[aaronleesander]

Description

This pull request updates the construction of Hamiltonian MPOs from Pauli Strings to use a more standard finite state machine construction.

This leads to a significant speedup, especially for larger systems.

Checklist:

• The pull request only contains commits that are focused and relevant to this change.
• I have added appropriate tests that cover the new/changed functionality.
• I have updated the documentation to reflect these changes.
• I have added entries to the changelog for any noteworthy additions, changes, fixes, or removals.
• I have added migration instructions to the upgrade guide (if needed).
• The changes follow the project's style guidelines and introduce no new warnings.
• The changes are fully tested and pass the CI checks.
• I have reviewed my own code changes.

[coderabbitai]

📝 Walkthrough

Summary by CodeRabbit

• Refactor
  • Reworked the internal Pauli-sum construction for improved efficiency and maintainability; public APIs remain backward compatible.
• Tests
  • Removed a unit test that validated internal term-construction calls (no behavioral changes).
• Documentation
  • Updated changelog with an entry describing the construction change and added the corresponding PR reference.

_{✏️ Tip: You can customize this high-level summary in your review settings.}

Walkthrough

Replaces the internal MPO.from_pauli_sum implementation: direct-sum construction is replaced by a finite state machine (FSM) based assembly; internal helper methods were removed. Changelog and a test that asserted internal behavior were updated accordingly.

Changes

Cohort / File(s)	Summary
Changelog CHANGELOG.md	Added Unreleased entry noting Pauli summation now uses FSM-based construction and linked PR #308.
Core Algorithm Refactor src/mqt/yaqs/core/data_structures/networks.py	Rewrote MPO.from_pauli_sum to use an FSM-driven construction: term parsing/validation, per-term trajectory/state assignment, left-to-right tensor assembly using FSM branches; removed _create_term and _sum_terms helpers; preserved public API surface.
Tests tests/core/data_structures/test_networks.py	Removed test test_from_pauli_sum_calls_create_term_for_each_term which asserted internal helper invocation; other tests left unchanged.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Possibly related PRs

• updates MPO class to allow construction of arbitrary Pauli Hamiltonians #216 — Earlier rewrite that introduced and later removal of _create_term/_sum_terms, closely related to the MPO.from_pauli_sum refactor.

Suggested labels

feature

Poem

🐰 I hop along the FSM trail,
Tracing Pauli steps without fail,
States align and transitions sing,
Tensors grow from each small spring,
Public doors stay bright — hooray, curtail no tale!

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately and concisely summarizes the main change: replacing Pauli summation with finite state machine construction.
Description check	✅ Passed	The description provides a clear summary of the change, indicates performance benefits, and confirms all checklist items are completed as required by the template.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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[coderabbitai]

Actionable comments posted: 2

🤖 Fix all issues with AI agents

In `@CHANGELOG.md`:
- Around line 14-15: The changelog entry "changed Pauli summation to use finite
state machine construction" is split across lines and not aligned with the
project template; update that entry so the title, PR link and author link are on
a single line using the template format: include the PR as
([`#308`](https://github.com/…/pull/308)) and the author as
([**aaronleesander**](https://github.com/aaronleesander)), removing any separate
issue references and combining the pieces into one line for the entry.

In `@src/mqt/yaqs/core/data_structures/networks.py`:
- Around line 1417-1431: The loop in the term-parsing block (iterating over
terms and calling self._parse_pauli_string) currently processes even
zero-coefficient terms; modify the loop in the method that fills parsed_terms so
it first checks the term coefficient (coeff) and skips appending or parsing when
coeff is exactly zero (or effectively zero using a small epsilon for
floats/complex), i.e., do not call self._parse_pauli_string or build op_list for
such terms; ensure the check happens before validating sites or using
self._VALID so parsed_terms only receives nonzero contributions.

[coderabbitai]

Actionable comments posted: 1

🤖 Fix all issues with AI agents

In `@CHANGELOG.md`:
- Line 14: The changelog entry line "Changed Pauli summation to use finite state
machine construction ([`#308`]) ([**@aaronleesander**])" has a broken PR
reference; replace the backticked reference with the proper PR reference format
and follow the project template: change "([`#308`])" to "([`#308`])" (or use an
inline link like
"([`#308`](https://github.com/munich-quantum-toolkit/yaqs/pull/308))") and ensure
the author remains as
"([**@aaronleesander**](https://github.com/aaronleesander))" to match other
entries (update the PR link URL if you choose the inline form).

[codecov]

Codecov Report

❌ Patch coverage is 97.87234% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/mqt/yaqs/core/data_structures/networks.py	97.8%	1 Missing ⚠️

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