#99 Medium: Precision Loss in Share Price Calculations

[coder45oj]

Description

Fixes precision loss in share price calculations by applying ceiling rounding on withdrawal, minimizing value leakage for liquidity providers. The deposit side remains floor rounding (conservative, benefits existing LPs).

Problem

Two integer division calculations cause silent value loss for LPs due to floor rounding:

1. Deposit (contracts/pool/src/lib.rs:130): usdc_amount * total_shares / total_deposits — LP receives slightly fewer shares than their deposit entitles them to.
2. Withdraw (contracts/pool/src/lib.rs:221): shares * total_deposits / total_shares — LP receives slightly less USDC than their shares are worth.

While each individual loss is tiny (often 1 unit), these accumulate over time across all LPs, creating systemic leakage.

Fix

Implemented ceiling division for the withdrawal calculation at contracts/pool/src/lib.rs:208-221:

let product = shares.checked_mul(total_deposits)?;
let floor = product.checked_div(total_shares)?;
let remainder = product % total_shares;
let usdc_to_return = if remainder > 0 {
    floor.checked_add(1)?   // ceiling when fractional
} else {
    floor                   // exact division, unchanged
};

This ensures that when the mathematical value of shares is not an integer number of USDC units, the withdrawing LP receives the ceiling (rounded up) instead of the floor, recovering value that would otherwise be lost.

Example

1. LP1 deposits 10B USDC → 10B shares
2. Pool generates 200M yield → share price = 1.02
3. LP2 deposits 1000 USDC → gets floor(1000 × 10B / 10.2B) = 980 shares (0.39 share lost)
4. LP2 withdraws 980 shares:
   • Before (floor): floor(980 × 10.2B / 10B) = 999 USDC ← 1 USDC loss
   • After (ceiling): ceil(980 × 10.2B / 10B) = 1000 USDC ← no loss

Precision Tests

Added 5 tests in contracts/pool/src/test.rs:

• test_precision_withdraw_ceil_recovers_value — single deposit + withdraw after yield, verifies full deposit recovered via ceiling
• test_precision_ceiling_gt_floor_when_remainder — dynamically confirms ceiling = floor + 1 when remainder exists
• test_precision_exact_division_unchanged — exact divisions unaffected
• test_precision_dust_share_gets_1_usdc — dust shares return ≥ 1 USDC instead of 0
• test_precision_multiple_withdrawals_accumulate — partial withdrawals each benefit from ceiling

Closes #99

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[K1NGD4VID]

Fix Ci error @coder45oj

[K1NGD4VID]

resolve Conflict @coder45oj