af146490bb
It is not needed due to the removal of the ctx field.
Reviewed-on: https://go-review.googlesource.com/16525
From-SVN: r229616
515 lines
14 KiB
Go
515 lines
14 KiB
Go
// Copyright 2009 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Package sort provides primitives for sorting slices and user-defined
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// collections.
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package sort
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// A type, typically a collection, that satisfies sort.Interface can be
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// sorted by the routines in this package. The methods require that the
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// elements of the collection be enumerated by an integer index.
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type Interface interface {
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// Len is the number of elements in the collection.
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Len() int
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// Less reports whether the element with
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// index i should sort before the element with index j.
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Less(i, j int) bool
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// Swap swaps the elements with indexes i and j.
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Swap(i, j int)
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}
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func min(a, b int) int {
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if a < b {
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return a
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}
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return b
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}
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// Insertion sort
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func insertionSort(data Interface, a, b int) {
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for i := a + 1; i < b; i++ {
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for j := i; j > a && data.Less(j, j-1); j-- {
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data.Swap(j, j-1)
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}
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}
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}
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// siftDown implements the heap property on data[lo, hi).
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// first is an offset into the array where the root of the heap lies.
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func siftDown(data Interface, lo, hi, first int) {
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root := lo
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for {
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child := 2*root + 1
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if child >= hi {
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break
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}
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if child+1 < hi && data.Less(first+child, first+child+1) {
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child++
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}
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if !data.Less(first+root, first+child) {
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return
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}
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data.Swap(first+root, first+child)
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root = child
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}
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}
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func heapSort(data Interface, a, b int) {
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first := a
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lo := 0
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hi := b - a
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// Build heap with greatest element at top.
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for i := (hi - 1) / 2; i >= 0; i-- {
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siftDown(data, i, hi, first)
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}
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// Pop elements, largest first, into end of data.
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for i := hi - 1; i >= 0; i-- {
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data.Swap(first, first+i)
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siftDown(data, lo, i, first)
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}
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}
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// Quicksort, following Bentley and McIlroy,
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// ``Engineering a Sort Function,'' SP&E November 1993.
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// medianOfThree moves the median of the three values data[m0], data[m1], data[m2] into data[m1].
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func medianOfThree(data Interface, m1, m0, m2 int) {
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// sort 3 elements
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if data.Less(m1, m0) {
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data.Swap(m1, m0)
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}
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// data[m0] <= data[m1]
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if data.Less(m2, m1) {
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data.Swap(m2, m1)
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// data[m0] <= data[m2] && data[m1] < data[m2]
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if data.Less(m1, m0) {
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data.Swap(m1, m0)
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}
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}
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// now data[m0] <= data[m1] <= data[m2]
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}
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func swapRange(data Interface, a, b, n int) {
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for i := 0; i < n; i++ {
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data.Swap(a+i, b+i)
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}
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}
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func doPivot(data Interface, lo, hi int) (midlo, midhi int) {
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m := lo + (hi-lo)/2 // Written like this to avoid integer overflow.
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if hi-lo > 40 {
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// Tukey's ``Ninther,'' median of three medians of three.
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s := (hi - lo) / 8
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medianOfThree(data, lo, lo+s, lo+2*s)
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medianOfThree(data, m, m-s, m+s)
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medianOfThree(data, hi-1, hi-1-s, hi-1-2*s)
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}
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medianOfThree(data, lo, m, hi-1)
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// Invariants are:
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// data[lo] = pivot (set up by ChoosePivot)
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// data[lo <= i < a] = pivot
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// data[a <= i < b] < pivot
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// data[b <= i < c] is unexamined
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// data[c <= i < d] > pivot
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// data[d <= i < hi] = pivot
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//
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// Once b meets c, can swap the "= pivot" sections
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// into the middle of the slice.
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pivot := lo
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a, b, c, d := lo+1, lo+1, hi, hi
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for {
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for b < c {
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if data.Less(b, pivot) { // data[b] < pivot
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b++
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} else if !data.Less(pivot, b) { // data[b] = pivot
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data.Swap(a, b)
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a++
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b++
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} else {
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break
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}
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}
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for b < c {
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if data.Less(pivot, c-1) { // data[c-1] > pivot
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c--
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} else if !data.Less(c-1, pivot) { // data[c-1] = pivot
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data.Swap(c-1, d-1)
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c--
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d--
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} else {
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break
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}
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}
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if b >= c {
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break
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}
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// data[b] > pivot; data[c-1] < pivot
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data.Swap(b, c-1)
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b++
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c--
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}
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n := min(b-a, a-lo)
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swapRange(data, lo, b-n, n)
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n = min(hi-d, d-c)
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swapRange(data, c, hi-n, n)
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return lo + b - a, hi - (d - c)
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}
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func quickSort(data Interface, a, b, maxDepth int) {
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for b-a > 7 {
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if maxDepth == 0 {
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heapSort(data, a, b)
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return
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}
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maxDepth--
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mlo, mhi := doPivot(data, a, b)
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// Avoiding recursion on the larger subproblem guarantees
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// a stack depth of at most lg(b-a).
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if mlo-a < b-mhi {
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quickSort(data, a, mlo, maxDepth)
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a = mhi // i.e., quickSort(data, mhi, b)
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} else {
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quickSort(data, mhi, b, maxDepth)
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b = mlo // i.e., quickSort(data, a, mlo)
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}
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}
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if b-a > 1 {
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insertionSort(data, a, b)
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}
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}
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// Sort sorts data.
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// It makes one call to data.Len to determine n, and O(n*log(n)) calls to
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// data.Less and data.Swap. The sort is not guaranteed to be stable.
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func Sort(data Interface) {
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// Switch to heapsort if depth of 2*ceil(lg(n+1)) is reached.
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n := data.Len()
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maxDepth := 0
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for i := n; i > 0; i >>= 1 {
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maxDepth++
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}
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maxDepth *= 2
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quickSort(data, 0, n, maxDepth)
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}
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type reverse struct {
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// This embedded Interface permits Reverse to use the methods of
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// another Interface implementation.
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Interface
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}
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// Less returns the opposite of the embedded implementation's Less method.
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func (r reverse) Less(i, j int) bool {
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return r.Interface.Less(j, i)
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}
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// Reverse returns the reverse order for data.
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func Reverse(data Interface) Interface {
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return &reverse{data}
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}
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// IsSorted reports whether data is sorted.
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func IsSorted(data Interface) bool {
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n := data.Len()
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for i := n - 1; i > 0; i-- {
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if data.Less(i, i-1) {
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return false
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}
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}
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return true
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}
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// Convenience types for common cases
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// IntSlice attaches the methods of Interface to []int, sorting in increasing order.
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type IntSlice []int
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func (p IntSlice) Len() int { return len(p) }
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func (p IntSlice) Less(i, j int) bool { return p[i] < p[j] }
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func (p IntSlice) Swap(i, j int) { p[i], p[j] = p[j], p[i] }
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// Sort is a convenience method.
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func (p IntSlice) Sort() { Sort(p) }
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// Float64Slice attaches the methods of Interface to []float64, sorting in increasing order.
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type Float64Slice []float64
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func (p Float64Slice) Len() int { return len(p) }
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func (p Float64Slice) Less(i, j int) bool { return p[i] < p[j] || isNaN(p[i]) && !isNaN(p[j]) }
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func (p Float64Slice) Swap(i, j int) { p[i], p[j] = p[j], p[i] }
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// isNaN is a copy of math.IsNaN to avoid a dependency on the math package.
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func isNaN(f float64) bool {
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return f != f
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}
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// Sort is a convenience method.
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func (p Float64Slice) Sort() { Sort(p) }
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// StringSlice attaches the methods of Interface to []string, sorting in increasing order.
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type StringSlice []string
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func (p StringSlice) Len() int { return len(p) }
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func (p StringSlice) Less(i, j int) bool { return p[i] < p[j] }
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func (p StringSlice) Swap(i, j int) { p[i], p[j] = p[j], p[i] }
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// Sort is a convenience method.
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func (p StringSlice) Sort() { Sort(p) }
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// Convenience wrappers for common cases
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// Ints sorts a slice of ints in increasing order.
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func Ints(a []int) { Sort(IntSlice(a)) }
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// Float64s sorts a slice of float64s in increasing order.
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func Float64s(a []float64) { Sort(Float64Slice(a)) }
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// Strings sorts a slice of strings in increasing order.
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func Strings(a []string) { Sort(StringSlice(a)) }
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// IntsAreSorted tests whether a slice of ints is sorted in increasing order.
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func IntsAreSorted(a []int) bool { return IsSorted(IntSlice(a)) }
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// Float64sAreSorted tests whether a slice of float64s is sorted in increasing order.
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func Float64sAreSorted(a []float64) bool { return IsSorted(Float64Slice(a)) }
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// StringsAreSorted tests whether a slice of strings is sorted in increasing order.
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func StringsAreSorted(a []string) bool { return IsSorted(StringSlice(a)) }
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// Notes on stable sorting:
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// The used algorithms are simple and provable correct on all input and use
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// only logarithmic additional stack space. They perform well if compared
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// experimentally to other stable in-place sorting algorithms.
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//
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// Remarks on other algorithms evaluated:
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// - GCC's 4.6.3 stable_sort with merge_without_buffer from libstdc++:
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// Not faster.
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// - GCC's __rotate for block rotations: Not faster.
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// - "Practical in-place mergesort" from Jyrki Katajainen, Tomi A. Pasanen
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// and Jukka Teuhola; Nordic Journal of Computing 3,1 (1996), 27-40:
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// The given algorithms are in-place, number of Swap and Assignments
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// grow as n log n but the algorithm is not stable.
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// - "Fast Stable In-Place Sorting with O(n) Data Moves" J.I. Munro and
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// V. Raman in Algorithmica (1996) 16, 115-160:
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// This algorithm either needs additional 2n bits or works only if there
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// are enough different elements available to encode some permutations
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// which have to be undone later (so not stable on any input).
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// - All the optimal in-place sorting/merging algorithms I found are either
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// unstable or rely on enough different elements in each step to encode the
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// performed block rearrangements. See also "In-Place Merging Algorithms",
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// Denham Coates-Evely, Department of Computer Science, Kings College,
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// January 2004 and the reverences in there.
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// - Often "optimal" algorithms are optimal in the number of assignments
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// but Interface has only Swap as operation.
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// Stable sorts data while keeping the original order of equal elements.
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//
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// It makes one call to data.Len to determine n, O(n*log(n)) calls to
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// data.Less and O(n*log(n)*log(n)) calls to data.Swap.
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func Stable(data Interface) {
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n := data.Len()
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blockSize := 20 // must be > 0
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a, b := 0, blockSize
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for b <= n {
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insertionSort(data, a, b)
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a = b
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b += blockSize
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}
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insertionSort(data, a, n)
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for blockSize < n {
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a, b = 0, 2*blockSize
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for b <= n {
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symMerge(data, a, a+blockSize, b)
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a = b
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b += 2 * blockSize
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}
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if m := a + blockSize; m < n {
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symMerge(data, a, m, n)
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}
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blockSize *= 2
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}
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}
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// SymMerge merges the two sorted subsequences data[a:m] and data[m:b] using
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// the SymMerge algorithm from Pok-Son Kim and Arne Kutzner, "Stable Minimum
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// Storage Merging by Symmetric Comparisons", in Susanne Albers and Tomasz
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// Radzik, editors, Algorithms - ESA 2004, volume 3221 of Lecture Notes in
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// Computer Science, pages 714-723. Springer, 2004.
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//
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// Let M = m-a and N = b-n. Wolog M < N.
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// The recursion depth is bound by ceil(log(N+M)).
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// The algorithm needs O(M*log(N/M + 1)) calls to data.Less.
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// The algorithm needs O((M+N)*log(M)) calls to data.Swap.
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//
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// The paper gives O((M+N)*log(M)) as the number of assignments assuming a
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// rotation algorithm which uses O(M+N+gcd(M+N)) assignments. The argumentation
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// in the paper carries through for Swap operations, especially as the block
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// swapping rotate uses only O(M+N) Swaps.
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//
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// symMerge assumes non-degenerate arguments: a < m && m < b.
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// Having the caller check this condition eliminates many leaf recursion calls,
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// which improves performance.
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func symMerge(data Interface, a, m, b int) {
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// Avoid unnecessary recursions of symMerge
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// by direct insertion of data[a] into data[m:b]
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// if data[a:m] only contains one element.
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if m-a == 1 {
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// Use binary search to find the lowest index i
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// such that data[i] >= data[a] for m <= i < b.
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// Exit the search loop with i == b in case no such index exists.
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i := m
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j := b
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for i < j {
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h := i + (j-i)/2
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if data.Less(h, a) {
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i = h + 1
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} else {
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j = h
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}
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}
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// Swap values until data[a] reaches the position before i.
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for k := a; k < i-1; k++ {
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data.Swap(k, k+1)
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}
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return
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}
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// Avoid unnecessary recursions of symMerge
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// by direct insertion of data[m] into data[a:m]
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// if data[m:b] only contains one element.
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if b-m == 1 {
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// Use binary search to find the lowest index i
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// such that data[i] > data[m] for a <= i < m.
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// Exit the search loop with i == m in case no such index exists.
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i := a
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j := m
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for i < j {
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h := i + (j-i)/2
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if !data.Less(m, h) {
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i = h + 1
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} else {
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j = h
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}
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}
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// Swap values until data[m] reaches the position i.
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for k := m; k > i; k-- {
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data.Swap(k, k-1)
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}
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return
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}
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mid := a + (b-a)/2
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n := mid + m
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var start, r int
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if m > mid {
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start = n - b
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r = mid
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} else {
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start = a
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r = m
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}
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p := n - 1
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for start < r {
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c := start + (r-start)/2
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if !data.Less(p-c, c) {
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start = c + 1
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} else {
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r = c
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}
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}
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end := n - start
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if start < m && m < end {
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rotate(data, start, m, end)
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}
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if a < start && start < mid {
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symMerge(data, a, start, mid)
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}
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if mid < end && end < b {
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symMerge(data, mid, end, b)
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}
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}
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// Rotate two consecutives blocks u = data[a:m] and v = data[m:b] in data:
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// Data of the form 'x u v y' is changed to 'x v u y'.
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// Rotate performs at most b-a many calls to data.Swap.
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// Rotate assumes non-degenerate arguments: a < m && m < b.
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func rotate(data Interface, a, m, b int) {
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i := m - a
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j := b - m
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for i != j {
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if i > j {
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swapRange(data, m-i, m, j)
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i -= j
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} else {
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swapRange(data, m-i, m+j-i, i)
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j -= i
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}
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}
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// i == j
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swapRange(data, m-i, m, i)
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}
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/*
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Complexity of Stable Sorting
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Complexity of block swapping rotation
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Each Swap puts one new element into its correct, final position.
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Elements which reach their final position are no longer moved.
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Thus block swapping rotation needs |u|+|v| calls to Swaps.
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This is best possible as each element might need a move.
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Pay attention when comparing to other optimal algorithms which
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typically count the number of assignments instead of swaps:
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E.g. the optimal algorithm of Dudzinski and Dydek for in-place
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rotations uses O(u + v + gcd(u,v)) assignments which is
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better than our O(3 * (u+v)) as gcd(u,v) <= u.
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Stable sorting by SymMerge and BlockSwap rotations
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SymMerg complexity for same size input M = N:
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Calls to Less: O(M*log(N/M+1)) = O(N*log(2)) = O(N)
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Calls to Swap: O((M+N)*log(M)) = O(2*N*log(N)) = O(N*log(N))
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(The following argument does not fuzz over a missing -1 or
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other stuff which does not impact the final result).
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Let n = data.Len(). Assume n = 2^k.
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Plain merge sort performs log(n) = k iterations.
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On iteration i the algorithm merges 2^(k-i) blocks, each of size 2^i.
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Thus iteration i of merge sort performs:
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Calls to Less O(2^(k-i) * 2^i) = O(2^k) = O(2^log(n)) = O(n)
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Calls to Swap O(2^(k-i) * 2^i * log(2^i)) = O(2^k * i) = O(n*i)
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In total k = log(n) iterations are performed; so in total:
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Calls to Less O(log(n) * n)
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Calls to Swap O(n + 2*n + 3*n + ... + (k-1)*n + k*n)
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= O((k/2) * k * n) = O(n * k^2) = O(n * log^2(n))
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Above results should generalize to arbitrary n = 2^k + p
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and should not be influenced by the initial insertion sort phase:
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Insertion sort is O(n^2) on Swap and Less, thus O(bs^2) per block of
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size bs at n/bs blocks: O(bs*n) Swaps and Less during insertion sort.
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Merge sort iterations start at i = log(bs). With t = log(bs) constant:
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Calls to Less O((log(n)-t) * n + bs*n) = O(log(n)*n + (bs-t)*n)
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= O(n * log(n))
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Calls to Swap O(n * log^2(n) - (t^2+t)/2*n) = O(n * log^2(n))
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*/
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