sort.go

package fgbase

import (
	"sort"
)

// These are sort.min, sort.medianOfThree, sort.swapRange, and sort.doPivot, borrowed
// under the GO-LICENSE from go 1.4.2.  Required to reuse sort.doPivot.

func min(a, b int) int {
	if a < b {
		return a
	}
	return b
}

// medianOfThree moves the median of the three values data[a], data[b], data[c] into data[a].
func medianOfThree(data sort.Interface, a, b, c int) {
	m0 := b
	m1 := a
	m2 := c
	// bubble sort on 3 elements
	if data.Less(m1, m0) {
		data.Swap(m1, m0)
	}
	if data.Less(m2, m1) {
		data.Swap(m2, m1)
	}
	if data.Less(m1, m0) {
		data.Swap(m1, m0)
	}
	// now data[m0] <= data[m1] <= data[m2]
}

func swapRange(data sort.Interface, a, b, n int) {
	for i := 0; i < n; i++ {
		data.Swap(a+i, b+i)
	}
}

func doPivot(data sort.Interface, lo, hi int) (midlo, midhi int) {
	m := lo + (hi-lo)/2 // Written like this to avoid integer overflow.
	if hi-lo > 40 {
		// Tukey's ``Ninther,'' median of three medians of three.
		s := (hi - lo) / 8
		medianOfThree(data, lo, lo+s, lo+2*s)
		medianOfThree(data, m, m-s, m+s)
		medianOfThree(data, hi-1, hi-1-s, hi-1-2*s)
	}
	medianOfThree(data, lo, m, hi-1)

	// Invariants are:
	//	data[lo] = pivot (set up by ChoosePivot)
	//	data[lo <= i < a] = pivot
	//	data[a <= i < b] < pivot
	//	data[b <= i < c] is unexamined
	//	data[c <= i < d] > pivot
	//	data[d <= i < hi] = pivot
	//
	// Once b meets c, can swap the "= pivot" sections
	// into the middle of the slice.
	pivot := lo
	a, b, c, d := lo+1, lo+1, hi, hi
	for {
		for b < c {
			if data.Less(b, pivot) { // data[b] < pivot
				b++
			} else if !data.Less(pivot, b) { // data[b] = pivot
				data.Swap(a, b)
				a++
				b++
			} else {
				break
			}
		}
		for b < c {
			if data.Less(pivot, c-1) { // data[c-1] > pivot
				c--
			} else if !data.Less(c-1, pivot) { // data[c-1] = pivot
				data.Swap(c-1, d-1)
				c--
				d--
			} else {
				break
			}
		}
		if b >= c {
			break
		}
		// data[b] > pivot; data[c-1] < pivot
		data.Swap(b, c-1)
		b++
		c--
	}

	n := min(b-a, a-lo)
	swapRange(data, lo, b-n, n)

	n = min(hi-d, d-c)
	swapRange(data, c, hi-n, n)

	return lo + b - a, hi - (d - c)
}