Tries really hard to read a string from a file. Brute forces encodings if necessary.

ReadingStrings.swift

// Douglas Hill, December 2019
// Made for https://douglashill.co/reading-app/

import Foundation

/// Tries really hard to read a string from a file.
///
/// Brute forces encodings if necessary. This will only fail if the file can’t be interpreted
/// in any encoding, or if some other error occurs like not being able to read from the file.
///
/// - Parameters:
///   - fileURL: The URL of the file to read from.
///   - suggestedEncoding: The expected encoding of the file or nil if no expectation can be given.
///
/// - Returns: The string ready from the file, and the encoding that was used to interpret the file.
func readStringFromFileAtURL(_ fileURL: URL, suggestedEncoding: String.Encoding?) throws -> (String, String.Encoding) {

    // First try the suggested encoding.
    if let encoding = suggestedEncoding {
        do {
            let contents = try String(contentsOf: fileURL, encoding: encoding)
            return (contents, encoding)
        } catch CocoaError.fileReadInapplicableStringEncoding {
            // The suggested encoding is wrong. Keep trying.
        } catch {
            // Bail on any other error. Might be something like  reading from the file failed.
            throw error
        }
    }

    // Try the inference API.
    // Inference seems to fail for ShiftJIS encoded text. For example from https://www.japan-guide.com/e/e2262.html
    let inferenceError: Error
    do {
        var encoding = String.Encoding.windowsCP1252
        let contents = try String(contentsOf: fileURL, usedEncoding: &encoding)
        return (contents, encoding)
    } catch {
        if case CocoaError.fileReadUnknownStringEncoding = error {
            // We’ll keep trying, but save this error because it will make more sense to return than an error from brute forcing if brute forcing fails.
            inferenceError = error
        } else {
            throw error
        }

    }

    // Brute force with all possible encodings.
    // This array is roughly sorted by likelihood based on https://w3techs.com/technologies/overview/character_encoding
    let encodings: [String.Encoding] = [
        .utf8,
        .isoLatin1,
        .windowsCP1251,
        .windowsCP1252,
        .shiftJIS,
        .japaneseEUC,
        .isoLatin2,
        .windowsCP1250,
        .windowsCP1254,
        // ASCII may seem redundant because it is a subset of UTF-8. However sometimes Foundation
        // is able to read data as ASCII but not as UTF-8, so it is still worth trying.
        // Example: https://signalvnoise.com/archives2/bezos_expeditions_invests_in_37signals.php
        .ascii,
        .nonLossyASCII,
        .windowsCP1253,
        .iso2022JP,
        .symbol,
        .nextstep,
        .macOSRoman,
        .unicode,
        .utf16BigEndian,
        .utf16LittleEndian,
        .utf32,
        .utf32BigEndian,
        .utf32LittleEndian,
    ].filter { $0 != suggestedEncoding }

    for encoding in encodings {
        do {
            let contents = try String(contentsOf: fileURL, encoding: encoding)
            return (contents, encoding)
        } catch CocoaError.fileReadInapplicableStringEncoding {
            // Keep trying.
        } catch {
            throw error
        }
    }

    // The inference error (fileReadUnknownStringEncoding) will make more sense than the one from brute forcing (fileReadInapplicableStringEncoding).
    // The error truly is that the text encoding can’t be determined, even after trying really hard.
    throw inferenceError
}