Grew tired of creating new types to sort...

sort.go

package util

import (
  "sort"
)

type sorter struct {
  n    int
  less func(i, j int) bool
  swap func(i, j int)
}

func (s *sorter) Less(i, j int) bool {
  return s.less(i, j)
}

func (s *sorter) Swap(i, j int) {
  s.swap(i, j)
}

func (s *sorter) Len() int {
  return s.n
}

func Sort(n int, less func(i, j int) bool, swap func(i, j int)) {
  sort.Sort(&sorter{
    n:    n,
    less: less,
    swap: swap,
  })
}

func SortIndex(n int, less func(i, j int) bool) []int {
  idx := make([]int, n)
  for i := 0; i < n; i++ {
    idx[i] = i
  }

  sort.Sort(&sorter{
    n: n,
    less: func(i, j int) bool {
      return less(idx[i], idx[j])
    },
    swap: func(i, j int) {
      idx[i], idx[j] = idx[j], idx[i]
    },
  })

  return idx
}