fix: honor explicit zero valence instead of hallucinating hydrogens

INCHI-1-SRC/INCHI_BASE/src/util.c

@@ -694,12 +694,17 @@ int get_num_H( const char* elname,
     {
         num_H = inp_num_H;
     }
-    else if (atom_input_valence && ( atom_input_valence != 15 || chem_bonds_valence ))
+    else if (atom_input_valence && atom_input_valence != 15)
     {
         num_H = inchi_max( 0, atom_input_valence - chem_bonds_valence );
     }
-    else if ( atom_input_valence == 15 && !chem_bonds_valence )
+    else if ( atom_input_valence == 15 )
     {
+        /* Explicit zero valence (V3000 VAL=-1 / V2000 valence field 15): the */
+        /* atom must carry neither implicit H nor (chemically) any bonds. Honour */
+        /* the zero valence regardless of whether bonds were drawn, instead of */
+        /* treating 15 as a literal target valence and adding 15 - chem_bonds_valence */
+        /* hydrogens (github #105). */
         num_H = 0;
     }
     else if (MIN_ATOM_CHARGE <= charge &&

INCHI-1-TEST/tests/test_executable/test_github_105.py

@@ -0,0 +1,56 @@
+"""Regression test for https://github.com/IUPAC-InChI/InChI/issues/105
+
+An explicit zero valence (``VAL=-1`` in V3000, valence field ``15`` in V2000)
+must not cause implicit hydrogens to be added. Before the fix, a terminal atom
+carrying a single bond was treated as having a target valence of 15, so
+``15 - 1 = 14`` hydrogens were "hallucinated" per atom, yielding
+``InChI=1S/C2H28/c1-2/h1-2H14`` for ethane.
+
+The correct behaviour is to honour the zero valence: no implicit hydrogens are
+added while the drawn bond is kept, giving ``InChI=1S/C2/c1-2``.
+"""
+
+import pytest
+
+from helpers import parse_inchi_from_executable_output
+
+EXPECTED_INCHI = "InChI=1S/C2/c1-2"
+
+ETHANE_V3000_ZERO_VALENCE = """
+  ChemDraw04232514542D
+
+  0  0  0     0  0              0 V3000
+M  V30 BEGIN CTAB
+M  V30 COUNTS 2 1 0 0 0
+M  V30 BEGIN ATOM
+M  V30 1 C 0.000000 0.000000 0.000000 0 VAL=-1
+M  V30 2 C 1.000000 0.000000 0.000000 0 VAL=-1
+M  V30 END ATOM
+M  V30 BEGIN BOND
+M  V30 1 1 1 2
+M  V30 END BOND
+M  V30 END CTAB
+M  END
+"""
+
+ETHANE_V2000_ZERO_VALENCE = """
+  test
+
+  2  1  0  0  0  0  0  0  0  0999 V2000
+    0.0000    0.0000    0.0000 C   0  0  0  0  0 15  0  0  0  0  0  0
+    1.0000    0.0000    0.0000 C   0  0  0  0  0 15  0  0  0  0  0  0
+  1  2  1  0  0  0  0
+M  END
+"""
+
+
+@pytest.mark.parametrize(
+    "molfile",
+    [ETHANE_V3000_ZERO_VALENCE, ETHANE_V2000_ZERO_VALENCE],
+    ids=["v3000", "v2000"],
+)
+def test_zero_valence_does_not_hallucinate_hydrogens(molfile, run_inchi_exe):
+    result = run_inchi_exe(molfile)
+    inchi = parse_inchi_from_executable_output(result.output)
+
+    assert inchi == EXPECTED_INCHI