Geohash

Geohash is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.

Geohashes[1] are used to represent standard latitude and longitude coordinates as single values in the form of a simple string -- using the digits (0-9) and the letters (B-Z excluding I, L, O). They can vary in length, with more characters in the string representing more precision.

Task

Generate a Geohash with a desired precision from a coordinate represented as an array of two doubles, latitude and longitude.

Example 1:

print (encodeGeohash (for: [51.433718, -0.214126], withPrecision: 2))

// Result: "gc" (all of Ireland, most of England and Wales, small part of Scotland)

Example 2:

print (encodeGeohash (for: [51.433718, -0.214126], withPrecision: 9))

// Result: "gcpue5hp4" (the umpire's chair on Center Court at Wimbledon)

From the Wikipedia page, geohashes can be "useful in database systems where queries on a single index are much easier or faster than multiple-index queries."

Swift

<lang Swift>let gBase32 = ["0", "1", "2", "3", "4", "5", "6", "7", "8", "9", "b", "c", "d", "e", "f", "g", "h", "j", "k", "m", "n", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z"]

func encodeGeohash (for location: Array<Double>, withPrecision precision: Int = 9) -> String {

 var latitudeRange = -90.0...90.0
 var longitudeRange = -180...180.0

 var hash = ""
 var hashVal = 0
 var bits = 0
 var even = true

 while (hash.count < precision) {
   let val     = even ? location[1]: location[0]
   var range   = even ? longitudeRange : latitudeRange
   let mid     = (range.lowerBound + range.upperBound) / 2

   if (val > mid) {
     hashVal = (hashVal << 1) + 1
     range = mid...range.upperBound

     if even { longitudeRange = mid...longitudeRange.upperBound }
     else    { latitudeRange	 = mid...latitudeRange.upperBound }
   } else {
     hashVal = (hashVal << 1) + 0
     if even	{ longitudeRange = longitudeRange.lowerBound...mid }
     else    { latitudeRange	 = latitudeRange.lowerBound...mid }
   }

   even = !even

   if (bits < 4) {
     bits += 1
   } else {
     bits = 0
     hash += gBase32[hashVal]
     hashVal = 0
   }
 }
 return hash

}