Fixes for iconfig-channel mappings in coloramps.h (obsolete, will be closed)

[valassi]

This is a clean WIP MR for iconfig-color mapping issues in coloramps.h. It starts from the current upstream/master, which already includes all relevant changes for rotxxx crashes.

It includes all of Olivier's commits in PR #852, as well as the relevant commits from my PR #853. It also supersedes the tests in WIP PR #870.

It is meant to address #856 (lhe color mismatches in ggttgg) and related issues, like possibly #845.

It is NOT meant to address #826 (no cross section in susy) and has most likely nothing to do with 826 (or susy), even if it includes fragments from 'fix_826" and 'gpucpp_826' and 'susy' branches.

[valassi]

Hi @oliviermattelaer I think that this is ready for review. Can you please have a look? Thanks!

(I still need to run some manual tests before merging, but the PR is ready for review as nothing will really change functionally).

[roiser]

* no cross section in susy_gg_t1t1 (the infamous [No cross section in SUSY gg_t1t1 log file #826](https://github.com/madgraph5/madgraph4gpu/issues/826), stilll there)

pls note I'm looking into this and I think its understood, will reply later in #826

[valassi]

pls note I'm looking into this and I think its understood, will reply later in #826

Thanks @roiser very good. If the issue (no cross section in susy_gg_t1t1) is actually caused by couplings, maybe discuss the technicalities in #862 (or as you want, I will add a note there if this is the case).

[valassi]

I have completed also all manual tests - this is ready to be merged

[oliviermattelaer]

Hi Andrea,

Changing the base since I need to see what you did change compare to my fix_826 branch.
Otherwise this is impossible to review since it includes too many things

[valassi]

Changing the base since I need to see what you did change compare to my fix_826 branch.

Hi Olivier, this is causing conflicts. And fix_826 itself has conflicts to the latest master now I guess.

I spent quite some effort fixing conflicts to make sure this can be merged into master. I will not do this again, sorry.

But if it helps you to review, ok. However when/if we decide to merge, this must merge into master, not into fix_826

Thanks Andrea

[valassi]

PS Please FIRST review mg5amcnlo/mg5amcnlo#115. This #873 depends on that.

[valassi]

Anyway, apart from logs, test scripts etc, the only relevant changes are the MG5AMC upgrade and this

git diff --no-ext-diff origin/color upstream/fix_826 CODEGEN/ > diff
...
diff --git a/epochX/cudacpp/CODEGEN/PLUGIN/CUDACPP_SA_OUTPUT/MG5aMC_patches/PROD/patch.P1 b/epochX/cudacpp/CODEGEN/PLUGIN/CUDACPP_SA_OUTPUT/MG5aMC_patches/PROD/patch.P1
index b90ef84b4..fc56de664 100644
--- a/epochX/cudacpp/CODEGEN/PLUGIN/CUDACPP_SA_OUTPUT/MG5aMC_patches/PROD/patch.P1
+++ b/epochX/cudacpp/CODEGEN/PLUGIN/CUDACPP_SA_OUTPUT/MG5aMC_patches/PROD/patch.P1
@@ -157,7 +157,7 @@ index 4fbb8e6ba..f9e2335de 100644
        END
  
 diff --git b/epochX/cudacpp/gg_tt.mad/SubProcesses/P1_gg_ttx/driver.f a/epochX/cudacpp/gg_tt.mad/SubProcesses/P1_gg_ttx/driver.f
-index 1124a9164..27a6e4674 100644
+index 71fbf2b25..0f1d199fc 100644
 --- b/epochX/cudacpp/gg_tt.mad/SubProcesses/P1_gg_ttx/driver.f
 +++ a/epochX/cudacpp/gg_tt.mad/SubProcesses/P1_gg_ttx/driver.f
 @@ -74,13 +74,77 @@ c      common/to_colstats/ncols,ncolflow,ncolalt,ic
@@ -239,7 +239,7 @@ index 1124a9164..27a6e4674 100644
  c
  c     Read process number
  c
-@@ -208,8 +272,33 @@ c      call sample_result(xsec,xerr)
+@@ -207,8 +271,33 @@ c      call sample_result(xsec,xerr)
  c      write(*,*) 'Final xsec: ',xsec
  
        rewind(lun)
@@ -274,7 +274,7 @@ index 1124a9164..27a6e4674 100644
        end
  
  c     $B$ get_user_params $B$ ! tag for MadWeight
-@@ -387,7 +476,7 @@ c
+@@ -386,7 +475,7 @@ c
        fopened=.false.
        tempname=filename 	 
        fine=index(tempname,' ') 	 
diff --git a/epochX/cudacpp/CODEGEN/PLUGIN/CUDACPP_SA_OUTPUT/madgraph/iolibs/template_files/gpu/coloramps.h b/epochX/cudacpp/CODEGEN/PLUGIN/CUDACPP_SA_OUTPUT/madgraph/iolibs/template_files/gpu/coloramps.h
index c8657394c..3d03a6063 100644
--- a/epochX/cudacpp/CODEGEN/PLUGIN/CUDACPP_SA_OUTPUT/madgraph/iolibs/template_files/gpu/coloramps.h
+++ b/epochX/cudacpp/CODEGEN/PLUGIN/CUDACPP_SA_OUTPUT/madgraph/iolibs/template_files/gpu/coloramps.h
@@ -1,38 +1,41 @@
 // Copyright (C) 2020-2024 CERN and UCLouvain.
 // Licensed under the GNU Lesser General Public License (version 3 or later).
 // Created by: A. Valassi (Dec 2022) for the MG5aMC CUDACPP plugin.
-// Further modified by: O. Mattelaer, A. Valassi (2022-2024) for the MG5aMC CUDACPP plugin.
+// Further modified by: A. Valassi (2022-2024) for the MG5aMC CUDACPP plugin.
 
 #ifndef COLORAMPS_H
 #define COLORAMPS_H 1
 
-namespace mgOnGpu /* clang-format off */
+#include <map>
+
+namespace mgOnGpu
 {
   // Summary of numbering and indexing conventions for the relevant concepts (see issue #826 and PR #852)
   // - Diagram number (no variable) in [1, N_diagrams]: all values are allowed (N_diagrams distinct values)
   //   => this number is displayed for information before each block of code in CPPProcess.cc
-  // - Channel number ("channelId" in C, CHANNEL_ID in F) in [1, N_diagrams]: not all values are allowed (N_config <= N_diagrams distinct values)
-  //   => this number (with F indexing as in ps/pdf output) is passed around as an API argument between cudacpp functions
-  //   Note: the old API passes around a single CHANNEL_ID (and uses CHANNEL_ID=0 to indicate no-multichannel mode, but this is not used in coloramps.h),
-  //   while the new API passes around an array of CHANNEL_ID's (and uses a NULL array pointer to indicate no-multichannel mode)
-  // - Channel number in C indexing: "channelIdC" = channelID - 1
-  //   => this number (with C indexing) is used as the index of the channelIdC_to_iconfig array below
-  // - Config number ("iconfig" in C, ICONFIG in F) in [1, N_config]: all values are allowed (N_config <= N_diagrams distinct values)
-  // - Config number in C indexing: "iconfigC" = iconfig - 1
-  //   => this number (with C indexing) is used as the index of the icolamp array below
-
-  // Map channelIdC (in C indexing, i.e. channelId-1) to iconfig (in F indexing)
-  // Note: iconfig=0 indicates invalid values, i.e. channels/diagrams with no single-diagram enhancement in the MadEvent sampling algorithm (presence of 4-point interaction?)
-  // This array has N_diagrams elements, but only N_config <= N_diagrams valid (non-zero) values
-  __device__ constexpr int channelIdC_to_iconfig[%(nb_diag)i] = { // note: a trailing comma in the initializer list is allowed
-%(channelc2iconfig_lines)s
+  // - Channel number (CHANNEL_ID) in [0, N_diagrams]: not all values are allowed (N_config <= N_diagrams distinct values)
+  //   => this number (with indexing like ps/pdf output) is passed around as an API argument between cudacpp functions
+  //   0 is allowed to fallback to no multi-channel mode.
+  // - Channel number in C indexing: "IconfiC", this is the equivalent of the Fortran iconfig
+  //   iconfigC = iconfig -1
+  //   provides a continuous index [0, N_config-1] for array
+  //  iconfigC = ChannelId_to_iconfigC[channelId]
+  //NOTE: All those ordering are event by event specific (with the intent to have those fix within a vector size/wrap   
+  
+  // Map channelId to iconfigC
+  // This array has N_diagrams+1 elements, but only N_config <= N_diagrams valid values
+  // unvalid values are set to -1
+  // The 0 entry is a fall back to still write events even if no multi-channel is setup (wrong color selected in that mode) 
+    __device__ constexpr int channelId_to_iconfigC[%(nb_diag_plus_one)i] = {
+     0, // channelId=0: This value means not multi-channel, color will be wrong anyway -> pick the first
+%(diag_to_channel)s
   };
 
   // Map iconfigC (in C indexing, i.e. iconfig-1) to the set of allowed colors
-  // This array has N_config <= N_diagrams elements
-  __device__ constexpr bool icolamp[%(nb_channel)s][%(nb_color)s] = { // note: a trailing comma in the initializer list is allowed
-%(icolamp_lines)s
-  }; /* clang-format on */
+  // This array has N_config <= N_diagrams elements    
+    __device__ constexpr bool icolamp[%(nb_channel)s][%(nb_color)s] = {
+%(is_LC)s
+  };
 
 }
 #endif // COLORAMPS_H
diff --git a/epochX/cudacpp/CODEGEN/PLUGIN/CUDACPP_SA_OUTPUT/madgraph/iolibs/template_files/gpu/process_sigmaKin_function.inc b/epochX/cudacpp/CODEGEN/PLUGIN/CUDACPP_SA_OUTPUT/madgraph/iolibs/template_files/gpu/process_sigmaKin_function.inc
index c3c7fd466..2ec052e3a 100644
--- a/epochX/cudacpp/CODEGEN/PLUGIN/CUDACPP_SA_OUTPUT/madgraph/iolibs/template_files/gpu/process_sigmaKin_function.inc
+++ b/epochX/cudacpp/CODEGEN/PLUGIN/CUDACPP_SA_OUTPUT/madgraph/iolibs/template_files/gpu/process_sigmaKin_function.inc
@@ -4,7 +4,7 @@
 ! Copyright (C) 2020-2024 CERN and UCLouvain.
 ! Licensed under the GNU Lesser General Public License (version 3 or later).
 ! Modified by: A. Valassi (Sep 2021) for the MG5aMC CUDACPP plugin.
-! Further modified by: O. Mattelaer, J. Teig, A. Valassi (2021-2024) for the MG5aMC CUDACPP plugin.
+! Further modified by: J. Teig, A. Valassi (2021-2024) for the MG5aMC CUDACPP plugin.
 !==========================================================================
 
 #include "GpuAbstraction.h"
@@ -72,9 +72,7 @@
     // Event-by-event random choice of color #402
     if( channelId != 0 ) // no event-by-event choice of color if channelId == 0 (fix FPE #783)
     {
-      const unsigned int channelIdC = channelId - 1;                           // channelIdC_to_iconfig in coloramps.h uses the C array indexing starting at 0
-      const unsigned int iconfig = mgOnGpu::channelIdC_to_iconfig[channelIdC]; // map N_diagrams to N_config <= N_diagrams configs (fix LHE color mismatch #856: see also #826, #852, #853)
-      const unsigned int iconfigC = iconfig - 1;                               // icolamp in coloramps.h uses the C array indexing starting at 0
+      const unsigned int iconfigC = mgOnGpu::channelId_to_iconfigC[channelId]; // coloramps.h uses a channel ordering not the diagram id
       fptype targetamp[ncolor] = { 0 };
       for( int icolC = 0; icolC < ncolor; icolC++ )
       {
@@ -117,7 +115,7 @@
     // - firstprivate: give each thread its own copy, and initialise with value from outside
 #define _OMPLIST0 allcouplings, allMEs, allmomenta, allrndcol, allrndhel, allselcol, allselhel, cGoodHel, cNGoodHel, npagV2
 #ifdef MGONGPU_SUPPORTS_MULTICHANNEL
-#define _OMPLIST1 , allDenominators, allNumerators, channelId, mgOnGpu::icolamp, mgOnGpu::channelIdC_to_iconfig
+#define _OMPLIST1 , allDenominators, allNumerators, channelId, mgOnGpu::icolamp, mgOnGpu::channelId_to_iconfigC
 #else
 #define _OMPLIST1
 #endif
@@ -189,9 +187,7 @@
       // Event-by-event random choice of color #402
       if( channelId != 0 ) // no event-by-event choice of color if channelId == 0 (fix FPE #783)
       {
-        const unsigned int channelIdC = channelId - 1;                           // channelIdC_to_iconfig in coloramps.h uses the C array indexing starting at 0
-        const unsigned int iconfig = mgOnGpu::channelIdC_to_iconfig[channelIdC]; // map N_diagrams to N_config <= N_diagrams configs (fix LHE color mismatch #856: see also #826, #852, #853)
-        const unsigned int iconfigC = iconfig - 1;                               // icolamp in coloramps.h uses the C array indexing starting at 0
+        const unsigned int iconfigC = mgOnGpu::channelId_to_iconfigC[channelId]; // coloramps.h uses a channel ordering not the diagram id
         fptype_sv targetamp[ncolor] = { 0 };
         for( int icolC = 0; icolC < ncolor; icolC++ )
         {
@@ -202,6 +198,7 @@
           if( mgOnGpu::icolamp[iconfigC][icolC] ) targetamp[icolC] += jamp2_sv[icolC];
         }
 #if defined MGONGPU_CPPSIMD and defined MGONGPU_FPTYPE_DOUBLE and defined MGONGPU_FPTYPE2_FLOAT
+        const unsigned int iconfigC = mgOnGpu::channelId_to_iconfigC[channelId]; // coloramps.h uses a channel ordering not the diagram id
         fptype_sv targetamp2[ncolor] = { 0 };
         for( int icolC = 0; icolC < ncolor; icolC++ )
         {
diff --git a/epochX/cudacpp/CODEGEN/PLUGIN/CUDACPP_SA_OUTPUT/model_handling.py b/epochX/cudacpp/CODEGEN/PLUGIN/CUDACPP_SA_OUTPUT/model_handling.py
index 5a275b566..a735a80dc 100644
--- a/epochX/cudacpp/CODEGEN/PLUGIN/CUDACPP_SA_OUTPUT/model_handling.py
+++ b/epochX/cudacpp/CODEGEN/PLUGIN/CUDACPP_SA_OUTPUT/model_handling.py
@@ -1548,28 +1548,27 @@ class PLUGIN_OneProcessExporter(PLUGIN_export_cpp.OneProcessExporterGPU):
         # will be smaller than the true number of diagram. This is fine for color
         # but maybe not for something else.
         nb_diag = max(config[0] for config in config_subproc_map)
-        import math
-        ndigits = str(int(math.log10(nb_diag))+1)
         # Output which diagrams correspond ot a channel to get information for valid color
         lines = []
+        # Note: line for index 0 (no multi-channel is hardcoded in the template, so here we fill the array from index 1)
         for diag in range(1, nb_diag+1):
-            channelidf = diag
-            channelidc = channelidf - 1 # C convention 
             if diag in diag_to_iconfig:
                 iconfigf = diag_to_iconfig[diag]
-                iconfigftxt = '%i'%iconfigf
+                iconfigc = iconfigf - 1 # C convention 
+                text = "     %(iconfigc)i, // channelId=%(diag)i (diagram=%(diag)i) --> iconfig=%(iconfigf)i (f77 conv) and iconfigC=%(iconfigc)i (c conv)" 
             else:
-                iconfigf = 0
-                iconfigftxt = 'None'
-            text = '    %(iconfigf)NNNNi, // CHANNEL_ID=%(channelidf)-NNNNi i.e. DIAGRAM=%(diag)-NNNNi --> ICONFIG=%(iconfigftxt)s'.replace('NNNN',ndigits)
-            lines.append(text % {'diag':diag, 'channelidf':channelidf, 'iconfigf':iconfigf, 'iconfigftxt':iconfigftxt})
-        replace_dict['channelc2iconfig_lines'] = '\n'.join(lines)
+                iconfigc = -1
+                iconfigf = -1
+                text = "     -1, // channelId=%(diag)i (diagram=%(diag)i): Not consider as a channel of integration (presence of 4 point interaction?)" 
+            lines.append(text % {'diag': diag, 'iconfigc':  iconfigc, 'iconfigf':iconfigf})  
+
+        replace_dict['diag_to_channel'] = '\n'.join(lines)
 
         if self.include_multi_channel: # NB unnecessary as edit_coloramps is not called otherwise...
             multi_channel = self.get_multi_channel_dictionary(self.matrix_elements[0].get('diagrams'), self.include_multi_channel)
-            replace_dict['icolamp_lines'] = self.get_icolamp_lines(multi_channel, self.matrix_elements[0], 1)
+            replace_dict['is_LC'] = self.get_icolamp_lines(multi_channel, self.matrix_elements[0], 1)
             replace_dict['nb_channel'] = len(multi_channel)
-            replace_dict['nb_diag'] = max(config[0] for config in config_subproc_map)
+            replace_dict['nb_diag_plus_one'] = max(config[0] for config in config_subproc_map)+1
             replace_dict['nb_color'] = max(1,len(self.matrix_elements[0].get('color_basis')))
             
             misc.sprint(multi_channel)
@@ -1577,14 +1576,18 @@ class PLUGIN_OneProcessExporter(PLUGIN_export_cpp.OneProcessExporterGPU):
             #raise Exception
             
             # AV extra formatting (e.g. gg_tt was "{{true,true};,{true,false};,{false,true};};")
-            ###misc.sprint(replace_dict['icolamp_lines'])
-            split = replace_dict['icolamp_lines'].replace('{{','{').replace('};};','}').split(';,')
-            text=', // ICONFIG=%-NNNNi <-- CHANNEL_ID=%i'.replace('NNNN',ndigits)
-            for iconfigc in range(len(split)): 
-                ###misc.sprint(split[iconfigc])
-                split[iconfigc] = '    ' + split[iconfigc].replace(',',', ').replace('true',' true').replace('{','{ ').replace('}',' }')
-                split[iconfigc] += text % (iconfigc+1, iconfig_to_diag[iconfigc+1])
-            replace_dict['icolamp_lines'] = '\n'.join(split)
+            split = replace_dict['is_LC'].split(';,') 
+            misc.sprint(replace_dict['is_LC'])
+            for i in range(len(split)): 
+                misc.sprint(split[i])
+                split[i] = '    ' + split[i].replace(',',', ').replace('{{', '{')
+                misc.sprint(split[i])
+                if '};};' in split[i]:
+                   split[i] = split[i][:-4] + '}, // iconfigC=%i, diag=%i' % (i, iconfig_to_diag[i+1]) 
+                elif 'false' in split[i] or 'true' in split[i]:
+                    split[i] += ', // iconfigC=%i, diag=%i' % (i, iconfig_to_diag[i+1])
+                misc.sprint(split[i])
+            replace_dict['is_LC'] = '\n'.join(split)
             ff.write(template % replace_dict)
         ff.close()
 
diff --git a/epochX/cudacpp/CODEGEN/checkFormatting.sh b/epochX/cudacpp/CODEGEN/checkFormatting.sh
index a6db39255..a016a7958 100755
--- a/epochX/cudacpp/CODEGEN/checkFormatting.sh
+++ b/epochX/cudacpp/CODEGEN/checkFormatting.sh
@@ -37,7 +37,7 @@ function checkProcdir()
   cd $TOPDIR
   if [ ! -d $procdir ]; then echo "ERROR! Directory not found $TOPDIR/$procdir"; exit 1; fi
   # Define the list of files to be checked
-  files=$(\ls $procdir/src/*.cc $procdir/src/*.h $procdir/SubProcesses/*.cc $procdir/SubProcesses/*.h $procdir/SubProcesses/P*/check_sa.cc $procdir/SubProcesses/P*/CPPProcess.cc $procdir/SubProcesses/P*/CPPProcess.h $procdir/SubProcesses/P*/epoch_process_id.h $procdir/SubProcesses/P*/coloramps.h)
+  files=$(\ls $procdir/src/*.cc $procdir/src/*.h $procdir/SubProcesses/*.cc $procdir/SubProcesses/*.h $procdir/SubProcesses/P*/check_sa.cc $procdir/SubProcesses/P*/CPPProcess.cc $procdir/SubProcesses/P*/CPPProcess.h $procdir/SubProcesses/P*/epoch_process_id.h)
   if [ "$files" == "" ]; then echo "ERROR! No files to check found in directory $TOPDIR/$procdir"; exit 1; fi
   # Check each file
   status=0

[valassi]

Essentially I went back to my three step mapping instead of your one step mapping. Given how complex this is (see the fact that even export_cpp and export_v4 disagree due to fortran/cpp indexing in mg5amcnlo/mg5amcnlo#115), I really think this makes it clearer.

And the generated code diffs are these

git diff --no-ext-diff 22328317716dee0b12e99547d4405d7d55a33e21..fc1b5cd75824f44e99253bc82a0dfecf14ab0f81 susy_gg_t1t1.mad/SubProcesses/ > diff
...
diff --git a/epochX/cudacpp/susy_gg_t1t1.mad/SubProcesses/P1_gg_t1t1x/CPPProcess.cc b/epochX/cudacpp/susy_gg_t1t1.mad/SubProcesses/P1_gg_t1t1x/CPPProcess.cc
index 4f46a8c4f..e5f1721f3 100644
--- a/epochX/cudacpp/susy_gg_t1t1.mad/SubProcesses/P1_gg_t1t1x/CPPProcess.cc
+++ b/epochX/cudacpp/susy_gg_t1t1.mad/SubProcesses/P1_gg_t1t1x/CPPProcess.cc
@@ -1009,7 +1009,9 @@ namespace mg5amcCpu
     // Event-by-event random choice of color #402
     if( channelId != 0 ) // no event-by-event choice of color if channelId == 0 (fix FPE #783)
     {
-      const unsigned int iconfigC = mgOnGpu::channelId_to_iconfigC[channelId]; // coloramps.h uses a channel ordering not the diagram id
+      const unsigned int channelIdC = channelId - 1;                           // channelIdC_to_iconfig in coloramps.h uses the C array indexing starting at 0
+      const unsigned int iconfig = mgOnGpu::channelIdC_to_iconfig[channelIdC]; // map N_diagrams to N_config <= N_diagrams configs (fix LHE color mismatch #856: see also #826, #852, #853)
+      const unsigned int iconfigC = iconfig - 1;                               // icolamp in coloramps.h uses the C array indexing starting at 0
       fptype targetamp[ncolor] = { 0 };
       for( int icolC = 0; icolC < ncolor; icolC++ )
       {
@@ -1052,7 +1054,7 @@ namespace mg5amcCpu
     // - firstprivate: give each thread its own copy, and initialise with value from outside
 #define _OMPLIST0 allcouplings, allMEs, allmomenta, allrndcol, allrndhel, allselcol, allselhel, cGoodHel, cNGoodHel, npagV2
 #ifdef MGONGPU_SUPPORTS_MULTICHANNEL
-#define _OMPLIST1 , allDenominators, allNumerators, channelId, mgOnGpu::icolamp, mgOnGpu::channelId_to_iconfigC
+#define _OMPLIST1 , allDenominators, allNumerators, channelId, mgOnGpu::icolamp, mgOnGpu::channelIdC_to_iconfig
 #else
 #define _OMPLIST1
 #endif
@@ -1124,7 +1126,9 @@ namespace mg5amcCpu
       // Event-by-event random choice of color #402
       if( channelId != 0 ) // no event-by-event choice of color if channelId == 0 (fix FPE #783)
       {
-        const unsigned int iconfigC = mgOnGpu::channelId_to_iconfigC[channelId]; // coloramps.h uses a channel ordering not the diagram id
+        const unsigned int channelIdC = channelId - 1;                           // channelIdC_to_iconfig in coloramps.h uses the C array indexing starting at 0
+        const unsigned int iconfig = mgOnGpu::channelIdC_to_iconfig[channelIdC]; // map N_diagrams to N_config <= N_diagrams configs (fix LHE color mismatch #856: see also #826, #852, #853)
+        const unsigned int iconfigC = iconfig - 1;                               // icolamp in coloramps.h uses the C array indexing starting at 0
         fptype_sv targetamp[ncolor] = { 0 };
         for( int icolC = 0; icolC < ncolor; icolC++ )
         {
@@ -1135,7 +1139,6 @@ namespace mg5amcCpu
           if( mgOnGpu::icolamp[iconfigC][icolC] ) targetamp[icolC] += jamp2_sv[icolC];
         }
 #if defined MGONGPU_CPPSIMD and defined MGONGPU_FPTYPE_DOUBLE and defined MGONGPU_FPTYPE2_FLOAT
-        const unsigned int iconfigC = mgOnGpu::channelId_to_iconfigC[channelId]; // coloramps.h uses a channel ordering not the diagram id
         fptype_sv targetamp2[ncolor] = { 0 };
         for( int icolC = 0; icolC < ncolor; icolC++ )
         {
diff --git a/epochX/cudacpp/susy_gg_t1t1.mad/SubProcesses/P1_gg_t1t1x/coloramps.h b/epochX/cudacpp/susy_gg_t1t1.mad/SubProcesses/P1_gg_t1t1x/coloramps.h
index 1a931324b..28a4d6d32 100644
--- a/epochX/cudacpp/susy_gg_t1t1.mad/SubProcesses/P1_gg_t1t1x/coloramps.h
+++ b/epochX/cudacpp/susy_gg_t1t1.mad/SubProcesses/P1_gg_t1t1x/coloramps.h
@@ -1,50 +1,47 @@
 // Copyright (C) 2020-2024 CERN and UCLouvain.
 // Licensed under the GNU Lesser General Public License (version 3 or later).
 // Created by: A. Valassi (Dec 2022) for the MG5aMC CUDACPP plugin.
-// Further modified by: A. Valassi (2022-2024) for the MG5aMC CUDACPP plugin.
+// Further modified by: O. Mattelaer, A. Valassi (2022-2024) for the MG5aMC CUDACPP plugin.
 
 #ifndef COLORAMPS_H
 #define COLORAMPS_H 1
 
-#include <map>
-
-namespace mgOnGpu
+namespace mgOnGpu /* clang-format off */
 {
   // Summary of numbering and indexing conventions for the relevant concepts (see issue #826 and PR #852)
   // - Diagram number (no variable) in [1, N_diagrams]: all values are allowed (N_diagrams distinct values)
   //   => this number is displayed for information before each block of code in CPPProcess.cc
-  // - Channel number (CHANNEL_ID) in [0, N_diagrams]: not all values are allowed (N_config <= N_diagrams distinct values)
-  //   => this number (with indexing like ps/pdf output) is passed around as an API argument between cudacpp functions
-  //   0 is allowed to fallback to no multi-channel mode.
-  // - Channel number in C indexing: "IconfiC", this is the equivalent of the Fortran iconfig
-  //   iconfigC = iconfig -1
-  //   provides a continuous index [0, N_config-1] for array
-  //  iconfigC = ChannelId_to_iconfigC[channelId]
-  //NOTE: All those ordering are event by event specific (with the intent to have those fix within a vector size/wrap   
-  
-  // Map channelId to iconfigC
-  // This array has N_diagrams+1 elements, but only N_config <= N_diagrams valid values
-  // unvalid values are set to -1
-  // The 0 entry is a fall back to still write events even if no multi-channel is setup (wrong color selected in that mode) 
-    __device__ constexpr int channelId_to_iconfigC[7] = {
-     0, // channelId=0: This value means not multi-channel, color will be wrong anyway -> pick the first
-     -1, // channelId=1 (diagram=1): Not consider as a channel of integration (presence of 4 point interaction?)
-     0, // channelId=2 (diagram=2) --> iconfig=1 (f77 conv) and iconfigC=0 (c conv)
-     1, // channelId=3 (diagram=3) --> iconfig=2 (f77 conv) and iconfigC=1 (c conv)
-     2, // channelId=4 (diagram=4) --> iconfig=3 (f77 conv) and iconfigC=2 (c conv)
-     3, // channelId=5 (diagram=5) --> iconfig=4 (f77 conv) and iconfigC=3 (c conv)
-     4, // channelId=6 (diagram=6) --> iconfig=5 (f77 conv) and iconfigC=4 (c conv)
+  // - Channel number ("channelId" in C, CHANNEL_ID in F) in [1, N_diagrams]: not all values are allowed (N_config <= N_diagrams distinct values)
+  //   => this number (with F indexing as in ps/pdf output) is passed around as an API argument between cudacpp functions
+  //   Note: the old API passes around a single CHANNEL_ID (and uses CHANNEL_ID=0 to indicate no-multichannel mode, but this is not used in coloramps.h),
+  //   while the new API passes around an array of CHANNEL_ID's (and uses a NULL array pointer to indicate no-multichannel mode)
+  // - Channel number in C indexing: "channelIdC" = channelID - 1
+  //   => this number (with C indexing) is used as the index of the channelIdC_to_iconfig array below
+  // - Config number ("iconfig" in C, ICONFIG in F) in [1, N_config]: all values are allowed (N_config <= N_diagrams distinct values)
+  // - Config number in C indexing: "iconfigC" = iconfig - 1
+  //   => this number (with C indexing) is used as the index of the icolamp array below
+
+  // Map channelIdC (in C indexing, i.e. channelId-1) to iconfig (in F indexing)
+  // Note: iconfig=0 indicates invalid values, i.e. channels/diagrams with no single-diagram enhancement in the MadEvent sampling algorithm (presence of 4-point interaction?)
+  // This array has N_diagrams elements, but only N_config <= N_diagrams valid (non-zero) values
+  __device__ constexpr int channelIdC_to_iconfig[6] = { // note: a trailing comma in the initializer list is allowed
+    0, // CHANNEL_ID=1 i.e. DIAGRAM=1 --> ICONFIG=None
+    1, // CHANNEL_ID=2 i.e. DIAGRAM=2 --> ICONFIG=1
+    2, // CHANNEL_ID=3 i.e. DIAGRAM=3 --> ICONFIG=2
+    3, // CHANNEL_ID=4 i.e. DIAGRAM=4 --> ICONFIG=3
+    4, // CHANNEL_ID=5 i.e. DIAGRAM=5 --> ICONFIG=4
+    5, // CHANNEL_ID=6 i.e. DIAGRAM=6 --> ICONFIG=5
   };
 
   // Map iconfigC (in C indexing, i.e. iconfig-1) to the set of allowed colors
-  // This array has N_config <= N_diagrams elements    
-    __device__ constexpr bool icolamp[5][2] = {
-    {true, true}, // iconfigC=0, diag=2
-    {true, true}, // iconfigC=1, diag=3
-    {true, false}, // iconfigC=2, diag=4
-    {true, false}, // iconfigC=3, diag=5
-    {false, true}, // iconfigC=4, diag=6
-  };
+  // This array has N_config <= N_diagrams elements
+  __device__ constexpr bool icolamp[5][2] = { // note: a trailing comma in the initializer list is allowed
+    {  true,  true }, // ICONFIG=1 <-- CHANNEL_ID=2
+    {  true, false }, // ICONFIG=2 <-- CHANNEL_ID=3
+    {  true, false }, // ICONFIG=3 <-- CHANNEL_ID=4
+    { false,  true }, // ICONFIG=4 <-- CHANNEL_ID=5
+    { false,  true }, // ICONFIG=5 <-- CHANNEL_ID=6
+  }; /* clang-format on */
 
 }
 #endif // COLORAMPS_H

[valassi]

Essentially I went back to my three step mapping instead of your one step mapping

And this means I remove the first element of the channel-to-config array, to just keep the fortran mappings: essentially this array does the fortran-to-fortran mapping, then some variables handle the -1 translation to C indexing. There are Ndiag+1 (7 here) entries in Olivier's code and Ndiag in mine (6 here), note that only Ndiag mappings are needed, not Ndiag+1.

-  // Map channelId to iconfigC
-  // This array has N_diagrams+1 elements, but only N_config <= N_diagrams valid values
-  // unvalid values are set to -1
-  // The 0 entry is a fall back to still write events even if no multi-channel is setup (wrong color selected in that mode) 
-    __device__ constexpr int channelId_to_iconfigC[7] = {
-     0, // channelId=0: This value means not multi-channel, color will be wrong anyway -> pick the first
-     -1, // channelId=1 (diagram=1): Not consider as a channel of integration (presence of 4 point interaction?)
-     0, // channelId=2 (diagram=2) --> iconfig=1 (f77 conv) and iconfigC=0 (c conv)
-     1, // channelId=3 (diagram=3) --> iconfig=2 (f77 conv) and iconfigC=1 (c conv)
-     2, // channelId=4 (diagram=4) --> iconfig=3 (f77 conv) and iconfigC=2 (c conv)
-     3, // channelId=5 (diagram=5) --> iconfig=4 (f77 conv) and iconfigC=3 (c conv)
-     4, // channelId=6 (diagram=6) --> iconfig=5 (f77 conv) and iconfigC=4 (c conv)
...
+  // Map channelIdC (in C indexing, i.e. channelId-1) to iconfig (in F indexing)
+  // Note: iconfig=0 indicates invalid values, i.e. channels/diagrams with no single-diagram enhancement in the MadEvent sampling algorithm (presence of 4-point interaction?)
+  // This array has N_diagrams elements, but only N_config <= N_diagrams valid (non-zero) values
+  __device__ constexpr int channelIdC_to_iconfig[6] = { // note: a trailing comma in the initializer list is allowed
+    0, // CHANNEL_ID=1 i.e. DIAGRAM=1 --> ICONFIG=None
+    1, // CHANNEL_ID=2 i.e. DIAGRAM=2 --> ICONFIG=1
+    2, // CHANNEL_ID=3 i.e. DIAGRAM=3 --> ICONFIG=2
+    3, // CHANNEL_ID=4 i.e. DIAGRAM=4 --> ICONFIG=3
+    4, // CHANNEL_ID=5 i.e. DIAGRAM=5 --> ICONFIG=4
+    5, // CHANNEL_ID=6 i.e. DIAGRAM=6 --> ICONFIG=5
   };

[valassi]

This has conflicts in branch color (because it was modified to be agains fix_826). I opened a new PR #877 with no conflicts, based on branch color2 (which can be both against fix_826 or against master)

Eventually this will be closed and #877 merged, I guess

[valassi]

This is obsolete because it has merge conflicts. It has been replaced by #877 which is functionally equivalent (and includes improvements suggested by Olivier). Closing as unmerged, at commit 93a547f - I will probably delete/change the color branch.

[valassi]

For the record I did delete the obsolete color branch