routing-service/encoding/polylines/decoding.go

package polylines

import (
	"math"

	"github.com/paulmach/orb"
)

// // Decode decodes coordinates to the "Encoded Polyline Algorithm Format"
// // More info: https://developers.google.com/maps/documentation/utilities/polylinealgorithm
// // polyline: polyline string in "Encoded Polyline Algorithm Format"
// // precision: usually 5 or 6; Google's original algorithm uses 5 digits of decimal precision,
// // which is accurate to about a meter. A precision of 6 gives you an accuracy of about 10cm
// // more info: https://mapzen.com/blog/polyline-precision/
// func Decode(polyline string, precision uint32) orb.LineString {

// 	group := ""
// 	coordinates := []float64{}

// 	for _, letter := range polyline {
// 		group += string(letter)

// 		if (int32(letter)-63)&0x20 == 0 {
// 			coordinates = append(coordinates, decodeElement(group, precision))
// 			group = ""
// 		}
// 	}

// 	points := orb.LineString{}
// 	for i := 1; i < len(coordinates); i += 2 {
// 		points = append(points, orb.Point{round(coordinates[i], precision), round(coordinates[i-1], precision)})
// 	}

// 	for i := range points {
// 		if i > 0 {
// 			points[i][1] = round(points[i-1].Lat()+points[i].Lat(), precision)
// 			points[i][0] = round(points[i-1].Lon()+points[i].Lon(), precision)
// 		}
// 	}

// 	return points
// }

// // Decode5 is a short call for Decode with precision set to 5
// // Accuracy is about one meter
// func Decode5(polyline string) orb.LineString {
// 	return Decode(polyline, 5)
// }

// // Decode6 is a short call for Decode with precision set to 6
// // Accuracy is about ten centimeters
// func Decode6(polyline string) orb.LineString {
// 	return Decode(polyline, 6)
// }

// // decodeElement decodes an coordinate element (i.e. latitude or longitude)
// // from the "Encoded Polyline Algorithm Format"
// func decodeElement(group string, precision uint32) float64 {

// 	var c chunks
// 	c.ParseLine(group)
// 	return c.Coordinate(precision)
// }

func Decode(encoded *string, precisionOptional ...int) orb.LineString {
	// default to 6 digits of precision
	precision := 6
	if len(precisionOptional) > 0 {
		precision = precisionOptional[0]
	}
	factor := math.Pow10(precision)

	// Coordinates have variable length when encoded, so just keep
	// track of whether we've hit the end of the string. In each
	// loop iteration, a single coordinate is decoded.
	lat, lng := 0, 0
	var coordinates orb.LineString
	index := 0
	for index < len(*encoded) {
		// Consume varint bits for lat until we run out
		var byte int = 0x20
		shift, result := 0, 0
		for byte >= 0x20 {
			byte = int((*encoded)[index]) - 63
			result |= (byte & 0x1f) << shift
			shift += 5
			index++
		}

		// check if we need to go negative or not
		if (result & 1) > 0 {
			lat += ^(result >> 1)
		} else {
			lat += result >> 1
		}

		// Consume varint bits for lng until we run out
		byte = 0x20
		shift, result = 0, 0
		for byte >= 0x20 {
			byte = int((*encoded)[index]) - 63
			result |= (byte & 0x1f) << shift
			shift += 5
			index++
		}

		// check if we need to go negative or not
		if (result & 1) > 0 {
			lng += ^(result >> 1)
		} else {
			lng += result >> 1
		}

		// scale the int back to floating point and storeLineString it
		coordinates = append(coordinates, orb.Point{float64(lng) / factor, float64(lat) / factor})
	}

	return coordinates
}