Removing elements at more than one index from an array?

Using Swift
13

This should be sufficient for trapping on duplicated indices, because shiftLeft() will be called with the same index twice in a row iff it was included at least twice in the array of indices to remove.

extension Array {
    mutating func remove(elementsAtIndices indicesToRemove: [Int]) -> [Element] {
        guard !indicesToRemove.isEmpty else {
            return []
        }
        
        // Copy the removed elements in the specified order.
        let removedElements = indicesToRemove.map { self[$0] }
        
        // Sort the indices to remove.
        let indicesToRemove = indicesToRemove.sorted()
        
        // Shift the elements we want to keep to the left.
        var destIndex = indicesToRemove.first!
        var srcIndex = destIndex + 1
        var prevRemovedIndex = destIndex
        func shiftLeft(untilIndex index: Int) {
            precondition(index != prevRemovedIndex)
            prevRemovedIndex = index
            while srcIndex < index {
                self[destIndex] = self[srcIndex]
                destIndex += 1
                srcIndex += 1
            }
            srcIndex += 1
        }
        for removeIndex in indicesToRemove[1...] {
            shiftLeft(untilIndex: removeIndex)
        }
        shiftLeft(untilIndex: self.endIndex)
        
        // Remove the extra elements from the end of the array.
        self.removeLast(indicesToRemove.count)
        
        return removedElements
    }
}

I’ll try to make it more generic.

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