Swift implementation of my interview code

main.swift

func render(text: String, entities: Set<Entity>) -> String {
    let entityArray = Array(entities).sorted()
    var sb = String()
    var pos = 0
    var posIndex = text.startIndex
    for entity in entityArray {
        let startIndex = text.index(posIndex, offsetBy: entity.start - pos)
        sb.append(contentsOf: text[posIndex ..< startIndex])
        sb += entity.html
        posIndex = text.index(startIndex, offsetBy: entity.end - entity.start)
        pos = entity.end
    }
    sb.append(contentsOf: text[posIndex...])
    return sb
}