2 files changed, 119 insertions, 10 deletions
diff --git a/internal/util/slices.go b/internal/util/xslices/slices.go
index 955fe8830..1c1c159b2 100644
--- a/internal/util/slices.go
+++ b/internal/util/xslices/slices.go
@@ -15,12 +15,53 @@
 // You should have received a copy of the GNU Affero General Public License
 // along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-package util
+package xslices
 
 import (
 	"slices"
 )
 
+// GrowJust increases slice capacity to guarantee
+// extra room 'size', where in the case that it does
+// need to allocate more it ONLY allocates 'size' extra.
+// This is different to typical slices.Grow behaviour,
+// 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
+}
+
+// AppendJust appends extra elements to slice,
+// ONLY allocating at most len(extra) elements. This
+// is different to the typical append behaviour which
+// 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 {
+	l := len(in)
+
+	if cap(in)-l < len(extra) {
+		// Reallocate enough for + extra.
+		in2 := make([]T, l+len(extra))
+		_ = copy(in2, in)
+		in = in2
+	} else {
+		// Reslice for + extra.
+		in = in[:l+len(extra)]
+	}
+
+	// Copy extra into slice.
+	_ = copy(in[l:], extra)
+	return in
+}
+
 // Deduplicate deduplicates entries in the given slice.
 func Deduplicate[T comparable](in []T) []T {
 	var (
diff --git a/internal/util/slices_test.go b/internal/util/xslices/slices_test.go
index c93e489f5..7c62ac77f 100644
--- a/internal/util/slices_test.go
+++ b/internal/util/xslices/slices_test.go
@@ -15,22 +15,90 @@
 // You should have received a copy of the GNU Affero General Public License
 // along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-package util_test
+package xslices_test
 
 import (
+	"math/rand"
 	"net/url"
 	"slices"
 	"testing"
 
-	"github.com/superseriousbusiness/gotosocial/internal/util"
+	"github.com/stretchr/testify/assert"
+	"github.com/superseriousbusiness/gotosocial/internal/util/xslices"
 )
 
-var (
-	testURLSlice = []*url.URL{}
-)
+func TestGrowJust(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)
+			for _, g := range []int{0, 2, 4, 8, 16, 32, 64} {
+				s2 := xslices.GrowJust(s, g)
+
+				// Slice length should not be different.
+				assert.Equal(t, len(s), len(s2))
+
+				switch {
+				// If slice already has capacity for
+				// 'g' then it should not be changed.
+				case cap(s) >= len(s)+g:
+					assert.Equal(t, cap(s), cap(s2))
+
+				// Else, returned slice should only
+				// have capacity for original length
+				// plus extra elements, NOTHING MORE.
+				default:
+					assert.Equal(t, cap(s2), len(s)+g)
+				}
+			}
+		}
+	}
+}
+
+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(s)+a, len(s2))
+
+				// Slice contents should be as expected.
+				assert.Equal(t, append(s, toAppend...), s2)
+
+				switch {
+				// If slice already has capacity for
+				// 'toAppend' then it should not change.
+				case cap(s) >= len(s)+a:
+					assert.Equal(t, cap(s), cap(s2))
+
+				// Else, returned slice should only
+				// have capacity for original length
+				// plus extra elements, NOTHING MORE.
+				default:
+					assert.Equal(t, len(s)+a, cap(s2))
+				}
+			}
+		}
+	}
+}
 
 func TestGather(t *testing.T) {
-	out := util.Gather(nil, []*url.URL{
+	out := xslices.Gather(nil, []*url.URL{
 		{Scheme: "https", Host: "google.com", Path: "/some-search"},
 		{Scheme: "http", Host: "example.com", Path: "/robots.txt"},
 	}, (*url.URL).String)
@@ -41,7 +109,7 @@ func TestGather(t *testing.T) {
 		t.Fatal("unexpected gather output")
 	}
 
-	out = util.Gather([]string{
+	out = xslices.Gather([]string{
 		"starting input string",
 		"another starting input",
 	}, []*url.URL{
@@ -59,7 +127,7 @@ func TestGather(t *testing.T) {
 }
 
 func TestGatherIf(t *testing.T) {
-	out := util.GatherIf(nil, []string{
+	out := xslices.GatherIf(nil, []string{
 		"hello world",
 		"not hello world",
 		"hello world",
@@ -73,7 +141,7 @@ func TestGatherIf(t *testing.T) {
 		t.Fatal("unexpected gatherif output")
 	}
 
-	out = util.GatherIf([]string{
+	out = xslices.GatherIf([]string{
 		"starting input string",
 		"another starting input",
 	}, []string{