fix: keep loop_Nc_power out of the process-wide colour memo

.github/workflows/acceptancetest.yml

@@ -491,6 +491,7 @@ jobs:
             cd $GITHUB_WORKSPACE
             ./tests/test_manager.py test_ML_check_brs_gd_gd -pA -t0 -l INFO
             ./tests/test_manager.py test_generate_output_semicolon_preserves_loop_process -pA -t0 -l INFO
+            ./tests/test_manager.py test_loop_nc_memo_not_poisoned_across_generations -pA -t0 -l INFO
 
 
 

.github/workflows/unittest.yml

@@ -171,7 +171,7 @@ jobs:
       - name: test one of the test test_color_flow_string_epsilon test_colorize_funny_model test_colorize_uu_gg test_colorize_uux_ggg test_color_matrix_Nc_restrictions test_color_matrix_fixed_indices test_color_matrix_multi_gluons test_color_matrix_multi_quarks test_helper_lcm_functions test_get_s_and_t_ub_tdg test_diagram_list test_diagram_list_nice_string test_representation test_setget_diagram_correct test_setget_diagram_exceptions test_values_for_prop test_make_unique test_generating_dict_to_0 test_generating_dict_to_1 test_interaction_list test_representation test_setget_interaction_correct test_setget_interaction_exceptions test_values_for_prop test_leg_list
         run: |
             cd $GITHUB_WORKSPACE
-            ./tests/test_manager.py test_color_flow_string_epsilon test_colorize_funny_model test_colorize_uu_gg test_colorize_uux_ggg test_color_matrix_Nc_restrictions test_color_matrix_fixed_indices test_color_matrix_multi_gluons test_color_matrix_multi_quarks test_helper_lcm_functions test_get_s_and_t_ub_tdg test_diagram_list test_diagram_list_nice_string test_representation test_setget_diagram_correct test_setget_diagram_exceptions test_values_for_prop test_make_unique test_generating_dict_to_0 test_generating_dict_to_1 test_interaction_list test_representation test_setget_interaction_correct test_setget_interaction_exceptions test_values_for_prop test_leg_list -t0
+            ./tests/test_manager.py test_color_flow_string_epsilon test_colorize_funny_model test_colorize_uu_gg test_colorize_uux_ggg test_color_matrix_Nc_restrictions test_color_matrix_fixed_indices test_color_matrix_multi_gluons test_color_matrix_multi_quarks test_helper_lcm_functions test_loop_Nc_power_survives_a_poisoned_memo test_memo_is_shared_across_loop_bases test_get_s_and_t_ub_tdg test_diagram_list test_diagram_list_nice_string test_representation test_setget_diagram_correct test_setget_diagram_exceptions test_values_for_prop test_make_unique test_generating_dict_to_0 test_generating_dict_to_1 test_interaction_list test_representation test_setget_interaction_correct test_setget_interaction_exceptions test_values_for_prop test_leg_list -t0
 
 
 

madgraph/core/color_amp.py

@@ -47,7 +47,10 @@ class ColorBasis(dict):
     diagram, coeff the corresponding coefficient (a fraction), is_imaginary
     if this contribution is real or complex, and Nc_power the Nc power."""
 
-    # Dictionary to save simplifications already done in a canonical form
+    # Dictionary to save simplifications already done in a canonical form.
+    # Keyed on the canonical color string alone, so nothing stored here may
+    # depend on anything else (see loop_Nc_power in update_color_basis).
+    # LoopColorBasis shares this class-level dict process-wide.
     _canonical_dict = {}
 
     # Dictionary store the raw colorize information
@@ -268,6 +271,9 @@ def update_color_basis(self, colorize_dict, index):
                 # Remove overall coefficient
                 for cs in canonical_col_fact:
                     cs.coeff = cs.coeff / col_str.coeff
+                    # loop_Nc_power is not a property of the canonical color
+                    # structure, so it must not be stored in the memo.
+                    cs.loop_Nc_power = 0
                 self._canonical_dict[canonical_rep] = canonical_col_fact
             else:
                 # If this representation has already been considered,
@@ -289,6 +295,11 @@ def update_color_basis(self, colorize_dict, index):
                 # in case we have K6 or K6bar Clebsch Gordan coefficients
                 for colstr in col_fact: colstr.order_summation()
 
+            # Restore loop_Nc_power, which the simplification above propagates
+            # unchanged from the incoming string but the memo does not carry.
+            for cs in col_fact:
+                cs.loop_Nc_power = col_str.loop_Nc_power
+
             # loop over color strings in the resulting color factor
             for col_str in col_fact:
                 immutable_col_str = col_str.to_immutable()

madgraph/loop/loop_color_amp.py

@@ -42,8 +42,13 @@ def __init__(self, compute_loop_nc = False):
         """ Defines the instance attribute compute_loop_nc.
         The compute_loop_nc sets wheter independent tracking of Nc power coming
         from the color loop trace is necessary or not (it is time consuming)."""
-        
+
         self.compute_loop_nc = compute_loop_nc
+        self._list_color_dict = []
+        # NB: ColorBasis.__init__ is deliberately not called, so that
+        # _canonical_dict stays the class-level (process-wide) memo. Loop color
+        # simplification is expensive and sharing it across matrix elements is
+        # what makes multi-subprocess loop generation tractable.
 
     def closeColorLoop(self, colorize_dict, lcut_charge, lcut_numbers):
         """ Add a color delta in the right representation (depending on the 

tests/acceptance_tests/test_cmd_madloop.py

@@ -14,6 +14,7 @@
 ################################################################################
 from __future__ import division
 from __future__ import absolute_import
+import glob
 import subprocess
 import unittest
 import os
@@ -200,6 +201,25 @@ def test_generate_output_semicolon_preserves_loop_process(self):
         self.assertNotIn('No processes generated', str(ctx.exception))
         self.assertTrue(self.interface._curr_amps)
 
+    def test_loop_nc_memo_not_poisoned_across_generations(self):
+        """Two loop generations in one process. The first one runs with
+        compute_loop_nc=False; it must not pin loop_Nc_power in the
+        process-wide color memo for the loop-induced generation that follows,
+        which needs the real value to write coloramps.inc."""
+
+        self.do('import model loop_sm')
+        self.do('generate g g > h [sqrvirt=QCD]')
+        self.do('output standalone %s -f' % pjoin(self.tmpdir, 'sqrvirt'))
+
+        self.do('generate g g > h [noborn=QCD]')
+        self.do('output madevent %s -f' % pjoin(self.tmpdir, 'noborn'))
+
+        coloramps = glob.glob(pjoin(self.tmpdir, 'noborn', 'SubProcesses',
+                                    'P*', 'coloramps.inc'))
+        self.assertTrue(coloramps)
+        for f in coloramps:
+            self.assertIn('ICOLAMP', open(f).read())
+
     def test_ML_check_full_epem_ttx(self):
         """ Test that check full e+ e- > t t~ works fine """
 

tests/unit_tests/core/test_color_amp.py

@@ -27,6 +27,7 @@
 
 import madgraph.core.color_amp as color_amp
 import madgraph.core.color_algebra as color
+import madgraph.loop.loop_color_amp as loop_color_amp
 
 import tests.unit_tests as unittest
 class ColorAmpTest(unittest.TestCase):
@@ -771,3 +772,45 @@ def test_helper_lcm_functions(self):
 
         self.assertEqual(color_amp.ColorMatrix.lcm(6, 3), 6)
         self.assertEqual(color_amp.ColorMatrix.lcmm(6, 3, 5, 2), 30)
+
+class LoopNcMemoTest(unittest.TestCase):
+    """The process-wide canonical color memo must not leak loop_Nc_power."""
+
+    def setUp(self):
+        self.saved = dict(color_amp.ColorBasis._canonical_dict)
+        color_amp.ColorBasis._canonical_dict.clear()
+
+    def tearDown(self):
+        color_amp.ColorBasis._canonical_dict.clear()
+        color_amp.ColorBasis._canonical_dict.update(self.saved)
+
+    @staticmethod
+    def _fill(compute_loop_nc, loop_Nc_power):
+        """Feed one T(-1,1,2)T(-1,3,4) chain through a fresh LoopColorBasis."""
+        basis = loop_color_amp.LoopColorBasis(compute_loop_nc=compute_loop_nc)
+        col_str = color.ColorString([color.T(-1, 1, 2), color.T(-1, 3, 4)])
+        col_str.loop_Nc_power = loop_Nc_power
+        basis.update_color_basis({(0,): col_str}, 0)
+        return basis
+
+    def test_loop_Nc_power_survives_a_poisoned_memo(self):
+        """A compute_loop_nc=False basis must not pin loop_Nc_power for the
+        next basis: the memo is keyed on the canonical color string only."""
+
+        # A [sqrvirt=]/standalone-style generation: loop_Nc_power is None.
+        self._fill(False, None)
+        # A later loop-induced madevent generation asks for the real value.
+        basis = self._fill(True, 1)
+
+        powers = [entry[5] for entries in basis.values() for entry in entries]
+        self.assertTrue(powers)
+        self.assertEqual(powers, [1] * len(powers))
+
+    def test_memo_is_shared_across_loop_bases(self):
+        """The memo must stay process-wide: that sharing is what makes
+        multi-subprocess loop generation tractable."""
+
+        self._fill(True, 1)
+        self.assertTrue(color_amp.ColorBasis._canonical_dict)
+        self.assertTrue(self._fill(True, 1)._canonical_dict is
+                        color_amp.ColorBasis._canonical_dict)