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

Fixes #4500

4 files changed, 113 insertions, 26 deletions

diff --git a/hugolib/pages_test.go b/hugolib/pages_test.go
new file mode 100644
index 000000000..371ffe8f2
--- /dev/null
+++ b/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)
+				}
+			})
+		}
+	}
+}
diff --git a/resources/page/pages_prev_next.go b/resources/page/pages_prev_next.go
index 9293c9874..dd87aa4ce 100644
--- a/resources/page/pages_prev_next.go
+++ b/resources/page/pages_prev_next.go
@@ -13,13 +13,12 @@
 
 package page
 
-// Prev returns the previous page reletive to the given
-func (p Pages) Prev(cur Page) Page {
+// Next returns the next page reletive to the given
+func (p Pages) Next(cur Page) Page {
 	for x, c := range p {
 		if c.Eq(cur) {
 			if x == 0 {
-				// TODO(bep) consider return nil here to get it line with the other Prevs
-				return p[len(p)-1]
+				return nil
 			}
 			return p[x-1]
 		}
@@ -27,15 +26,14 @@ func (p Pages) Prev(cur Page) Page {
 	return nil
 }
 
-// Next returns the next page reletive to the given
-func (p Pages) Next(cur Page) Page {
+// Prev returns the previous page reletive to the given
+func (p Pages) Prev(cur Page) Page {
 	for x, c := range p {
 		if c.Eq(cur) {
 			if x < len(p)-1 {
 				return p[x+1]
 			}
-			// TODO(bep) consider return nil here to get it line with the other Nexts
-			return p[0]
+			return nil
 		}
 	}
 	return nil
diff --git a/resources/page/pages_prev_next_test.go b/resources/page/pages_prev_next_test.go
index a75335f00..689e102c2 100644
--- a/resources/page/pages_prev_next_test.go
+++ b/resources/page/pages_prev_next_test.go
@@ -38,18 +38,20 @@ func TestPrev(t *testing.T) {
 	t.Parallel()
 	c := qt.New(t)
 	pages := preparePageGroupTestPages(t)
-	c.Assert(pages[4], qt.Equals, pages.Prev(pages[0]))
-	c.Assert(pages[0], qt.Equals, pages.Prev(pages[1]))
-	c.Assert(pages[3], qt.Equals, pages.Prev(pages[4]))
+
+	c.Assert(pages.Prev(pages[3]), qt.Equals, pages[4])
+	c.Assert(pages.Prev(pages[1]), qt.Equals, pages[2])
+	c.Assert(pages.Prev(pages[4]), qt.IsNil)
 }
 
 func TestNext(t *testing.T) {
 	t.Parallel()
 	c := qt.New(t)
 	pages := preparePageGroupTestPages(t)
-	c.Assert(pages[1], qt.Equals, pages.Next(pages[0]))
-	c.Assert(pages[2], qt.Equals, pages.Next(pages[1]))
-	c.Assert(pages[0], qt.Equals, pages.Next(pages[4]))
+
+	c.Assert(pages.Next(pages[0]), qt.IsNil)
+	c.Assert(pages.Next(pages[1]), qt.Equals, pages[0])
+	c.Assert(pages.Next(pages[4]), qt.Equals, pages[3])
 }
 
 func prepareWeightedPagesPrevNext(t *testing.T) WeightedPages {
@@ -72,16 +74,20 @@ func TestWeightedPagesPrev(t *testing.T) {
 	t.Parallel()
 	c := qt.New(t)
 	w := prepareWeightedPagesPrevNext(t)
-	c.Assert(w[4].Page, qt.Equals, w.Prev(w[0].Page))
-	c.Assert(w[0].Page, qt.Equals, w.Prev(w[1].Page))
-	c.Assert(w[3].Page, qt.Equals, w.Prev(w[4].Page))
+
+	c.Assert(w.Prev(w[0].Page), qt.Equals, w[1].Page)
+	c.Assert(w.Prev(w[1].Page), qt.Equals, w[2].Page)
+	c.Assert(w.Prev(w[4].Page), qt.IsNil)
+
 }
 
 func TestWeightedPagesNext(t *testing.T) {
 	t.Parallel()
 	c := qt.New(t)
 	w := prepareWeightedPagesPrevNext(t)
-	c.Assert(w[1].Page, qt.Equals, w.Next(w[0].Page))
-	c.Assert(w[2].Page, qt.Equals, w.Next(w[1].Page))
-	c.Assert(w[0].Page, qt.Equals, w.Next(w[4].Page))
+
+	c.Assert(w.Next(w[0].Page), qt.IsNil)
+	c.Assert(w.Next(w[1].Page), qt.Equals, w[0].Page)
+	c.Assert(w.Next(w[4].Page), qt.Equals, w[3].Page)
+
 }
diff --git a/resources/page/weighted.go b/resources/page/weighted.go
index 48ed736ce..b2fc8b67b 100644
--- a/resources/page/weighted.go
+++ b/resources/page/weighted.go
@@ -95,13 +95,13 @@ func (wp WeightedPages) Pages() Pages {
 	return pages
 }
 
-// Prev returns the previous Page relative to the given Page in
+// Next returns the next Page relative to the given Page in
 // this weighted page set.
-func (wp WeightedPages) Prev(cur Page) Page {
+func (wp WeightedPages) Next(cur Page) Page {
 	for x, c := range wp {
 		if c.Page == cur {
 			if x == 0 {
-				return wp[len(wp)-1].Page
+				return nil
 			}
 			return wp[x-1].Page
 		}
@@ -109,15 +109,15 @@ func (wp WeightedPages) Prev(cur Page) Page {
 	return nil
 }
 
-// Next returns the next Page relative to the given Page in
+// Prev returns the previous Page relative to the given Page in
 // this weighted page set.
-func (wp WeightedPages) Next(cur Page) Page {
+func (wp WeightedPages) Prev(cur Page) Page {
 	for x, c := range wp {
 		if c.Page == cur {
 			if x < len(wp)-1 {
 				return wp[x+1].Page
 			}
-			return wp[0].Page
+			return nil
 		}
 	}
 	return nil