Swift port of Apple's one-way hashing function for setting SKMutablePayment's applicationUsername

one-way-hashing-function.swift

// Objective-C version available here:
// https://developer.apple.com/library/archive/documentation/NetworkingInternet/Conceptual/StoreKitGuide/Chapters/RequestPayment.html#//apple_ref/doc/uid/TP40008267-CH4-SW6

// Usage:
//
// 	let userID = "146001"
// 	print(hashedValueForAccountName(userID))
// 
// Output:
//
// 	b0f66adc-83641586-65686681-3fd9dd0b-8ebb6379-6075661b-a45d1aa8-089e1d44

// Custom method to calculate the SHA-256 hash using Common Crypto
func hashedValueForAccountName(_ userAccountName: String) -> String? {
	let HASH_SIZE = Int(32)

	let hashedChars = UnsafeMutablePointer<CUnsignedChar>.allocate(capacity: HASH_SIZE)
	let accountName = UnsafePointer<CChar>(userAccountName.cString(using: .utf8))
	let accountNameLen = userAccountName.count

	// Confirm that the length of the user name is small enough
	// to be recast when calling the hash function.
	if accountNameLen > UInt32.max {
		print("Account name too long to hash: \(userAccountName)")
		return nil
	}
	CC_SHA256(accountName, CC_LONG(accountNameLen), hashedChars)

	// Convert the array of bytes into a string showing its hex representation.
	var userAccountHash = String()
	for i in 0..<HASH_SIZE {
		// Add a dash every four bytes, for readability.
		if (i != 0 && i%4 == 0) {
			userAccountHash += "-"
		}
		userAccountHash += String(format: "%02x", arguments: [hashedChars[i]])
	}

	return userAccountHash
}