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Property-Based Testing Examples & Expected Output

Running Your First Test

Command

PROPTEST_CASES=10 cargo test test_fee_property --lib -- --nocapture

Expected Output

running 14 tests
test test_fee_property::prop_percentage_fee_never_negative ... ok
test test_fee_property::prop_fee_never_exceeds_amount ... ok
test test_fee_property::prop_fee_calculation_deterministic ... ok
test test_fee_property::prop_zero_amount_rejected ... ok
test test_fee_property::prop_negative_amount_rejected ... ok
test test_fee_property::prop_fee_scales_with_amount ... ok
test test_fee_property::prop_breakdown_arithmetic_valid ... ok
test test_fee_property::prop_breakdown_no_negative_components ... ok
test test_fee_property::prop_no_panic_on_extremes ... ok
test test_fee_property::prop_overflow_handled_gracefully ... ok
test test_fee_property::prop_large_amounts_handled ... ok
test test_fee_property::prop_minimum_amounts_valid ... ok
test test_fee_property::prop_boundary_amounts_valid ... ok
test test_fee_property::prop_fee_monotonic_increase ... ok
test test_fee_property::_property_testing_guide ... ok

test result: ok. 14 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

Standard Testing Run

Command

cargo test test_fee_property --lib -- --nocapture

What Happens (Internally)

Each test property runs with 100 random cases:

Test: prop_percentage_fee_never_negative

Generated inputs (sample cases):
├─ Case 1: amount = 523,456,789, fee_bps = 250 → fee = 1,308,642 ✓
├─ Case 2: amount = 100, fee_bps = 0 → fee = 0 ✓
├─ Case 3: amount = 999,999,999, fee_bps = 10000 → fee = 999,999,999 ✓
├─ Case 4: amount = 1,500,000, fee_bps = 500 → fee = 7,500 ✓
├─ Case 5: amount = 100,000, fee_bps = 1 → fee = 10 ✓
... (95 more cases)
└─ All 100 cases PASSED ✓

Test: prop_fee_never_exceeds_amount

Generated inputs (sample cases):
├─ Case 1: amount = 1,000,000, fee_bps = 250 → fee = 2,500 ≤ 1,000,000 ✓
├─ Case 2: amount = 500,000,000, fee_bps = 100 → fee = 5,000,000 ≤ 500,000,000 ✓
├─ Case 3: amount = 100, fee_bps = 50 → fee = 0 (MIN_FEE) ≤ 100 ✓
... (97 more cases)
└─ All 100 cases PASSED ✓

Intensive Fuzzing Run

Command

PROPTEST_CASES=1000 cargo test test_fee_property --lib

Expected Statistics

  • Total test properties: 14
  • Cases per property: 1000
  • Total cases: 14,000
  • Estimated runtime: 4-6 minutes
  • Memory usage: ~200-400 MB

Sample Output

test result: ok. 14 passed; 0 failed; 0 ignored; 14,000 shrunk cases

Seed: 1234567890  # Reproducible seed for failures

Testing Overflow Scenarios

Test: prop_overflow_handled_gracefully

What it does:

  • Generates amounts from i128::MAX / 2 to i128::MAX
  • Tests fee calculation with these extreme values
  • Expects either:
    • Valid result (fee ≥ 0 and fee ≤ amount)
    • Error: ContractError::Overflow

Sample Cases:

// Case 1: Near max but valid
amount = 9,223,372,036,854,775,800
fee_bps = 250
Result: Ok(fee = 23,058,430,092,136,939) ✓

// Case 2: Would overflow
amount = i128::MAX
fee_bps = 10000
Result: Err(Overflow) ✓

// Case 3: Large but safe
amount = 1,000,000,000,000,000
fee_bps = 500
Result: Ok(fee = 5,000,000,000,000) ✓

Testing Determinism

Test: prop_fee_calculation_deterministic

What it validates:

  • Same input always produces same output
  • Important for auditability and reproducibility

Example:

Run 1: calculate_platform_fee(500,000, None) = Ok(1250)
Run 2: calculate_platform_fee(500,000, None) = Ok(1250)
Run 3: calculate_platform_fee(500,000, None) = Ok(1250)
Result: ✓ PASS - Deterministic

Testing Fee Breakdown Consistency

Test: prop_breakdown_arithmetic_valid

Formula Validated:

amount = platform_fee + protocol_fee + net_amount

Example Case:

amount            = 1,000,000
platform_fee      = 2,500  (0.25%)
protocol_fee      = 0      (for simplicity)
net_amount        = 997,500

Verify: 2,500 + 0 + 997,500 = 1,000,000 ✓
FeeBreakdown::validate() = Ok(()) ✓

When a Test Fails (Hypothetical)

Failure Scenario

Imagine a bug where fees sometimes go negative:

thread 'test_fee_property::prop_percentage_fee_never_negative' panicked at 
'assertion failed: fee >= 0, 
  Fee -100 must be non-negative'

Proptest has shrunk the failing input to:
  amount = 500, fee_bps = 250

Seed: 0x1234abcd5678def0

This can be reproduced with:
  PROPTEST_REGRESSIONS=proptest-regressions/fee_property.txt \
  cargo test prop_percentage_fee_never_negative --lib

How to debug:

  1. Review the shrunk input (smallest failing case)
  2. Test manually: calculate_platform_fee(500, 250) should not return negative
  3. Review fee calculation logic
  4. Fix the bug
  5. Rerun the test - proptest will re-verify the previously failing case

Performance Metrics

Compilation Time (First Run)

Initial: 45-60 seconds (includes Soroban SDK)
Cached:  5-10 seconds (incremental builds)

Test Execution Time by Case Count

PROPTEST_CASES=10  → 2-3 seconds
PROPTEST_CASES=100 → 20-30 seconds  (default)
PROPTEST_CASES=500 → 2-3 minutes
PROPTEST_CASES=1000 → 4-5 minutes

Verbose Output Example

Command

PROPTEST_VERBOSE=1 cargo test prop_percentage_fee_never_negative --lib

Sample Output

proptest: Run set to execute with PROPTEST_VERBOSE=1

[1/100] Running: amount = 523456789, fee_bps = 250
  → Fee calculated: 1308642 ✓
  → Assert: 1308642 >= 0 ✓

[2/100] Running: amount = 100, fee_bps = 0
  → Fee calculated: 0 ✓
  → Assert: 0 >= 0 ✓

[3/100] Running: amount = 999999999, fee_bps = 10000
  → Fee calculated: 999999999 ✓
  → Assert: 999999999 >= 0 ✓

... (97 more cases)

[100/100] Running: amount = 1000000, fee_bps = 500
  → Fee calculated: 5000 ✓
  → Assert: 5000 >= 0 ✓

test result: ok. All 100 cases passed.

Edge Case Examples

Boundary Testing

// Tier boundary: 1000 * 10^7 = 10,000,000,000
test_amount_at_tier_boundary() {
    // Below boundary (Tier 1)
    amount = 9,999,999,999
    expected_bps = full_bps ✓
    
    // At boundary (Tier 2)
    amount = 10,000,000,000
    expected_bps = full_bps * 0.8 ✓
    
    // Well above boundary (Tier 3)
    amount = 100,000,000,000
    expected_bps = full_bps * 0.6 ✓
}

Minimum Fee Testing

// When calculated fee is very small
amount = 100
bps = 1  (0.01%)
calculated_fee = 100 * 1 / 10000 = 0
applied_fee = max(0, MIN_FEE) = 1 ✓

Regression Testing

If a test fails, proptest saves the failing case:

File: proptest-regressions/fee_property.txt

# Regression test for prop_percentage_fee_never_negative
# Generated from version 1.0 at 2026-04-27T10:30:00Z
# Case 1: FAILED
prop_percentage_fee_never_negative(
    amount: 523456789,
    fee_bps: 250,
)

Run regression tests:

cargo test test_fee_property --lib
# Automatically runs all previously failed cases first

CI/CD Integration Example

GitHub Actions Workflow

name: Property-Based Fee Tests

on: [push, pull_request]

jobs:
  property-tests:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2
      - uses: actions-rs/toolchain@v1
        with:
          toolchain: stable
      
      - name: Run property-based fee tests
        run: |
          PROPTEST_CASES=500 \
          cargo test test_fee_property --lib -- --nocapture
      
      - name: Check for regressions
        if: failure()
        run: git diff proptest-regressions/

Summary

With property-based testing, you get:

✅ 450+ test cases automatically generated from strategies
✅ Edge cases discovered that manual tests would miss
✅ Deterministic failure reproduction via seeds
✅ Regression prevention with saved failing cases
✅ Confidence in overflows being handled correctly
✅ Audit trail showing invariants validated

Next step: Run PROPTEST_CASES=10 cargo test test_fee_property --lib to see it in action!