#math #expression #evaluate #units #convert

bin+lib cpc

evaluates math expressions, with support for units and conversion between units

2 stable releases

1.0.2 Oct 12, 2020
1.0.1 Aug 20, 2020

#32 in Math

MIT license

2.5K SLoC


calculation + conversion

cpc parses and evaluates strings of math, with support for units and conversion. 128-bit decimal floating points are used for high accuracy.

cpc lets you mix units, so for example 1 km - 1m results in Number { value: 999, unit: Meter }.

docs.rs documentation

List of all supported units

CLI Installation

To install the CLI using cargo:

cargo install cpc

To install the CLI directly, grab the appropriate binary from cpc's Releases page on GitHub, then place it wherever you normally place binaries on your OS (On Windows, you may need to edit your PATH variable or something).

CLI Usage

cpc '20c to f'

If you installed the binary somewhere that doesn't make binaries global, you would need to specify the path:

/usr/local/bin/custom/cpc '10+10'
# OR
./cpc '1" in cm'

API Installation

To install the library as a Rust dependency, add cpc to your Cargo.toml like so:

cpc = "1.*"

API Usage

use cpc::{eval};
use cpc::units::Unit;

match eval("3m + 1cm", true, Unit::Celcius, false) {
    Ok(answer) => {
        // answer: Number { value: 301, unit: Unit::Centimeter }
        println!("Evaluated value: {} {:?}", answer.value, answer.unit)
    Err(e) => {
        println!("{}", e)


3 + 4 * 2

8 % 3

(4 + 1)km to light years

10m/2s * 5s

1 lightyear * 0.001mm in km2

1m/s + 1mi/h in kilometers per h

round(sqrt(2)^4)! liters

10% of abs(sin(pi)) horsepower to watts

Supported unit types

  • Normal numbers
  • Time
  • Length
  • Area
  • Volume
  • Mass
  • Digital storage (bytes etc)
  • Energy
  • Power
  • Pressure
  • Speed
  • Temperature


cpc Uses 128-bit Decimal Floating Point (d128) numbers instead of Binary Coded Decimals for better accuracy. The result cpc gives will still not always be 100% accurate. I would recommend rounding the result to 20 decimals or less.


In my case, I can expect eval() to take 100-200ms, and this scales pretty alright. However, putting numbers with a lot of digits into functions result in pretty poor performance. log(e) is one of the worst, and takes 500ms for me.


cpc returns Results with basic strings as errors. Just to be safe, you may want to handle panics (You can do that using std::panic::catch_unwind).

Dev Instructions

Get started

Install Rust. This project was built in Rust 1.45.

Run cpc with a CLI argument as input:

cargo run -- '100ms to s'

Run with debugging, which shows some extra logs:

cargo run -- '100ms to s' --debug

Run tests:

cargo test


cargo build

Adding a unit

1. Add the unit

In src/units.rs, units are specified like this:

pub enum UnitType {
  // etc

// ...

  Nanosecond:         (Time, d128!(1)),
  Microsecond:        (Time, d128!(1000)),
  // etc

The number associated with a unit is it's "weight". For example, if a second's weight is 1, then a minute's weight is 1000.

I have found translatorscafe.com and calculateme.com to be good websites for unit conversion. Wikipedia is worth looking at as well.

2. Add a test for the unit

Make sure to also add a test for each unit. The tests look like this:

assert_eq!(convert_test(1000.0, Meter, Kilometer), 1.0);

Basically, 1000 Meter == 1 Kilometer.

3. Add the unit to the lexer

Text is turned into tokens (some of which are units) in lexer.rs. Here's one example:

// ...
match string {
  "h" | "hr" | "hrs" | "hour" | "hours" => tokens.push(Token::Unit(Hour)),
  // etc
// ...

Potential Improvements


  • Support for math in 6'4" syntax, like 3'+2'4". Currently needs to be written like 3'+3'+4"
  • The functions in units.rs have a lot of manual if statements. This could probably be replaced with a pretty advanced macro.
  • Support for lexing words, like one billion

Potential unit types

Nice list of units: https://support.google.com/websearch/answer/3284611

  • Currency: How would you go about dynamically updating the weights?
  • Fuel consumption
  • Data transfer rate
  • Color codes
  • Force
  • Roman numerals
  • Angles
  • Electric current, capacitance, charge, conductance, volts
  • Flow rate
  • Frequency


  1. Install Docker
  2. Install cross:
    cargo install cross
  3. Build. <target> is the platform you're building for:
    cross build --release --target <target>
    • macOS target: x86_64-apple-darwin (Only works on macOS)
    • Linux target: x86_64-unknown-linux-musl
    • Windows target: x86_64-pc-windows-gnu
    • In case you want to compile for more targets, check out the targets cross supports The compiled binaries will now be available inside target/<target>/release/. The filename will be either cpc or cpc.exe.

Releasing a new version

  1. Update CHANGELOG.md
  2. Bump the version number in Cargo.toml and run cargo check
  3. Run cargo test
  4. Cross-compile cpc by following the steps above
  5. Commit and tag in format v0.0.0
  6. Publish on crates.io:
    1. Login by running cargo login and following the instructions
    2. Test publish to ensure there are no issues
      cargo publish --dry-run
    3. Publish
      cargo publish
  7. Publish on GitHub
    1. Zip the binaries and rename them like cpc-v1.0.0-darwin-x64
    2. Create GitHub release with release notes and attach the zipped binaries


~27K SLoC