Golang Sort package - Fuzzy sorting error - sorting

Question

I tried to modify standard sorting approach and add certain randomness to sorting Less interface.
when
if (u[i] - u[j]) <= 0
or
if u[i] < u[j]
it works as expected
But
if (u[i] - u[j]) <= rv
condition produces panic after several executions
package main
import (
"crypto/rand"
"fmt"
"math/big"
"sort"
)
type FuzzySorter []float64
func (u FuzzySorter) Len() int {
return len(u)
}
func (u FuzzySorter) Swap(i, j int) {
u[i], u[j] = u[j], u[i]
}
func (u FuzzySorter) Less(i, j int) bool {
pom, _ := rand.Int(rand.Reader, big.NewInt(int64(2)))
rv := float64(pom.Int64())
if (u[i] - u[j]) <= rv {
return true
} else {
return false
}
}
func (u FuzzySorter) Sort() FuzzySorter {
sort.Sort(u)
return u
}
func main() {
unsorted := FuzzySorter{
0,
1,
1,
1,
1,
6,
0,
4,
6,
1,
1,
1,
0,
2,
8,
1,
5,
4,
6,
6,
6,
16,
12,
6,
1,
1,
1,
0,
0,
11,
2,
14,
16,
6,
12,
0,
4,
1,
0,
16,
2,
6,
0,
0,
0,
0,
1,
11,
1,
0,
2,
1,
1,
1,
1,
0,
1,
12,
10,
1,
5,
2,
6,
4,
1,
0,
0,
11,
1,
1,
2,
2,
1,
0,
0,
1,
0,
1,
17,
2,
1,
1,
2,
0,
3,
7,
1,
5,
1,
0,
1,
0,
0,
0,
1,
3,
1,
1,
1,
2,
1,
0,
3,
1,
6,
1,
1,
0,
1,
12,
0,
1,
1,
0,
1,
0,
0,
6,
1,
2,
2,
0,
0,
2,
1,
1,
0,
4,
4,
1,
1,
1,
0,
1,
1,
1,
2,
0,
0,
1,
0,
1,
2,
1,
2,
1,
1,
0,
0,
4,
1,
0,
1,
0,
1,
1,
3,
1,
0,
}
unsorted.Sort()
fmt.Println(unsorted)
}
https://play.golang.org/p/4AxNRN4VD7
panic message
panic: runtime error: index out of range
goroutine 1 [running]:
panic(0x176ba0, 0x1040a010)
/usr/local/go/src/runtime/panic.go:464 +0x700
main.FuzzySorter.Less(0x10456000, 0x9f, 0x9f, 0x19, 0xffffffff, 0x4, 0x1, 0xd)
/tmp/sandbox201242525/main.go:21 +0x140
main.(*FuzzySorter).Less(0x10434140, 0x19, 0xffffffff, 0x5c, 0x1, 0x10434140)
<autogenerated>:3 +0xc0
sort.doPivot(0xfef741b0, 0x10434140, 0x19, 0x9f, 0x7, 0x19)
/usr/local/go/src/sort/sort.go:128 +0x280
sort.quickSort(0xfef741b0, 0x10434140, 0x19, 0x9f, 0xe, 0xfef741b0)
/usr/local/go/src/sort/sort.go:195 +0xa0
sort.Sort(0xfef741b0, 0x10434140)
/usr/local/go/src/sort/sort.go:229 +0x80
main.FuzzySorter.Sort(0x10456000, 0x9f, 0x9f, 0x1, 0x0, 0x0, 0x0, 0x1777a0)
/tmp/sandbox201242525/main.go:29 +0xa0
main.main()
/tmp/sandbox201242525/main.go:195 +0xc0

Answer 1

As far as I can understand Go sort implementation it expect two negative comparisons eg. Less(i, j) and Less(j, i) both return false, which it treats as equality, but not positive. E.g Less(i, j) and Less(j, i) can't both return true. So you can easily achieve desired result logically correct and deterministic way, just
if (u[i] - u[j]) < -1 {
return true
} else {
return false
}
https://play.golang.org/p/VcKI9uzcM9

Answer 2

As of Go 1.8, there is an easier way to sort a slice that does not require you to define new types. You simply create a Less (anonymous) lambda.
a := []int{5, 3, 4, 7, 8, 9}
sort.Slice(a, func(i, j int) bool {
return a[i] < a[j]
})
for _, v := range a {
fmt.Println(v)
}
This will sort in ascending order, if you want the opposite, simply write a[i] < a[j]