geokit - v0.1.0

geokit/polyline

Google Encoded Polyline algorithm.

Encodes a sequence of LatLng points as a compact ASCII string using delta encoding, ZigZag transform, base-32 chunking, and a + 0x3F ASCII offset. Used by the Google Maps Directions API, OSRM, Mapbox Directions, and Valhalla.

Specification: https://developers.google.com/maps/documentation/utilities/polylinealgorithmformat.

Types

PolylineError

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Errors returned by encode_with, decode, and decode_with.

pub type PolylineError {
  TruncatedInput
  InvalidCharacter(char: String, position: Int)
  PrecisionOutOfRange(precision: Int)
}

Constructors 

  • TruncatedInput

    The input string was malformed — typically a continuation byte without a corresponding stop byte (high bit set on the last chunk).

  • InvalidCharacter(char: String, position: Int)

    A character outside the printable ASCII range used by the encoding appeared in the input.

  • PrecisionOutOfRange(precision: Int)

    precision was outside the supported range [1, 11]. The Google reference encoding uses 5; Valhalla and OSRM use 6. Precisions above 11 overflow the safe integer range on the JavaScript target.

Values

decode

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pub fn decode(
  input input: String,
) -> Result(List(latlng.LatLng), PolylineError)

Decode a polyline string with the default precision (5).

import geokit/polyline

let assert Ok(points) = polyline.decode("_p~iF~ps|U_ulLnnqC_mqNvxq`@")
// points == [(38.5, -120.2), (40.7, -120.95), (43.252, -126.453)]

decode_with

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pub fn decode_with(
  input input: String,
  precision precision: Int,
) -> Result(List(latlng.LatLng), PolylineError)

Decode a polyline string with the given precision. Must match the precision used at encode time. precision must be in [1, 11], matching encode_with.

encode

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pub fn encode(points points: List(latlng.LatLng)) -> String

Encode a list of points using the default precision of 5 (1e-5 degrees, ~1 m at the equator). This matches Google’s original algorithm.

import geokit/polyline
import geokit/latlng

let assert Ok(p1) = latlng.new(lat: 38.5, lng: -120.2)
let assert Ok(p2) = latlng.new(lat: 40.7, lng: -120.95)
let assert Ok(p3) = latlng.new(lat: 43.252, lng: -126.453)
polyline.encode([p1, p2, p3])
// == "_p~iF~ps|U_ulLnnqC_mqNvxq`@"

encode_with

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pub fn encode_with(
  points points: List(latlng.LatLng),
  precision precision: Int,
) -> Result(String, PolylineError)

Encode a list of points with the given precision (number of decimal digits to preserve). precision must be in [1, 11]; precision 6 is used by Valhalla and the Open Source Routing Machine for higher accuracy than the Google default of 5.

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