Make Pages.Prev/Next work like the other Prev/Next methods

Fixes #4500

hugolib/pages_test.go

@@ -0,0 +1,83 @@
+package hugolib
+
+import (
+	"fmt"
+	"math/rand"
+	"testing"
+
+	"github.com/gohugoio/hugo/resources/page"
+
+	qt "github.com/frankban/quicktest"
+)
+
+func newPagesPrevNextTestSite(t testing.TB, numPages int) *sitesBuilder {
+	pageTemplate := `
+---
+title: "Page %d"
+weight: %d
+---
+
+`
+	b := newTestSitesBuilder(t)
+
+	for i := 1; i <= numPages; i++ {
+		b.WithContent(fmt.Sprintf("page%d.md", i), fmt.Sprintf(pageTemplate, i, rand.Intn(numPages)))
+	}
+
+	return b
+}
+
+func TestPagesPrevNext(t *testing.T) {
+	b := newPagesPrevNextTestSite(t, 100)
+	b.Build(BuildCfg{SkipRender: true})
+
+	pages := b.H.Sites[0].RegularPages()
+
+	b.Assert(pages, qt.HasLen, 100)
+
+	for _, p := range pages {
+		msg := qt.Commentf("w=%d", p.Weight())
+		b.Assert(pages.Next(p), qt.Equals, p.Next(), msg)
+		b.Assert(pages.Prev(p), qt.Equals, p.Prev(), msg)
+	}
+}
+
+func BenchmarkPagesPrevNext(b *testing.B) {
+	type Variant struct {
+		name         string
+		preparePages func(pages page.Pages) page.Pages
+		run          func(p page.Page, pages page.Pages)
+	}
+
+	shufflePages := func(pages page.Pages) page.Pages {
+		rand.Shuffle(len(pages), func(i, j int) { pages[i], pages[j] = pages[j], pages[i] })
+		return pages
+	}
+
+	for _, variant := range []Variant{
+		Variant{".Next", nil, func(p page.Page, pages page.Pages) { p.Next() }},
+		Variant{".Prev", nil, func(p page.Page, pages page.Pages) { p.Prev() }},
+		Variant{"Pages.Next", nil, func(p page.Page, pages page.Pages) { pages.Next(p) }},
+		Variant{"Pages.Prev", nil, func(p page.Page, pages page.Pages) { pages.Prev(p) }},
+		Variant{"Pages.Shuffled.Next", shufflePages, func(p page.Page, pages page.Pages) { pages.Next(p) }},
+		Variant{"Pages.Shuffled.Prev", shufflePages, func(p page.Page, pages page.Pages) { pages.Prev(p) }},
+		Variant{"Pages.ByTitle.Next", func(pages page.Pages) page.Pages { return pages.ByTitle() }, func(p page.Page, pages page.Pages) { pages.Next(p) }},
+	} {
+		for _, numPages := range []int{100, 300, 900, 5000} {
+			b.Run(fmt.Sprintf("%s-pages-%d", variant.name, numPages), func(b *testing.B) {
+				b.StopTimer()
+				builder := newPagesPrevNextTestSite(b, numPages)
+				builder.Build(BuildCfg{SkipRender: true})
+				pages := builder.H.Sites[0].RegularPages()
+				if variant.preparePages != nil {
+					pages = variant.preparePages(pages)
+				}
+				b.StartTimer()
+				for i := 0; i < b.N; i++ {
+					p := pages[rand.Intn(len(pages))]
+					variant.run(p, pages)
+				}
+			})
+		}
+	}
+}