Swift x-www-form-urlencoded

URLRequest+FormURLEncoded.swift

// Public domain - https://gist.github.com/nolanw/14b277903a2ba446f75202a6bfd55977

import Foundation

extension URLRequest {
    
    /**
        Configures the URL request for `application/x-www-form-urlencoded` data. The request's `httpBody` is set, and values are set for HTTP header fields `Content-Type` and `Content-Length`.
     
        - Parameter queryItems: The (name, value) pairs to encode and set as the request's body.
     
        - Note: The default `httpMethod` is `GET`, and `GET` requests do not typically have a body. Remember to set the `httpMethod` to e.g. `POST` before sending the request.
     
        - Warning: It is a programmer error if any name or value in `queryItems` contains an unpaired UTF-16 surrogate.
     */
    mutating func setFormURLEncoded(_ queryItems: [URLQueryItem]) {
        setValue("application/x-www-form-urlencoded", forHTTPHeaderField: "Content-Type")
        
        func serialize(_ s: String) -> String {
            return s
                // Force-unwrapping because only known failure case is unpaired surrogate, which we've documented above as an error.
                .addingPercentEncoding(withAllowedCharacters: formURLEncodedAllowedCharacters)!
                .replacingOccurrences(of: " ", with: "+")
        }
        
        // https://url.spec.whatwg.org/#concept-urlencoded-serializer
        let output = queryItems.lazy
            .map { ($0.name, $0.value ?? "") }
            .map { (serialize($0), serialize($1)) }
            .map { "\($0)=\($1)" }
            .joined(separator: "&")
        
        let data = output.data(using: .utf8)
        httpBody = data
        
        if let contentLength = data?.count {
            setValue(String(contentLength), forHTTPHeaderField: "Content-Length")
        }
    }
}

private let formURLEncodedAllowedCharacters: CharacterSet = {
    typealias c = UnicodeScalar
    
    // https://url.spec.whatwg.org/#urlencoded-serializing
    var allowed = CharacterSet()
    allowed.insert(c(0x2A))
    allowed.insert(charactersIn: c(0x2D)...c(0x2E))
    allowed.insert(charactersIn: c(0x30)...c(0x39))
    allowed.insert(charactersIn: c(0x41)...c(0x5A))
    allowed.insert(c(0x5F))
    allowed.insert(charactersIn: c(0x61)...c(0x7A))
    
    // and we'll deal with ` ` later…
    allowed.insert(" ")
    
    return allowed
}()