"""REGRESSION, 2026-09-21: sign tests by product underflowed on flat functions.

`FA * fnew < 0.0` with FA ~ -1e-200 and fnew ~ 1e-150 is -0.0, and '-0.0 < 0.0' is
False: the bracket update moved the wrong end, the root left the bracket, and the run
still reported convergence - on fun6.5 (x^19 over [-10, 100]) the proposed rule returned
6.7e-9 and a lengthening mode 1.27e-6 with a claimed 1e-12 bracket.  The same product
test sat in our bisection, false-position family, Ridders, Brent and ITP.
"""
from __future__ import annotations

import pytest

from simroot.baselines import (_opposite, _same_sign, bisection, illinois, itp,
                               regula_falsi, ridders)
from simroot.pure import pure_root

XTOL, MAXFEV = 1e-12, 200


def test_the_product_underflows_and_the_sign_test_does_not():
    u, v = -1e-200, 1e-150
    assert u * v == 0.0                  # the defect, pinned
    assert _opposite(u, v) and not _same_sign(u, v)
    assert _same_sign(-1e-200, -1e-150) and not _opposite(-1e-200, -1e-150)
    assert not _opposite(0.0, 1.0) and not _same_sign(0.0, 1.0)


def power19(t):
    return t ** 19


@pytest.mark.parametrize("guard", ("project", "safe", "credit", "none"))
@pytest.mark.parametrize("extrapolate", ("none", "confirmed"))
def test_the_method_keeps_the_root_in_the_bracket_on_x19(guard, extrapolate):
    for sim in ("ratio", "cbrt"):
        x, n, ok = pure_root(power19, -10.0, 100.0, XTOL, MAXFEV, similarity=sim,
                             scale="derived3", guard=guard, extrapolate=extrapolate)
        if ok:
            assert abs(x) <= XTOL, (sim, guard, extrapolate, x)


@pytest.mark.parametrize("method", (bisection, regula_falsi, illinois, ridders, itp))
def test_the_classical_methods_keep_the_root_on_x19(method):
    x, n, ok = method(power19, -10.0, 100.0, XTOL, MAXFEV)
    if ok:
        assert abs(x) <= XTOL, (method.__name__, x)