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An Elixir data structure that encapsulates a decimal value with a unit

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# Quantity

![](https://github.com/wise-home/quantity/workflows/CI/badge.svg)

A `Quantity` is an Elixir data structure that encapsulates values with units. This data structure can be used to carry values and units through calculations using the calculation functions in the Quantity library.

```elixir
~Q[200.345 kWh] |> Quantity.add(~Q[249.23 kWh])
# {:ok, ~Q[449.575 kWh]}

[
  ~Q[13.49 second],
  ~Q[12.39 second],
  ~Q[4.49 second]
]
|> Quantity.sum()
# {:ok, ~Q[30.37 second]}

cost = ~Q[0.40 $/banana]
amount = ~Q[2000 banana]
Quantity.mult(cost, amount)
# ~Q[800.00 $]
```

## Usage

#### Creating Quantities

A `Quantity` can be created in several ways, one is `Quantity.parse/1` or `Quantity.parse!/1`, which parses a Quantity from a string:

```elixir
Quantity.parse("200.345 EUR")
# {:ok, ~Q[200.345 EUR]}

Quantity.parse!("200.345 EUR")
# ~Q[200.345 EUR]

Quantity.parse!("20.67 EUR/beer")
# ~Q[20.67 EUR/beer] or the same as
# Quantity.new(~d[20.67], {:div, "EUR", "beer"})

Quantity.parse!("15.0 m*m")
# ~Q[15.0 m*m] or the same as
# Quantity.new(~d[15.0], {:mult, "m", "m"})
```

Note: When complex units are used, the parse function allows for :div and :mult of the form `a*b*c/d*e*f`, which
mathematically is interpretted as (a*b*c)/(d*e*f).

```elixir
~Q[1 a*b*c/d*e*f].unit
# {:div, {:mult, "a", {:mult, "b", "c"}}, {:mult, "d", {:mult, "e", "f"}}}
```

Quantity supports unit-less quantities, which acts as decimals. Technically they have the unit `1`
~Q[42].unit
# 1

The `Quantity` module also has `new/2` which creates a `Quantity` from a `Decimal.t()` and a `String.t()`:

```elixir
Quantity.new(Decimal.new("300.445"), "km")
# ~Q[300.445 km]
```

And lastly a `Quantity` can be created from a value, an exponent and a unit:

```elixir
Quantity.new(6578, -2, "second")
# ~Q[65.78 second]
```

#### Addition and subtraction

Basic math can be done using the `add/2` and `sub/2` functions along with their `add!/2` and `sub!/2` siblings:

```elixir
Quantity.add(~Q[500 mL coffee], ~Q[250 mL coffee])
# {:ok, ~Q[750 mL coffee]}

Quantity.add!(~Q[500 mL coffee], ~Q[250 mL coffee])
# ~Q[750 mL coffee]

Quantity.sub(~Q[250 gram], ~Q[125 gram])
# {:ok, ~Q[125 gram]}

Quantity.sub!(~Q[250 gram], ~Q[125 gram])
# ~Q[125 gram]
```

#### Summation

To sum non-empty lists with identical units, `sum/1` and `sum!/1` can be used:

```elixir
lap_times = [
  ~Q[4.24 second],
  ~Q[4.59 second],
  ~Q[5.22 second],
  ~Q[3.99 second]
]

lap_times |> Quantity.sum()
# {:ok, ~Q[18.04 second]}

lap_times |> Quantity.sum!()
# ~Q[18.04 second]

[] |> Quantity.sum()
# :error
```

When a list is empty, `Quantity` doesn't know what precision and unit the result should have. If a `Quantity` with value 0 is desired when summing an empty list, one can use `sum/3` and `sum!/3`:

```elixir
sales_2018 = []

sales_2019 = [
  ~Q[1234.39 EUR],
  ~Q[228.50 EUR]
]

sales_2018 |> Quantity.sum(-2, "EUR")
# {:ok, ~Q[0.00 EUR]}

sales_2019 |> Quantity.sum(-2, "EUR")
# {:ok, ~Q[1462.89 EUR]}
```

#### Multiplication and division

Quantity has support for arbitrary complex units with :div and :mult

```elixir
cost = ~Q[20.00 $]
amount = ~Q[50 banana]
cost_per_banana = Quantity.div(cost, amount)
# ~Q[0.40 $/banana]

Quantity.div(~Q[50 bananas], ~Q[5 bananas])
# ~Q[10]

new_amount = ~Q[2000 banana]
Quantity.mult(cost_per_banana, new_amount)
# ~Q[800.00 $]

Quantity.mult(~Q[5 m], ~Q[4.3 m])
# ~Q[21.5 m*m]

Quantity.mult(~Q[5 m], ~d[20.01])
# ~Q[100.05 m]

Quantity.mult(~Q[10 m/s*s], ~Q[5 s])
# ~Q[50 m/s]
```

#### The ~Q and ~d sigils

`Quantity` has a special `sigil_Q/2` sigil, as seen above. Quantities can be created using the `~Q` sigil, and they are printed as `~Q` when inspected.

```elixir
import Quantity.Sigils, only: [sigil_Q: 2]

~Q[500 bananas]
# ~Q[500 bananas]

# Supports :mult and :div units
~Q[0.54 $/banana]
~Q[13.2 m*m]

```

Additionally a `sigil_d/2` sigil is also present to easily create decimals:

```elixir
import Quantity.Sigils, only: [sigil_d: 2]

~d[123.456]
#Decimal<123.456>
```

#### Data structure

Underneath, a `Quantity` is simply a map consisting of `value` and `unit`, where `value` is a `Decimal.t()` and `unit` a `String.t()`:

```elixir
~Q[99 problems] |> IO.inspect(structs: false)
%{
  __struct__: Quantity,
  unit: "problems",
  value: %{__struct__: Decimal, coef: 99, exp: 0, sign: 1}
}
```

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