Implemented CSS Tokenizer in Swift. Original Rust code is https://www.youtube.com/watch?v=8Y0-lrFf3Qk

CSSTokenizer.swift

import Foundation

public struct CSSTokenizer {
    var position: String.Index
    let input: String

    public init(input: String) {
        self.position = input.startIndex
        self.input = input
    }
}

private extension CSSTokenizer {
    mutating func consumeName() -> Token {

        func check(_ char: Character) -> Bool {
            switch char {
            case "a"..."z", "A"..."Z", "0"..."9", "-", "_":
                return true
            default:
                return false
            }
        }

        let start: String.Index = position
        var end: String.Index = position

        repeat {
            let cursor = input.index(after: position)
            let c = input[cursor]
            if check(c) {
                position = cursor
                end = position
            } else {
                break
            }
        } while position < input.endIndex

        let sub = input[start...end]
        return .ident(String(sub))
    }
}

extension CSSTokenizer: IteratorProtocol {
    public typealias Element = Token

    mutating public func next() -> Self.Element? {
        guard position < input.endIndex else {
            return nil
        }

        defer {
            position = input.index(after: position)
        }

        let c = input[position]
        switch c {
        case ":":
            return .colon
        case ";":
            return .semiColon
        case "{":
            return .openCurly
        case "}":
            return .closeCurly
        case "a"..."z", "A"..."Z":
            return consumeName()
        default:
            fatalError("Something went wrong. Maybe invalid character.")
        }
    }
}

public enum Token {
    case colon
    case semiColon
    case openCurly
    case closeCurly
    case ident(String)
    case eof
}

extension Token: Equatable {

}


let input = "h1{background-color:red;}"
var tokenizer = CSSTokenizer(input: input)

print("--------------------")
print("style: \(input)")
print("--------------------")

var tokens: [Token] = []
while let token = tokenizer.next() {
    print("token \(token)")
    tokens.append(token)
}