Comparing performance between Java and Swift

render.cpp

string render(string text, set<Entity> *entitySet) {
    string result = "";
    auto entityList = list<Entity>(entitySet->begin(), entitySet->end());
    entityList.sort();
    int pos = 0;
    for (list<Entity>::const_iterator entity = entityList.begin(), end = entityList.end(); entity != end; ++entity) {
        result.append(text.substr(pos, entity->start - pos));
        result.append(entity->html);
        pos = entity->end;
    }
    result.append(text.substr(pos));
    return result;
}

Render.java

  public CharSequence render(String text, Set<Entity> entities) {
    StringBuilder sb = new StringBuilder(256);
    Entity[] array = entities.toArray(new Entity[entities.size()]);
    Arrays.sort(array);
    int pos = 0;
    int codePointPosition = 0;
    for (Entity entity : array) {
      int start = text.offsetByCodePoints(pos, entity.start - codePointPosition);
      sb.append(text, pos, start);
      sb.append(entity.html);
      codePointPosition = entity.end;
      pos = text.offsetByCodePoints(start, entity.end - entity.start);
    }
    sb.append(text, pos, text.length());
    return sb;
  }

Render.swift

func render(text: String, entities: Set<Entity>) -> String {
    let entityArray: Array<Entity> = Array(entities).sorted(by: {(e1: Entity, e2: Entity) -> Bool in return e1.start - e2.start < 0})
    var sb = String()
    var pos = 0
    var posIndex = text.startIndex
    for entity in entityArray {
        let startIndex = text.index(posIndex, offsetBy: entity.start - pos)
        let endIndex = text.index(startIndex, offsetBy: entity.end - entity.start)
        sb.append(text.substring(with: posIndex ..< startIndex))
        sb.append(entity.html)
        posIndex = endIndex
        pos = entity.end
    }
    sb.append(text.substring(from: posIndex))
    return sb
}

Render2.swift

func render2(text: String, entities: Set<Entity>) -> String {
    var sb: Array<Character> = Array()
    sb.reserveCapacity(256)
    let chars = ArraySlice(text.characters)
    let entityArray: Array<Entity> = Array(entities).sorted(by: {(e1: Entity, e2: Entity) -> Bool in return e1.start - e2.start < 0})
    var pos = 0
    for entity in entityArray {
        sb.append(contentsOf: chars[pos..<entity.start])
        sb.append(contentsOf: entity.html.characters)
        pos = entity.end
    }
    sb.append(contentsOf: chars.suffix(from: pos))
    return String(sb)
}