Cleaning malformed UTF-8 data - ios

Question

Background
With Swift, I'm trying to fetch HTML via URLSession rather than by loading it into a WKWebView first as I only need the HTML and none of the subresources. I'm running into a problem with certain pages that work when loaded into WKWebView but when loaded via URLSession (or even a simple NSString(contentsOf: url, encoding String.Encoding.utf8.rawValue)) the UTF-8 conversion fails.
How to reproduce
This fails (prints "nil"):
print(try? NSString(contentsOf: URL(string: "http://www.huffingtonpost.jp/techcrunch-japan/amazon-is-gobbling-whole-foods-for-a-reported-13-7-billion_b_17171132.html?utm_hp_ref=japan&ir=Japan")!, encoding: String.Encoding.utf8.rawValue))
But changing the URL to the site's homepage, it succeeds:
print(try? NSString(contentsOf: URL(string: "http://www.huffingtonpost.jp")!, encoding: String.Encoding.utf8.rawValue))
Question
How can I "clean" the data returned by a URL that contains malformed UTF-8? I'd like to either remove or replace any invalid sequences in the malformed UTF-8 so that the rest of it can be viewed. WKWebView is able to render the page just fine (and claims it's UTF-8 content as well), as you can see by visiting the URL: http://www.huffingtonpost.jp/techcrunch-japan/amazon-is-gobbling-whole-foods-for-a-reported-13-7-billion_b_17171132.html?utm_hp_ref=japan&ir=Japan

Answer 1

Here is an approach to create a String from (possibly) malformed
UTF-8 data:
Read the website contents into a Data object.
Append a 0 byte to make it a "C string"
Use String(cString:) for the conversion. This initializer replaces ill-formed UTF-8 code unit sequences with the Unicode replacement character ("\u{FFFD}").
Optionally: Remove all occurrences of the replacement character.
Example for the "cleaning" process:
var data = Data(bytes: [65, 66, 200, 67]) // malformed UTF-8
data.append(0)
let s = data.withUnsafeBytes { (p: UnsafePointer<CChar>) in String(cString: p) }
let clean = s.replacingOccurrences(of: "\u{FFFD}", with: "")
print(clean) // ABC
Swift 5:
var data = Data([65, 66, 200, 67]) // malformed UTF-8
data.append(0)
let s = data.withUnsafeBytes { p in
String(cString: p.bindMemory(to: CChar.self).baseAddress!)
}
let clean = s.replacingOccurrences(of: "\u{FFFD}", with: "")
print(clean) // ABC
Of course this can be defined as a custom init method:
extension String {
init(malformedUTF8 data: Data) {
var data = data
data.append(0)
self = data.withUnsafeBytes { (p: UnsafePointer<CChar>) in
String(cString: p).replacingOccurrences(of: "\u{FFFD}", with: "")
}
}
}
Swift 5:
extension String {
init(malformedUTF8 data: Data) {
var data = data
data.append(0)
self = data.withUnsafeBytes{ p in
String(cString: p.bindMemory(to: CChar.self).baseAddress!)
}.replacingOccurrences(of: "\u{FFFD}", with: "")
}
}
Usage:
let data = Data(bytes: [65, 66, 200, 67])
let s = String(malformedUTF8: data)
print(s) // ABC
The cleaning can be done more "directly" using transcode with
extension String {
init(malformedUTF8 data: Data) {
var utf16units = [UInt16]()
utf16units.reserveCapacity(data.count) // A rough estimate
_ = transcode(data.makeIterator(), from: UTF8.self, to: UTF16.self,
stoppingOnError: false) { code in
if code != 0xFFFD {
utf16units.append(code)
}
}
self = String(utf16CodeUnits: utf16units, count: utf16units.count)
}
}
This is essentially what String(cString:)
does, compare
CString.swift and
StringCreate.swift.
Yet another option is to use the UTF8 codecs decode() method
and ignore errors:
extension String {
init(malformedUTF8 data: Data) {
var str = ""
var iterator = data.makeIterator()
var utf8codec = UTF8()
var done = false
while !done {
switch utf8codec.decode(&iterator) {
case .emptyInput:
done = true
case let .scalarValue(val):
str.unicodeScalars.append(val)
case .error:
break // ignore errors
}
}
self = str
}
}