make AppendJust() test check for slice contents, fix AppendJust() final copying behaviour

internal/util/xslices/slices.go

@@ -28,11 +28,14 @@
 // which simply guarantees extra through append() which
 // may allocate more than necessary extra size.
 func GrowJust[T any](in []T, size int) []T {
+
 	if cap(in)-len(in) < size {
+		// Reallocate enough for in + size.
 		in2 := make([]T, len(in), len(in)+size)
 		_ = copy(in2, in)
 		in = in2
 	}
+
 	return in
 }
 
@@ -42,14 +45,20 @@ func GrowJust[T any](in []T, size int) []T {
 // will append extra, in a manner to reduce the need
 // for new allocations on every call to append.
 func AppendJust[T any](in []T, extra ...T) []T {
-	if cap(in)-len(in) < len(extra) {
+	l := len(in)
-		in2 := make([]T, len(in)+len(extra))
+
+	if cap(in)-l < len(extra) {
+		// Reallocate enough for + extra.
+		in2 := make([]T, l+len(extra))
 		_ = copy(in2, in)
 		in = in2
 	} else {
-		in = in[:len(in)+len(extra)]
+		// Reslice for + extra.
+		in = in[:l+len(extra)]
 	}
-	_ = copy(in[len(in):], extra)
+
+	// Copy extra into slice.
+	_ = copy(in[l:], extra)
 	return in
 }
 

internal/util/xslices/slices_test.go

@@ -18,6 +18,7 @@
 package xslices_test
 
 import (
+	"math/rand"
 	"net/url"
 	"slices"
 	"testing"
@@ -57,12 +58,27 @@ func TestAppendJust(t *testing.T) {
 	for _, l := range []int{0, 2, 4, 8, 16, 32, 64} {
 		for _, x := range []int{0, 2, 4, 8, 16, 32, 64} {
 			s := make([]int, l, l+x)
+
+			// Randomize slice.
+			for i := range s {
+				s[i] = rand.Int()
+			}
+
 			for _, a := range []int{0, 2, 4, 8, 16, 32, 64} {
 				toAppend := make([]int, a)
+
+				// Randomize appended vals.
+				for i := range toAppend {
+					toAppend[i] = rand.Int()
+				}
+
 				s2 := xslices.AppendJust(s, toAppend...)
 
 				// Slice length should be as expected.
-				assert.Equal(t, len(s2), len(s)+a)
+				assert.Equal(t, len(s)+a, len(s2))
+
+				// Slice contents should be as expected.
+				assert.Equal(t, append(s, toAppend...), s2)
 
 				switch {
 				// If slice already has capacity for
@@ -74,7 +90,7 @@ func TestAppendJust(t *testing.T) {
 				// have capacity for original length
 				// plus extra elements, NOTHING MORE.
 				default:
-					assert.Equal(t, cap(s2), len(s)+a)
+					assert.Equal(t, len(s)+a, cap(s2))
 				}
 			}
 		}