geohash.swift

//
//  Geohash.swift
//  mn_ios
//
//  Created by Alex Bosworth on 11/26/14.
//  Copyright (c) 2014 adylitica. All rights reserved.
//

import Foundation
import MapKit

typealias GeoInterval = (CLLocationDegrees, CLLocationDegrees)

class Geohash {
    private class func _joinInterval(interval: GeoInterval) -> Double {
        return (interval.0 + interval.1) / 2
    }
    
    private class func _refineInterval(interval: GeoInterval, cd: Int, mask: Int) -> GeoInterval {
        let joined = _joinInterval(interval)
        
        return cd & mask != 0 ? (joined, interval.1) : (interval.0, joined)
    }
    
    private struct Constants {
        static let interval = (lat: GeoInterval(-90, 90), long: GeoInterval(-180, 180))
        static let base32 = "0123456789bcdefghjkmnpqrstuvwxyz"
        static let bits = [16, 8, 4, 2, 1]
    }
    
    /** Convert a CLLocation into a geohash
    */
    class func encode(coordinate: CLLocationCoordinate2D) -> String {
        var isEven = true
        var bit = 0
        var ch = 0
        var precision = 12
        var geohash = [String]()
        var mid: Double = 0
        
        var lat = Constants.interval.lat
        var lon = Constants.interval.long
        
        while geohash.count < precision {
            if isEven {
                mid = _joinInterval(lon)
                
                if coordinate.longitude > mid {
                    ch |= Constants.bits[bit]
                    lon.0 = mid
                }
                else {
                    lon.1 = mid
                }
            }
            else {
                mid = _joinInterval(lat)
                
                if coordinate.latitude > mid {
                    ch |= Constants.bits[bit]
                    lat.0 = mid
                }
                else {
                    lat.1 = mid
                }
            }
            
            isEven = !isEven
            
            if bit < 4 {
                bit++
            }
            else {
                geohash += [String(Constants.base32[advance(Constants.base32.startIndex, ch)])]
                bit = 0
                ch = 0
            }
        }

        return join("", geohash.map { String($0) })
    }

    /** Convert a geohash into a CLLocation
    */
    class func decode(geohash: String) -> CLLocationCoordinate2D? {
        var isEven = true
        var interval = Constants.interval

        let base32 = Constants.base32
        let bits = Constants.bits
        
        for char in geohash {
            let index = find(base32, char)

            // Exit early when the geohash is invalid
            if index == nil { return nil }
            
            let value = distance(base32.startIndex, index!)

            for mask in bits {
                if isEven {
                    interval.long = _refineInterval(interval.long, cd: value, mask: mask)
                }
                else {
                    interval.lat = _refineInterval(interval.lat, cd: value, mask: mask)
                }
            
                isEven = !isEven
            }
        }
        
        return CLLocationCoordinate2D(latitude: _joinInterval(interval.lat), longitude: _joinInterval(interval.long))
    }
}