# orb [![CI](https://github.com/paulmach/orb/workflows/CI/badge.svg)](https://github.com/paulmach/orb/actions?query=workflow%3ACI+event%3Apush) [![codecov](https://codecov.io/gh/paulmach/orb/branch/master/graph/badge.svg?token=NuuTjLVpKW)](https://codecov.io/gh/paulmach/orb) [![Go Report Card](https://goreportcard.com/badge/github.com/paulmach/orb)](https://goreportcard.com/report/github.com/paulmach/orb) [![Go Reference](https://pkg.go.dev/badge/github.com/paulmach/orb.svg)](https://pkg.go.dev/github.com/paulmach/orb)

Package `orb` defines a set of types for working with 2d geo and planar/projected geometric data in Golang.
There are a set of sub-packages that use these types to do interesting things.
They each provide their own README with extra info.

## Interesting features

-   **Simple types** - allow for natural operations using the `make`, `append`, `len`, `[s:e]` builtins.
-   **GeoJSON** - support as part of the [`geojson`](geojson) sub-package.
-   **Mapbox Vector Tile** - encoding and decoding as part of the [`encoding/mvt`](encoding/mvt) sub-package.
-   **Direct to type from DB query results** - by scanning WKB data directly into types.
-   **Rich set of sub-packages** - including [`clipping`](clip), [`simplifying`](simplify), [`quadtree`](quadtree) and more.

## Type definitions

```go
type Point [2]float64
type MultiPoint []Point

type LineString []Point
type MultiLineString []LineString

type Ring LineString
type Polygon []Ring
type MultiPolygon []Polygon

type Collection []Geometry

type Bound struct { Min, Max Point }
```

Defining the types as slices allows them to be accessed in an idiomatic way
using Go's built-in functions such at `make`, `append`, `len`
and with slice notation like `[s:e]`. For example:

```go
ls := make(orb.LineString, 0, 100)
ls = append(ls, orb.Point{1, 1})
point := ls[0]
```

### Shared `Geometry` interface

All of the base types implement the `orb.Geometry` interface defined as:

```go
type Geometry interface {
    GeoJSONType() string
    Dimensions() int // e.g. 0d, 1d, 2d
    Bound() Bound
}
```

This interface is accepted by functions in the sub-packages which then act on the
base types correctly. For example:

```go
l := clip.Geometry(bound, geom)
```

will use the appropriate clipping algorithm depending on if the input is 1d or 2d,
e.g. a `orb.LineString` or a `orb.Polygon`.

Only a few methods are defined directly on these type, for example `Clone`, `Equal`, `GeoJSONType`.
Other operation that depend on geo vs. planar contexts are defined in the respective sub-package.
For example:

-   Computing the geo distance between two point:

    ```go
    p1 := orb.Point{-72.796408, -45.407131}
    p2 := orb.Point{-72.688541, -45.384987}

    geo.Distance(p1, p2)
    ```

-   Compute the planar area and centroid of a polygon:

    ```go
    poly := orb.Polygon{...}
    centroid, area := planar.CentroidArea(poly)
    ```

## GeoJSON

The [geojson](geojson) sub-package implements Marshalling and Unmarshalling of GeoJSON data.
Features are defined as:

```go
type Feature struct {
    ID         interface{}  `json:"id,omitempty"`
    Type       string       `json:"type"`
    Geometry   orb.Geometry `json:"geometry"`
    Properties Properties   `json:"properties"`
}
```

Defining the geometry as an `orb.Geometry` interface along with sub-package functions
accepting geometries allows them to work together to create easy to follow code.
For example, clipping all the geometries in a collection:

```go
fc, err := geojson.UnmarshalFeatureCollection(data)
for _, f := range fc {
    f.Geometry = clip.Geometry(bound, f.Geometry)
}
```

The library supports third party "encoding/json" replacements
such [github.com/json-iterator/go](https://github.com/json-iterator/go).
See the [geojson](geojson) readme for more details.

The types also support BSON so they can be used directly when working with MongoDB.

## Mapbox Vector Tiles

The [encoding/mvt](encoding/mvt) sub-package implements Marshalling and
Unmarshalling [MVT](https://www.mapbox.com/vector-tiles/) data.
This package uses sets of `geojson.FeatureCollection` to define the layers,
keyed by the layer name. For example:

```go
collections := map[string]*geojson.FeatureCollection{}

// Convert to a layers object and project to tile coordinates.
layers := mvt.NewLayers(collections)
layers.ProjectToTile(maptile.New(x, y, z))

// In order to be used as source for MapboxGL geometries need to be clipped
// to max allowed extent. (uncomment next line)
// layers.Clip(mvt.MapboxGLDefaultExtentBound)

// Simplify the geometry now that it's in tile coordinate space.
layers.Simplify(simplify.DouglasPeucker(1.0))

// Depending on use-case remove empty geometry, those too small to be
// represented in this tile space.
// In this case lines shorter than 1, and areas smaller than 2.
layers.RemoveEmpty(1.0, 2.0)

// encoding using the Mapbox Vector Tile protobuf encoding.
data, err := mvt.Marshal(layers) // this data is NOT gzipped.

// Sometimes MVT data is stored and transferred gzip compressed. In that case:
data, err := mvt.MarshalGzipped(layers)
```

## Decoding WKB/EWKB from a database query

Geometries are usually returned from databases in WKB or EWKB format. The [encoding/ewkb](encoding/ewkb)
sub-package offers helpers to "scan" the data into the base types directly.
For example:

```go
db.Exec(
  "INSERT INTO postgis_table (point_column) VALUES (ST_GeomFromEWKB(?))",
  ewkb.Value(orb.Point{1, 2}, 4326),
)

row := db.QueryRow("SELECT ST_AsBinary(point_column) FROM postgis_table")

var p orb.Point
err := row.Scan(ewkb.Scanner(&p))
```

For more information see the readme in the [encoding/ewkb](encoding/ewkb) package.

## List of sub-package utilities

-   [`clip`](clip) - clipping geometry to a bounding box
-   [`encoding/mvt`](encoding/mvt) - encoded and decoding from [Mapbox Vector Tiles](https://www.mapbox.com/vector-tiles/)
-   [`encoding/wkb`](encoding/wkb) - well-known binary as well as helpers to decode from the database queries
-   [`encoding/ewkb`](encoding/ewkb) - extended well-known binary format that includes the SRID
-   [`encoding/wkt`](encoding/wkt) - well-known text encoding
-   [`geojson`](geojson) - working with geojson and the types in this package
-   [`maptile`](maptile) - working with mercator map tiles and quadkeys
-   [`project`](project) - project geometries between geo and planar contexts
-   [`quadtree`](quadtree) - quadtree implementation using the types in this package
-   [`resample`](resample) - resample points in a line string geometry
-   [`simplify`](simplify) - linear geometry simplifications like Douglas-Peucker
