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Generate Property Test
gen-property-test
Generate fast-check property tests following project conventions
SKILL.md
Full skill instructions
Generate Property Test
Generate a property-based test for the {{ method }} method on the Decimal class.
Project Conventions
This project uses fast-check for property-based testing. Follow these patterns:
Custom Arbitrary
Use the existing decimalArb pattern from src/core/decimal.property.test.ts:
import * as fc from "fast-check";
import { Decimal } from "./decimal";
// Arbitrary for valid decimal values
const decimalArb = fc
.tuple(
fc.bigInt({ min: -10n ** 18n, max: 10n ** 18n }),
fc.integer({ min: 0, max: 8 })
)
.map(([significand, scale]) => {
const str = significand.toString();
if (scale === 0) return Decimal.from(str);
const insertPoint = str.length - scale;
if (insertPoint <= 0) {
return Decimal.from(`0.${"0".repeat(-insertPoint)}${str.replace("-", "")}`);
}
return Decimal.from(`${str.slice(0, insertPoint)}.${str.slice(insertPoint)}`);
});
Test Structure
import { describe, test, expect } from "bun:test";
import * as fc from "fast-check";
describe("{{ method }} properties", () => {
test("property name", () => {
fc.assert(
fc.property(decimalArb, (a) => {
// Return boolean for invariant
return /* invariant expression */;
})
);
});
});
Mathematical Invariants to Consider
For arithmetic operations, consider these properties:
| Property | Formula | Applies To |
|---|---|---|
| Identity | a.op(identity) = a | add(0), multiply(1) |
| Commutativity | a.op(b) = b.op(a) | add, multiply |
| Associativity | (a.op(b)).op(c) = a.op(b.op(c)) | add, multiply |
| Inverse | a.op(a.inverse()) = identity | add/negate, multiply/reciprocal |
| Distributivity | a.mul(b.add(c)) = a.mul(b).add(a.mul(c)) | multiply over add |
| Idempotence | op(a, a) = a | min, max |
| Absorption | min(a, max(a, b)) = a | min/max |
Output Location
Add tests to: src/core/decimal.property.test.ts
Your Task
- Read
src/core/decimal.property.test.tsto understand existing patterns - Read
src/core/decimal.tsto understand the{{ method }}implementation - Generate property tests for
{{ method }}covering relevant invariants - Add tests to the existing property test file in the appropriate describe block
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