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Fee Service API Reference

Public Contract Methods

Fee Calculation

calculate_fee_breakdown

Calculates complete fee breakdown for a transaction amount.

pub fn calculate_fee_breakdown(
    env: Env,
    amount: i128,
) -> Result<FeeBreakdown, ContractError>

Parameters:

  • env - Contract environment
  • amount - Transaction amount to calculate fees for

Returns:

  • FeeBreakdown - Complete breakdown of all fees

Example:

let breakdown = client.calculate_fee_breakdown(&10000);
assert_eq!(breakdown.amount, 10000);
assert_eq!(breakdown.platform_fee, 250);  // 2.5%
assert_eq!(breakdown.protocol_fee, 100);  // 1%
assert_eq!(breakdown.total_fees, 350);
assert_eq!(breakdown.net_amount, 9650);

calculate_fee_breakdown_with_corridor

Calculates fees using corridor-specific configuration.

pub fn calculate_fee_breakdown_with_corridor(
    env: Env,
    amount: i128,
    corridor: FeeCorridor,
) -> Result<FeeBreakdown, ContractError>

Parameters:

  • env - Contract environment
  • amount - Transaction amount
  • corridor - Corridor configuration with country codes and fee rules

Returns:

  • FeeBreakdown - Breakdown using corridor-specific rates

Example:

let corridor = FeeCorridor {
    from_country: String::from_str(&env, "US"),
    to_country: String::from_str(&env, "MX"),
    strategy: FeeStrategy::Percentage(150),  // 1.5%
    protocol_fee_bps: Some(50),              // 0.5%
};

let breakdown = client.calculate_fee_breakdown_with_corridor(
    &10000,
    &corridor,
);
assert_eq!(breakdown.platform_fee, 150);
assert_eq!(breakdown.protocol_fee, 50);
assert_eq!(breakdown.net_amount, 9800);

Corridor Management

set_fee_corridor

Configures fee rules for a country-to-country corridor.

pub fn set_fee_corridor(
    env: Env,
    caller: Address,
    corridor: FeeCorridor,
) -> Result<(), ContractError>

Parameters:

  • env - Contract environment
  • caller - Admin address (requires authentication)
  • corridor - Corridor configuration to set

Authorization: Admin only

Example:

let corridor = FeeCorridor {
    from_country: String::from_str(&env, "US"),
    to_country: String::from_str(&env, "MX"),
    strategy: FeeStrategy::Percentage(150),
    protocol_fee_bps: Some(50),
};

client.set_fee_corridor(&admin, &corridor)?;

get_fee_corridor

Retrieves corridor configuration for a country pair.

pub fn get_fee_corridor(
    env: Env,
    from_country: String,
    to_country: String,
) -> Option<FeeCorridor>

Parameters:

  • env - Contract environment
  • from_country - Source country code (ISO 3166-1 alpha-2)
  • to_country - Destination country code (ISO 3166-1 alpha-2)

Returns:

  • Some(FeeCorridor) - Corridor configuration if exists
  • None - No corridor configured for this pair

Example:

let corridor = client.get_fee_corridor(
    &String::from_str(&env, "US"),
    &String::from_str(&env, "MX"),
);

if let Some(corr) = corridor {
    println!("Found corridor: {} -> {}", corr.from_country, corr.to_country);
}

remove_fee_corridor

Removes corridor configuration for a country pair.

pub fn remove_fee_corridor(
    env: Env,
    caller: Address,
    from_country: String,
    to_country: String,
) -> Result<(), ContractError>

Parameters:

  • env - Contract environment
  • caller - Admin address (requires authentication)
  • from_country - Source country code
  • to_country - Destination country code

Authorization: Admin only

Example:

client.remove_fee_corridor(
    &admin,
    &String::from_str(&env, "US"),
    &String::from_str(&env, "MX"),
)?;

Data Structures

FeeBreakdown

Complete breakdown of all fees for a transaction.

pub struct FeeBreakdown {
    pub amount: i128,
    pub platform_fee: i128,
    pub protocol_fee: i128,
    pub total_fees: i128,
    pub net_amount: i128,
    pub strategy_used: FeeStrategy,
    pub corridor_applied: Option<FeeCorridor>,
}

Fields:

  • amount - Original transaction amount
  • platform_fee - Platform fee charged
  • protocol_fee - Protocol fee charged (sent to treasury)
  • total_fees - Sum of platform_fee + protocol_fee
  • net_amount - Amount after all fees (amount - total_fees)
  • strategy_used - Fee strategy that was applied
  • corridor_applied - Corridor configuration if used

Validation: The breakdown includes a validate() method that ensures:

  • total_fees = platform_fee + protocol_fee
  • net_amount = amount - total_fees
  • All amounts are non-negative

FeeCorridor

Configuration for country-to-country fee rules.

pub struct FeeCorridor {
    pub from_country: String,
    pub to_country: String,
    pub strategy: FeeStrategy,
    pub protocol_fee_bps: Option<u32>,
}

Fields:

  • from_country - Source country code (ISO 3166-1 alpha-2, e.g., "US")
  • to_country - Destination country code (ISO 3166-1 alpha-2, e.g., "MX")
  • strategy - Fee calculation strategy for this corridor
  • protocol_fee_bps - Optional protocol fee override (if None, uses global setting)

FeeStrategy

Fee calculation strategy enum.

pub enum FeeStrategy {
    Percentage(u32),  // Basis points (250 = 2.5%)
    Flat(i128),       // Fixed amount
    Dynamic(u32),     // Tiered based on amount
}

Variants:

  1. Percentage(bps) - Fee as percentage of amount

    • bps - Basis points (1 bps = 0.01%, max 10000 = 100%)
    • Example: Percentage(250) = 2.5%

  2. Flat(amount) - Fixed fee regardless of transaction size

    • amount - Fixed fee amount
    • Example: Flat(100) = 100 units

  3. Dynamic(base_bps) - Tiered fees based on amount ranges

    • base_bps - Base fee in basis points
    • Tiers:
      • Amount < 1000: base_bps
      • 1000 ≤ Amount < 10000: base_bps / 2
      • Amount ≥ 10000: base_bps / 4
    • Example: Dynamic(400) = 4% for small, 2% for medium, 1% for large

Internal Service Functions

These functions are used internally by the contract but not exposed as public methods.

calculate_platform_fee

Calculates only the platform fee (backward compatible).

pub fn calculate_platform_fee(
    env: &Env,
    amount: i128,
) -> Result<i128, ContractError>

Used by create_remittance() to calculate the fee when creating a new remittance.

calculate_batch_fees

Aggregates fees for multiple transactions.

pub fn calculate_batch_fees(
    env: &Env,
    amounts: &[i128],
    corridor: Option<&FeeCorridor>,
) -> Result<FeeBreakdown, ContractError>

Used for batch settlement operations to calculate total fees across multiple remittances.

Error Handling

The fee service returns standard ContractError variants:

  • InvalidAmount - Amount is zero, negative, or invalid
  • InvalidFeeBps - Fee basis points exceed maximum (10000)
  • Overflow - Arithmetic overflow in calculation
  • NotInitialized - Contract not properly initialized

Usage Patterns

Pattern 1: Display Fees to User

// Before user commits to transaction, show fee breakdown
let breakdown = client.calculate_fee_breakdown(&amount);

display_to_user(&format!(
    "Amount: {}\n\
     Platform Fee: {}\n\
     Protocol Fee: {}\n\
     Total Fees: {}\n\
     You will receive: {}",
    breakdown.amount,
    breakdown.platform_fee,
    breakdown.protocol_fee,
    breakdown.total_fees,
    breakdown.net_amount
));

Pattern 2: Corridor-Based Pricing

// Check if corridor exists for this country pair
let corridor = client.get_fee_corridor(&from_country, &to_country);

let breakdown = if let Some(corr) = corridor {
    // Use corridor-specific rates
    client.calculate_fee_breakdown_with_corridor(&amount, &corr)?
} else {
    // Use default rates
    client.calculate_fee_breakdown(&amount)?
};

Pattern 3: Admin Configuration

// Set up multiple corridors
let corridors = vec![
    FeeCorridor {
        from_country: String::from_str(&env, "US"),
        to_country: String::from_str(&env, "MX"),
        strategy: FeeStrategy::Percentage(150),
        protocol_fee_bps: Some(50),
    },
    FeeCorridor {
        from_country: String::from_str(&env, "US"),
        to_country: String::from_str(&env, "PH"),
        strategy: FeeStrategy::Percentage(200),
        protocol_fee_bps: Some(75),
    },
];

for corridor in corridors {
    client.set_fee_corridor(&admin, &corridor)?;
}

Migration Guide

From Old API

Old way (before refactor):

// Fee was calculated internally, no breakdown available
let remittance_id = client.create_remittance(&sender, &agent, &amount, &None);
let remittance = client.get_remittance(&remittance_id);
// Only remittance.fee was available

New way (after refactor):

// Get complete breakdown before creating remittance
let breakdown = client.calculate_fee_breakdown(&amount);
// Show breakdown to user
display_fees(&breakdown);
// Then create remittance
let remittance_id = client.create_remittance(&sender, &agent, &amount, &None);

Best Practices

  1. Always show fee breakdown to users before they commit to a transaction
  2. Use corridors for cross-border transactions to optimize fees
  3. Cache corridor lookups if making multiple calculations for same country pair
  4. Validate amounts before calling fee calculation functions
  5. Handle errors gracefully and provide clear error messages to users

Performance Considerations

  • Fee calculations are O(1) - constant time
  • Corridor lookups are O(1) - single storage read
  • Batch calculations are O(n) where n is number of amounts
  • No network calls or external dependencies

Security Notes

  • All arithmetic uses checked operations to prevent overflow
  • Corridor management requires admin authentication
  • Fee breakdowns self-validate for consistency
  • Input validation at service boundary
  • Type-safe Rust implementation prevents common errors

Version History

v1.0.0 (Current)

  • Initial release of centralized fee service
  • Support for corridor-based configurations
  • Complete fee breakdown functionality
  • Three fee strategies: Percentage, Flat, Dynamic