Common (CMN)

121 mathematical and physical constants for Rust. Zero runtime cost. no_std compatible.

What is CMN?

CMN gives you accurate, well-documented mathematical and physical constants as compile-time const values in Rust. Every constant resolves at compile time with zero runtime allocation.

One line to install. Zero configuration. 295 tests. 100% code coverage.

cargo add cmn

Why CMN?

Need	Without CMN	With CMN
PI, E, TAU	Hand-copy from `std::f64::consts` or Wikipedia	`use cmn::constants::PI;`
Physical constants (Avogadro, Planck, Boltzmann)	No stdlib equivalent; copy-paste from NIST	Pre-validated, sourced from CODATA 2018
Boson masses (W, Z, Higgs)	Look up PDG tables manually	`W_BOSON_MASS_GEV`, `Z_BOSON_MASS_GEV`, `HIGGS_BOSON_MASS_GEV`
Typed constant lookup at runtime	Build your own HashMap	`constants.get_value("PI")` returns `ConstantValue::Float`
Utility macros (min, max, range-check)	Write boilerplate or pull in a macro crate	`cmn_max!(3, 7, 2)` — done
Datetime without chrono/time	Pull in a heavy dependency	`DateTime::parse("2026-04-05T14:30:00Z")` — zero deps
Word list for passphrase generation	Find a dictionary crate or embed your own	`Words::default()` — curated, deduplicated, sorted

How CMN compares to other constants crates

	`cmn`	`physical_constants`	`natural_constants`	`std::f64::consts`
Constants	121	354	370+	11
Runtime typed lookup	`ConstantValue` enum	--	--	--
Category filtering	`Category` enum	--	--	--
`no_std` support	Yes	No	No	Yes
WASM support	Yes	Unknown	Unknown	Yes
Utility macros	15 (7 no_std + 8 std)	--	--	--
Datetime module	Built-in	--	--	--
Word list	Built-in	--	--	--
License	MIT / Apache-2.0	GPL-3.0	MIT	stdlib
Test coverage	100% (295 tests)	Unknown	Unknown	N/A
Documentation	100%	100%	29%	stdlib
MSRV	1.72	Unspecified	Unspecified	N/A

physical_constants has the most values but is GPL-3.0 — incompatible with MIT/Apache projects. natural_constants spans the most disciplines but is 29% documented and stale since 2022. CMN is the only crate combining constants, typed runtime lookup, category filtering, no_std/WASM support, utility macros, a datetime module, and a word list under a permissive license with 100% test coverage.

Install

Add to Cargo.toml:

[dependencies]
cmn = "0.0.6"

For no_std (constants + 7 macros only, zero dependencies):

cmn = { version = "0.0.6", default-features = false }

Requires Rust 1.72+. Works on macOS, Linux, Windows, and WASM.

Quick Start

Constants (compile-time)

use cmn::constants::{PI, TAU, EULER, SPEED_OF_LIGHT};

fn main() {
    println!("PI = {PI}");
    println!("TAU = {TAU}");
    println!("e = {EULER}");
    println!("c = {SPEED_OF_LIGHT} m/s");
}

Constants (runtime lookup)

use cmn::constants::{Constants, ConstantValue};

let constants = Constants::new();
if let Some(ConstantValue::Float(pi)) = constants.get_value("PI") {
    println!("PI = {pi}");
}

Category Filtering (no_std)

use cmn::constants::{CONSTANTS_TABLE, Category};

let physical: Vec<_> = CONSTANTS_TABLE.iter()
    .filter(|(_, _, cat)| *cat == Category::Physical)
    .collect();
println!("{} physical constants", physical.len());

Macros (7 no_std + 8 std)

// These work in no_std:
use cmn::{cmn_max, cmn_min, cmn_in_range};

let max = cmn_max!(3, 7, 2);       // 7
let min = cmn_min!(3, 7, 2);       // 2
let ok  = cmn_in_range!(5, 0, 10); // true

// These require the std feature (default):
use cmn::{cmn_vec, cmn_map, cmn_join};

let v = cmn_vec!(1, 2, 3);
let m = cmn_map!("a" => 1, "b" => 2);
let s = cmn_join!("hello", " ", "world");

Word List (std)

use cmn::Words;

let words = Words::default();
println!("{} words loaded", words.count());
println!("First: {}", words.words_list()[0]); // "aboard"

Datetime (std)

use cmn::datetime::DateTime;

let dt = DateTime::parse("2026-04-05T14:30:00Z").unwrap();
let dt2 = DateTime::parse("2026-04-05T16:30:00Z").unwrap();
println!("{}", dt2.duration_since(&dt).whole_hours()); // 2
println!("{}", dt.relative_to(&dt2));                  // "2 hours ago"

let now = DateTime::now();
let tomorrow = now.add_days(1);

Available Constants

Mathematical — Core (11)

Constant	Symbol	Value
`PI`	pi	3.14159265358979...
`TAU`	tau = 2pi	6.28318530717958...
`EULER`	e	2.71828182845904...
`PHI`	phi = (1+sqrt5)/2	1.61803398874989...
`GAMMA`	gamma (Euler-Mascheroni)	0.57721566490153...
`SQRT2`, `SQRT3`, `SQRT5`	Square roots	Exact f64 values
`APERY`	zeta(3)	1.20205690315959...
`CATALAN`	C	0.91596559417721...
`KHINCHIN`	K	2.68545200106530...
`GLAISHER_KINKELIN`	A	1.28242712910062...
`SILVER_RATIO`	delta_s = 1+sqrt2	2.41421356237309...

Mathematical — Logarithmic & Pi Fractions (13)

Constant	Symbol	Value
`LN_2`, `LN_10`	ln(2), ln(10)	0.693..., 2.302...
`LOG2_E`, `LOG10_E`	log2(e), log10(e)	1.442..., 0.434...
`FRAC_1_SQRT_2`	1/sqrt(2)	0.70710678...
`FRAC_1_PI`, `FRAC_2_PI`	1/pi, 2/pi	0.318..., 0.636...
`FRAC_2_SQRT_PI`	2/sqrt(pi)	1.12837916...
`FRAC_PI_2`	pi/2 (90 deg)	1.57079632...
`FRAC_PI_3`	pi/3 (60 deg)	1.04719755...
`FRAC_PI_4`	pi/4 (45 deg)	0.78539816...
`FRAC_PI_6`	pi/6 (30 deg)	0.52359877...
`FRAC_PI_8`	pi/8 (22.5 deg)	0.39269908...

Physical — Fundamental (10)

Constant	Symbol	Value	Unit
`SPEED_OF_LIGHT`	c	299,792,458	m/s
`PLANCK`	h	6.62607015e-34	J s
`PLANCK_REDUCED`	h-bar	h / 2pi	J s
`ELEMENTARY_CHARGE`	e	1.602176634e-19	C
`BOLTZMANN`	k_B	1.380649e-23	J/K
`AVOGADRO`	N_A	6.02214076e23	1/mol
`GAS_CONSTANT`	R	8.314462618	J/(mol K)
`FARADAY`	F	96,485.33212	C/mol
`GRAVITATIONAL_CONSTANT`	G	6.67430e-11	m^3/(kg s^2)
`FINE_STRUCTURE`	alpha	7.2973525693e-3	dimensionless

Physical — Electromagnetic & Vacuum (9)

Constant	Symbol	Value	Unit
`COULOMB`	k_e	8.9875517923e9	N m^2/C^2
`VACUUM_PERMEABILITY`	mu_0	1.25663706212e-6	N/A^2
`VACUUM_PERMITTIVITY`	eps_0	8.8541878128e-12	F/m
`MAGNETIC_FLUX_QUANTUM`	Phi_0	2.067833848e-15	Wb
`CONDUCTANCE_QUANTUM`	G_0	7.748091729e-5	S
`IMPEDANCE_OF_FREE_SPACE`	Z_0	376.730313668	Ohm
`INVERSE_FINE_STRUCTURE`	1/alpha	137.035999084	dimensionless
`JOSEPHSON_CONSTANT`	K_J	4.835978484e14	Hz/V
`VON_KLITZING_CONSTANT`	R_K	25,812.80745	Ohm

Physical — Particle Masses (10)

Constant	Symbol	Value	Unit
`ELECTRON_MASS`	m_e	9.1093837015e-31	kg
`PROTON_MASS`	m_p	1.67262192369e-27	kg
`NEUTRON_MASS`	m_n	1.67492749804e-27	kg
`MUON_MASS`	m_mu	1.883531627e-28	kg
`TAU_PARTICLE_MASS`	m_tau	3.16754e-27	kg
`DEUTERON_MASS`	m_d	3.3435837724e-27	kg
`TRITON_MASS`	m_t	5.0073567446e-27	kg
`HELION_MASS`	m_h	5.0064127796e-27	kg
`ALPHA_PARTICLE_MASS`	m_alpha	6.6446573357e-27	kg
`ATOMIC_MASS_UNIT`	u	1.66053906660e-27	kg

Physical — Masses in MeV/c² & Boson Masses (7)

Constant	Value	Source
`ELECTRON_MASS_MEV`	0.51099895 MeV	CODATA 2018
`PROTON_MASS_MEV`	938.272 MeV	CODATA 2018
`NEUTRON_MASS_MEV`	939.565 MeV	CODATA 2018
`MUON_MASS_MEV`	105.658 MeV	CODATA 2018
`W_BOSON_MASS_GEV`	80.377 GeV	PDG 2022
`Z_BOSON_MASS_GEV`	91.1876 GeV	PDG 2022
`HIGGS_BOSON_MASS_GEV`	125.25 GeV	PDG 2022

Physical — Mass Ratios (5)

Constant	Value
`ELECTRON_PROTON_MASS_RATIO`	5.44617021487e-4
`PROTON_ELECTRON_MASS_RATIO`	1,836.15267343
`MUON_ELECTRON_MASS_RATIO`	206.7682830
`NEUTRON_PROTON_MASS_RATIO`	1.00137841931
`DEUTERON_PROTON_MASS_RATIO`	1.99900750139

Physical — Magnetic Moments & g-Factors (7)

Constant	Symbol	Value	Unit
`BOHR_MAGNETON`	mu_B	9.2740100783e-24	J/T
`NUCLEAR_MAGNETON`	mu_N	5.0507837461e-27	J/T
`ELECTRON_MAGNETIC_MOMENT`	mu_e	-9.2847647043e-24	J/T
`PROTON_MAGNETIC_MOMENT`	mu_p	1.41060679736e-26	J/T
`NEUTRON_MAGNETIC_MOMENT`	mu_n	-9.6623651e-27	J/T
`ELECTRON_G_FACTOR`	g_e	-2.00231930436256
`PROTON_G_FACTOR`	g_p	5.5856946893

Physical — eV Equivalents (6)

Constant	Value	Unit
`ELECTRON_VOLT`	1.602176634e-19	J
`EV_TO_KG`	1.782661921e-36	kg
`EV_TO_AMU`	1.07354410233e-9	u
`EV_TO_HZ`	2.417989242e14	Hz
`EV_TO_KELVIN`	1.160451812e4	K
`EV_TO_INVERSE_METER`	8.065543937e5	1/m

Physical — Atomic & Nuclear (13)

Constant	Symbol	Value	Unit
`BOHR_RADIUS`	a_0	5.29177210903e-11	m
`CLASSICAL_ELECTRON_RADIUS`	r_e	2.8179403262e-15	m
`ELECTRON_COMPTON_WAVELENGTH`	lambda_C	2.42631023867e-12	m
`PROTON_COMPTON_WAVELENGTH`	lambda_C,p	1.32140985539e-15	m
`NEUTRON_COMPTON_WAVELENGTH`	lambda_C,n	1.31959090581e-15	m
`ELECTRON_REDUCED_COMPTON`	lambda-bar_e	3.8615926796e-13	m
`PROTON_REDUCED_COMPTON`	lambda-bar_p	2.10308910336e-16	m
`NEUTRON_REDUCED_COMPTON`	lambda-bar_n	2.10019415600e-16	m
`THOMSON_CROSS_SECTION`	sigma_T	6.6524587321e-29	m^2
`RYDBERG`	R_inf	10,973,731.568160	1/m
`HARTREE_ENERGY`	E_h	4.3597447222071e-18	J
`HARTREE_ENERGY_EV`	E_h	27.211386245988	eV
`FIRST_RADIATION_CONSTANT`	c_1	3.741771852e-16	W m^2

Physical — Thermodynamic & Molar (8)

Constant	Symbol	Value	Unit
`STEFAN_BOLTZMANN`	sigma	5.670374419e-8	W/(m^2 K^4)
`WIEN_DISPLACEMENT`	b	2.897771955e-3	m K
`SECOND_RADIATION_CONSTANT`	c_2	1.438776877e-2	m K
`STANDARD_GRAVITY`	g	9.80665	m/s^2
`STANDARD_ATMOSPHERE`	atm	101,325	Pa
`GAS_CONSTANT_L_ATM`	R	0.08205736608	L atm/(mol K)
`MOLAR_MASS_CONSTANT`	M_u	0.99999999965e-3	kg/mol
`MOLAR_VOLUME_IDEAL_GAS`	V_m	22.71095e-3	m^3/mol

Physical — Planck Units (5)

Constant	Symbol	Value	Unit
`PLANCK_MASS`	m_P	2.176434e-8	kg
`PLANCK_LENGTH`	l_P	1.616255e-35	m
`PLANCK_TIME`	t_P	5.391247e-44	s
`PLANCK_TEMPERATURE`	T_P	1.416784e32	K
`PLANCK_CHARGE`	q_P	1.875546e-18	C

Physical — Charge-to-Mass & Atomic Units (10)

Constant	Value	Unit
`ELECTRON_CHARGE_TO_MASS`	-1.75882001076e11	C/kg
`PROTON_CHARGE_TO_MASS`	9.5788332e7	C/kg
`ATOMIC_UNIT_OF_LENGTH`	5.29177210903e-11	m
`ATOMIC_UNIT_OF_TIME`	2.4188843265857e-17	s
`ATOMIC_UNIT_OF_VELOCITY`	2.18769126364e6	m/s
`ATOMIC_UNIT_OF_FORCE`	8.2387234983e-8	N
`ATOMIC_UNIT_OF_ELECTRIC_FIELD`	5.14220674763e11	V/m
`ATOMIC_UNIT_OF_POLARIZABILITY`	1.64877727436e-41	C^2 m^2/J
`LOSCHMIDT_CONSTANT`	2.6867774e25	1/m^3
`MOLAR_PLANCK_CONSTANT`	3.990312712e-10	J s/mol

Cryptographic & Utility (4)

Constant	Type	Value
`HASH_ALGORITHM`	`&str`	`"Blake3"`
`HASH_COST`	`u32`	`8`
`HASH_LENGTH`	`usize`	`32`
`SPECIAL_CHARS`	`&[char]`	29 symbols

Full API reference: docs.rs/cmn

Architecture

graph LR
    subgraph "cmn crate"
        B["constants.rs<br/>121 const values<br/>Constants lookup API<br/>Category filtering"]
        C["words.rs<br/>Words HashSet<br/>WORD_LIST dictionary"]
        D["macros.rs<br/>15 utility macros<br/>(7 no_std + 8 std)"]
        E["datetime.rs<br/>ISO 8601 parsing<br/>Duration & relative time"]
        A["lib.rs<br/>Common struct<br/>JSON serde bridge"]
    end

    A --> B
    A --> C
    A --> D
    A --> E

    U["Your Code"] --> B
    U --> C
    U --> D
    U --> E
    U --> A

Module	What it does	When to use it
`constants`	121 compile-time `const` values + `Constants` runtime API + `ConstantValue` enum + `Category` filtering	You need a mathematical or physical constant
`words`	`Words` struct backed by `HashSet<String>` with add/remove/contains + `WORD_LIST`	Passphrase generation, word games, text processing
`macros`	15 macros: 7 `no_std` (`cmn_max!`, `cmn_min!`, `cmn_in_range!`, `cmn_assert!`, `cmn_contains!`, `cmn_to_num!`, `cmn_constants!`) + 8 `std` (`cmn_vec!`, `cmn_map!`, `cmn_join!`, `cmn_split!`, `cmn_print!`, `cmn_print_vec!`, `cmn!`, `cmn_parse!`)	Quick utilities without writing boilerplate
`datetime`	ISO 8601 parsing, `now()`, arithmetic, duration, relative formatting, timezone offsets	Timestamps, "3 hours ago", duration calculations — no external crate
`Common`	JSON-backed bridge connecting constants + words via `serde`	Deserializing configuration that includes constants or words

Feature Flags

Feature	Default	Enables
`std`	Yes	`Constants` struct, `ConstantValue`, `Words`, `Common`, `datetime`, serde, 8 std macros

Without std: all 121 const values, CONSTANTS_TABLE with Category, and 7 no_std macros — with zero dependencies.

FAQ

How accurate are the constants? Mathematical constants use core::f64::consts where available (PI, E, TAU, SQRT2). Physical constants are sourced from CODATA 2018 recommended values. Boson masses are from PDG 2022. All values are validated by 295 tests including mathematical identity checks (e.g., SQRT2^2 == 2, R == k_B * N_A, Phi_0 == h/(2e)).

Does CMN support no_std? Yes. Disable default features to get all 121 const values, CONSTANTS_TABLE with Category filtering, and 7 macros with zero dependencies:

cmn = { version = "0.0.6", default-features = false }

The Constants runtime API, Words, Common, datetime, and 8 std macros require the std feature (enabled by default).

Does CMN compile to WASM? Yes. cargo build --target wasm32-unknown-unknown --no-default-features compiles cleanly.

What is the MSRV? Rust 1.72. Tested on stable. No nightly features required.

How does CMN compare to other Rust constants crates? physical_constants has 354 values but is GPL-3.0 — incompatible with MIT/Apache projects. natural_constants covers more disciplines but is only 29% documented and unmaintained since 2022. std::f64::consts provides 11 math constants with no physical values. CMN is the only crate combining 121 constants with typed runtime lookup, category filtering, no_std/WASM support, 15 utility macros, a datetime module, and a word list under a permissive license with 100% test coverage. See the comparison table above.

Development

Prerequisites

Platform	Install Rust
macOS	`curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs \| sh`
Linux / WSL	Same as above, plus `sudo apt-get install -y build-essential`
Windows	Download rustup-init.exe and install the MSVC toolchain

Build, Test, Verify

git clone https://github.com/sebastienrousseau/cmn.git
cd cmn
cargo build        # Compile
cargo test         # 295 tests, 100% coverage
cargo clippy       # Zero warnings
cargo fmt --check  # Verify formatting
cargo doc --open   # Browse API docs locally

Run Examples

cargo run --example cmn
cargo run --example constants_math
cargo run --example constants_physical
cargo run --example constants_lookup
cargo run --example datetime_demo
cargo run --example words_demo
cargo run --example macros_demo

Run Benchmarks

cargo bench

Troubleshooting

Symptom	Fix
`rustup: command not found`	Install Rust via rustup.rs
`error[E0658]: unstable library feature`	`rustup update stable` (MSRV is 1.72)
`linker 'cc' not found` (Linux/WSL)	`sudo apt-get install -y build-essential`
`cargo test` fails on fresh clone	Open an issue with `rustc --version`

Contributing

See CONTRIBUTING.md for signed-commit setup and PR guidelines. See CHANGELOG.md for version history.

License

Dual-licensed under Apache 2.0 or MIT, at your option.

Built by Sebastien Rousseau.

Name		Name	Last commit message	Last commit date
Latest commit History 101 Commits
.github		.github
benches		benches
examples		examples
src		src
tests		tests
.deepsource.toml		.deepsource.toml
.gitignore		.gitignore
AUTHORS.md		AUTHORS.md
CHANGELOG.md		CHANGELOG.md
CONTRIBUTING.md		CONTRIBUTING.md
Cargo.lock		Cargo.lock
Cargo.toml		Cargo.toml
LICENSE-APACHE		LICENSE-APACHE
LICENSE-MIT		LICENSE-MIT
README.md		README.md
TEMPLATE.md		TEMPLATE.md
build.rs		build.rs
deny.toml		deny.toml
rustfmt.toml		rustfmt.toml

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Common (CMN)

What is CMN?

Why CMN?

How CMN compares to other constants crates

Install

Quick Start

Constants (compile-time)

Constants (runtime lookup)

Category Filtering (no_std)

Macros (7 no_std + 8 std)

Word List (std)

Datetime (std)

Available Constants

Mathematical — Core (11)

Mathematical — Logarithmic & Pi Fractions (13)

Physical — Fundamental (10)

Physical — Electromagnetic & Vacuum (9)

Physical — Particle Masses (10)

Physical — Masses in MeV/c² & Boson Masses (7)

Physical — Mass Ratios (5)

Physical — Magnetic Moments & g-Factors (7)

Physical — eV Equivalents (6)

Physical — Atomic & Nuclear (13)

Physical — Thermodynamic & Molar (8)

Physical — Planck Units (5)

Physical — Charge-to-Mass & Atomic Units (10)

Cryptographic & Utility (4)

Architecture

Feature Flags

FAQ

Development

Prerequisites

Build, Test, Verify

Run Examples

Run Benchmarks

Troubleshooting

Contributing

License

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