hugolib: Implement "related content"

This closes #98, even if this commit does not do full content text search.

We may revisit that problem in the future, but that deserves its own issue.

Fixes #98

common/types/types.go

@@ -0,0 +1,44 @@
+// Copyright 2017-present The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package types contains types shared between packages in Hugo.
+package types
+
+import (
+	"fmt"
+
+	"github.com/spf13/cast"
+)
+
+// KeyValues holds an key and a slice of values.
+type KeyValues struct {
+	Key    interface{}
+	Values []interface{}
+}
+
+// KeyString returns the key as a string, an empty string if conversion fails.
+func (k KeyValues) KeyString() string {
+	return cast.ToString(k.Key)
+}
+
+func (k KeyValues) String() string {
+	return fmt.Sprintf("%v: %v", k.Key, k.Values)
+}
+
+func NewKeyValuesStrings(key string, values ...string) KeyValues {
+	iv := make([]interface{}, len(values))
+	for i := 0; i < len(values); i++ {
+		iv[i] = values[i]
+	}
+	return KeyValues{Key: key, Values: iv}
+}

common/types/types_test.go

@@ -0,0 +1,29 @@
+// Copyright 2017-present The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package types
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestKeyValues(t *testing.T) {
+	assert := require.New(t)
+
+	kv := NewKeyValuesStrings("key", "a1", "a2")
+
+	assert.Equal("key", kv.KeyString())
+	assert.Equal([]interface{}{"a1", "a2"}, kv.Values)
+}

hugolib/page.go

@@ -20,6 +20,8 @@ import (
 	"reflect"
 	"unicode"
 
+	"github.com/gohugoio/hugo/related"
+
 	"github.com/bep/gitmap"
 
 	"github.com/gohugoio/hugo/helpers"
@@ -54,6 +56,9 @@ var (
 	// Assert that it implements the Eqer interface.
 	_ compare.Eqer = (*Page)(nil)
 	_ compare.Eqer = (*PageOutput)(nil)
+
+	// Assert that it implements the interface needed for related searches.
+	_ related.Document = (*Page)(nil)
 )
 
 const (
@@ -231,6 +236,28 @@ type Page struct {
 	targetPathDescriptorPrototype *targetPathDescriptor
 }
 
+// SearchKeywords implements the related.Document interface needed for fast page searches.
+func (p *Page) SearchKeywords(cfg related.IndexConfig) ([]related.Keyword, error) {
+
+	v, err := p.Param(cfg.Name)
+	if err != nil {
+		return nil, err
+	}
+
+	return cfg.ToKeywords(v)
+}
+
+// PubDate is when this page was or will be published.
+// NOTE: This is currently used for search only and is not meant to be used
+// directly in templates. We need to consolidate the dates in this struct.
+// TODO(bep) see https://github.com/gohugoio/hugo/issues/3854
+func (p *Page) PubDate() time.Time {
+	if !p.PublishDate.IsZero() {
+		return p.PublishDate
+	}
+	return p.Date
+}
+
 func (p *Page) RSSLink() template.URL {
 	f, found := p.outputFormats.GetByName(output.RSSFormat.Name)
 	if !found {
@@ -329,6 +356,21 @@ func (ps Pages) findPagePosByFilePath(inPath string) int {
 	return -1
 }
 
+func (ps Pages) removeFirstIfFound(p *Page) Pages {
+	ii := -1
+	for i, pp := range ps {
+		if pp == p {
+			ii = i
+			break
+		}
+	}
+
+	if ii != -1 {
+		ps = append(ps[:ii], ps[ii+1:]...)
+	}
+	return ps
+}
+
 func (ps Pages) findFirstPagePosByFilePathPrefix(prefix string) int {
 	if prefix == "" {
 		return -1

hugolib/pageCache.go

@@ -36,7 +36,7 @@ func (c *pageCache) get(key string, p Pages, apply func(p Pages)) (Pages, bool)
 	c.RLock()
 	if cached, ok := c.m[key]; ok {
 		for _, ps := range cached {
-			if probablyEqualPages(p, ps[0]) {
+			if fastEqualPages(p, ps[0]) {
 				c.RUnlock()
 				return ps[1], true
 			}
@@ -51,7 +51,7 @@ func (c *pageCache) get(key string, p Pages, apply func(p Pages)) (Pages, bool)
 	// double-check
 	if cached, ok := c.m[key]; ok {
 		for _, ps := range cached {
-			if probablyEqualPages(p, ps[0]) {
+			if fastEqualPages(p, ps[0]) {
 				return ps[1], true
 			}
 		}
@@ -73,10 +73,10 @@ func (c *pageCache) get(key string, p Pages, apply func(p Pages)) (Pages, bool)
 
 }
 
-// "probably" as in: we do not compare every element for big slices, but that is
-// good enough for our use case.
+// "fast" as in: we do not compare every element for big slices, but that is
+// good enough for our use cases.
 // TODO(bep) there is a similar method in pagination.go. DRY.
-func probablyEqualPages(p1, p2 Pages) bool {
+func fastEqualPages(p1, p2 Pages) bool {
 	if p1 == nil && p2 == nil {
 		return true
 	}

hugolib/pageCache_test.go

@@ -56,8 +56,8 @@ func TestPageCache(t *testing.T) {
 				l1.Unlock()
 				p2, c2 := c1.get("k1", p, nil)
 				assert.True(t, c2)
-				assert.True(t, probablyEqualPages(p, p2))
-				assert.True(t, probablyEqualPages(p, pages))
+				assert.True(t, fastEqualPages(p, p2))
+				assert.True(t, fastEqualPages(p, pages))
 				assert.NotNil(t, p)
 
 				l2.Lock()

hugolib/pageGroup.go

@@ -24,8 +24,8 @@ import (
 // PageGroup represents a group of pages, grouped by the key.
 // The key is typically a year or similar.
 type PageGroup struct {
-	Key   interface{}
-	Pages Pages
+	Key interface{}
+	Pages
 }
 
 type mapKeyValues []reflect.Value

hugolib/pageSort_test.go

@@ -115,7 +115,7 @@ func TestPageSortReverse(t *testing.T) {
 	assert.Equal(t, 9, p2[0].fuzzyWordCount)
 	assert.Equal(t, 0, p2[9].fuzzyWordCount)
 	// cached
-	assert.True(t, probablyEqualPages(p2, p1.Reverse()))
+	assert.True(t, fastEqualPages(p2, p1.Reverse()))
 }
 
 func TestPageSortByParam(t *testing.T) {

hugolib/pages_related.go

@@ -0,0 +1,191 @@
+// Copyright 2017-present The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package hugolib
+
+import (
+	"sync"
+
+	"github.com/gohugoio/hugo/common/types"
+	"github.com/gohugoio/hugo/related"
+	"github.com/spf13/cast"
+)
+
+var (
+	// Assert that Pages and PageGroup implements the PageGenealogist interface.
+	_ PageGenealogist = (Pages)(nil)
+	_ PageGenealogist = PageGroup{}
+)
+
+// A PageGenealogist finds related pages in a page collection. This interface is implemented
+// by Pages and PageGroup, which makes it available as `{{ .RegularPages.Related . }}` etc.
+type PageGenealogist interface {
+
+	// Template example:
+	// {{ $related := .RegularPages.Related . }}
+	Related(doc related.Document) (Pages, error)
+
+	// Template example:
+	// {{ $related := .RegularPages.RelatedIndices . "tags" "date" }}
+	RelatedIndices(doc related.Document, indices ...interface{}) (Pages, error)
+
+	// Template example:
+	// {{ $related := .RegularPages.RelatedTo ( keyVals "tags" "hugo", "rocks")  ( keyVals "date" .Date ) }}
+	RelatedTo(args ...types.KeyValues) (Pages, error)
+}
+
+// Related searches all the configured indices with the search keywords from the
+// supplied document.
+func (p Pages) Related(doc related.Document) (Pages, error) {
+	page, err := unwrapPage(doc)
+	if err != nil {
+		return nil, err
+	}
+
+	result, err := p.searchDoc(page)
+	if err != nil {
+		return nil, err
+	}
+
+	return result.removeFirstIfFound(page), nil
+}
+
+// RelatedIndices searches the given indices with the search keywords from the
+// supplied document.
+func (p Pages) RelatedIndices(doc related.Document, indices ...interface{}) (Pages, error) {
+	page, err := unwrapPage(doc)
+	if err != nil {
+		return nil, err
+	}
+
+	indicesStr, err := cast.ToStringSliceE(indices)
+	if err != nil {
+		return nil, err
+	}
+
+	result, err := p.searchDoc(page, indicesStr...)
+	if err != nil {
+		return nil, err
+	}
+
+	return result.removeFirstIfFound(page), nil
+
+}
+
+// RelatedTo searches the given indices with the corresponding values.
+func (p Pages) RelatedTo(args ...types.KeyValues) (Pages, error) {
+	if len(p) == 0 {
+		return nil, nil
+	}
+
+	return p.search(args...)
+
+}
+
+func (p Pages) search(args ...types.KeyValues) (Pages, error) {
+	return p.withInvertedIndex(func(idx *related.InvertedIndex) ([]related.Document, error) {
+		return idx.SearchKeyValues(args...)
+	})
+
+}
+
+func (p Pages) searchDoc(doc related.Document, indices ...string) (Pages, error) {
+	return p.withInvertedIndex(func(idx *related.InvertedIndex) ([]related.Document, error) {
+		return idx.SearchDoc(doc, indices...)
+	})
+}
+
+func (p Pages) withInvertedIndex(search func(idx *related.InvertedIndex) ([]related.Document, error)) (Pages, error) {
+	if len(p) == 0 {
+		return nil, nil
+	}
+
+	cache := p[0].s.relatedDocsHandler
+
+	searchIndex, err := cache.getOrCreateIndex(p)
+	if err != nil {
+		return nil, err
+	}
+
+	result, err := search(searchIndex)
+	if err != nil {
+		return nil, err
+	}
+
+	if len(result) > 0 {
+		mp := make(Pages, len(result))
+		for i, match := range result {
+			mp[i] = match.(*Page)
+		}
+		return mp, nil
+	}
+
+	return nil, nil
+}
+
+type cachedPostingList struct {
+	p Pages
+
+	postingList *related.InvertedIndex
+}
+
+type relatedDocsHandler struct {
+	// This is configured in site or langugage config.
+	cfg related.Config
+
+	postingLists []*cachedPostingList
+	mu           sync.RWMutex
+}
+
+func newSearchIndexHandler(cfg related.Config) *relatedDocsHandler {
+	return &relatedDocsHandler{cfg: cfg}
+}
+
+// This assumes that a lock has been aquired.
+func (s *relatedDocsHandler) getIndex(p Pages) *related.InvertedIndex {
+	for _, ci := range s.postingLists {
+		if fastEqualPages(p, ci.p) {
+			return ci.postingList
+		}
+	}
+	return nil
+}
+
+func (s *relatedDocsHandler) getOrCreateIndex(p Pages) (*related.InvertedIndex, error) {
+	s.mu.RLock()
+	cachedIndex := s.getIndex(p)
+	if cachedIndex != nil {
+		s.mu.RUnlock()
+		return cachedIndex, nil
+	}
+	s.mu.RUnlock()
+
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	if cachedIndex := s.getIndex(p); cachedIndex != nil {
+		return cachedIndex, nil
+	}
+
+	searchIndex := related.NewInvertedIndex(s.cfg)
+
+	for _, page := range p {
+		if err := searchIndex.Add(page); err != nil {
+			return nil, err
+		}
+	}
+
+	s.postingLists = append(s.postingLists, &cachedPostingList{p: p, postingList: searchIndex})
+
+	return searchIndex, nil
+}

hugolib/pages_related_test.go

@@ -0,0 +1,75 @@
+// Copyright 2017-present The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package hugolib
+
+import (
+	"fmt"
+	"path/filepath"
+	"testing"
+
+	"github.com/gohugoio/hugo/common/types"
+	"github.com/gohugoio/hugo/deps"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestRelated(t *testing.T) {
+	assert := require.New(t)
+
+	t.Parallel()
+
+	var (
+		cfg, fs = newTestCfg()
+		//th      = testHelper{cfg, fs, t}
+	)
+
+	pageTmpl := `---
+title: Page %d
+keywords: [%s]
+date: %s
+---
+
+Content
+`
+
+	writeSource(t, fs, filepath.Join("content", "page1.md"), fmt.Sprintf(pageTmpl, 1, "hugo, says", "2017-01-03"))
+	writeSource(t, fs, filepath.Join("content", "page2.md"), fmt.Sprintf(pageTmpl, 2, "hugo, rocks", "2017-01-02"))
+	writeSource(t, fs, filepath.Join("content", "page3.md"), fmt.Sprintf(pageTmpl, 3, "bep, says", "2017-01-01"))
+
+	s := buildSingleSite(t, deps.DepsCfg{Fs: fs, Cfg: cfg}, BuildCfg{SkipRender: true})
+	assert.Len(s.RegularPages, 3)
+
+	result, err := s.RegularPages.RelatedTo(types.NewKeyValuesStrings("keywords", "hugo", "rocks"))
+
+	assert.NoError(err)
+	assert.Len(result, 2)
+	assert.Equal("Page 2", result[0].Title)
+	assert.Equal("Page 1", result[1].Title)
+
+	result, err = s.RegularPages.Related(s.RegularPages[0])
+	assert.Len(result, 2)
+	assert.Equal("Page 2", result[0].Title)
+	assert.Equal("Page 3", result[1].Title)
+
+	result, err = s.RegularPages.RelatedIndices(s.RegularPages[0], "keywords")
+	assert.Len(result, 2)
+	assert.Equal("Page 2", result[0].Title)
+	assert.Equal("Page 3", result[1].Title)
+
+	result, err = s.RegularPages.RelatedTo(types.NewKeyValuesStrings("keywords", "bep", "rocks"))
+	assert.Len(result, 2)
+	assert.Equal("Page 2", result[0].Title)
+	assert.Equal("Page 3", result[1].Title)
+
+}

hugolib/site.go

@@ -42,6 +42,7 @@ import (
 	"github.com/gohugoio/hugo/helpers"
 	"github.com/gohugoio/hugo/output"
 	"github.com/gohugoio/hugo/parser"
+	"github.com/gohugoio/hugo/related"
 	"github.com/gohugoio/hugo/source"
 	"github.com/gohugoio/hugo/tpl"
 	"github.com/gohugoio/hugo/transform"
@@ -135,6 +136,8 @@ type Site struct {
 	// The func used to title case titles.
 	titleFunc func(s string) string
 
+	relatedDocsHandler *relatedDocsHandler
+
 	siteStats *siteStats
 }
 
@@ -176,6 +179,7 @@ func (s *Site) reset() *Site {
 		layoutHandler:       output.NewLayoutHandler(s.PathSpec.ThemeSet()),
 		disabledKinds:       s.disabledKinds,
 		titleFunc:           s.titleFunc,
+		relatedDocsHandler:  newSearchIndexHandler(s.relatedDocsHandler.cfg),
 		outputFormats:       s.outputFormats,
 		outputFormatsConfig: s.outputFormatsConfig,
 		mediaTypesConfig:    s.mediaTypesConfig,
@@ -231,6 +235,21 @@ func newSite(cfg deps.DepsCfg) (*Site, error) {
 		return nil, err
 	}
 
+	var relatedContentConfig related.Config
+
+	if cfg.Language.IsSet("related") {
+		relatedContentConfig, err = related.DecodeConfig(cfg.Language.Get("related"))
+		if err != nil {
+			return nil, err
+		}
+	} else {
+		relatedContentConfig = related.DefaultConfig
+		taxonomies := cfg.Language.GetStringMapString("taxonomies")
+		if _, found := taxonomies["tag"]; found {
+			relatedContentConfig.Add(related.IndexConfig{Name: "tags", Weight: 80})
+		}
+	}
+
 	titleFunc := helpers.GetTitleFunc(cfg.Language.GetString("titleCaseStyle"))
 
 	s := &Site{
@@ -239,6 +258,7 @@ func newSite(cfg deps.DepsCfg) (*Site, error) {
 		Language:            cfg.Language,
 		disabledKinds:       disabledKinds,
 		titleFunc:           titleFunc,
+		relatedDocsHandler:  newSearchIndexHandler(relatedContentConfig),
 		outputFormats:       outputFormats,
 		outputFormatsConfig: siteOutputFormatsConfig,
 		mediaTypesConfig:    siteMediaTypesConfig,
@@ -1607,6 +1627,7 @@ func (s *Site) assembleTaxonomies() {
 // Prepare site for a new full build.
 func (s *Site) resetBuildState() {
 
+	s.relatedDocsHandler = newSearchIndexHandler(s.relatedDocsHandler.cfg)
 	s.PageCollections = newPageCollectionsFromPages(s.rawAllPages)
 	// TODO(bep) get rid of this double
 	s.Info.PageCollections = s.PageCollections

related/inverted_index.go

@@ -0,0 +1,450 @@
+// Copyright 2017-present The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package related holds code to help finding related content.
+package related
+
+import (
+	"errors"
+	"fmt"
+	"math"
+	"sort"
+	"strings"
+	"time"
+
+	"github.com/gohugoio/hugo/common/types"
+	"github.com/mitchellh/mapstructure"
+)
+
+var (
+	_        Keyword = (*StringKeyword)(nil)
+	zeroDate         = time.Time{}
+
+	// DefaultConfig is the default related config.
+	DefaultConfig = Config{
+		Threshold: 80,
+		Indices: IndexConfigs{
+			IndexConfig{Name: "keywords", Weight: 100},
+			IndexConfig{Name: "date", Weight: 10},
+		},
+	}
+)
+
+/*
+Config is the top level configuration element used to configure how to retrieve
+related content in Hugo.
+
+An example site config.toml:
+
+	[related]
+	threshold = 1
+	[[related.indices]]
+	name = "keywords"
+	weight = 200
+	[[related.indices]]
+	name  = "tags"
+	weight = 100
+	[[related.indices]]
+	name  = "date"
+	weight = 1
+	pattern = "2006"
+*/
+type Config struct {
+	// Only include matches >= threshold, a normalized rank between 0 and 100.
+	Threshold int
+
+	// To get stable "See also" sections we, by default, exclude newer related pages.
+	IncludeNewer bool
+
+	// Will lower case all string values and queries to the indices.
+	// May get better results, but at a slight performance cost.
+	ToLower bool
+
+	Indices IndexConfigs
+}
+
+func (c *Config) Add(index IndexConfig) {
+	if c.ToLower {
+		index.ToLower = true
+	}
+	c.Indices = append(c.Indices, index)
+}
+
+// IndexConfigs holds a set of index configurations.
+type IndexConfigs []IndexConfig
+
+// IndexConfig configures an index.
+type IndexConfig struct {
+	// The index name. This directly maps to a field or Param name.
+	Name string
+
+	// Contextual pattern used to convert the Param value into a string.
+	// Currently only used for dates. Can be used to, say, bump posts in the same
+	// time frame when searching for related documents.
+	// For dates it follows Go's time.Format patterns, i.e.
+	// "2006" for YYYY and "200601" for YYYYMM.
+	Pattern string
+
+	// This field's weight when doing multi-index searches. Higher is "better".
+	Weight int
+
+	// Will lower case all string values in and queries tothis index.
+	// May get better accurate results, but at a slight performance cost.
+	ToLower bool
+}
+
+// Document is the interface an indexable document in Hugo must fulfill.
+type Document interface {
+	// SearchKeywords returns a list of keywords for the given index config.
+	SearchKeywords(cfg IndexConfig) ([]Keyword, error)
+
+	// When this document was or will be published.
+	PubDate() time.Time
+}
+
+// InvertedIndex holds an inverted index, also sometimes named posting list, which
+// lists, for every possible search term, the documents that contain that term.
+type InvertedIndex struct {
+	cfg   Config
+	index map[string]map[Keyword][]Document
+
+	minWeight int
+	maxWeight int
+}
+
+func (idx *InvertedIndex) getIndexCfg(name string) (IndexConfig, bool) {
+	for _, conf := range idx.cfg.Indices {
+		if conf.Name == name {
+			return conf, true
+		}
+	}
+
+	return IndexConfig{}, false
+}
+
+// NewInvertedIndex creates a new InvertedIndex.
+// Documents to index must be added in Add.
+func NewInvertedIndex(cfg Config) *InvertedIndex {
+	idx := &InvertedIndex{index: make(map[string]map[Keyword][]Document), cfg: cfg}
+	for _, conf := range cfg.Indices {
+		idx.index[conf.Name] = make(map[Keyword][]Document)
+		if conf.Weight < idx.minWeight {
+			// By default, the weight scale starts at 0, but we allow
+			// negative weights.
+			idx.minWeight = conf.Weight
+		}
+		if conf.Weight > idx.maxWeight {
+			idx.maxWeight = conf.Weight
+		}
+	}
+	return idx
+}
+
+// Add documents to the inverted index.
+// The value must support == and !=.
+func (idx *InvertedIndex) Add(docs ...Document) error {
+	var err error
+	for _, config := range idx.cfg.Indices {
+		if config.Weight == 0 {
+			// Disabled
+			continue
+		}
+		setm := idx.index[config.Name]
+
+		for _, doc := range docs {
+			var words []Keyword
+			words, err = doc.SearchKeywords(config)
+			if err != nil {
+				continue
+			}
+
+			for _, keyword := range words {
+				setm[keyword] = append(setm[keyword], doc)
+			}
+		}
+	}
+
+	return err
+
+}
+
+// queryElement holds the index name and keywords that can be used to compose a
+// search for related content.
+type queryElement struct {
+	Index    string
+	Keywords []Keyword
+}
+
+func newQueryElement(index string, keywords ...Keyword) queryElement {
+	return queryElement{Index: index, Keywords: keywords}
+}
+
+type ranks []*rank
+
+type rank struct {
+	Doc     Document
+	Weight  int
+	Matches int
+}
+
+func (r *rank) addWeight(w int) {
+	r.Weight += w
+	r.Matches++
+}
+
+func newRank(doc Document, weight int) *rank {
+	return &rank{Doc: doc, Weight: weight, Matches: 1}
+}
+
+func (r ranks) Len() int      { return len(r) }
+func (r ranks) Swap(i, j int) { r[i], r[j] = r[j], r[i] }
+func (r ranks) Less(i, j int) bool {
+	if r[i].Weight == r[j].Weight {
+		return r[i].Doc.PubDate().After(r[j].Doc.PubDate())
+	}
+	return r[i].Weight > r[j].Weight
+}
+
+// SearchDoc finds the documents matching any of the keywords in the given indices
+// against the given document.
+// The resulting document set will be sorted according to number of matches
+// and the index weights, and any matches with a rank below the configured
+// threshold (normalize to 0..100) will be removed.
+// If an index name is provided, only that index will be queried.
+func (idx *InvertedIndex) SearchDoc(doc Document, indices ...string) ([]Document, error) {
+	var q []queryElement
+
+	var configs IndexConfigs
+
+	if len(indices) == 0 {
+		configs = idx.cfg.Indices
+	} else {
+		configs = make(IndexConfigs, len(indices))
+		for i, indexName := range indices {
+			cfg, found := idx.getIndexCfg(indexName)
+			if !found {
+				return nil, fmt.Errorf("index %q not found", indexName)
+			}
+			configs[i] = cfg
+		}
+	}
+
+	for _, cfg := range configs {
+		keywords, err := doc.SearchKeywords(cfg)
+		if err != nil {
+			return nil, err
+		}
+
+		q = append(q, newQueryElement(cfg.Name, keywords...))
+
+	}
+
+	return idx.searchDate(doc.PubDate(), q...)
+}
+
+func (cfg IndexConfig) ToKeywords(v interface{}) ([]Keyword, error) {
+	var (
+		keywords []Keyword
+		toLower  = cfg.ToLower
+	)
+	switch vv := v.(type) {
+	case string:
+		if toLower {
+			vv = strings.ToLower(vv)
+		}
+		keywords = append(keywords, StringKeyword(vv))
+	case []string:
+		if toLower {
+			for i := 0; i < len(vv); i++ {
+				vv[i] = strings.ToLower(vv[i])
+			}
+		}
+		keywords = append(keywords, StringsToKeywords(vv...)...)
+	case time.Time:
+		layout := "2006"
+		if cfg.Pattern != "" {
+			layout = cfg.Pattern
+		}
+		keywords = append(keywords, StringKeyword(vv.Format(layout)))
+	case nil:
+		return keywords, nil
+	default:
+		return keywords, fmt.Errorf("indexing currently not supported for for index %q and type %T", cfg.Name, vv)
+	}
+
+	return keywords, nil
+}
+
+// SearchKeyValues finds the documents matching any of the keywords in the given indices.
+// The resulting document set will be sorted according to number of matches
+// and the index weights, and any matches with a rank below the configured
+// threshold (normalize to 0..100) will be removed.
+func (idx *InvertedIndex) SearchKeyValues(args ...types.KeyValues) ([]Document, error) {
+	q := make([]queryElement, len(args))
+
+	for i, arg := range args {
+		var keywords []Keyword
+		key := arg.KeyString()
+		if key == "" {
+			return nil, fmt.Errorf("index %q not valid", arg.Key)
+		}
+		conf, found := idx.getIndexCfg(key)
+		if !found {
+			return nil, fmt.Errorf("index %q not found", key)
+		}
+
+		for _, val := range arg.Values {
+			k, err := conf.ToKeywords(val)
+			if err != nil {
+				return nil, err
+			}
+			keywords = append(keywords, k...)
+		}
+
+		q[i] = newQueryElement(conf.Name, keywords...)
+
+	}
+
+	return idx.search(q...)
+}
+
+func (idx *InvertedIndex) search(query ...queryElement) ([]Document, error) {
+	return idx.searchDate(zeroDate, query...)
+}
+
+func (idx *InvertedIndex) searchDate(upperDate time.Time, query ...queryElement) ([]Document, error) {
+	matchm := make(map[Document]*rank, 200)
+	applyDateFilter := !idx.cfg.IncludeNewer && !upperDate.IsZero()
+
+	for _, el := range query {
+		setm, found := idx.index[el.Index]
+		if !found {
+			return []Document{}, fmt.Errorf("index for %q not found", el.Index)
+		}
+
+		config, found := idx.getIndexCfg(el.Index)
+		if !found {
+			return []Document{}, fmt.Errorf("index config for %q not found", el.Index)
+		}
+
+		for _, kw := range el.Keywords {
+			if docs, found := setm[kw]; found {
+				for _, doc := range docs {
+					if applyDateFilter {
+						// Exclude newer than the limit given
+						if doc.PubDate().After(upperDate) {
+							continue
+						}
+					}
+					r, found := matchm[doc]
+					if !found {
+						matchm[doc] = newRank(doc, config.Weight)
+					} else {
+						r.addWeight(config.Weight)
+					}
+				}
+			}
+		}
+	}
+
+	if len(matchm) == 0 {
+		return []Document{}, nil
+	}
+
+	matches := make(ranks, 0, 100)
+
+	for _, v := range matchm {
+		avgWeight := v.Weight / v.Matches
+		weight := norm(avgWeight, idx.minWeight, idx.maxWeight)
+		threshold := idx.cfg.Threshold / v.Matches
+
+		if weight >= threshold {
+			matches = append(matches, v)
+		}
+	}
+
+	sort.Stable(matches)
+
+	result := make([]Document, len(matches))
+
+	for i, m := range matches {
+		result[i] = m.Doc
+	}
+
+	return result, nil
+}
+
+// normalizes num to a number between 0 and 100.
+func norm(num, min, max int) int {
+	if min > max {
+		panic("min > max")
+	}
+	return int(math.Floor((float64(num-min) / float64(max-min) * 100) + 0.5))
+}
+
+// DecodeConfig decodes a slice of map into Config.
+func DecodeConfig(in interface{}) (Config, error) {
+	if in == nil {
+		return Config{}, errors.New("no related config provided")
+	}
+
+	m, ok := in.(map[string]interface{})
+	if !ok {
+		return Config{}, fmt.Errorf("expected map[string]interface {} got %T", in)
+	}
+
+	if len(m) == 0 {
+		return Config{}, errors.New("empty related config provided")
+	}
+
+	var c Config
+
+	if err := mapstructure.WeakDecode(m, &c); err != nil {
+		return c, err
+	}
+
+	if c.Threshold < 0 || c.Threshold > 100 {
+		return Config{}, errors.New("related threshold must be between 0 and 100")
+	}
+
+	if c.ToLower {
+		for i, _ := range c.Indices {
+			c.Indices[i].ToLower = true
+		}
+	}
+
+	return c, nil
+}
+
+// StringKeyword is a string search keyword.
+type StringKeyword string
+
+func (s StringKeyword) String() string {
+	return string(s)
+}
+
+// Keyword is the interface a keyword in the search index must implement.
+type Keyword interface {
+	String() string
+}
+
+// StringsToKeywords converts the given slice of strings to a slice of Keyword.
+func StringsToKeywords(s ...string) []Keyword {
+	kw := make([]Keyword, len(s))
+
+	for i := 0; i < len(s); i++ {
+		kw[i] = StringKeyword(s[i])
+	}
+
+	return kw
+}

related/inverted_index_test.go

@@ -0,0 +1,276 @@
+// Copyright 2017-present The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package related
+
+import (
+	"fmt"
+	"math/rand"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+)
+
+type testDoc struct {
+	keywords map[string][]Keyword
+	date     time.Time
+}
+
+func (k *testDoc) String() string {
+	s := "\n"
+	for k, v := range k.keywords {
+		s += k + ":\t\t"
+		for _, vv := range v {
+			s += "  " + vv.String()
+		}
+		s += "\n"
+	}
+	return s
+}
+
+func newTestDoc(name string, keywords ...string) *testDoc {
+	km := make(map[string][]Keyword)
+
+	time.Sleep(1 * time.Millisecond)
+	kw := &testDoc{keywords: km, date: time.Now()}
+
+	kw.addKeywords(name, keywords...)
+	return kw
+}
+
+func (t *testDoc) addKeywords(name string, keywords ...string) *testDoc {
+	keywordm := createTestKeywords(name, keywords...)
+
+	for k, v := range keywordm {
+		keywords := make([]Keyword, len(v))
+		for i := 0; i < len(v); i++ {
+			keywords[i] = StringKeyword(v[i])
+		}
+		t.keywords[k] = keywords
+	}
+	return t
+}
+
+func createTestKeywords(name string, keywords ...string) map[string][]string {
+	return map[string][]string{
+		name: keywords,
+	}
+}
+
+func (k *testDoc) SearchKeywords(cfg IndexConfig) ([]Keyword, error) {
+	return k.keywords[cfg.Name], nil
+}
+
+func (k *testDoc) PubDate() time.Time {
+	return k.date
+}
+
+func TestSearch(t *testing.T) {
+
+	config := Config{
+		Threshold:    90,
+		IncludeNewer: false,
+		Indices: IndexConfigs{
+			IndexConfig{Name: "tags", Weight: 50},
+			IndexConfig{Name: "keywords", Weight: 65},
+		},
+	}
+
+	idx := NewInvertedIndex(config)
+	//idx.debug = true
+
+	docs := []Document{
+		newTestDoc("tags", "a", "b", "c", "d"),
+		newTestDoc("tags", "b", "d", "g"),
+		newTestDoc("tags", "b", "h").addKeywords("keywords", "a"),
+		newTestDoc("tags", "g", "h").addKeywords("keywords", "a", "b"),
+	}
+
+	idx.Add(docs...)
+
+	t.Run("count", func(t *testing.T) {
+		assert := require.New(t)
+		assert.Len(idx.index, 2)
+		set1, found := idx.index["tags"]
+		assert.True(found)
+		// 6 tags
+		assert.Len(set1, 6)
+
+		set2, found := idx.index["keywords"]
+		assert.True(found)
+		assert.Len(set2, 2)
+
+	})
+
+	t.Run("search-tags", func(t *testing.T) {
+		assert := require.New(t)
+		m, err := idx.search(newQueryElement("tags", StringsToKeywords("a", "b", "d", "z")...))
+		assert.NoError(err)
+		assert.Len(m, 2)
+		assert.Equal(docs[0], m[0])
+		assert.Equal(docs[1], m[1])
+	})
+
+	t.Run("search-tags-and-keywords", func(t *testing.T) {
+		assert := require.New(t)
+		m, err := idx.search(
+			newQueryElement("tags", StringsToKeywords("a", "b", "z")...),
+			newQueryElement("keywords", StringsToKeywords("a", "b")...))
+		assert.NoError(err)
+		assert.Len(m, 3)
+		assert.Equal(docs[3], m[0])
+		assert.Equal(docs[2], m[1])
+		assert.Equal(docs[0], m[2])
+	})
+
+	t.Run("searchdoc-all", func(t *testing.T) {
+		assert := require.New(t)
+		doc := newTestDoc("tags", "a").addKeywords("keywords", "a")
+		m, err := idx.SearchDoc(doc)
+		assert.NoError(err)
+		assert.Len(m, 2)
+		assert.Equal(docs[3], m[0])
+		assert.Equal(docs[2], m[1])
+	})
+
+	t.Run("searchdoc-tags", func(t *testing.T) {
+		assert := require.New(t)
+		doc := newTestDoc("tags", "a", "b", "d", "z").addKeywords("keywords", "a", "b")
+		m, err := idx.SearchDoc(doc, "tags")
+		assert.NoError(err)
+		assert.Len(m, 2)
+		assert.Equal(docs[0], m[0])
+		assert.Equal(docs[1], m[1])
+	})
+
+	t.Run("searchdoc-keywords-date", func(t *testing.T) {
+		assert := require.New(t)
+		doc := newTestDoc("tags", "a", "b", "d", "z").addKeywords("keywords", "a", "b")
+		// This will get a date newer than the others.
+		newDoc := newTestDoc("keywords", "a", "b")
+		idx.Add(newDoc)
+
+		m, err := idx.SearchDoc(doc, "keywords")
+		assert.NoError(err)
+		assert.Len(m, 2)
+		assert.Equal(docs[3], m[0])
+	})
+
+}
+
+func BenchmarkRelatedNewIndex(b *testing.B) {
+
+	pages := make([]*testDoc, 100)
+	numkeywords := 30
+	allKeywords := make([]string, numkeywords)
+	for i := 0; i < numkeywords; i++ {
+		allKeywords[i] = fmt.Sprintf("keyword%d", i+1)
+	}
+
+	for i := 0; i < len(pages); i++ {
+		start := rand.Intn(len(allKeywords))
+		end := start + 3
+		if end >= len(allKeywords) {
+			end = start + 1
+		}
+
+		kw := newTestDoc("tags", allKeywords[start:end]...)
+		if i%5 == 0 {
+			start := rand.Intn(len(allKeywords))
+			end := start + 3
+			if end >= len(allKeywords) {
+				end = start + 1
+			}
+			kw.addKeywords("keywords", allKeywords[start:end]...)
+		}
+
+		pages[i] = kw
+	}
+
+	cfg := Config{
+		Threshold: 50,
+		Indices: IndexConfigs{
+			IndexConfig{Name: "tags", Weight: 100},
+			IndexConfig{Name: "keywords", Weight: 200},
+		},
+	}
+
+	b.Run("singles", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			idx := NewInvertedIndex(cfg)
+			for _, doc := range pages {
+				idx.Add(doc)
+			}
+		}
+	})
+
+	b.Run("all", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			idx := NewInvertedIndex(cfg)
+			docs := make([]Document, len(pages))
+			for i := 0; i < len(pages); i++ {
+				docs[i] = pages[i]
+			}
+			idx.Add(docs...)
+		}
+	})
+
+}
+
+func BenchmarkRelatedMatchesIn(b *testing.B) {
+
+	q1 := newQueryElement("tags", StringsToKeywords("keyword2", "keyword5", "keyword32", "asdf")...)
+	q2 := newQueryElement("keywords", StringsToKeywords("keyword3", "keyword4")...)
+
+	docs := make([]*testDoc, 1000)
+	numkeywords := 20
+	allKeywords := make([]string, numkeywords)
+	for i := 0; i < numkeywords; i++ {
+		allKeywords[i] = fmt.Sprintf("keyword%d", i+1)
+	}
+
+	cfg := Config{
+		Threshold: 20,
+		Indices: IndexConfigs{
+			IndexConfig{Name: "tags", Weight: 100},
+			IndexConfig{Name: "keywords", Weight: 200},
+		},
+	}
+
+	idx := NewInvertedIndex(cfg)
+
+	for i := 0; i < len(docs); i++ {
+		start := rand.Intn(len(allKeywords))
+		end := start + 3
+		if end >= len(allKeywords) {
+			end = start + 1
+		}
+
+		index := "tags"
+		if i%5 == 0 {
+			index = "keywords"
+		}
+
+		idx.Add(newTestDoc(index, allKeywords[start:end]...))
+	}
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		if i%10 == 0 {
+			idx.search(q2)
+		} else {
+			idx.search(q1)
+		}
+	}
+}

tpl/collections/collections.go

@@ -23,6 +23,7 @@ import (
 	"strings"
 	"time"
 
+	"github.com/gohugoio/hugo/common/types"
 	"github.com/gohugoio/hugo/deps"
 	"github.com/gohugoio/hugo/helpers"
 	"github.com/spf13/cast"
@@ -641,3 +642,8 @@ func (ns *Namespace) Uniq(l interface{}) (interface{}, error) {
 	}
 	return ret.Interface(), nil
 }
+
+// KeyVals creates a key and values wrapper.
+func (ns *Namespace) KeyVals(key interface{}, vals ...interface{}) (types.KeyValues, error) {
+	return types.KeyValues{Key: key, Values: vals}, nil
+}

tpl/collections/init.go

@@ -63,6 +63,13 @@ func init() {
 			[][2]string{},
 		)
 
+		ns.AddMethodMapping(ctx.KeyVals,
+			[]string{"keyVals"},
+			[][2]string{
+				{`{{ keyVals "key" "a" "b" }}`, `key: [a b]`},
+			},
+		)
+
 		ns.AddMethodMapping(ctx.In,
 			[]string{"in"},
 			[][2]string{