Fix #140 (#141)

* Fix #140

+ Fix SortIndex()
+ Add SortIndexStable()

* `any` is not supported in Go1.15

---------

Co-authored-by: Chewxy <chewxy@gmail.com>

api_utils.go

@@ -1,7 +1,6 @@
 package tensor
 
 import (
-	"log"
 	"math"
 	"math/rand"
 	"reflect"
@@ -10,41 +9,49 @@ import (
 	"github.com/chewxy/math32"
 )
 
-// SortIndex is similar to numpy's argsort
-// TODO: tidy this up
+// SortIndex: Similar to numpy's argsort.
+// Returns indices for sorting a slice in increasing order.
+// Input slice remains unchanged.
+// SortIndex may not be stable; for stability, use SortIndexStable.
 func SortIndex(in interface{}) (out []int) {
+	return sortIndex(in, sort.Slice)
+}
+
+// SortIndexStable: Similar to SortIndex, but stable.
+// Returns indices for sorting a slice in increasing order.
+// Input slice remains unchanged.
+func SortIndexStable(in interface{}) (out []int) {
+	return sortIndex(in, sort.SliceStable)
+}
+
+func sortIndex(in interface{}, sortFunc func(x interface{}, less func(i int, j int) bool)) (out []int) {
 	switch list := in.(type) {
 	case []int:
-		orig := make([]int, len(list))
 		out = make([]int, len(list))
-		copy(orig, list)
-		sort.Ints(list)
-		for i, s := range list {
-			for j, o := range orig {
-				if o == s {
-					out[i] = j
-					break
-				}
-			}
+		for i := 0; i < len(list); i++ {
+			out[i] = i
 		}
+		sortFunc(out, func(i, j int) bool {
+			return list[out[i]] < list[out[j]]
+		})
 	case []float64:
-		orig := make([]float64, len(list))
 		out = make([]int, len(list))
-		copy(orig, list)
-		sort.Float64s(list)
-
-		for i, s := range list {
-			for j, o := range orig {
-				if o == s {
-					out[i] = j
-					break
-				}
-			}
+		for i := 0; i < len(list); i++ {
+			out[i] = i
 		}
+		sortFunc(out, func(i, j int) bool {
+			return list[out[i]] < list[out[j]]
+		})
 	case sort.Interface:
-		sort.Sort(list)
-
-		log.Printf("TODO: SortIndex for sort.Interface not yet done.")
+		out = make([]int, list.Len())
+		for i := 0; i < list.Len(); i++ {
+			out[i] = i
+		}
+		sortFunc(out, func(i, j int) bool {
+			return list.Less(out[i], out[j])
+		})
+	default:
+		panic("The slice type is not currently supported.")
 	}
 
 	return

api_utils_test.go

@@ -0,0 +1,62 @@
+package tensor
+
+import (
+	"testing"
+)
+
+type testInt []int
+
+func (m testInt) Less(i, j int) bool { return m[i] < m[j] }
+func (m testInt) Len() int           { return len(m) }
+func (m testInt) Swap(i, j int)      { m[i], m[j] = m[j], m[i] }
+
+func TestSortIndexInts(t *testing.T) {
+	in := []int{9, 8, 7, 6, 5, 4, 10, -1, -2, -4, 11, 13, 15, 100, 99}
+	inCopy := make([]int, len(in))
+	copy(inCopy, in)
+	out := SortIndex(in)
+	for i := 1; i < len(out); i++ {
+		if inCopy[out[i]] < inCopy[out[i-1]] {
+			t.Fatalf("Unexpected output")
+		}
+	}
+	for i := range in {
+		if in[i] != inCopy[i] {
+			t.Fatalf("The input slice should not be changed")
+		}
+	}
+}
+
+func TestSortIndexFloats(t *testing.T) {
+	in := []float64{.9, .8, .7, .6, .5, .4, .10, -.1, -.2, -.4, .11, .13, .15, .100, .99}
+	inCopy := make([]float64, len(in))
+	copy(inCopy, in)
+	out := SortIndex(in)
+	for i := 1; i < len(out); i++ {
+		if inCopy[out[i]] < inCopy[out[i-1]] {
+			t.Fatalf("Unexpected output")
+		}
+	}
+	for i := range in {
+		if in[i] != inCopy[i] {
+			t.Fatalf("The input slice should not be changed")
+		}
+	}
+}
+
+func TestSortIndexSortInterface(t *testing.T) {
+	in := testInt{9, 8, 7, 6, 5, 4, 10, -1, -2, -4, 11, 13, 15, 100, 99}
+	inCopy := make(testInt, len(in))
+	copy(inCopy, in)
+	out := SortIndex(in)
+	for i := 1; i < len(out); i++ {
+		if inCopy[out[i]] < inCopy[out[i-1]] {
+			t.Fatalf("Unexpected output")
+		}
+	}
+	for i := range in {
+		if in[i] != inCopy[i] {
+			t.Fatalf("The input slice should not be changed")
+		}
+	}
+}