remove_where.hylo

extension RangeReplaceableCollection {

  public fun remove<E>(where exclude: [E](Element)->Bool) inout {
    let end = end_position()
    var i = first(where: exclude)
    if i == end { return }
    let include = 
    var j = self[self.position(after: i)...]
      .first(where: x => !exclude(x) )
    var k = j
    // invariant: i <= j <= k
    // - self[..<i] is all included elements
    // - self[i..<j] is all excluded elements
    // - self[j..<k] is uninitialized
    while k != end {
      if exclude(self[k]) {
        unsafe_deinitialize(at: k)
      }
      else {
        if i != j {
          unsafe_move_assign(from: k, to: i)
        }
        else {
          unsafe_move_initialize(from: k, to: i)
          &j = index(after: j)
        }
        &i = index(after: i)
      }
      &k = index(after: k)
    }
    let z = i.copy()
    
    while i != j {
      unsafe_deinitialize(at: i)
      &i = index(after: i)
    }
    unsafe_remove_uninitialized(z...)
  }

}