Add negative income edge cases and bracket threshold patterns #27
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Adds critical edge case pattern discovered from WV homestead credit bug: - Negative household income combined with zero deductible expenses - This combination often reveals incorrect eligibility calculations - Tax credits need to handle this case explicitly Real-world example: WV homestead excess property tax credit incorrectly calculated $120 credit when income was negative and property tax was $0. The bug was in eligibility logic that didn't properly handle negative income thresholds. 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <noreply@anthropic.com>
Add critical guidance for bracket-based parameters where negative input values are possible (e.g., AGI can be negative). The first bracket threshold must be -.inf rather than 0 to correctly handle all cases. Based on learning from PolicyEngine/policyengine-us#6953 (Hawaii Food/Excise Tax Credit), where using 0 as the first threshold incorrectly excluded taxpayers with negative AGI. 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <noreply@anthropic.com>
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Documents critical pattern: state income taxes should reference federal income sources (like adjusted_gross_income) rather than redefining income sources at the state level. This prevents bugs like PolicyEngine/policyengine-us#6964 where Mississippi incorrectly allowed unlimited capital losses because it bypassed federal capital loss limits. 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <noreply@anthropic.com>
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Additional learning: State income tax conformityAdded new guidance on state income tax conformity to federal rules. Key pattern documented: Context: Changes:
This will help prevent similar bugs in future state income tax implementations. |
Discovered from MT income tax bug fix: when subtracting values and flooring at zero, must wrap entire subtraction in max_(), not just one operand. Pattern prevents "phantom income" bugs where max_(income, 0) - loss can produce negative results when loss > income. 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <noreply@anthropic.com>
Captures learning from Montana income tax bug fix (PolicyEngine/policyengine-us#6947): - Phantom non-zero tax despite zero taxable income - Root causes: type conversion issues and missing zero-income short-circuits - Debugging steps for tracing calculation chains This pattern will help future sessions diagnose similar state tax calculation bugs more quickly. 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <noreply@anthropic.com>
When implementing state tax credits and income-based programs, statutes often specify modified income definitions (e.g., "AGI plus exemptions") rather than standard measures. This is a common source of bugs. Added warnings and examples to: - implementation-validator: validation checklist for income measures - document-collector: guidance on capturing exact income definitions Real-world example: Arizona Family Tax Credit uses "AGI plus exemptions" per ARS 43-1073, not just AGI alone. 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <noreply@anthropic.com>
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Summary
Adds three critical patterns learned from recent PolicyEngine-US bug fixes:
Changes
Testing Patterns (from #6948)
>instead of>=Parameter Patterns (from #6953)
-.inf(not0) when negative values are valid-.infvs when0is appropriateVectorization Patterns (from #6965)
max_()when subtracting and flooring at zeromax_(A - B, 0)NOTmax_(A, 0) - BContext
Testing edge case (#6948): WV homestead excess property tax credit incorrectly calculated $120 refund when household income was negative AND property tax paid was $0. TaxAct correctly showed $0, highlighting the importance of this edge case.
Bracket threshold pattern (#6953): Hawaii Food/Excise Tax Credit used
0as first bracket threshold, incorrectly excluding taxpayers with negative AGI. Using-.infcorrectly handles all cases including business losses.Subtraction flooring pattern (#6965): Montana income tax used
max_(income, 0) - capital_gainswhich created phantom taxable income when capital losses occurred. The correct patternmax_(income - capital_gains, 0)properly floors the entire subtraction result at zero.All three are subtle but important patterns that will help prevent similar bugs in future implementations.
🤖 Generated with Claude Code