Improve coverage on Peirce criterion (#46)

* Improve coverage on Peirce criterion

* Increase `n` such that `ldiv` goes to zero numerically

Author: trappitsch

tests/func/utilities/test_peirce.py

@@ -1,12 +1,45 @@
 """Test for Peirce criterion data rejection."""
 
 
+from hypothesis import given, strategies as st
 import pytest
 import numpy as np
 
 from rimseval.utilities import peirce
 
 
+def test_peirce_criterion_ldiv_zero():
+    """Choose a minimum ldiv if it is zero (erfc turns 0 at large values)."""
+    # define values such that it turns out to be zero
+    n_tot = 100
+    n = 1e3  # this brings `ldiv` numerically to zero
+    m = 1
+
+    _ = peirce.peirce_criterion(n_tot, n, m)
+
+
+@given(n_hyp=st.integers(), m_hyp=st.integers())
+def test_peirce_criterion_n_tot_zero(n_hyp, m_hyp):
+    """Check that setting number of observations to zero returns zero always."""
+    n_tot = 0
+    expected = 0
+    received = peirce.peirce_criterion(n_tot, n_hyp, m_hyp)
+    assert expected == received
+
+
+def test_peirce_criterion_x2_less_zero():
+    """Ensure that in while-loop, x2 is set to zero if it is smaller than zero."""
+    # define values that get x2 < 0
+    n_tot = 2
+    n = 1
+    m = 5
+
+    expected = 0
+    received = peirce.peirce_criterion(n_tot, n, m)
+
+    assert expected == received
+
+
 def test_peirce_ross_2003_example():
     """Run example in Ross (2003) through Peirce criterion."""
     data = np.array([102.2, 90, 99, 102, 103, 100.2, 89, 98.1, 101.5, 102])