MaxBetween panics on a view and scalar

[aglyzov]

Here is a minimal demonstration code:

package main

import (
	"fmt"
	"os"

	"gorgonia.org/tensor"
)

func main() {
	a := tensor.New(
		tensor.WithShape(3, 2),
		tensor.WithBacking([]float64{1, 2, 3, 4, 5, 6}),
	)

	x, err := a.Slice(nil, tensor.S(0))
	if err != nil {
		fmt.Println(err)
		os.Exit(2)
	}

	var scalar = float64(3.5)

	_, err = tensor.MaxBetween(x, scalar, tensor.UseUnsafe()) // <-- this panics: "unreachable"
	if err != nil {
		fmt.Println(err)
		os.Exit(1)
	}
}

[aglyzov]

I think the following patch fixes the issue. Could you please check if I am not missing anything:

diff --git a/defaultengine_minmax.go b/defaultengine_minmax.go
index 56ac432..a8383da 100644
--- a/defaultengine_minmax.go
+++ b/defaultengine_minmax.go
@@ -169,7 +169,7 @@ func (e StdEng) MinBetweenScalar(t Tensor, s interface{}, leftTensor bool, opts
 	}
 
 	// check to see if anything needs to be created
-	if reuse == nil {
+	if safe && reuse == nil {
 		reuse = NewDense(a.Dtype(), a.Shape().Clone(), WithEngine(e))
 		dataReuse = reuse.hdr()
 		if useIter {
@@ -275,7 +275,7 @@ func (e StdEng) MaxBetweenScalar(t Tensor, s interface{}, leftTensor bool, opts
 	}
 
 	// check to see if anything needs to be created
-	if reuse == nil {
+	if safe && reuse == nil {
 		reuse = NewDense(a.Dtype(), a.Shape().Clone(), WithEngine(e))
 		dataReuse = reuse.hdr()
 		if useIter {

[aglyzov]

Explanation of the fix:
The code in {Min,Max}BetweenScalar assumes it should always have a reuse (*Dense) populated:

	// check to see if anything needs to be created
	if reuse == nil {
		reuse = NewDense(a.Dtype(), a.Shape().Clone(), WithEngine(e))
		dataReuse = reuse.hdr()
		if useIter {
			iit = IteratorFromDense(reuse)
		}
	}

Which is not true for the case when there is an unsafe option. If the unsafe option is given then we should output into the first Tensor.

Otherwise the following switch fails to pick the first case:

		switch {
		case !safe && reuse == nil:
			err = e.E.MaxBetweenIter(typ, dataA, dataB, ait, bit)
			retVal = a
		case safe && reuse != nil && !leftTensor:
			storage.CopyIter(typ, dataReuse, dataB, iit, bit)
			bit.Reset()
			iit.Reset()
			err = e.E.MaxBetweenIter(typ, dataA, dataReuse, ait, bit)
			retVal = reuse
		case safe && reuse != nil && leftTensor:
			storage.CopyIter(typ, dataReuse, dataA, iit, ait)
			ait.Reset()
			iit.Reset()
			err = e.E.MaxBetweenIter(typ, dataReuse, dataB, iit, bit)
			retVal = reuse
		default: // safe && bool
			panic("Unreachable")
		}