# finance-solution
**finance-solution** is a Rust library for time-value-of-money (TVM), cashflow, amortization, and related finance formulas — with **detailed solution structs**, period-by-period series, and pretty-printed tables. Scalar math executes in nanoseconds, making it suitable for quant systems. Solution structs execute in microseconds, providing observability with speed.
## finance-solution is a financial library for solving time-value-of-money problems and financial math, including common quant calculations. 💸
People who will find this crate helpful include:
- **Students of Finance** who want to solve their financial problems using something better than a crude handheld calculator. Using this library also reduces the chance of human error, and provides better output and data displays than Excel in less time thereby providing a better surface for internalizing financial math concepts.
- **New developers** who want to learn Rust using Finance as a topic.
- **Experienced Rust developers** who want to learn more about finance and/or incorporate safe financial calculations into their projects.
- **Serious Rust developers** who want to build financial software, and prefer to rely on a rigourously tested library instead of reinventing the wheel and spending hundreds of hours to develop and test their own library of financial calculations.
In the v0.0.0 release, this library was geared only towards the basic financial equations, regarding:
- **Simple Time-Value-of-Money formulas** -- this includes `present_value`, `future_value`, `rate`, and `periods` (known as NPER in Excel).
- **Cashflow Time-Value-of-Money formulas** -- this includes `present_value_annuity`, `future_value_annuity`, `net_present_value`, and `payment` (known as PMT in Excel).
- **Rate conversions** -- this includes all conversions between `apr`, `ear`, and `epr`, and also includes conversion for continuous compounding (`apr_continuous`, `ear_continuous`).
> **Current Status:** revived as **0.1.0** (edition 2021). Previous v0.0.0 release relied on panicking APIs (by design, asserts were placed in the functions to immediately inform the user of incorrect input values) which was great for teaching finance to students, but not ideal for library use in applications. In `v0.1` all methods return *only* `Result`. This is a breaking change from v0.0.0 which provided both panicking APIs (a compromise made for teaching) and `try_<methodname>` as a non-panicking approach, but the dual API leads to duplication, documentation bloat, and could confuse crate users and AI Agents implementing the finance-solution crate. By consolidating all library features to return `Result`, we separate "bad data inputs" from "true bugs" and allows serious rust engineers to utilize the library ergnomically and decide how they want their system to handle the `Result` (log, skip and report, etc., without crashing the whole system). `v0.1` also adds `amortization` and `stocks` modules.
## Modules (0.1 layout)
| Module | Contents |
|--------|----------|
| `tvm` | Present/future value, rate, periods (simple + continuous, fixed + schedule) |
| `cashflow` | Payment (PMT), annuities, NPV, NPER |
| `convert_rate` | APR ↔ EPR ↔ EAR (+ continuous) |
| `amortization` | Solution/series/tables + PPMT, IPMT, CUMPRINC, CUMIPMT |
| `returns` | Doubling rules: solution + comparison tables (72/70/69 vs exact) |
| `stocks` | Price-path solution/series/tables; returns, vol, Sharpe, Sortino, beta, drawdowns |
| `util` | `FinanceError`, `FinanceResult` |
| `round` | Rounding + test assert helpers |
## Quick start
```rust
use finance_solution::*;
// Present value of $4,000 in 3 years at 5% (simple compounding)
// Fixture rates/amounts are valid; production code should use `?` or match.
let pv = present_value(0.05, 3, 4_000.0, false).expect("fixture: 5% PV");
assert_rounded_2!(pv, -3_455.35); // sign: opposite of future value
// Prefer the solution API for formulas + period series
let answer = present_value_solution(0.05, 3, 4_000.0, false).expect("fixture");
println!("{}", answer.formula());
answer.series().print_table();
// Handle bad input without panicking
match present_value(-1.5, 3, 4_000.0, false) {
Ok(v) => println!("pv = {v}"),
Err(e) => eprintln!("error: {e}"),
}
```
### Payment + amortization table
```rust
use finance_solution::*;
let rate = 0.08 / 12.0;
let periods = 60;
let principal = 13_000.0;
let solution = payment_solution(rate, periods, principal, 0.0, false).expect("fixture loan");
solution.print_table();
// Excel-style period components (1-based period index)
let interest_m1 = ipmt(rate, 1, periods, principal, 0.0, false).expect("period 1");
let principal_m1 = ppmt(rate, 1, periods, principal, 0.0, false).expect("period 1");
let year1_interest = cumipmt(rate, periods, principal, 0.0, 1, 12, false).expect("range");
```
### Rate conversion
```rust
use finance_solution::*;
let r = apr(0.034, 12).expect("fixture: 3.4% APR monthly");
println!("EPR = {}, EAR = {}", r.epr(), r.ear());
```
### Doubling rules & price paths
```rust
use finance_solution::*;
let d = doubling_solution(0.08).expect("fixture: 8%");
assert_rounded_2!(d.rule_of_72(), 9.0);
d.print_table();
let prices = [100.0, 110.0, 105.0, 120.0];
let path = price_path_solution(&prices, PricePathOptions::new(12.0)).expect("fixture prices");
path.print_summary();
path.series().print_table();
```
## Design notes
1. **Result-only public math** — domain failures are `FinanceResult` / `FinanceError`. There is **no** dual panicking + `try_*` pair; the ordinary name *is* the fallible function.
2. **Solution + series** — `*_solution` types carry inputs, symbolic/numeric formulas, and `.series()` for period detail.
3. **Excel-compatible signs** — loans/payments follow spreadsheet conventions (positive principal → negative payment, etc.).
4. **Enums for flags** — prefer [`Compounding`] and [`PaymentTiming`]; `bool` still converts via `From` for ergonomics.
5. **Tables** — `.print_table()` / `.print_table_locale()` for teaching and debugging (copy/paste friendly).
6. **Hot path vs solution path** — scalar APIs (`payment`, `present_value`, …) are the nanosecond-class production path (`FinanceResult<f64>`). Solution/series APIs are the microsecond teaching/observability path. Same formulas; different packaging. See [`benches/RESULTS.md`](benches/RESULTS.md).
### Parameter order (when possible)
`rate, periods, present_value, future_value, …`
Money arguments often accept any `Into<f64>` (`i32`, `f32`, …). Rates stay `f64`; period counts are `u32` (NPER returns `f64` for fractional periods).
## Error handling
**Why.** Libraries used from web handlers, batch jobs, or CLIs must not abort when a user types `-150%` interest or period `0`.
**Canonical pattern** (structured match + `?`):
```rust
use finance_solution::{payment, FinanceError, FinanceResult};
fn loan_payment(principal: f64) -> FinanceResult<f64> {
payment(0.09 / 12.0, 60, principal, 0.0, false)
}
match loan_payment(20_000.0) {
Ok(pmt) => println!("payment = {pmt}"),
Err(FinanceError::InvalidRate { rate }) => {
eprintln!("interest rate {rate} is not usable");
}
Err(FinanceError::NonFinite { field, value }) => {
eprintln!("{field} must be finite; got {value}");
}
Err(e) => eprintln!("finance error: {e}"),
}
```
**Affordability example** (compose with `?`):
```rust
use finance_solution::{payment, present_value, FinanceResult};
fn affordability(
future_goal: f64,
discount_rate: f64,
loan_apr: f64,
years: u32,
loan_months: u32,
) -> FinanceResult<f64> {
// Present value of the goal (Excel-style sign: positive FV → negative PV).
let pv = present_value(discount_rate, years, future_goal, false)?;
// Positive principal → negative payment (borrower cash out).
let principal = pv.abs();
payment(loan_apr / 12.0, loan_months, principal, 0.0, false)
}
// Call from a fallible main or handler:
// let pmt = affordability(50_000.0, 0.05, 0.06, 10, 60)?;
// pmt ≈ -593.43
```
| Quantity | Value | Meaning |
|----------|-------|---------|
| `pv` | **≈ −30,695.66** | Today’s value of $50k in 10 years at 5% |
| Loan principal | **≈ +30,695.66** | `pv.abs()` |
| Monthly payment | **≈ −593.43** | Negative = cash you pay |
| 60 × payment | **≈ −35,606** | Total cash paid over the loan |
**Early payoff / mortgage what-ifs** use the same Result APIs — see `cargo run --example early_payoff_what_if` and the amortization tables below.
**Benefits:**
1. **No surprise panics** on bad external input
2. **Matchable variants** for UI / metrics (`FinanceError::code()`)
3. **`std::error::Error` + `Display`** for logging and `?` into app error types
4. **One name per formula** — fallibility lives in the return type
## Amortization: solution / series / tables
```text
amortization_solution → .series() → .print_table()
↓ ↓
formulas + payment period rows (principal, interest, balance, formulas)
```
```rust
use finance_solution::*;
let rate = 0.08 / 12.0;
let periods = 12;
let principal = 10_000.0;
let solution = amortization_solution(rate, periods, principal, 0.0, false)
.expect("fixture: 12-month demo loan");
println!("{}", solution.formula());
let series = solution.series();
assert_eq!(series.len(), periods as usize);
series.print_table(true, true);
assert_approx_equal!(solution.ipmt(1).unwrap(), series[0].interest());
assert_approx_equal!(solution.ppmt(1).unwrap(), series[0].principal());
let _first_half = solution.cumipmt(1, 6).unwrap();
```
Sample `print_table(true, true)` (12-month $10k loan at 8% APR monthly):
```text
period payment principal interest balance principal_to_date interest_to_date payments_to_date principal_remaining interest_remaining payments_remaining
------ --------- --------- -------- ---------- ----------------- ---------------- ---------------- ------------------- ------------------ ------------------
1 -869.8843 -803.2176 -66.6667 9_196.7824 -803.2176 -66.6667 -869.8843 -9_196.7824 -371.9448 -9_568.7272
2 -869.8843 -808.5724 -61.3119 8_388.2100 -1_611.7900 -127.9785 -1_739.7686 -8_388.2100 -310.6329 -8_698.8429
...
12 -869.8843 -864.1235 -5.7608 -0.0000 -10_000.0000 -438.6115 -10_438.6115 0.0000 -0.0000 0.0000
```
| Need | API |
|------|-----|
| One Excel cell (`IPMT` month 3) | `ipmt(...)?` / `solution.ipmt(3)?` |
| Full schedule | `amortization_solution(...)?.series()` |
| Terminal table | `.print_table()` / `.print_table_locale()` |
| Safe construction | `amortization_solution` → `FinanceResult` |
## Returns & stocks
```rust
use finance_solution::*;
let s = doubling_solution(0.08).expect("fixture");
s.print_table();
doubling_compare_rates(&[0.01, 0.05, 0.08, 0.12])
.expect("fixture rates")
.print_table();
let prices = [100.0, 102.0, 101.0, 108.0, 105.0, 112.0, 110.0, 118.0, 115.0, 125.0];
let path = price_path_solution(
&prices,
PricePathOptions::new(12.0).with_years(prices.len() as f64 / 12.0),
)
.expect("fixture prices");
path.print_summary();
path.series().print_table();
```
## Examples
```bash
cargo run --example common_word_problems
cargo run --example amortization_table_demo
cargo run --example early_payoff_what_if
cargo run --example doubling_rules
cargo run --example price_path_analysis
```
## Testing & CI
```bash
cargo test
cargo test --doc
cargo clippy --all-targets -- -D warnings
cargo bench # optional Criterion suites (local / on demand)
```
Symmetry tests, Excel golden values, and integration tests cover core TVM, cashflow, amortization, returns, and stocks paths. CI runs build, tests, doctests, clippy, and fmt on stable and MSRV 1.70.
## License
MIT — see [LICENSE](LICENSE).