Record all mothers of an MCParticle - #34
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A single motherId cannot represent a generator record. Pythia encodes up to two mother indices in six documented combinations: a carbon copy of the mother, one mother, an inclusive range of string-fragmentation mothers, two genuinely distinct mothers, and an order-reversed variant. GENIE's rootracker carries its own first/last pair. Keeping only the first mother drops half the incoming edges of the truth graph, which matters now that aegir can write the full record rather than only final-state particles: tracing a final-state hadron back to the string or to the hard process it came from is most of the reason to store that record at all. Store the resolved list rather than the raw pair. Pythia8's Particle::motherList() provides exactly it, derived from mother1, mother2 *and* the native status — and the native status is precisely what distinguishes an inclusive range from two distinct mothers, so a (first, last) pair would bake that ambiguity into the format permanently. A list also normalises Pythia's and GENIE's differing conventions into one unambiguous representation, and matches EDM4hep's parents relation. motherId stays as the first element, both to keep existing files readable and as the portable single-mother accessor. status was documented as "Status code (1 = stable)", which is wrong on both counts: the field carries the HepMC status set produced by statusHepMC() and used by EDM4hep as generatorStatus, and 1 means "not decayed by the generator", which says nothing about stability — a final-state K_S is status 1 and is expected to decay in the detector. The full set is now spelled out in the header. The default of 1 is unchanged and still correct. Additive schema change: the four frozen reference files stay readable with mothers reading back empty, so the mask table gains a v0.5.0 entry following the RecParticle::hits precedent. Verified by disabling the mask, which makes all four compat_read_v* tests fail. The class version is bumped to 3 and the schema snapshot and reference_head.root are regenerated here, as the policy requires. test_utils.hpp sets the new member too, so the TTree and RNTuple round-trip tests cover it; the geometryNodeId precedent left its member untested there. Assisted-by: Claude Code:claude-opus-5
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ChangesMCParticle mother tracking
Estimated code review effort: 3 (Moderate) | ~25 minutes Merge Risk: ⚪ Minimal · up to MCParticle now preserves complete mother relationships while retaining motherId compatibility. Legacy files retain empty mother lists, and current data is covered by consistency and serialization checks, leaving no identified merge-blocking risk. 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
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In `@tests/test_utils.hpp`:
- Line 29: The canonical MCParticle::mothers recipe must use only valid
same-collection indices: update tests/test_utils.hpp lines 29-29 and
tests/reference_values.hpp lines 46-46 so records with no mothers use an empty
list and multi-mother records contain only in-range indices, removing the -1 and
300 + 5 * i + offset values; document this corrected recipe and invariant in
tests/data/README.md lines 78-79.
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include/SHiP/LinkDef.hinclude/SHiP/MCParticle.hpptests/data/README.mdtests/data/reference_head.roottests/data/schema_snapshot.txttests/reference_values.hpptests/test_read_reference.cpptests/test_utils.hpp
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The recipe introduced in 21c6f0d contradicted the contract it was meant to illustrate. For the first entry it produced motherId = -1 together with a non-empty mothers = {-1, 300 + offset}, although the header states that an entry without a mother carries an empty list and that the -1 sentinel belongs to motherId alone; every entry's second element, 300 to 310 plus the offset, sat far outside the valid index range of a three-element collection. Nothing failed, because every test compares read-back data against the generator that produced it. It matters because tests/data/README.md declares the recipe part of the on-disk contract that "a future non-C++ reader can check against the same formulas", and asserts the very invariant the values broke. Entry 0 now carries an empty list, and later entries lead with motherId and add one further in-range index, giving {}, {0, 2} and {1, 0}. The offset dependence is dropped deliberately: an index into a fixed three-element collection cannot legitimately vary with an arbitrary offset. No frozen reference file exists for v0.5.0 yet, and the mask table clears mothers below it, so the four frozen files are unaffected; only reference_head.root is rewritten. The schema is untouched, so the snapshot needs no regeneration. Assisted-by: Claude Code:claude-opus-5
The mother-index contract lived only in prose, and the suite could not have caught a violation of it: every test compares read-back data against the generator that produced it, so a recipe that drifts out of spec still agrees with itself. The preceding commit repairs exactly such a drift, which review caught and ctest did not. Ship the contract as two predicates beside the struct. mothersAreConsistent requires motherId to be -1 or a valid index, mothers to hold only valid and distinct indices none of which is the entry itself, and a non-empty list to start with motherId. An empty list stays acceptable, because files written before v0.5.0 have no such field and read back empty next to a perfectly valid motherId. mothersArePopulated adds the requirement that current-schema data actually fill the list, and is therefore applied to round-tripped data and to head, while the frozen files take the tolerant form. Free functions sit outside the dictionary — LinkDef.h disables them wholesale, which fromSimParticle already relies on — so this is neither a schema change nor a class-version bump, and the snapshot is untouched. The header now also states the two rules that were left implicit, that indices are distinct and that no entry is its own mother, and records what is deliberately not required: mothers need not precede their daughters, since a producer that filters a generator record and remaps indices may reorder it. The new mc_mothers test pins both recipes at every offset the suite uses and rejects one hand-built violation per rule, so a vacuously true predicate cannot pass unnoticed. Restoring the old recipe makes mc_mothers, ttree_io and rntuple_io fail, which is the regression this exists to catch. Assisted-by: Claude Code:claude-opus-5
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A single motherId cannot represent a general generator record faithfully. Pythia encodes up to two mother indices in six documented combinations:
GENIE's rootracker carries its own first/last pair.
Keeping only the first mother drops half the incoming edges of the truth graph, which matters now that aegir can write the full record rather than only final-state particles: tracing a final-state hadron back to the string or to the hard process it came from is most of the reason to store that record at all.
Store the resolved list rather than the raw pair. Pythia8's Particle::motherList() provides exactly it, derived from mother1, mother2 and the native status — and the native status is precisely what distinguishes an inclusive range from two distinct mothers, so a (first, last) pair would bake that ambiguity into the format permanently. A list also normalises Pythia's and GENIE's differing conventions into one unambiguous representation, and matches EDM4hep's parents relation. motherId stays as the first element, both to keep existing files readable and as the portable single-mother accessor.
statuswas documented as "Status code (1 = stable)", which is wrong: the field carries the HepMC status set produced by statusHepMC() and used by EDM4hep as generatorStatus, and 1 means "not decayed by the generator", which says nothing about stability — a final-state K_S is status 1 and is expected to decay in the detector. The full set is now spelled out in the header. The default of 1 is unchanged and still correct.Additive schema change: the four frozen reference files stay readable with mothers reading back empty, so the mask table gains a v0.5.0 entry following the RecParticle::hits precedent. Verified by disabling the mask, which makes all four compat_read_v* tests fail. The class version is bumped to 3 and the schema snapshot and reference_head.root are regenerated here, as the policy requires.
test_utils.hpp sets the new member too, so the TTree and RNTuple round-trip tests cover it.
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Follow-up: the recipe was invalid, and now it is checked
Review caught the recipe committed here contradicting the contract it was meant to illustrate. For the first entry it produced
motherId = -1alongside a non-emptymothers = {-1, 300 + offset}, although the header states that an entry without a mother carries an empty list and that the-1sentinel belongs tomotherIdalone; every entry's second element (300–310plus the offset) sat far outside the valid index range of a three-element collection.The recipe is now
{}fori == 0and{i - 1, (i + 1) % 3}otherwise, giving{},{0, 2},{1, 0}. Theoffsetdependence is dropped deliberately: an index into a fixed three-element collection cannot legitimately vary with an arbitrary offset. No frozen reference file exists for v0.5.0 yet and the mask table clearsmothersbelow it, so the four frozen files are unaffected — onlyreference_head.rootis rewritten.Nothing failed at the time, and that is the real problem: every test compares read-back data against the generator that produced it, so a recipe that drifts out of spec still agrees with itself. So the contract is now a callable predicate rather than prose:
SHiP::mothersAreConsistent—motherIdis-1or in[0, N); every element ofmothersis in[0, N); a non-empty list starts withmotherId; elements are distinct and none is the entry itself. An empty list stays acceptable, since pre-v0.5.0 files have no such field and read back empty beside a validmotherId.SHiP::mothersArePopulated— the above plusmotherId >= 0implying a non-empty list. Only current-schema data can satisfy it, so it is applied to round-tripped data and tohead, while the frozen files take the tolerant form.Free functions are outside the dictionary (
LinkDef.hdisables them wholesale, asfromSimParticlealready relies on), so this is neither a schema change nor a further class-version bump, andschema_snapshot.txtis untouched.The header additionally states the two rules that were implicit — indices are distinct, and no entry is its own mother — and records what is deliberately not required: mothers need not precede their daughters, because a producer that filters a record and remaps indices may reorder it.
A new
mc_motherstest pins both recipes at every offset the suite uses and rejects one hand-built violation per rule, so a vacuously true predicate cannot pass unnoticed. Restoring the old recipe makesmc_mothers,ttree_ioandrntuple_iofail — the regression this exists to catch.compat_read_*correctly stay green there, since they read the on-disk file rather than that generator.Summary by CodeRabbit
New Features
MCParticlerecords now support multiple mother-particle indices through themothersfield.motherscorresponds to the existingmotherIdvalue.Backward Compatibility
mothersfield.