package polylines

import (
	"math"

	"github.com/paulmach/orb"
)

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(lat) / factor, float64(lng) / factor})
	}

	return coordinates
}