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// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Package sort provides primitives for sorting slices and user-defined
// collections.
package sort
// A type, typically a collection, that satisfies sort.Interface can be
// sorted by the routines in this package. The methods require that the
// elements of the collection be enumerated by an integer index.
type Interface interface {
// Len is the number of elements in the collection.
Len() int
// Less returns whether the element with index i should sort
// before the element with index j.
Less(i, j int) bool
// Swap swaps the elements with indexes i and j.
Swap(i, j int)
}
func min(a, b int) int {
if a < b {
return a
}
return b
}
// Insertion sort
func insertionSort(data Interface, a, b int) {
for i := a + 1; i < b; i++ {
for j := i; j > a && data.Less(j, j-1); j-- {
data.Swap(j, j-1)
}
}
}
// Quicksort, following Bentley and McIlroy,
// ``Engineering a Sort Function,'' SP&E November 1993.
// Move the median of the three values data[a], data[b], data[c] into data[a].
func medianOfThree(data 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 Interface, a, b, n int) {
for i := 0; i < n; i++ {
data.Swap(a+i, b+i)
}
}
func doPivot(data 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 b < c {
if data.Less(b, pivot) { // data[b] < pivot
b++
continue
}
if !data.Less(pivot, b) { // data[b] = pivot
data.Swap(a, b)
a++
b++
continue
}
if data.Less(pivot, c-1) { // data[c-1] > pivot
c--
continue
}
if !data.Less(c-1, pivot) { // data[c-1] = pivot
data.Swap(c-1, d-1)
c--
d--
continue
}
// 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)
}
func quickSort(data Interface, a, b int) {
for b-a > 7 {
mlo, mhi := doPivot(data, a, b)
// Avoiding recursion on the larger subproblem guarantees
// a stack depth of at most lg(b-a).
if mlo-a < b-mhi {
quickSort(data, a, mlo)
a = mhi // i.e., quickSort(data, mhi, b)
} else {
quickSort(data, mhi, b)
b = mlo // i.e., quickSort(data, a, mlo)
}
}
if b-a > 1 {
insertionSort(data, a, b)
}
}
func Sort(data Interface) { quickSort(data, 0, data.Len()) }
func IsSorted(data Interface) bool {
n := data.Len()
for i := n - 1; i > 0; i-- {
if data.Less(i, i-1) {
return false
}
}
return true
}
// Convenience types for common cases
// IntSlice attaches the methods of Interface to []int, sorting in increasing order.
type IntSlice []int
func (p IntSlice) Len() int { return len(p) }
func (p IntSlice) Less(i, j int) bool { return p[i] < p[j] }
func (p IntSlice) Swap(i, j int) { p[i], p[j] = p[j], p[i] }
// Sort is a convenience method.
func (p IntSlice) Sort() { Sort(p) }
// Float64Slice attaches the methods of Interface to []float64, sorting in increasing order.
type Float64Slice []float64
func (p Float64Slice) Len() int { return len(p) }
func (p Float64Slice) Less(i, j int) bool { return p[i] < p[j] }
func (p Float64Slice) Swap(i, j int) { p[i], p[j] = p[j], p[i] }
// Sort is a convenience method.
func (p Float64Slice) Sort() { Sort(p) }
// StringSlice attaches the methods of Interface to []string, sorting in increasing order.
type StringSlice []string
func (p StringSlice) Len() int { return len(p) }
func (p StringSlice) Less(i, j int) bool { return p[i] < p[j] }
func (p StringSlice) Swap(i, j int) { p[i], p[j] = p[j], p[i] }
// Sort is a convenience method.
func (p StringSlice) Sort() { Sort(p) }
// Convenience wrappers for common cases
// Ints sorts a slice of ints in increasing order.
func Ints(a []int) { Sort(IntSlice(a)) }
// Float64s sorts a slice of float64s in increasing order.
func Float64s(a []float64) { Sort(Float64Slice(a)) }
// Strings sorts a slice of strings in increasing order.
func Strings(a []string) { Sort(StringSlice(a)) }
// IntsAreSorted tests whether a slice of ints is sorted in increasing order.
func IntsAreSorted(a []int) bool { return IsSorted(IntSlice(a)) }
// Float64sAreSorted tests whether a slice of float64s is sorted in increasing order.
func Float64sAreSorted(a []float64) bool { return IsSorted(Float64Slice(a)) }
// StringsAreSorted tests whether a slice of strings is sorted in increasing order.
func StringsAreSorted(a []string) bool { return IsSorted(StringSlice(a)) }
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