gen-property-test

Testing & Quality

Generate fast-check property tests following project conventions

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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.ts to understand existing patterns
Read src/core/decimal.ts to 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