Numeric conversions for engineering notation and RKM code.

Overview

In engineering applications it is common to express quantities relative to the next-lower power of 1000, described by an SI (metric) prefix.

This is normally done by writing the SI multiplier after the quantity. In the "RKM code" variant, the SI multiplier replaces the decimal point.

For example:

And so on going up the SI prefixes, including the new ones R (10²⁷) and Q (10³⁰) which were added in 2022.

This crate exists to support convenient conversion of numbers to/from engineering and RKM notation. The intended use case is for parsing user-entered configuration.

Detail

This crate is centred around the EngineeringQuantity<T> type. This type supports comparisons via PartialEq, Eq, PartialOrd and Ord, though if you want to perform actual maths you are probably better off converting to int or Ratio.

Storage

• The generic parameter T specifies the storage type to use for the significand. This can be any primitive integer except for i8 or u8, which are too small to be useful.
  • For example, EngineeringQuantity<u64>.
• The exponent is always stored as an i8. This can range from -10 (q) to +10 (Q); going beyond that will likely cause Overflow or Underflow errors.

Conversions

You can convert an EngineeringQuantity to:

• integer types, truncating any fraction:
  • directly into type T, or a larger integer type (one which implements From<T>);
  • any integer type using the num_traits::ToPrimitive trait (to_i32() and friends, which apply an overflow check);
• String, optionally via the DisplayAdapter type to control the formatting;
• another EngineeringQuantity (convert if the destination storage type is larger; try_convert if it is smaller);
• f32 and f64 (with an over/underflow check);
• num_rational::Ratio (with an over/underflow check);
• its component parts, as a tuple (<T>, i8) (see to_raw).

You can create an EngineeringQuantity from:

• type T, or a smaller integer type (one which implements Into<T>);
• String or &str, which autodetects both standard and RKM code variants;
• num_rational::Ratio, which requires the denominator be a power of 1000;
• its component parts (<T>, i8) (see from_raw), which will overflow if the converted number cannot fit into T.

Supported integer types may be converted directly to string via the EngineeringRepr convenience trait.

Or, if you prefer, here are the type relations in diagram form:

                                            ┌────────────────────┐
                                            │      integers      │
                                            └────────────────────┘
                                              ▲                I
                                              ╵                I [impl]
                                              ╵                I
                                              ▼                ▼
          ┌───────────────────────────────────────────┐  ┌─────────────────────┐
          │           EngineeringQuantity<T>          │  │   EngineeringRepr   │
          │                                           │  │ (convenience trait) │
          └───────────────────────────────────────────┘  └─────────────────────┘
            ▲             ▲            ╵        ▲    │        │
            ╵             ╵            ╵        ╵    │        │ to_eng()
            ╵             ╵            ╵        ╵    │        │ to_rkm()
            ▼             ▼            ▼        ╵    ▼        ▼
┌─────────────┐  ┌────────────────┐  ┌───────┐  ╵   ┌───────────────────┐
│ "raw" tuple │  │ num_rational:: │  │  f32  │  ╵   │ DisplayAdapter<T> │
│   (T, i8)   │  │    Ratio<T>    │  │  f64  │  ╵   │                   │
└─────────────┘  └────────────────┘  └───────┘  ╵   └───────────────────┘
                                                ╵       │
                                                ╵       │
                                                ▼       ▼
                                           ┌───────────────────┐
                                           │      String       │
                                           └───────────────────┘

Serialization

The serde feature flag adds support for EngineeringQuantity:

• Serialization as a String (only);
• Deserialization from a String or an integer

Deserialization is subject to range checks and will fail if, for example, the number does not fit into the underlying storage type.

If you need more control than this, you may wish to specify a custom serializer / deserializer.

Examples

String to number

use engineering_repr::EngineeringQuantity as EQ;
use std::str::FromStr as _;
use num_rational::Ratio;

// Standard notation
let eq = EQ::<i64>::from_str("1.5k").unwrap();
assert_eq!(i64::try_from(eq).unwrap(), 1500);

// RKM style notation
let eq2 = EQ::<i64>::from_str("1k5").unwrap();
assert_eq!(eq, eq2);

// Conversion to the nearest integer
let eq3 = EQ::<i32>::from_str("3m").unwrap();
assert_eq!(i32::try_from(eq3).unwrap(), 0);
// Convert to Ratio
let r : Ratio<i32> = eq3.try_into().unwrap();
assert_eq!(r, Ratio::new(3, 1000)); // => 3 / 1000
// Convert to float
let f : f64 = eq3.try_into().unwrap();
assert_eq!(f, 0.003); // caution, not all float conversions will work out exactly

Number to string

use engineering_repr::EngineeringQuantity as EQ;

// default precision (3 places, "sloppy" omitting trailing zeroes)
let ee1 = EQ::<i32>::from(1200);
assert_eq!(ee1.to_string(), "1.2k");
// strict precision
assert_eq!(ee1.with_strict_precision(3).to_string(), "1.20k");
// explicit precision
let ee2 = EQ::<i32>::from(1234567);
assert_eq!(ee2.with_precision(2).to_string(), "1.2M");

// RKM style
assert_eq!(ee2.rkm_with_precision(2).to_string(), "1M2");

// Zero precision means "automatic, lossless"
assert_eq!(ee2.with_precision(0).to_string(), "1.234567M");
assert_eq!(ee2.rkm_with_precision(0).to_string(), "1M234567");

Integer directly to string via convenience trait

use engineering_repr::EngineeringRepr as _;
assert_eq!("123.45k", 123456.to_eng(5));
assert_eq!("123.456k", 123456.to_eng(0)); // automatic precision
assert_eq!("123k4", 123456.to_rkm(4));

Serialization

#[cfg(feature="serde")] // This functionality requires the `serde` feature flag
{
use engineering_repr::EngineeringQuantity as EQ;
let eq1 = EQ::<i32>::from(1200);
// Serialization to string
assert_eq!(serde_json::to_string(&eq1).unwrap(), "\"1.2k\"");
// Deserialization from string
assert_eq!(serde_json::from_str::<EQ<i32>>("\"1.2k\"").unwrap(), eq1);
// Deserialization from integer
assert_eq!(serde_json::from_str::<EQ<i32>>("1200").unwrap(), eq1);
}

Limitations

• Multipliers which are not a power of 1000 (da, h, d, c) are not supported.

Alternatives

• human-repr is great for converting numbers to human-friendly representations.
• humanize-rs is great for converting some human-friendly representations to numbers, though engineering-repr offers more flexibility.