mastodon-ios/NotificationService/Decode85.swift

import Foundation

private let DecodeTable: [UInt32] = [
    0xff, 0x44, 0xff, 0x54, 0x53, 0x52, 0x48, 0xff,
    0x4b, 0x4c, 0x46, 0x41, 0xff, 0x3f, 0x3e, 0x45,
    0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
    0x08, 0x09, 0x40, 0xff, 0x49, 0x42, 0x4a, 0x47,
    0x51, 0x24, 0x25, 0x26, 0x27, 0x28, 0x29, 0x2a,
    0x2b, 0x2c, 0x2d, 0x2e, 0x2f, 0x30, 0x31, 0x32,
    0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39, 0x3a,
    0x3b, 0x3c, 0x3d, 0x4d, 0xff, 0x4e, 0x43, 0xff,
    0xff, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f, 0x10,
    0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18,
    0x19, 0x1a, 0x1b, 0x1c, 0x1d, 0x1e, 0x1f, 0x20,
    0x21, 0x22, 0x23, 0x4f, 0xff, 0x50, 0xff, 0xff
]

extension String {
    func decode85() -> Data {
        var data = Data()
        var block: UInt32 = 0
        var n = 0
        for c in utf8 {
            if c >= 32, c < 128, DecodeTable[Int(c - 32)] != 0xff {
                let value = DecodeTable[Int(c - 32)]
                block = block * 85 + value
                n += 1
                if n == 5 {
                    data.append(UInt8(block >> 24))
                    data.append(UInt8((block >> 16) & 0xff))
                    data.append(UInt8((block >> 8) & 0xff))
                    data.append(UInt8(block & 0xff))
                    block = 0
                    n = 0
                }
            }
        }

        if n >= 4 { data.append(UInt8((block >> 16) & 0xff)) }
        if n >= 3 { data.append(UInt8((block >> 8) & 0xff)) }
        if n >= 2 { data.append(UInt8(block & 0xff)) }

        return data
    }
}