Add container.FilterSlice function (#30339)

Many places have the following logic:
```go
func (jobs ActionJobList) GetRunIDs() []int64 {
	ids := make(container.Set[int64], len(jobs))
	for _, j := range jobs {
		if j.RunID == 0 {
			continue
		}
		ids.Add(j.RunID)
	}
	return ids.Values()
}
```

this introduces a `container.FilterMapUnique` function, which reduces
the code above to:
```go
func (jobs ActionJobList) GetRunIDs() []int64 {
	return container.FilterMapUnique(jobs, func(j *ActionRunJob) (int64, bool) {
		return j.RunID, j.RunID != 0
	})
}
```

2 files changed, 49 insertions, 0 deletions

diff --git a/modules/container/filter.go b/modules/container/filter.go
new file mode 100644
index 0000000000..37ec7c3d56
--- /dev/null
+++ b/modules/container/filter.go
@@ -0,0 +1,21 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package container
+
+import "slices"
+
+// FilterSlice ranges over the slice and calls include() for each element.
+// If the second returned value is true, the first returned value will be included in the resulting
+// slice (after deduplication).
+func FilterSlice[E any, T comparable](s []E, include func(E) (T, bool)) []T {
+	filtered := make([]T, 0, len(s)) // slice will be clipped before returning
+	seen := make(map[T]bool, len(s))
+	for i := range s {
+		if v, ok := include(s[i]); ok && !seen[v] {
+			filtered = append(filtered, v)
+			seen[v] = true
+		}
+	}
+	return slices.Clip(filtered)
+}
diff --git a/modules/container/filter_test.go b/modules/container/filter_test.go
new file mode 100644
index 0000000000..ad304e5abb
--- /dev/null
+++ b/modules/container/filter_test.go
@@ -0,0 +1,28 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package container
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestFilterMapUnique(t *testing.T) {
+	result := FilterSlice([]int{
+		0, 1, 2, 3, 4, 5, 6, 7, 8, 9,
+	}, func(i int) (int, bool) {
+		switch i {
+		case 0:
+			return 0, true // included later
+		case 1:
+			return 0, true // duplicate of previous (should be ignored)
+		case 2:
+			return 2, false // not included
+		default:
+			return i, true
+		}
+	})
+	assert.Equal(t, []int{0, 3, 4, 5, 6, 7, 8, 9}, result)
+}