algorithms/sorting/merge.go
2021-12-29 23:59:49 +02:00

117 lines
2.3 KiB
Go

package sorting
import (
"sync"
)
func merge[T any](items []T, low, mid, hig int, less func(a, b T) bool, aux []T) {
i, j := low, mid+1
for k := low; k <= hig; k++ {
aux[k] = items[k]
}
for k := low; k <= hig; k++ {
if i > mid {
items[k] = aux[j]
j++
} else if j > hig {
items[k] = aux[i]
i++
} else if less(aux[i], aux[j]) {
items[k] = aux[i]
i++
} else {
items[k] = aux[j]
j++
}
}
}
func doMergeSort[T any](items []T, low, hig int, less func(a, b T) bool, aux []T) {
if hig <= low {
return
}
// Optimization 2: if array length is less than 15
// it's imperically prooven that insertion sort
// for such arrays can speed up merge sort up to 15%
if hig-low <= 15 {
Insertion(items[low:hig+1], less)
return
}
mid := low + (hig-low)/2
doMergeSort(items, low, mid, less, aux)
doMergeSort(items, mid+1, hig, less, aux)
// Optimization 1: if two subarrays already sorted
// we can skip merging them
if less(items[mid], items[mid+1]) {
return
}
merge(items, low, mid, hig, less, aux)
}
func doParallelSort[T any](items []T, low, hig int, less func(a, b T) bool, aux []T) {
if hig <= low {
return
}
if hig-low <= 15 {
Insertion(items[low:hig+1], less)
return
}
mid := low + (hig-low)/2
// Optimization 3: we can run sorting in parallel.
// This is costly optimization because we increase memory
// usage by creating WaitGroup instances
wg := sync.WaitGroup{}
wg.Add(2)
go func() { doParallelSort(items, low, mid, less, aux); wg.Done() }()
go func() { doParallelSort(items, mid+1, hig, less, aux); wg.Done() }()
wg.Wait()
if less(items[mid], items[mid+1]) {
return
}
merge(items, low, mid, hig, less, aux)
}
func Merge[T any](items []T, less func(a, b T) bool) {
len := len(items)
doMergeSort(items, 0, len-1, less, make([]T, len))
}
func ParallelMerge[T any](items []T, less func(a, b T) bool) {
len := len(items)
doParallelSort(items, 0, len-1, less, make([]T, len))
}
func BottomUpMerge[T any](items []T, less func(a, b T) bool) {
len := len(items)
aux := make([]T, len)
min := func(x, y int) int {
if x < y {
return x
}
return y
}
for sz := 1; sz < len; sz = sz + sz {
for low := 0; low < len-sz; low += sz + sz {
merge(items, low, low+sz-1, min(low+sz+sz-1, len-1), less, aux)
}
}
}