# Demo

```elixir
Mix.install([
  {:measurements, path: "."},
  {:vega_lite, "~> 0.1.6"},
  {:kino_vega_lite, "~> 0.1.7"}
])

alias VegaLite, as: Vl
```

## Introduction

`measurements` is a package helping with manipulation of real physics quantities.

## Usage

A measurement is just a struct that can be easily created like so:

<!-- livebook:{"reevaluate_automatically":true} -->

```elixir
Measurements.new(4, :kilometer)
```

If desired, an associated error can be added onto that measurement

<!-- livebook:{"reevaluate_automatically":true} -->

```elixir
Measurements.new(300, :second) |> Measurements.add_error(4, :millisecond)
```

A measurement can be converted to another unit of the same dimension

<!-- livebook:{"reevaluate_automatically":true} -->

```elixir
Measurements.new(300, :second, 5) |> Measurements.best_convert(:microsecond)
```

The best unit is chosen to avoid loosing precision. If the target unit is not better than the current one, the conversion is simply ignored.

<!-- livebook:{"reevaluate_automatically":true} -->

```elixir
Measurements.new(300, :millisecond, 5) |> Measurements.best_convert(:second)
```

This allows to operate on measurements without knowing their precision, but remaining confident no precision in lost in various computations.

## Arithmetic

### Addition

`Measurements` support addition with automated conversion, provided the unit dimension matches.

<!-- livebook:{"reevaluate_automatically":true} -->

```elixir
m1 = Measurements.new(4, :millimeter)
m2 = Measurements.new(543, :micrometer, 2)

Measurements.sum(m1, m2)
```

Otherwise an explicit error with clean measurement representation is displayed:

<!-- livebook:{"reevaluate_automatically":true} -->

```elixir
m1 = Measurements.new(4, :second) |> Measurements.add_error(5, :millisecond)
m2 = Measurements.new(543, :micrometer)
Measurements.sum(m1, m2)
```

### Scaling

Scaling the `Measurement` by a constant is also supported. Note the scale also applies to the error.

<!-- livebook:{"reevaluate_automatically":true} -->

```elixir
m1 = Measurements.new(4, :second) |> Measurements.add_error(5, :millisecond)
Measurements.scale(m1, 60)
```

### Difference

Therefore difference of two measurements with unit of same dimension is also supported.

A more convenient `delta/2` function is provided for this purpose.

<!-- livebook:{"reevaluate_automatically":true} -->

```elixir
m1 = Measurements.new(4, :millimeter)
m2 = Measurements.new(543, :micrometer, 2)

Measurements.delta(m1, m2)
```

### Ratio

The ratio of two measurement is also supported.

The result will be a measurement, with its unit adjusted.
Therefore the `nil` Unit is just a constant, with potentially an error...

<!-- livebook:{"reevaluate_automatically":true} -->

```elixir
m1 = Measurements.new(4, :millimeter)
m2 = Measurements.new(543, :micrometer, 2)
Measurements.ratio(m1, m2)
```

We can verify this result, and maybe get an exact match, even if the value is a float:

```elixir
[
  Measurements.ratio(m1, m2).value * 0.543 == 4,
  # relative error recovery is a bit more involved.
  # Refer to Error propagation theory when in doubt (TODO link?)
  Measurements.ratio(m1, m2).error * 543 / 2 == Measurements.ratio(m1, m2).value
]
```

### TODO : Product

This will require more involved unit dimension manipulation...

## Discrete Calculus

With the capabilities in Measurements so far, we can already implement some basic discrete calculus on physical quantities.

<!-- livebook:{"break_markdown":true} -->

### Local Discrete Derivative

TODO

<!-- livebook:{"break_markdown":true} -->

### Local Discrete Integral

TODO

## Iterative Control