ArraySliceを上手に使ったバイナリサーチ #CodePiece

CodePiece.swift

import Foundation
extension Array {
  func binarySearch(_ f: (Element) -> ComparisonResult) -> Element? {
    
    self[...].binarySearch(f)
  }
}
extension ArraySlice {
  func binarySearch(_ f: (Element) -> ComparisonResult) -> Element? {
     
     guard count != 0 else { return nil }
     
     let midIdx = startIndex + ((endIndex - 1) - startIndex) / 2
     let mid = self[midIdx]
          
     switch f(mid) {
     case .orderedSame:
        return mid
        
     case .orderedAscending:
        return suffix(from: midIdx + 1).binarySearch(f)
        
     case .orderedDescending:
        return prefix(upTo: midIdx).binarySearch(f)
     }
  }
}