Struct Unit

Source

pub struct Unit<ExponentsType = DynDimensionExponents> {
    pub name: &'static str,
    pub symbols: &'static [&'static str],
    pub scale: ScaleExponents,
    pub conversion_factor: f64,
    pub affine_offset: f64,
    pub exponents: ExponentsType,
    pub system: System,
}

Expand description

A unit.

Each unit is assigned a “storage unit”. The storage unit is the unit of a value stored with this unit. Storage units are always a well defined multiple of an SI base unit.

The logarithmic scale encoding in the type system uses only powers of 2, 3, 5, and pi. This means that the storage unit must be a multiple of an SI base unit and a product of powers of 2, 3, 5, and pi. For example,

“kilometer” has a scale factor of 10^3 = 2^3 * 5^3
“degree” has a scale factor of π/180 = 2^-2 * 3^-2 * 5^-1 * pi^1

Units that differ from identity in their conversion_factor are “non-storage” units. Non-storage units are not stored in their native scale; upon declaration they are converted to their “nearest neighbor” power-of-10 multiple of a SI base unit. For example

“inch” is multiplied by 2.54 and stored as “centimeters”
“yard” is multiplied by 0.9144 and stored as “meters”
“mile” is multiplied by 1.609344 and stored as “kilometers”

Fields§

§name: &'static str

Name of the unit.

§symbols: &'static [&'static str]

Symbols associated with the unit.

Symbols are also used for lookup, so they must be unique within the unit system.

Because SI has a systematic prefixing semantics, symbols must be kept from colliding not just with the SI base symbols, but with any legal prefixing thereof. “SI base symbols” are defined as the first symbol of the first unit in each dimension, by declaration order.

§scale: ScaleExponents

Base unit per storage unit.

To convert from a value in the storage unit to a SI base unit value, multiply it by scale.

For example, a scale of 1000 in dimension length would store the value as kilometers. If we have the value 123 stored then that means in meters it is 123 * 1000 = 123,000 meters.

§conversion_factor: f64

Storage unit per this unit.

Difference from identity canonically identifies a unit as a “non-storage” unit. Non-storage units are not stored in their native scale, as arbitrary float scaling factors are not part of the logarithmic scale encoding in the type system.

To convert from a value in this unit to a storage unit value, multiply it by conversion_factor.

For example, the “inch” unit has a conversion_factor of 2.54, which means that a value of 1 in inches is stored as 2.54 (accordingly, the scale is 10^-2).

Non-storage units are always given a storage scale of their “nearest neighbor” power-of-10 multiple of a SI base unit. For example,

“inch” is multiplied by 2.54 and stored as “centimeters”
“yard” is multiplied by 0.9144 and stored as “meters”
“mile” is multiplied by 1.609344 and stored as “kilometers”

§affine_offset: f64

The “zero point offset” of this unit’s measurement scale from the numerical zero of the storage unit.

To convert from a value in the unit to the storage unit, add the affine offset to the value.

For example, the “celsius” unit has an affine offset of 273.15, which means that a value of 0 in celsius is stored as 273.15 in kelvin.

§exponents: ExponentsType

Dimensional exponent vector of the dimension this unit belongs to.

§system: System

Which “unit system” this unit belongs to. This determines the name of the declarator trait in which this unit’s nominal declarators will live (e.g. “ImperialLength” or “MetricMass”).