Modify quadratic slightly, add tests

pooltool/ptmath/roots/quadratic.py

@@ -1,5 +1,6 @@
 from typing import Tuple
 
+import numpy as np
 from numba import jit
 
 import pooltool.constants as const
@@ -9,10 +10,19 @@
 def solve(a: float, b: float, c: float) -> Tuple[float, float]:
     """Solve a quadratic equation At^2 + Bt + C = 0 (just-in-time compiled)"""
     if a == 0:
-        u = -c / b
-        return u, u
+        if b == 0:
+            # c=0 => infinite solutions, c≠0 => no solutions
+            return np.nan, np.nan
+        else:
+            # Linear: b * t + c = 0 => t = -c/b
+            return -c / b, np.nan
+
     bp = b / 2
     delta = bp * bp - a * c
+
+    if delta < 0:
+        return np.nan, np.nan
+
     u1 = (-bp - delta**0.5) / a
     u2 = -u1 - b / a
     return u1, u2

tests/ptmath/roots/test_quadratic.py

@@ -57,3 +57,27 @@ def test_solve_large_values():
     # required relative tolerance required to pass this test is pretty large (1e-2).
     assert pytest.approx(solutions[0], rel=1e-2) == 1e-7
     assert pytest.approx(solutions[1], rel=1e-2) == 1e7
+
+
+def test_solve_linear_equation():
+    # a=0, b≠0 => linear equation b*t + c = 0 => t=-c/b
+    # e.g. 2t + 4 = 0 => t=-2
+    r1, r2 = solve(0.0, 2.0, 4.0)
+    assert r1 == -2.0
+    assert math.isnan(r2)  # The second "root" should be NaN for a linear equation
+
+
+def test_solve_degenerate_no_solution():
+    # a=0, b=0, c≠0 => no solutions
+    # e.g. 0*t^2 + 0*t + 5 = 0 => no real solution
+    r1, r2 = solve(0.0, 0.0, 5.0)
+    assert math.isnan(r1)
+    assert math.isnan(r2)
+
+
+def test_solve_degenerate_infinite_solutions():
+    # a=0, b=0, c=0 => infinite solutions
+    # e.g. 0*t^2 + 0*t + 0 = 0 => t can be anything
+    r1, r2 = solve(0.0, 0.0, 0.0)
+    assert math.isnan(r1)
+    assert math.isnan(r2)