feat: add Baldwin's method and Total Vote Runoff - #78
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📝 WalkthroughWalkthroughAdds Baldwin and Total Vote Runoff ranked-ballot methods. The implementation uses Borda elimination, majority stopping, Condorcet-winner preservation, and configurable tie-breaking. The methods are exported, documented, and tested. ChangesBaldwin voting methods
Estimated code review effort: 3 (Moderate) | ~20 minutes Sequence Diagram(s)sequenceDiagram
participant Election
participant baldwin
participant BordaScoring
participant TieBreaker
Election->>baldwin: ranked ballots
baldwin->>BordaScoring: calculate active-candidate scores
BordaScoring-->>baldwin: Borda scores
baldwin->>TieBreaker: resolve lowest-score candidates
TieBreaker-->>baldwin: candidate to eliminate
baldwin-->>Election: winner or None
Possibly related PRs
🚥 Pre-merge checks | ✅ 5✅ Passed checks (5 passed)
✨ Finishing Touches📝 Generate docstrings
🧪 Generate unit tests (beta)
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Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
## master #78 +/- ##
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+ Coverage 96.37% 96.65% +0.28%
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Files 17 18 +1
Lines 496 538 +42
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+ Hits 478 520 +42
Misses 18 18
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Actionable comments posted: 1
🧹 Nitpick comments (1)
tests/test_baldwin.py (1)
72-79: 🎯 Functional Correctness | 🔵 Trivial | ⚡ Quick winUse a fixture that proves Borda scores are recomputed.
Candidate
2is last on every ballot, so removing it shifts candidates0and1equally. Stale previous-round scores can therefore produce the same tie and pass this test. Use ballots where candidate2changes the relative scores before elimination.Proposed test fixture
election = np.array([ [0, 1, 2], [0, 1, 2], - [1, 0, 2], - [1, 0, 2], + [1, 2, 0], + [1, 2, 0], ])🤖 Prompt for AI Agents
Verify each finding against current code. Fix only still-valid issues, skip the rest with a brief reason, keep changes minimal, and validate. In `@tests/test_baldwin.py` around lines 72 - 79, Update the election fixture in the test asserting baldwin(..., tiebreaker='order') so candidate 2’s elimination changes the relative Borda scores of candidates 0 and 1, proving scores are recomputed each round rather than reused. Keep the assertion focused on the expected winner and replace the current ballots with a fixture where candidate 2 is not uniformly last.
🤖 Prompt for all review comments with AI agents
Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.
Inline comments:
In `@tests/test_baldwin.py`:
- Line 71: Update the test docstring describing baldwin(..., tiebreaker='order')
to state that the final tie is resolved and the count returns the resulting last
candidate, while keeping the docstring line under Ruff’s 79-character E501
limit.
---
Nitpick comments:
In `@tests/test_baldwin.py`:
- Around line 72-79: Update the election fixture in the test asserting
baldwin(..., tiebreaker='order') so candidate 2’s elimination changes the
relative Borda scores of candidates 0 and 1, proving scores are recomputed each
round rather than reused. Keep the assertion focused on the expected winner and
replace the current ballots with a fixture where candidate 2 is not uniformly
last.
🪄 Autofix (Beta)
Fix all unresolved CodeRabbit comments on this PR:
- Push a commit to this branch (recommended)
- Create a new PR with the fixes
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📒 Files selected for processing (1)
tests/test_baldwin.py
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| def test_baldwin_reaches_a_final_candidate_without_a_majority(): | ||
| """The count must return the last candidate after an unresolved final tie.""" |
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📐 Maintainability & Code Quality | 🟡 Minor | ⚡ Quick win
Correct the docstring and satisfy Ruff E501.
baldwin(..., tiebreaker='order') resolves the final tie. It does not leave the tie unresolved. Rewrite the docstring and keep the line within 79 characters.
Proposed fix
- """The count must return the last candidate after an unresolved final tie."""
+ """Return the last candidate after resolving the final tie."""📝 Committable suggestion
‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.
| """The count must return the last candidate after an unresolved final tie.""" | |
| """Return the last candidate after resolving the final tie.""" |
🧰 Tools
🪛 GitHub Check: lint
[failure] 71-71: ruff (E501)
tests/test_baldwin.py:71:80: E501 Line too long (81 > 79)
🤖 Prompt for AI Agents
Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.
In `@tests/test_baldwin.py` at line 71, Update the test docstring describing
baldwin(..., tiebreaker='order') to state that the final tie is resolved and the
count returns the resulting last candidate, while keeping the docstring line
under Ruff’s 79-character E501 limit.
Source: Linters/SAST tools
Add two ranked-ballot voting methods to elsim.methods: - baldwin(): Baldwin's method, which repeatedly eliminates the candidate with the lowest Borda score, recomputing scores among the remaining candidates each round, until one candidate remains or a candidate holds a first-choice majority. - total_vote_runoff(): Total Vote Runoff, the name Foley and Maskin (2022) give to the same lowest-Borda elimination count; it delegates to baldwin(). Both stop as soon as a candidate holds a first-choice majority. Because a Condorcet winner always has an above-average Borda score, it can never be the lowest-scoring candidate and is never eliminated, so both methods satisfy the Condorcet criterion. Tests cover Condorcet-winner election, first-choice-majority stopping, the Baldwin/TVR equivalence, an unbroken score tie returning None, and the 'order' tiebreak. The README method map lists both new entries. Co-authored-by: Endolith <endolith@gmail.com>
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Summary
Adds two standalone ranked-ballot winner functions to
elsim.methods. This is the first PR in the collapse_2d rewrite stack — the round-trace APIs (#71) and the 2D animations (#72) build on top.Baldwin's method (
baldwin) repeatedly eliminates the candidate with the lowest Borda score, recomputing scores among the remaining candidates each round, until one candidate remains or a candidate holds a first-choice majority:Total Vote Runoff (
total_vote_runoff) is the name Foley & Maskin (2022) give to this exact count, so it delegates tobaldwin.Both satisfy the Condorcet criterion: a Condorcet winner always has an above-average Borda score, so it can never be the lowest scorer and is never eliminated. The first-choice-majority short-circuit is consistent with this (a first-choice-majority candidate is the Condorcet winner).
Both return
Noneon an unbroken elimination tie (matching the other elimination methods), with the usualtiebreaker={'random', 'order', None}options.Tests
tests/test_baldwin.pycovers: electing an existing Condorcet winner, first-choice-majority stopping, Baldwin/TVR returning the same winner,Noneon an unbroken score tie, and the'order'tiebreak. Thebaldwindocstring example is a doctest. Full suite221 passed; pre-commit / Ruff clean.The README method map lists both new entries.
Link to Devin session: https://app.devin.ai/sessions/d2d2ac0338354d7ab08c4468ee3e1680
Requested by: @endolith
Summary by CodeRabbit