Decode metar weather string

metarweatherdecoder.swift

import Foundation

extension String {
    func index(from: Int) -> Index {
        return self.index(startIndex, offsetBy: from)
    }
    
    func substring(from: Int) -> String {
        let fromIndex = index(from: from)
        return String(self[fromIndex...])
    }
    
    func substring(to: Int) -> String {
        let toIndex = index(from: to)
        return String(self[..<toIndex])
    }
    
    func substring(with r: Range<Int>) -> String {
        let startIndex = index(from: r.lowerBound)
        let endIndex = index(from: r.upperBound)
        return String(self[startIndex..<endIndex])
    }
}

// An example of a full length metar
let fullMetar: String = "FM2000 30015G25KT 1SM SHRA OVC15CB PROB40 2022 1/2SM TSRA +FZFG OVC008CB -RA RERA"

// The pattern extracts only the weather part, ie. winds, clouds, etc are discarded
let pattern = #"[+-]?\S{0,2}(?:RA|SN|SG|IC|PL|GR|GS|UP|FG|FU|VA|DU|SA|HZ|PY|SQ|FC|SS|SS)"#
let regex = try NSRegularExpression(pattern: pattern, options: [])

let regularExpression = try! NSRegularExpression(pattern: pattern, options: [])
let matchesArray = regularExpression.matches(in: fullMetar, options: [], range: NSRange(location: 0, length: fullMetar.utf16.count))

for match in matchesArray {
    for i in 0..<match.numberOfRanges {
        let range = match.range(at: i)
        if range.location == NSNotFound {continue} 
        print(readWeather((fullMetar as NSString).substring(with: range)))
    }
}

// The function parses the weather string which can be either 2,3,4 or five chars
// It composes a strict just adding each decoded part
func readWeather (_ codedMetar:String) -> String {
    var decoded: String = ""
    switch codedMetar.count {
    case 5: decoded += decodeWeather(codedMetar.substring(with: 0..<1)) + " "
        decoded += decodeWeather(codedMetar.substring(with: 1..<3)) + " "
        decoded += decodeWeather(codedMetar.substring(with: 3..<5))
    case 4: decoded += decodeWeather(codedMetar.substring(with: 0..<2)) + " "
        decoded += decodeWeather(codedMetar.substring(with: 2..<4))
    case 3:        
        decoded += decodeWeather(codedMetar.substring(with: 0..<1)) + " "
        decoded += decodeWeather(codedMetar.substring(with: 1..<3))
    case 2:        
        decoded += decodeWeather(codedMetar.substring(with: 0..<2))
    default: decoded = ""
}
    return decoded
}

// The decoder of weather codes
func decodeWeather (_ code:String) -> String {
    switch code {
    case "VC": return "In the vicinity"
    case "MI": return "Shallow"
    case "PR": return "Partial"
    case "BC": return "Patches"
    case "DR": return "Low Drifting"
    case "BL": return "Blowing"
    case "SH": return "Shower"
    case "TS": return "Thunderstorm"
    case "FZ": return "Freezing"
    case "DZ": return "Drizzle"
    case "RA": return "Rain"
    case "SN": return "Snow"
    case "SG": return "Snow Grains"
    case "IC": return "Ice Crystals"
    case "PL": return "Ice Pellets"
    case "GR": return "Hail"
    case "GS": return "Small Hail"
    case "UP": return "Unknown Precipitation"
    case "BR": return "Mist"
    case "FG": return "Fog"
    case "FU": return "Smoke"
    case "VA": return "Volcanic Ash"
    case "DU": return "Widespread Dust"
    case "SA": return "Sand"
    case "HZ": return "Haze"
    case "PY": return "Spray"
    case "PO": return "Well- Developed Dust/Sand Whirls"
    case "SQ": return "Squalls"
    case "FC": return "Funnel Cloud Tornado Waterspout"
    case "SS": return "Sandstorm"
    case "SS": return "Duststorm"
    case "RE": return "Recent"
    case "+": return "Heavy"
    case "-": return "Light"
    default: return ""
    }
}