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Fee Arithmetic Invariants

This document captures the arithmetic guarantees for platform-fee calculation in contracts/predictify-hybrid/src/fees.rs.

Core Rules

  1. All fee percentage math uses checked arithmetic.
  2. Any overflow in fee multiplication, addition, subtraction, or division returns Error::FeeArithmeticOverflow.
  3. Basis-point calculations round down using integer division.
  4. Fee breakdowns must reconcile exactly:
platform_fee + user_payout_amount == total_staked

Basis-Point Rounding

Platform fees are computed with checked multiply-then-divide logic:

floor(total_staked * fee_bps / 10_000)

Because all supported fee inputs are non-negative, integer division is equivalent to floor rounding. This intentionally favors reconciliation over accidental over-collection.

Overflow Handling

The following operations must never wrap:

  • FeeCalculator::calculate_platform_fee
  • FeeCalculator::calculate_user_payout_after_fees
  • FeeCalculator::calculate_fee_breakdown
  • dynamic fee multiplier math
  • FeeManager::collect_fees through its fee calculation path
  • fee vault and creation-fee total accumulation in FeeTracker

If an intermediate value exceeds i128, the operation fails with Error::FeeArithmeticOverflow and no fee state is updated.

Regression Coverage

Regression tests cover:

  • zero pool rejection
  • 1-bps floor rounding
  • overflow-adjacent i128::MAX stake pools
  • fee breakdown reconciliation after floor rounding
  • no mutation when collect_fees encounters overflow
  • fee vault accumulation overflow rejection