsing to "up vs not-up" — the natural directional signal
            # for §6b symmetric panels — and persist it as auc_roc so the
            # symmetric panel is a pure registry query regardless of
            # whether the label is binary or 3-class.
            if len(class_values) > 2 and "auc_roc" not in cls_m:
                from sklearn.metrics import roc_auc_score

                yb01 = (yt_fold[valid_all] > 0).astype(int)
                yp_v = yp_fold[valid_all]
                if 0 < yb01.sum() < len(yb01):
                    cls_m["auc_roc"] = float(roc_auc_score(yb01, yp_v))
            fold_m.update(cls_m)

        fold_results[fold_id] = fold_m

    # Headline aggregates over the folds that produced a *defined* IC.
    #
    # A fold whose scores are constant has no cross-sectional rank correlation, so
    # `cross_sectional_ic` returns NaN for it — an L1 config that zeroes every
    # coefficient on one fold is the case that surfaced this. Aggregating with plain
    # `np.mean`/`np.std` propagates that NaN into every headline value, and because
    # `np.nan > 0` is False the `ic_t` guard fell through to a sentinel `0.0`. A
    # stored `ic_t = 0.0` reads as "this IC is indistinguishable from zero", which is
    # a claim; "not computable" is not. Aggregate over the defined folds, count them
    # in `n_folds_ic` so partial coverage is visible next to `n_folds`, and return
    # None (SQL NULL) for a t statistic that does not exist.
    #
    # `ic_std` keeps its 0.0-when-undefined convention: `_verify_cached_config` in
    # `case_studies/utils/gbm.py` reads the stored value through `float()`, which a
    # NULL would raise on.
    fold_ics = [fm["ic"] for fm in fold_results.values()]
    defined_ics = [float(ic) for ic in fold_ics if ic is not None and np.isfinite(ic)]
    n_ic = len(defined_ics)
    ic_dispersion = float(np.std(defined_ics)) if n_ic > 1 else 0.0
    headline: dict[str, float | str | None] = {
        "ic_mean": float(np.mean(defined_ics)) if n_ic else None,
        "ic_std": ic_dispersion,
        "ic_t": float(np.mean(defined_ics) / (ic_dispersion / np.sqrt(n_ic)))
        if n_ic > 1 and ic_dispersion > 0
        else None,
        "n_folds": len(folds),
        "n_folds_ic": n_ic,
        "pct_positive": float(np.mean([ic > 0 for ic in defined_ics])) if n_ic else None,
        "task_type": "classification" if task_type == "classification" else "regression",
    }

    # Aggregate classification headline metrics (mean across folds)
    if task_type == "classification":
        cls_metric_names = [
            "auc_roc",
            "auc_pr",
            "log_loss",
            "brier_score",
            "accuracy",
            "balanced_accuracy",
        ]
        for m_name in cls_metric_names:
            vals = [fm[m_name] for fm in fold_results.values() if m_name in fm]
            if vals:
                headline[m_name] = float(np.mean(vals))

    # ---- Daily-pooled uncertainty (HAC + block bootstrap) -----------------
    # The unit of observation is the date, not the asset prediction. Pool all
    # OOS dates across folds into a single series and compute HAC SE +
    # stationary block-bootstrap CI. This is what `model_analysis` notebooks
    # use for headline IC/AUC and CIs.
    horizon = horizon_in_observations(
        label_buffer, predictions[date_col] if date_col in predictions.columns else None
    )
    if horizon is None:
        horizon = _infer_horizon_from_label(label)
    _entity = entity_col if entity_col and entity_col in predictions.columns else None

    daily_ic = cross_sectional_ic_series(
        predictions,
        predictions,
        pred_col=y_score_col,
        ret_col=ic_target_col,
        date_col=date_col,
        entity_col=_entity,
        method="spearman",
        min_obs=5,
    )
    defined_daily_ic = defined_ic(daily_ic) if isinstance(daily_ic, pl.DataFrame) else None
    if defined_daily_ic is not None and defined_daily_ic.height >= 3:
        ic_unc = compute_ic_uncertainty(
            defined_daily_ic.select("ic"),
            horizon=int(max(1, horizon)),
            n_boot=1000,
        )
        headline.update(
            {
                "ic_mean_daily": ic_unc["mean_ic"],
                "ic_std_daily": ic_unc["std_ic"],
                "ic_n_days": float(ic_unc["n_days"]),
                "ic_pct_positive": ic_unc["pct_positive"],
                "ic_se_naive": ic_unc["se_naive"],
                "ic_naive_lo": ic_unc["ci_naive_lower"],
                "ic_naive_hi": ic_unc["ci_naive_upper"],
                "ic_se_hac": ic_unc["se_hac"],
                "ic_ci_lo": ic_unc["ci_hac_lower"],
                "ic_ci_hi": ic_unc["ci_hac_upper"],
                "ic_t_hac": ic_unc["t_hac"],
                "ic_p_hac": ic_unc["p_hac"],
                "ic_hac_lag": float(ic_unc["hac_lag"]),
                "ic_boot_lo": ic_unc["ci_boot_lower"],
                "ic_boot_hi": ic_unc["ci_boot_upper"],
                "ic_boot_block": ic_unc["boot_block_size"],
            }
        )

    if is_classification:
        # Daily AUC + uncertainty when the label is binary 0/1.
        unique_classes = predictions[y_true_col].drop_nulls().unique().sort().to_list()
        if set(int(v) for v in unique_classes) <= {0, 1} and len(unique_classes) == 2:
            daily_auc = cross_sectional_auc_series(
                predictions,
                predictions,
                pred_col=y_score_col,
                label_col=y_true_col,
                date_col=date_col,
                entity_col=_entity,
                min_obs=5,
            )
            if isinstance(daily_auc, pl.DataFrame) and daily_auc.drop_nulls("auc").height >= 3:
                auc_unc = compute_auc_uncertainty(
                    daily_auc.drop_nulls("auc").select("auc"),
                    horizon=int(max(1, horizon)),
                    n_boot=1000,
                )
                headline.update(
                    {
                        "auc_mean_daily": auc_unc["mean_auc"],
                        "auc_std_daily": auc_unc["std_auc"],
                        "auc_n_days": float(auc_unc["n_days"]),
                        "auc_pct_above_null": auc_unc["pct_above_null"],
                        "auc_se_naive": auc_unc["se_naive"],
                        "auc_naive_lo": auc_unc["ci_naive_lower"],
                        "auc_naive_hi": auc_unc["ci_naive_upper"],
                        "auc_se_hac": auc_unc["se_hac"],
                        "auc_ci_lo": auc_unc["ci_hac_lower"],
                        "auc_ci_hi": auc_unc["ci_hac_upper"],
                        "auc_t_hac": auc_unc["t_hac"],
                        "auc_p_hac": auc_unc["p_hac"],
                        "auc_hac_lag": float(auc_unc["hac_lag"]),
                        "auc_boot_lo": auc_unc["ci_boot_lower"],
                        "auc_boot_hi": auc_unc["ci_boot_upper"],
                        "auc_boot_block": auc_unc["boot_block_size"],
                    }
                )

    # The mirror of the classification path above. A classification model is scored by IC
    # against the continuous return its label was cut from; a regression model is scored by
    # AUC against the direction label cut from its own return. Both directions are optional -
    # five of the nine case studies declare no classification label at all - and neither is
    # the selection criterion, which stays validation backtest Sharpe.
    if not is_classification and direction_labels is not None and direction_col:
        # Never fatal. This is a secondary reading of a run whose own metrics are already
        # computed above, so a schema surprise in a sibling label must not lose the run.
        #
        # But a failure has to survive somewhere a reader looks. It used to reach only
        # `logger.warning`, into a papermill log the harness deletes when the run succeeds,
        # and the row was left with a NULL `direction_label` - identical to the NULL that
        # means "this label declares no direction sibling", which `fwd_ret_24h` legitimately
        # produces in every family. The two are opposite claims wearing the same face, and
        # the ambiguity hid a real result: every `deep_learning` run in
        # `crypto_perps_funding` raised here on a time-unit mismatch, and once the join was
        # repaired 20 of the 21 significant sets turned out to sit BELOW 0.5 with confidence
        # intervals excluding it.
        #
        # So the reason is written beside the absent value. Cleared on the success path
        # rather than only set on the failure one, so a stored error cannot outlive the
        # cause it names: a row that computes gets an explicit NULL over whatever a previous
        # attempt left there, and nobody has to remember to clear it.
        try:
            computed = compute_cross_sectional_direction_auc(
                predictions,
                direction_labels,
                y_score_col=y_score_col,
                direction_col=direction_col,
                date_col=date_col,
                entity_col=_entity,
                horizon=int(max(1, horizon)),
            )
        except Exception as exc:  # noqa: BLE001
            logger.warning("direction AUC against %s not computed: %s", direction_col, exc)
            # Truncated because this is a diagnostic in a metrics row, not a log: the type
            # and the first line are what identify the failure, and a polars schema error
            # carries several hundred characters of frame detail behind them.
            detail = " ".join(str(exc).split())
            headline["direction_label_error"] = f"{type(exc).__name__}: {detail}"[:300]
        else:
            headline.update(computed)
            # An empty return is the third state and it was as silent as the raise. The
            # function declines rather than raises when the joined frame is too small, the
            # label is degenerate, or fewer than three dates carry a defined AUC - all of
            # which leave the same NULL. Reaching here at all means a direction sibling WAS
            # declared and requested, so "declined" is a different claim from "none exists"
            # and is worth the row it is written on.
            headline["direction_label_error"] = (
                None
                if computed
                else (
                    "not computable: fewer than 100 joined rows, a degenerate direction "
                    "label, or fewer than 3 dates carrying a defined AUC"
                )
            )

    return headline, fold_results


# Case matters: `M` is a month and `min` is a minute, which is the collision that put a
# 314-lag HAC bandwidth on a 15-minute label. A bare `m` is deliberately absent - it is
# the one token that could mean either, and no declaration in the nine case studies uses
# it, so refusing it costs nothing and removes the ambiguity at the source.
_SUB_DAILY_UNITS = {
    "min": 60.0,
    "mins": 60.0,
    "minute": 60.0,
    "minutes": 60.0,
    "T": 60.0,
    "h": 3600.0,
    "H": 3600.0,
    "hour": 3600.0,
    "hours": 3600.0,
}


def _duration_seconds(text: str | None) -> float | None:
    """Seconds in a declared duration such as ``15min``, ``60_minute`` or ``8H``.

    Only sub-daily units resolve. A day, a week or a month has no fixed length in
    seconds on a trading calendar, and treating one as though it did is the mistake
    this helper exists to avoid making.
    """
    if not text:
        return None
    import re

    match = re.fullmatch(r"\s*(\d+)\s*[_-]?\s*([A-Za-z]+)\s*", str(text))
    if match is None:
        return None
    unit = match.group(2)
    seconds = _SUB_DAILY_UNITS.get(unit) or _SUB_DAILY_UNITS.get(unit.lower())
    if seconds is None:
        return None
    return int(match.group(1)) * seconds


def _observation_step_seconds(dates: Any) -> float | None:
    """Seconds between one observation of the IC series and the next.

    The series is one IC per distinct decision timestamp, so its step is the thing a
    lag count is expressed in. It is measured from the timestamps themselves rather
    than taken from a declaration, because the two need not agree: nasdaq100 declares
    a 15-minute rebalance cadence and registers predictions on the one-minute grid the
    features are built on.

    The modal positive gap, not the mean: an overnight or weekend gap between sessions
    is not a step of the grid, and averaging it in would inflate the step and shrink
    every lag that follows from it.
    """
    import polars as pl

    if dates is None or not isinstance(dates, pl.Series):
        return None
    stamps = dates.drop_nulls().unique().sort()
    if stamps.len() < 3:
        return None
    if stamps.dtype == pl.Date:
        stamps = stamps.cast(pl.Datetime("us"))
    if stamps.dtype.base_type() != pl.Datetime:
        return None
    gaps = stamps.diff().drop_nulls().dt.total_microseconds()
    gaps = gaps.filter(gaps > 0)
    if gaps.is_empty():
        return None
    return float(gaps.mode().min()) / 1e6


def horizon_in_observations(label_buffer: str | None, dates: Any) -> int | None:
    """How many IC observations a sub-daily label's holding period covers.

    A label name cannot say this. ``fwd_ret_15m`` is fifteen minutes and ``fwd_ret_1m``
    is one month, and both parse as a number followed by ``m``; reading the first as
    months resolved a fifteen-minute label to 315 and set the HAC bandwidth to 314 lags.
    What disambiguates them is the buffer the case study declares - ``15min`` against
    ``1M``. Dividing it by the step of the series the lag count applies to turns a
    duration into the overlap ``compute_ic_uncertainty`` expects.

    Returns ``None`` unless the buffer is sub-daily, which leaves every daily, weekly and
    monthly label to :func:`_infer_horizon_from_label`. A day, a week and a month have no
    fixed length in seconds on a trading calendar, so the same division cannot be made
    for them without a calendar this function does not have.
    """
    buffer_s = _duration_seconds(label_buffer)
    step_s = _observation_step_seconds(dates)
    if buffer_s is None or step_s is None or step_s <= 0:
        return None
    return max(1, math.ceil(buffer_s / step_s))


def _infer_horizon_from_label(label: str | None) -> int:
    """Resolve forward-return horizon (in label-step units) from a label name.

    `fwd_ret_5d` -> 5, `fwd_dir_21d` -> 21, `fwd_class_1m` -> 21,
    `fwd_carry_8h` -> 1 (one 8h bar). Defaults to 1 when label is missing.
    Callers should always pass `label=` so the HAC lag matches horizon-1.

    Some deployed labels carry no `<number><unit>` token. `ret_to_expiry`
    (S&P 500 options, hold-to-expiry) overlaps ~35 trading days per its
    `buffer: 35D` setup — resolving it to 1 silently under-lags the HAC
    bandwidth (this was the Ch13 §13.9 exposure). It is resolved by name here.
    Any *other* non-parsing label triggers a warning and a conservative
    fallback of 1; the caller should pass an explicit horizon at the call site.
    """
    if not label:
        return 1
    import re

    s = label.lower()
    # Named horizons for labels that do not carry a <number><unit> token.
    if "to_expiry" in s:
        return 35
    m = re.search(r"(\d+)\s*([dhwm])", s)
    if not m:
        import warnings

        warnings.warn(
            f"_infer_horizon_from_label: label {label!r} has no <number><unit> "
            "token and is not a named horizon; defaulting to horizon=1, which "
            "under-lags the HAC bandwidth for any overlapping label. Pass an "
            "explicit horizon at the call site.",
            stacklevel=2,
        )
        return 1
    n = int(m.group(1))
    unit = m.group(2)
    if unit == "d":
        return n
    if unit == "h":
        return max(1, n // 8)
    if unit == "w":
        return n * 5
    if unit == "m":
        return n * 21
    return n


def compute_backtest_fold_metrics(
    daily_returns,
    case_study_id: str,
    label: str = "",
    *,
    periods_per_year: int = 0,
) -> dict[int, dict[str, float]]:
    """Compute per-fold backtest metrics by slicing daily returns at fold boundaries.

    Uses the evaluation config from setup.yaml to determine fold boundaries,
    then computes PortfolioAnalysis metrics on each fold's return slice.

    Parameters
    ----------
    daily_returns : pl.DataFrame
        [timestamp, daily_return] — full backtest return series.
    case_study_id : str
        Case study identifier for loading setup.yaml.
    label : str
        Label name (e.g., "fwd_ret_21d") — used to compute label buffer
        for fold boundary calculation.
    periods_per_year : int
        Annualization factor. If 0, auto-detected from data frequency.

    Returns
    -------
    dict[int, dict[str, float]]
        {fold_id: {metric: value, ...}, ...}
    """
    import re

    import polars as pl

    from case_studies.utils.backtest_runner import compute_portfolio_metrics
    from case_studies.utils.cv_window import fold_boundaries

    if not isinstance(daily_returns, pl.DataFrame):
        daily_returns = pl.from_pandas(daily_returns)

    # Determine periods_per_year: the declared convention of the daily_returns
    # grid first, then the exchange calendar, then the observed data frequency.
    # `evaluation.periods_per_year` is the only one of the three that knows the
    # difference between a genuinely monthly series (us_firm, 12) and a monthly-
    # rebalanced strategy marked to market daily (etfs, 252).
    # Callers that omit it — `BacktestExplorer.backfill_fold_metrics` is the one
    # in the tree — get the same reconciliation `run_backtest` applies, so a
    # backfilled thinned grid is not annualized at the declared daily rate.
    if periods_per_year == 0 or periods_per_year is None:
        try:
            from case_studies.utils.backtest_runner import reconcile_periods_per_year
            from case_studies.utils.uncertainty import periods_per_year_from_setup

            declared = int(periods_per_year_from_setup(case_study_id))
            periods_per_year = (
                reconcile_periods_per_year(declared, daily_returns, case_study=case_study_id)
                if declared
                else 0
            )
        except (KeyError, FileNotFoundError, ImportError):
            periods_per_year = 0
    if periods_per_year == 0 or periods_per_year is None:
        # Try to get calendar from setup.yaml → exchange_calendars
        try:
            from case_studies.utils.backtest_runner import calendar_periods_per_year
            from utils.cv_splits import load_evaluation_config

            eval_cfg = load_evaluation_config(case_study_id)
            calendar = eval_cfg.get("calendar", "NYSE")
            periods_per_year = calendar_periods_per_year(calendar)
        except (KeyError, FileNotFoundError, ImportError):
            # Fallback: estimate from data frequency
            n_obs = len(daily_returns)
            if n_obs > 1:
                ts = daily_returns["timestamp"].unique().sort()
                span_days = (
                    (ts[-1] - ts[0]).total_seconds() / 86400
                    if hasattr(ts[-1] - ts[0], "total_seconds")
                    else float(
                        (ts[-1] - ts[0]).cast(pl.Duration("ms")).dt.total_milliseconds() / 86400000
                    )
                )
                span_years = span_days / 365.25
                obs_per_year = n_obs / span_years if span_years > 0.01 else 252
                if obs_per_year > 350:
                    periods_per_year = 365
                elif obs_per_year > 200:
                    periods_per_year = 252
                elif obs_per_year > 40:
                    periods_per_year = 52
                elif obs_per_year > 8:
                    periods_per_year = 12
                else:
                    periods_per_year = max(1, int(obs_per_year))
            else:
                periods_per_year = 252

    # Infer label buffer from label name (e.g., "fwd_ret_21d" → "21D")
    label_buffer = "0D"
    if label:
        m = re.search(r"(\d+)[dD]", label)
        if m:
            label_buffer = f"{m.group(1)}D"
        elif "8h" in label.lower():
            label_buffer = "1D"

    # Fold boundaries derived from the modeling dataset (same source as
    # canonical_window). Avoids passing val-only daily_returns to the
    # walk-forward splitter — that fails when train_size > val window length
    # (e.g., 10Y train_size on an 8Y val window).
    splits = fold_boundaries(case_study_id, label)
    if not splits:
        logger.warning(
            "Cannot load fold boundaries for %s/%s — skipping fold metrics", case_study_id, label
        )
        return {}

    fold_results: dict[int, dict[str, float]] = {}

    # Cast timestamps to Date for uniform comparison with fold boundaries
    ts_dtype = daily_returns["timestamp"].dtype
    if ts_dtype != pl.Date:
        daily_returns = daily_returns.with_columns(pl.col("timestamp").cast(pl.Date))

    # Where the account ended, read off the whole series before it is cut up.
    # A fold that begins after ruin is all zeros, and a slice of zeros is
    # indistinguishable from a fold in which the strategy simply did not trade:
    # `compute_portfolio_metrics` on it reports `ruin=0`, a Sharpe of 0 and no
    # drawdown, which is a clean row for a period in which the book did not exist
    # . The fold that contains the ruin finds it for
    # itself; the folds after it are told.
    from case_studies.utils.backtest_runner import first_ruin_index

    ruin_index = first_ruin_index(daily_returns["daily_return"].to_numpy())
    ruin_date = None if ruin_index is None else daily_returns["timestamp"].to_list()[ruin_index]

    for split in splits:
        fold_id = split["fold"]
        val_start = str(split["val_start"])[:10]  # "YYYY-MM-DD"
        val_end = str(split["val_end"])[:10]

        from datetime import date as date_cls

        start_date = date_cls.fromisoformat(val_start)
        end_date = date_cls.fromisoformat(val_end)

        mask = (daily_returns["timestamp"] >= start_date) & (daily_returns["timestamp"] <= end_date)

        fold_returns = daily_returns.filter(mask)

        if len(fold_returns) < 5:
            logger.debug("Fold %d has only %d returns — skipping", fold_id, len(fold_returns))
            continue

        returns_arr = fold_returns["daily_return"].to_numpy()
        fold_metrics = compute_portfolio_metrics(returns_arr, periods_per_year=periods_per_year)
        if ruin_date is not None and start_date > ruin_date:
            from case_studies.utils.backtest_runner import _apply_ruin_semantics

            # The account was already gone when this fold opened. Report the ruin
            # rather than the zeros it leaves behind, and carry the whole path's
            # index so the fold rows agree with the overall row about where it
            # happened.
            fold_metrics = _apply_ruin_semantics(fold_metrics, ruin_index)

        # Add fold metadata
        fold_metrics["n_days"] = len(fold_returns)

        fold_results[fold_id] = fold_metrics

    return fold_results


def compute_classification_metrics_from_predictions(
    predictions,
    *,
    y_true_col: str = "actual",
    y_score_col: str = "prediction",
    fold_col: str = "fold",
    eval_col: str | None = None,
    label: str | None = None,
    date_col: str = "timestamp",
    entity_col: str = "symbol",
    class_values: list | None = None,
) -> tuple[dict[str, float], dict[int, dict[str, float]]]:
    """Compute classification metrics (AUC/log_loss/brier/accuracy) for an
    existing predictions DataFrame.

    This is a thin wrapper around :func:`compute_prediction_fold_metrics`
    pinned to ``task_type="classification"`` that auto-derives
    ``class_values`` from the unique values of ``y_true_col`` when not
    provided. It exists so notebooks and the registry-AUC backfill script
    invoke the same code path: a future training run that registers a
    classification pred-set populates ``auc_roc`` via
    ``compute_prediction_fold_metrics``; a backfill of pre-existing
    pred-sets populates ``auc_roc`` via this function — the same metric
    function (``compute_classification_metrics``) underlies both.

    When ``eval_col`` (continuous return) is missing from the predictions
    parquet, IC-vs-returns cannot be computed; only the classification
    metrics (AUC/log_loss/etc.) are returned. The headline IC fields are
    populated as 0.0 / NaN to keep the schema stable.

    Returns (headline_metrics, fold_metrics).
    """
    import polars as pl

    if not isinstance(predictions, pl.DataFrame):
        predictions = pl.from_pandas(predictions)

    if class_values is None:
        class_values = (
            predictions[y_true_col].drop_nulls().unique().sort().cast(pl.Float64).to_list()
        )
        # Cast to int when the float values are whole numbers (typical for
        # categorical labels stored as float32). Preserves the {-1, 0, 1}
        # tri-state ordering used by `compute_classification_metrics`.
        if all(float(v).is_integer() for v in class_values):
            class_values = [int(v) for v in class_values]

    # If eval_col is missing, fall back to passing y_true as the IC target;
    # ic computed against the binary label is meaningless but the upstream
    # function still produces classification metrics. Headline IC will be
    # collapsed to 2*(AUC-0.5) — we discard the IC fields downstream when
    # eval_col is absent and only persist the AUC family.
    have_eval = eval_col and eval_col in predictions.columns
    if have_eval:
        return compute_prediction_fold_metrics(
            predictions,
            y_true_col=y_true_col,
            y_score_col=y_score_col,
            fold_col=fold_col,
            date_col=date_col,
            entity_col=entity_col,
            task_type="classification",
            class_values=class_values,
            eval_col=eval_col,
            label=label,
        )

    # No eval_col — compute classification metrics per-fold by hand.
    import numpy as np

    from utils.modeling import compute_classification_metrics

    folds = sorted(predictions[fold_col].unique().drop_nulls().to_list())
    fold_results: dict[int, dict[str, float]] = {}
    for fold_id in folds:
        fold_preds = predictions.filter(pl.col(fold_col) == fold_id)
        yt = fold_preds[y_true_col].to_numpy().astype(float)
        yp = fold_preds[y_score_col].to_numpy().astype(float)
        cls_m = compute_classification_metrics(yt, yp, class_values)
        # Multiclass ordinal labels (e.g. {-1, 0, 1}) don't get a single
        # auc_roc from compute_classification_metrics. Derive one by
        # collapsing to "up vs not-up" — the natural directional signal
        # for §6b symmetric panels — and persist it as auc_roc.
        if len(class_values) > 2 and "auc_roc" not in cls_m:
            from sklearn.metrics import roc_auc_score

            valid = np.isfinite(yt) & np.isfinite(yp)
            yb01 = (yt[valid] > 0).astype(int)
            if 0 < yb01.sum() < len(yb01):
                cls_m["auc_roc"] = float(roc_auc_score(yb01, yp[valid]))
        fold_results[fold_id] = cls_m

    # Headline = mean across folds for each metric that all folds produced.
    headline: dict[str, float] = {"task_type": "classification"}
    if fold_results:
        keys = set().union(*(fm.keys() for fm in fold_results.values()))
        for k in keys:
            vals = [fm[k] for fm in fold_results.values() if k in fm]
            if vals:
                headline[k] = float(np.mean(vals))

    return headline, fold_results


_CLOCK_UNIT_NS = {"ms": 1_000_000, "us": 1_000, "ns": 1}


def _cast_to_label_clock(frame, date_col: str, target):
    """Put a prediction stamp on the label's clock, refusing a cast that would truncate.

    Only ever applied to the prediction side. The label panel is authoritative because
    `value_digest` is time-unit sensitive: a prediction frame rewritten to a different unit
    no longer reproduces the `expected_prediction_keys.digest` its own spec records, which is
    why `registry/store.py::_timestamps_as_utc` normalised the zone and deliberately left the
    unit. Casting at read time costs nothing and changes no stored bytes.

    A narrowing cast is silent in polars, so precision that the label genuinely cannot hold is
    checked here rather than discovered as a join that quietly matches fewer rows.
    """
    import polars as pl

    source_unit = getattr(frame.schema[date_col], "time_unit", None)
    target_unit = getattr(target, "time_unit", None)
    source_ns = _CLOCK_UNIT_NS.get(source_unit or "", 0)
    target_ns = _CLOCK_UNIT_NS.get(target_unit or "", 0)

    if source_ns and target_ns and source_ns < target_ns:
        remainder = target_ns // source_ns
        lost = frame.select(
            (pl.col(date_col).dt.timestamp(source_unit) % remainder != 0).sum()
        ).item()
        if lost:
            raise ValueError(
                f"cannot put {date_col!r} on the label's clock without losing precision: "
                f"{lost:,} of {frame.height:,} prediction stamps carry sub-{target_unit} "
                f"detail that {target_unit!r} cannot hold. The label panel is the authority "
                "on the decision clock, so this is a producer that is stamping finer than "
                "the panel it is scored against."
            )
    return frame.with_columns(pl.col(date_col).cast(target))


def compute_cross_sectional_direction_auc(
    predictions,
    direction_labels,
    *,
    y_score_col: str = "y_score",
    direction_col: str,
    date_col: str = "timestamp",
    entity_col: str | None = "symbol",
    horizon: int = 1,
    min_obs: int = 5,
    n_boot: int = 1000,
) -> dict[str, float]:
    """AUC of a regression score against the sibling direction label, per date.

    The mirror of what ``compute_prediction_fold_metrics`` already does for a classification
    model, which is scored by IC against the continuous return its label was cut from. Here a
    regression model is scored by AUC against the direction label cut from *its* return, so a
    regression and a classification model on the same horizon can be compared on both metrics.

    **The AUC is cross-sectional, computed within each date and then averaged**, exactly as IC
    is. Pooling every ``(entity, date)`` row into one ROC instead measures something else: on a
    date when most of the cross-section moved up, a high score is more likely to sit on a
    positive outcome whatever its rank within that date, so the pooled figure pays the model
    for the base rate moving. Measured on the classification rows that carry both, the two
    agree to 0.0002 in ``us_firm_characteristics``, whose monthly cross-section is close to
    balanced, and diverge in ``sp500_equity_option_analytics`` to 0.5308 pooled against 0.5063
    cross-sectional - an edge above one half of 0.0308 against 0.0063, five times over.

    The score is used as a ranking signal: higher implies "more likely up". A label with more
    than two levels is collapsed to up against not-up on ``> 0``, which is the same convention
    the classification path already applies to an ordinal panel.

    Returns the ``auc_*`` block keyed exactly as the classification path writes it, plus
    ``direction_label`` naming the sibling it scored against, so both
    task types populate the same columns and a query does not have to know which it is reading.
    Returns ``{}`` when the join is too small, the label is degenerate, or fewer than three
    dates carry a defined AUC - "not computable" is not a value.
    """
    import polars as pl
    from ml4t.diagnostic.metrics import compute_auc_uncertainty, cross_sectional_auc_series

    if not isinstance(predictions, pl.DataFrame):
        predictions = pl.from_pandas(predictions)
    if not isinstance(direction_labels, pl.DataFrame):
        direction_labels = pl.from_pandas(direction_labels)

    join_keys = [date_col] + ([entity_col] if entity_col else [])
    missing = [k for k in (*join_keys, direction_col) if k not in direction_labels.columns]
    if missing:
        raise KeyError(f"direction label frame is missing {missing}")
    if y_score_col not in predictions.columns:
        raise KeyError(f"predictions are missing {y_score_col!r}")

    left = predictions.select([*join_keys, y_score_col])
    right = direction_labels.select([*join_keys, direction_col])
    # A label parquet keyed by calendar date meets predictions stamped `datetime[ms]`, and
    # polars refuses that join outright rather than coercing. Narrowing the prediction stamp to
    # its date is the direction that cannot invent precision the label never had; it is right
    # exactly when the label is daily or coarser, which is when the dtypes differ at all. An
    # intraday label is Datetime on both sides and nothing is cast.
    left_dt, right_dt = left.schema[date_col], right.schema[date_col]
    if left_dt != right_dt:
        if right_dt == pl.Date and left_dt in (
            pl.Datetime,
            *(pl.Datetime(u) for u in ("ms", "us", "ns")),
        ):
            left = left.with_columns(pl.col(date_col).cast(pl.Date))
        elif left_dt == pl.Date:
            right = right.with_columns(pl.col(date_col).cast(pl.Date))
        elif isinstance(left_dt, pl.Datetime) and isinstance(right_dt, pl.Datetime):
            # Two Datetimes that differ only in time unit or zone are the same instants,
            # and this is the case that lost the metric entirely: `deep_learning` and
            # `latent_factors` reach the registry through `flush_fold_predictions`, whose
            # dates come from a numpy `datetime64` array, so their artifacts carry `us`
            # where the label panel carries `ms`. 1,548 artifacts across seven case studies
            # are on the other side of this line.
            #
            # The label is authoritative and the cast goes rightwards onto it, never the
            # other way: `value_digest` is time-unit sensitive, so rewriting a prediction
            # frame's unit moves `computation.expected_prediction_keys.digest` and the
            # artifact stops reproducing the digest its own spec records - which is why
            # `registry/store.py::_timestamps_as_utc` fixed the zone and says in as many
            # words that it leaves the unit alone. Reading is the only side that may cast.
            #
            # Lossless here, and checked rather than assumed: across all 200 `us`
            # artifacts in `crypto_perps_funding`, no row carries a sub-millisecond
            # component, so narrowing to the label's unit moves no instant. A stamp that
            # genuinely carried finer precision than its label would be a different
            # problem, and `assert_no_precision_loss` below refuses it rather than
            # silently truncating.
            left = _cast_to_label_clock(left, date_col, right_dt)
        else:
            raise TypeError(
                f"cannot align join key {date_col!r}: predictions are {left_dt}, "
                f"direction label {direction_col!r} is {right_dt}"
            )

    joined = left.join(right, on=join_keys, how="inner")
    # An empty join is a key mismatch, not an absent label, and the two must not look alike.
    # `us_firm_characteristics` stores a dense 1..2712 entity re-index on its prediction rows
    # while its label parquet carries the real permno, so the keys share a dtype and overlap in
    # nothing; a quiet `{}` there reads as "this case study declares no direction label", which
    # is false. Say which side is which and let the caller log it.
    if joined.height == 0:
        raise ValueError(
            f"no rows join predictions to {direction_col!r} on {join_keys}: "
            f"{predictions.height:,} prediction rows and {direction_labels.height:,} label "
            f"rows share no key. The prediction entity id is probably not the label's."
        )
    if joined.height < 100:
        return {}

    joined = joined.filter(
        pl.col(y_score_col).is_finite() & pl.col(direction_col).is_not_null()
    ).with_columns((pl.col(direction_col) > 0).cast(pl.Int8).alias("__up"))
    if joined.height < 100:
        return {}
    positives = int(joined.get_column("__up").sum())
    if positives == 0 or positives == joined.height:
        return {}

    daily = cross_sectional_auc_series(
        joined,
        joined,
        pred_col=y_score_col,
        label_col="__up",
        date_col=date_col,
        entity_col=entity_col,
        min_obs=min_obs,
    )
    if not isinstance(daily, pl.DataFrame) or daily.drop_nulls("auc").height < 3:
        return {}

    unc = compute_auc_uncertainty(
        daily.drop_nulls("auc").select("auc"), horizon=int(max(1, horizon)), n_boot=n_boot
    )
    return {
        "auc_mean_daily": unc["mean_auc"],
        "auc_std_daily": unc["std_auc"],
        "auc_n_days": float(unc["n_days"]),
        "auc_pct_above_null": unc["pct_above_null"],
        "auc_se_naive": unc["se_naive"],
        "auc_naive_lo": unc["ci_naive_lower"],
        "auc_naive_hi": unc["ci_naive_upper"],
        "auc_se_hac": unc["se_hac"],
        "auc_ci_lo": unc["ci_hac_lower"],
        "auc_ci_hi": unc["ci_hac_upper"],
        "auc_t_hac": unc["t_hac"],
        "auc_p_hac": unc["p_hac"],
        "auc_hac_lag": float(unc["hac_lag"]),
        "auc_boot_lo": unc["ci_boot_lower"],
        "auc_boot_hi": unc["ci_boot_upper"],
        "auc_boot_block": unc["boot_block_size"],
        "direction_label": direction_col,
    }


def compute_fold_metrics_from_predictions(
    all_predictions,
    best_config: str,
    best_epoch: int,
    date_col: str = "timestamp",
    entity_col: str = "symbol",
    eval_col: str | None = None,
):
    """Compute per-fold cross-sectional IC from a registered predictions table.

    Filters to the best (config, epoch) and groups by fold_id, returning a
    polars DataFrame with [fold_id, ic_mean, n_test, n_entities].

    Used by deep_learning / tabular_dl / darts_forecasting runners to assemble
    a fold_metrics summary at the end of CV.
    """
    import polars as pl
    from ml4t.diagnostic.metrics import cross_sectional_ic

    if all_predictions.height == 0 or best_config is None:
        return pl.DataFrame()

    best_preds = all_predictions.filter(
        (pl.col("config") == best_config) & (pl.col("epoch") == best_epoch)
    )
    if best_preds.height == 0:
        return pl.DataFrame()

    actual_col = eval_col if eval_col and eval_col in best_preds.columns else "y_true"
    rows = []
    for fold_id in sorted(best_preds["fold_id"].unique().to_list()):
        fold_df = best_preds.filter(pl.col("fold_id") == fold_id)
        _entity = entity_col if entity_col and entity_col in fold_df.columns else None
        result = cross_sectional_ic(
            fold_df,
            fold_df,
            pred_col="y_score",
            ret_col=actual_col,
            date_col=date_col,
            entity_col=_entity,
            method="spearman",
            min_obs=5,
        )
        rows.append(
            {
                "fold_id": fold_id,
                "ic_mean": result["ic_mean"],
                "n_test": fold_df.height,
                "n_entities": (
                    fold_df[entity_col].n_unique() if entity_col in fold_df.columns else 0
                ),
            }
        )
    return pl.DataFrame(rows) if rows else pl.DataFrame()

```