"""Full-sample and LEFT-TAIL correlation. Left-tail = correlation on the book's worst days (crisis-relevant)."""
from __future__ import annotations

import numpy as np

from fxhnt.domain.diversification.correlation import full_correlation, left_tail_correlation


def test_full_correlation_aligned() -> None:
    a = np.array([1.0, -1.0, 2.0, -2.0, 0.5])
    assert abs(full_correlation(a, a) - 1.0) < 1e-9
    assert abs(full_correlation(a, -a) + 1.0) < 1e-9


def test_left_tail_picks_book_worst_days() -> None:
    # book: last two days are the worst; sleeve co-moves ONLY on those days -> high left-tail corr,
    # near-zero full-sample corr.
    rng = np.random.default_rng(0)
    n = 200
    book = rng.normal(0, 0.01, n)
    book[-2:] = [-0.05, -0.06]                    # crisis days
    sleeve = rng.normal(0, 0.01, n)
    sleeve[-2:] = [-0.05, -0.07]                  # sleeve crashes WITH the book
    lt = left_tail_correlation(sleeve, book, tail_frac=0.02)   # worst ~4 days
    fs = full_correlation(sleeve, book)
    assert lt > 0.5                                # co-moves in the tail
    assert lt > fs                                 # tail corr exceeds full-sample (the crisis-hiding case)


def test_near_constant_returns_nan() -> None:
    """A series with std ~1e-15 (float noise around a constant) must return NaN, not a garbage value."""
    rng = np.random.default_rng(7)
    # near-constant: tiny float noise well below the 1e-12 epsilon
    near_const = np.ones(200) + rng.normal(0, 1e-15, 200)
    normal = rng.normal(0, 0.01, 200)
    result = full_correlation(near_const, normal)
    assert np.isnan(result), f"expected NaN for near-constant input, got {result}"


def test_empty_input_is_nan_not_raise() -> None:
    """Empty input must return NaN without raising — _aligned's a[-0:] footgun is guarded."""
    result = full_correlation(np.array([]), np.array([1.0, 2.0]))
    assert np.isnan(result), f"expected NaN for empty input, got {result}"


def test_left_tail_unestimable_returns_nan() -> None:
    book = np.array([0.01, 0.02])                 # too few days for a tail
    sleeve = np.array([0.0, 0.0])
    assert np.isnan(left_tail_correlation(sleeve, book, tail_frac=0.1))