## fpdec

Decimal fixed-point arithmetic

### 5 unstable releases

 0.5.4 Aug 17, 2022 Jun 21, 2022 May 22, 2022 Jan 2, 2022 Nov 29, 2021

#64 in Math

Used in 2 crates

250KB
6K SLoC

This crate provides a fast implementation of `Decimal` fixed-point arithmetics. It is targeted at typical business applications, dealing with numbers representing quantities, money and the like, not at scientific computations, for which the accuracy of floating point math is - in most cases - sufficient.

### Objectives

• "Exact" representation of decimal numbers (no deviation as with binary floating point numbers)
• No hidden rounding errors (as inherent to floating point math)
• Very fast operations (by mapping them to integer ops)
• Range of representable decimal numbers sufficient for typical business applications

At the binary level a `Decimal` number is represented as a coefficient (stored as an `i128` value) combined with a value specifying the number of fractional decimal digits (stored as a `u8`). The latter is limited to a value given by the constant `MAX_N_FRAC_DIGITS` = 18.

### Status

Work in progess, but most of the API is stable.

## Getting started

Add `fpdec` to your `Cargo.toml`:

``````[dependencies]
fpdec = "0.5"
``````

## Usage

A `Decimal` number can be created in different ways.

The easiest method is to use the procedural macro `Dec`:

``````# use fpdec::{Dec, Decimal};
let d = Dec!(-17.5);
assert_eq!(d.to_string(), "-17.5");
``````

Alternatively you can convert an integer, a float or a string to a `Decimal`:

``````# use fpdec::Decimal;
let d = Decimal::from(297_i32);
assert_eq!(d.to_string(), "297");
``````
``````# use fpdec::{Decimal, DecimalError};
# use core::convert::TryFrom;
let d = Decimal::try_from(83.25_f64)?;
assert_eq!(d.to_string(), "83.25");
# Ok::<(), DecimalError>(())
``````
``````# use fpdec::{Decimal, ParseDecimalError};
# use core::str::FromStr;
let d = Decimal::from_str("38.2070")?;
assert_eq!(d.to_string(), "38.2070");
# Ok::<(), ParseDecimalError>(())
``````

The sign of a `Decimal` can be inverted using the unary minus operator and a `Decimal` instance can be compared to other instances of type `Decimal` or all basic types of integers (besides u128):

``````# use fpdec::{Dec, Decimal};
let x = Dec!(129.24);
let y = -x;
assert_eq!(y.to_string(), "-129.24");
assert!(-129_i64 > y);
let z = -y;
assert_eq!(x, z);
let z = Dec!(0.00097);
assert!(x > z);
assert!(y <= z);
assert!(z != 7_u32);
assert!(7_u32 == Dec!(7.00));
``````

`Decimal` supports all five binary numerical operators +, -, *, /, and %, with two `Decimal`s or with a `Decimal` and a basic integer (besides u128):

``````# use fpdec::{Dec, Decimal};
let x = Dec!(17.5);
let y = Dec!(6.40);
let z = x + y;
assert_eq!(z.to_string(), "23.90");
let z = x - y;
assert_eq!(z.to_string(), "11.10");
let z = x * y;
assert_eq!(z.to_string(), "112.000");
let z = x / y;
assert_eq!(z.to_string(), "2.734375");
let z = x % y;
assert_eq!(z.to_string(), "4.70");
``````
``````# use fpdec::{Dec, Decimal};
let x = Dec!(17.5);
let y = -5_i64;
let z = x + y;
assert_eq!(z.to_string(), "12.5");
let z = x - y;
assert_eq!(z.to_string(), "22.5");
let z = y * x;
assert_eq!(z.to_string(), "-87.5");
let z = x / y;
assert_eq!(z.to_string(), "-3.5");
let z = x % y;
assert_eq!(z.to_string(), "2.5");
``````

The results of Multiplication or Division are not exact in any case. If the number of fractional decimal digits of the exact result would exceed `MAX_N_FRAC_DIGITS` fractional decimal digits, the result given is rounded to fit this limit.

``````# use fpdec::{Dec, Decimal};
let x = Dec!(1e-10);
let y = Dec!(75e-9);
let z = x * y;
assert_eq!(z.to_string(), "0.000000000000000008");
let x = Dec!(1.);
let y = Dec!(3.);
let z = x / y;
assert_eq!(z.to_string(), "0.333333333333333333");
``````

All these binary numeric operators panic if the result is not representable as a `Decimal` according to the constraints stated above. In addition, there are functions implementing "checked" variants of the operators which return `Option::None` instead of panicking.

For Multiplication and Division there are also functions which return a result rounded to a given number of fractional digits:

``````# use fpdec::{Dec, Decimal, DivRounded, MulRounded};
let x = Dec!(17.5);
let y = Dec!(6.47);
let z: Decimal = x.mul_rounded(y, 1);
assert_eq!(z.to_string(), "113.2");
let z: Decimal = x.div_rounded(y, 3);
assert_eq!(z.to_string(), "2.705");
``````

~140KB