bitchat/bitchat/Protocols/Geohash.swift

import Foundation

/// Lightweight Geohash encoder used for Location Channels.
/// Encodes latitude/longitude to base32 geohash with a fixed precision.
enum Geohash {
    private static let base32Chars = Array("0123456789bcdefghjkmnpqrstuvwxyz")
    private static let base32Map: [Character: Int] = {
        var map: [Character: Int] = [:]
        for (i, c) in base32Chars.enumerated() { map[c] = i }
        return map
    }()

    /// Encodes the provided coordinates into a geohash string.
    /// - Parameters:
    ///   - latitude: Latitude in degrees (-90...90)
    ///   - longitude: Longitude in degrees (-180...180)
    ///   - precision: Number of geohash characters (2-12 typical). Values <= 0 return an empty string.
    /// - Returns: Base32 geohash string of length `precision`.
    static func encode(latitude: Double, longitude: Double, precision: Int) -> String {
        guard precision > 0 else { return "" }

        var latInterval: (Double, Double) = (-90.0, 90.0)
        var lonInterval: (Double, Double) = (-180.0, 180.0)

        var isEven = true
        var bit = 0
        var ch = 0
        var geohash: [Character] = []

        let lat = max(-90.0, min(90.0, latitude))
        let lon = max(-180.0, min(180.0, longitude))

        while geohash.count < precision {
            if isEven {
                let mid = (lonInterval.0 + lonInterval.1) / 2
                if lon >= mid {
                    ch |= (1 << (4 - bit))
                    lonInterval.0 = mid
                } else {
                    lonInterval.1 = mid
                }
            } else {
                let mid = (latInterval.0 + latInterval.1) / 2
                if lat >= mid {
                    ch |= (1 << (4 - bit))
                    latInterval.0 = mid
                } else {
                    latInterval.1 = mid
                }
            }

            isEven.toggle()
            if bit < 4 {
                bit += 1
            } else {
                geohash.append(base32Chars[ch])
                bit = 0
                ch = 0
            }
        }

        return String(geohash)
    }

    /// Decodes a geohash into the center latitude/longitude of its bounding box.
    /// - Parameter geohash: Base32 geohash string.
    /// - Returns: (lat, lon) center coordinate.
    static func decodeCenter(_ geohash: String) -> (lat: Double, lon: Double) {
        var latInterval: (Double, Double) = (-90.0, 90.0)
        var lonInterval: (Double, Double) = (-180.0, 180.0)

        var isEven = true
        for ch in geohash.lowercased() {
            guard let cd = base32Map[ch] else { continue }
            for mask in [16, 8, 4, 2, 1] {
                if isEven {
                    let mid = (lonInterval.0 + lonInterval.1) / 2
                    if (cd & mask) != 0 { lonInterval.0 = mid } else { lonInterval.1 = mid }
                } else {
                    let mid = (latInterval.0 + latInterval.1) / 2
                    if (cd & mask) != 0 { latInterval.0 = mid } else { latInterval.1 = mid }
                }
                isEven.toggle()
            }
        }
        let lat = (latInterval.0 + latInterval.1) / 2
        let lon = (lonInterval.0 + lonInterval.1) / 2
        return (lat, lon)
    }
}