breez_sdk_core/swap_out/
mod.rs

1pub(crate) mod boltzswap;
2pub(crate) mod error;
3pub(crate) mod reverseswap;
4
5/// Calculate the service fee from the `invoice_amount_sat`.
6///
7/// The fee is a percentage of the invoice amount, rounded up.
8pub(crate) fn get_service_fee_sat(invoice_amount_sat: u64, fees_percentage: f64) -> u64 {
9    ((invoice_amount_sat as f64) * fees_percentage / 100.0).ceil() as u64
10}
11
12/// Calculate the `invoice_amount_sat` from `invoice_amount_minus_service_fee`.
13///
14/// This calculates the initial amount going in reverse, e.g. from the resulting sum.
15pub(crate) fn get_invoice_amount_sat(
16    invoice_amount_minus_service_fee: u64,
17    fees_percentage: f64,
18) -> u64 {
19    // The resulting invoice amount contains the service fee, which is rounded up with ceil()
20    // Therefore, when calculating the invoice amount, we must also round it up with ceil()
21    (invoice_amount_minus_service_fee as f64 * 100.0 / (100.0 - fees_percentage)).ceil() as u64
22}
23
24#[cfg(test)]
25mod tests {
26    use crate::swap_out::{get_invoice_amount_sat, get_service_fee_sat};
27
28    #[test]
29    fn test_get_service_fee_sat() {
30        // Round values, so rounding up plays no role
31        assert_eq!(250, get_service_fee_sat(50_000, 0.5));
32        assert_eq!(300, get_service_fee_sat(50_000, 0.6));
33        assert_eq!(750, get_service_fee_sat(100_000, 0.75));
34
35        // Odd values, where rounding up kicks in
36        assert_eq!(251, get_service_fee_sat(50_001, 0.5));
37        assert_eq!(301, get_service_fee_sat(50_001, 0.6));
38        assert_eq!(751, get_service_fee_sat(100_001, 0.75));
39    }
40
41    #[test]
42    fn test_get_invoice_amount_sat() {
43        // Round values, so rounding up plays no role
44        test_invoice_amount_calculation_in_reverse(50_000, 0.5);
45        test_invoice_amount_calculation_in_reverse(51_000, 0.5);
46        test_invoice_amount_calculation_in_reverse(52_000, 0.5);
47        test_invoice_amount_calculation_in_reverse(53_000, 0.5);
48        test_invoice_amount_calculation_in_reverse(54_000, 0.5);
49        test_invoice_amount_calculation_in_reverse(60_000, 0.6);
50        test_invoice_amount_calculation_in_reverse(100_000, 0.75);
51
52        // Odd values, where the rounding up in the service fee calculation kicks in
53        test_invoice_amount_calculation_in_reverse(50_001, 0.5);
54        test_invoice_amount_calculation_in_reverse(50_999, 0.5);
55        test_invoice_amount_calculation_in_reverse(51_001, 0.5);
56        test_invoice_amount_calculation_in_reverse(52_001, 0.5);
57        test_invoice_amount_calculation_in_reverse(53_001, 0.5);
58        test_invoice_amount_calculation_in_reverse(54_001, 0.5);
59        test_invoice_amount_calculation_in_reverse(60_001, 0.6);
60        test_invoice_amount_calculation_in_reverse(75_001, 0.75);
61    }
62
63    fn test_invoice_amount_calculation_in_reverse(invoice_amount_sat: u64, fees_percentage: f64) {
64        let service_fee_sat = get_service_fee_sat(invoice_amount_sat, fees_percentage);
65        let calculated_invoice_amount_sat =
66            get_invoice_amount_sat(invoice_amount_sat - service_fee_sat, fees_percentage);
67
68        // The rounding up of the service fee means there will be a precision loss when we try to calculate in reverse.
69        //
70        // For example:
71        // - invoice_amount_sat = 50_000, service_fee_sat = 250
72        // - invoice_amount_sat = 50_001, service_fee_sat = 251
73        // both lead to an onchain_amount_sat of 49_750 and an identical receive_amount_sat of 46_040.
74        //
75        // This is not case anymore for invoice_amount_sat of 50_002, as service_fee_sat stays 251, and
76        // therefore the received amount has to increase by 1 sat.
77        //
78        // Trying to find the invoice_amount_sat in reverse can result in either one or two valid results.
79        assert!(
80            (calculated_invoice_amount_sat == invoice_amount_sat)
81                || (calculated_invoice_amount_sat == invoice_amount_sat - 1)
82        );
83    }
84}
breez_sdk_core/swap_out/mod.rs

breez_sdk_core/swap_out/
mod.rs
