Stable Swift merge sort

mergesort.swift

extension Array where Element: Comparable
{
    mutating func mergesortInPlace() {
        
        var tmp: [Generator.Element] = []
        tmp.reserveCapacity(numericCast(self.count))
        
        func merge(lo: Int, _ mi: Int, _ hi: Int) {
            tmp.removeAll(keepCapacity: true)
            tmp.extend(self[lo..<hi])
            var i = 0, var j = mi - lo
            let imax = j, jmax = tmp.count
            for k in lo..<hi {
                     if i == imax       { self[k] = tmp[j++] }
                else if j == jmax       { self[k] = tmp[i++] }
                else if tmp[j] < tmp[i] { self[k] = tmp[j++] }
                else                    { self[k] = tmp[i++] }
            }
        }
        
        let N = self.count
        for var sz = 1 ; sz < N ; sz += sz {
            for var lo = 0; lo < N-sz ; lo += sz + sz {
                merge(lo, (lo+sz), min(lo+sz+sz,N))
            }
        }
    }
}