shellsort.swift

// The MIT License (MIT)

// Copyright (c) 2016 Mike Taghavi (mitghi[at]me.com)

// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:

// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.

// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.

public func insertionSort(_ list: inout [Int], start: Int, gap: Int) {
  for i in stride(from: (start + gap), to: list.count, by: gap) {
    let currentValue = list[i]
    var pos = i
    while pos >= gap && list[pos - gap] > currentValue {
      list[pos] = list[pos - gap]
      pos -= gap
    }
    list[pos] = currentValue
  }
}

public func shellSort(_ list: inout [Int]) {
  var sublistCount = list.count / 2
  while sublistCount > 0 {
    for pos in 0..<sublistCount {
      insertionSort(&list, start: pos, gap: sublistCount)
    }
    sublistCount = sublistCount / 2
  }
}