Add AbsoluteListOptions (#17028)

This PR adds a `ListOptions` type which is not paged but uses absolute values. It is implemented as discussed in Discord.
Extracted from #16510 to clean that PR.

models/list_options_test.go

@@ -0,0 +1,62 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package models
+
+import (
+	"testing"
+
+	"code.gitea.io/gitea/modules/setting"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestPaginator(t *testing.T) {
+	cases := []struct {
+		Paginator
+		Skip  int
+		Take  int
+		Start int
+		End   int
+	}{
+		{
+			Paginator: &ListOptions{Page: -1, PageSize: -1},
+			Skip:      0,
+			Take:      setting.API.DefaultPagingNum,
+			Start:     0,
+			End:       setting.API.DefaultPagingNum,
+		},
+		{
+			Paginator: &ListOptions{Page: 2, PageSize: 10},
+			Skip:      10,
+			Take:      10,
+			Start:     10,
+			End:       20,
+		},
+		{
+			Paginator: NewAbsoluteListOptions(-1, -1),
+			Skip:      0,
+			Take:      setting.API.DefaultPagingNum,
+			Start:     0,
+			End:       setting.API.DefaultPagingNum,
+		},
+		{
+			Paginator: NewAbsoluteListOptions(2, 10),
+			Skip:      2,
+			Take:      10,
+			Start:     2,
+			End:       12,
+		},
+	}
+
+	for _, c := range cases {
+		skip, take := c.Paginator.GetSkipTake()
+		start, end := c.Paginator.GetStartEnd()
+
+		assert.Equal(t, c.Skip, skip)
+		assert.Equal(t, c.Take, take)
+		assert.Equal(t, c.Start, start)
+		assert.Equal(t, c.End, end)
+	}
+}