Improve the overall speed and memory use of libCellML

[agarny]

Fixes #1368.

[Copilot]

Pull request overview

This PR focuses on improving libCellML runtime performance and memory usage (issue #1368) by reducing allocations/copies, introducing caching, and replacing some heavier utilities/containers with lighter alternatives.

Changes:

• Reduced copying/allocation across core code paths (e.g., reserve() on hot strings/vectors, fewer temporary objects, std::unordered_set/std::unordered_map in places).
• Added/expanded caching for variable equivalence and internal/analyser variable lookup to speed up repeated queries.
• Reworked some XML/Math handling to avoid std::regex overhead (e.g., printer math normalisation and libxml error string cleanup), and updated coverage tests accordingly.

Reviewed changes

Copilot reviewed 57 out of 59 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
CMakeLists.txt	Sets global C++ standard to C++20.
src/CMakeLists.txt	Adds CODE_COVERAGE_ENABLED define for coverage builds.
src/analyser.cpp	Performance-oriented refactors (caching, sets, reduced repeated work) and MathML element name comparisons.
src/analyser_p.h	Adds caches/sets to accelerate internal analysis bookkeeping.
src/analyserexternalvariable.cpp	Minor lambda capture optimization; dependencies accessor returns by reference.
src/analyserequation.cpp	Reduces allocations via reserve(); several accessors return by reference.
src/analyserequationast.cpp	Returns AST value by reference to avoid copies.
src/analysermodel.cpp	Adds caches for analyser variable lookup and variable equivalence checks; returns vectors by reference with static empties for invalid models.
src/analysermodel_p.h	Replaces equivalence cache keying structure with hashed pointer-pair key.
src/analyservariable.cpp	Reserves capacity when materializing weak dependencies.
src/annotator.cpp	Reduces string allocations and uses std::format for some formatting.
src/commonutils.cpp	Simplifies/optimizes owningModel() by walking parent chain directly.
src/component.cpp	Avoids copies in lambdas; math() returns by reference.
src/componententity.cpp	Avoids copies in lambdas; encapsulationId() returns by reference.
src/debug.cpp	Reserves output string capacity.
src/entity.cpp	id() returns by reference.
src/generator_p.h	Updates generator internals to use sets and addCode() helper.
src/generatorprofile.cpp	Many getters return by reference to avoid string copies.
src/generatorprofiletools.cpp	Reserves buffer for profile serialization.
src/generatorvariabletracker.cpp	Avoids intermediate vector copies and reserves capacity.
src/importedentity.cpp	importReference() returns by reference.
src/importer.cpp	Uses unordered containers and iterator-based lookups to reduce repeated map searches/copies.
src/importsource.cpp	url() returns by reference.
src/internaltypes.h	Switches several internal maps to unordered variants for speed.
src/issue.cpp	description() returns by reference.
src/model.cpp	Avoids copies in lambdas; uses set to avoid duplicate import URLs.
src/namedentity.cpp	name() returns by reference.
src/parser.cpp	Avoids string copies; reserves description capacity.
src/printer.cpp	Replaces regex-based Math normalization/whitespace cleanup; reduces duplicates via sets; adds reserves.
src/reset.cpp	Reset value/id getters return by reference.
src/units.cpp	Avoids temporary copies for unit attributes and updates by in-place mutation.
src/utilities.cpp	Reserves in hot helpers; uses std::format for numeric conversion; optimizes equivalence traversal with seen set; returns analyser-variable lists by reference.
src/utilities.h	analyserVariables(AnalyserVariablePtr) now returns by reference.
src/validator.cpp	Replaces repeated linear searches with unordered_sets; removes regex formatting; uses std::format; speeds MathML element membership checks.
src/xmlattribute.cpp	Avoids allocating namespace URI strings repeatedly; handles null namespace pointers explicitly.
src/xmldoc.cpp	Replaces regex newline replacement with std::replace.
src/xmlnode.cpp	Speeds element checks; removes hasAttribute() and adjusts attribute handling; introduces rawName().
src/xmlnode.h	Replaces hasAttribute() with rawName() API.
src/api/libcellml/analyserexternalvariable.h	dependencies() returns by reference.
src/api/libcellml/analyserequation.h	Several vector accessors return by reference.
src/api/libcellml/analyserequationast.h	value() returns by reference.
src/api/libcellml/analysermodel.h	Several vector accessors return by reference.
src/api/libcellml/component.h	math() returns by reference.
src/api/libcellml/componententity.h	encapsulationId() returns by reference.
src/api/libcellml/entity.h	id() returns by reference.
src/api/libcellml/generatorprofile.h	Many string getters return by reference.
src/api/libcellml/importedentity.h	importReference() returns by reference.
src/api/libcellml/importsource.h	url() returns by reference.
src/api/libcellml/issue.h	description() returns by reference.
src/api/libcellml/namedentity.h	name() returns by reference.
src/api/libcellml/reset.h	Several string getters return by reference.
src/api/libcellml/variable.h	initialValue() / interfaceType() return by reference.
tests/bindings/javascript/generator.test.js	Adds (commented-out) stress-test scaffold for repeated big-model runs.
tests/bindings/python/test_generator.py	Adds (commented-out) stress-test scaffold for repeated big-model runs.
tests/coverage/coverage.cpp	Adds coverage tests for generator version strings and printer math parsing.
tests/generator/generator.cpp	Adds (commented-out) stress-test scaffold for repeated big-model runs.

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