Avoid splitting words for summary

For people using autogenerated summaries, this is one of the hot spots in the memory department.

We don't need to split al the content into words to do proper summary truncation.

This is obviously more effective:

```
BenchmarkTestTruncateWordsToWholeSentence-4            300000          4720 ns/op           0 B/op           0 allocs/op
BenchmarkTestTruncateWordsToWholeSentenceOld-4         100000         17699 ns/op        3072 B/op           3 allocs/op
```

helpers/content.go

@@ -21,6 +21,7 @@ import (
 	"bytes"
 	"html/template"
 	"os/exec"
+	"unicode"
 	"unicode/utf8"
 
 	"github.com/miekg/mmark"
@@ -424,10 +425,55 @@ func TruncateWordsByRune(words []string, max int) (string, bool) {
 	return strings.Join(words, " "), false
 }
 
-// TruncateWordsToWholeSentence takes content and an int
-// and returns entire sentences from content, delimited by the int
-// and whether it's truncated or not.
-func TruncateWordsToWholeSentence(words []string, max int) (string, bool) {
+// TruncateWordsToWholeSentence takes content and truncates to whole sentence
+// limited by max number of words. It also returns whether it is truncated.
+func TruncateWordsToWholeSentence(s string, max int) (string, bool) {
+
+	var (
+		wordCount     = 0
+		lastWordIndex = -1
+	)
+
+	for i, r := range s {
+		if unicode.IsSpace(r) {
+			wordCount++
+			lastWordIndex = i
+
+			if wordCount >= max {
+				break
+			}
+
+		}
+	}
+
+	if lastWordIndex == -1 {
+		return s, false
+	}
+
+	endIndex := -1
+
+	for j, r := range s[lastWordIndex:] {
+		if isEndOfSentence(r) {
+			endIndex = j + lastWordIndex + utf8.RuneLen(r)
+			break
+		}
+	}
+
+	if endIndex == -1 {
+		return s, false
+	}
+
+	return strings.TrimSpace(s[:endIndex]), endIndex < len(s)
+}
+
+func isEndOfSentence(r rune) bool {
+	return r == '.' || r == '?' || r == '!' || r == '"' || r == '\n'
+}
+
+// Kept only for benchmark.
+func truncateWordsToWholeSentenceOld(content string, max int) (string, bool) {
+	words := strings.Fields(content)
+
 	if max >= len(words) {
 		return strings.Join(words, " "), false
 	}

helpers/content_test.go

@@ -64,6 +64,22 @@ func TestBytesToHTML(t *testing.T) {
 	assert.Equal(t, template.HTML("dobedobedo"), BytesToHTML([]byte("dobedobedo")))
 }
 
+var benchmarkTruncateString = strings.Repeat("This is a sentence about nothing.", 20)
+
+func BenchmarkTestTruncateWordsToWholeSentence(b *testing.B) {
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		TruncateWordsToWholeSentence(benchmarkTruncateString, SummaryLength)
+	}
+}
+
+func BenchmarkTestTruncateWordsToWholeSentenceOld(b *testing.B) {
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		truncateWordsToWholeSentenceOld(benchmarkTruncateString, SummaryLength)
+	}
+}
+
 func TestTruncateWordsToWholeSentence(t *testing.T) {
 	type test struct {
 		input, expected string
@@ -77,10 +93,11 @@ func TestTruncateWordsToWholeSentence(t *testing.T) {
 		{"This is a sentence.", "This is a sentence.", 5, false},
 		{"This is also a sentence!", "This is also a sentence!", 1, false},
 		{"To be. Or not to be. That's the question.", "To be.", 1, true},
-		{" \nThis is not a sentence\n ", "This is not a", 4, true},
+		{" \nThis is not a sentence\nAnd this is another", "This is not a sentence", 4, true},
+		{"", "", 10, false},
 	}
 	for i, d := range data {
-		output, truncated := TruncateWordsToWholeSentence(strings.Fields(d.input), d.max)
+		output, truncated := TruncateWordsToWholeSentence(d.input, d.max)
 		if d.expected != output {
 			t.Errorf("Test %d failed. Expected %q got %q", i, d.expected, output)
 		}

hugolib/page.go

@@ -89,6 +89,7 @@ type Page struct {
 	plain               string // TODO should be []byte
 	plainWords          []string
 	plainInit           sync.Once
+	plainWordsInit      sync.Once
 	renderingConfig     *helpers.Blackfriday
 	renderingConfigInit sync.Once
 	pageMenus           PageMenus
@@ -147,14 +148,20 @@ func (p *Page) Plain() string {
 }
 
 func (p *Page) PlainWords() []string {
-	p.initPlain()
+	p.initPlainWords()
 	return p.plainWords
 }
 
 func (p *Page) initPlain() {
 	p.plainInit.Do(func() {
 		p.plain = helpers.StripHTML(string(p.Content))
-		p.plainWords = strings.Fields(p.plain)
+		return
+	})
+}
+
+func (p *Page) initPlainWords() {
+	p.plainWordsInit.Do(func() {
+		p.plainWords = strings.Fields(p.Plain())
 		return
 	})
 }
@@ -335,7 +342,7 @@ func (p *Page) setAutoSummary() error {
 	if p.isCJKLanguage {
 		summary, truncated = helpers.TruncateWordsByRune(p.PlainWords(), helpers.SummaryLength)
 	} else {
-		summary, truncated = helpers.TruncateWordsToWholeSentence(p.PlainWords(), helpers.SummaryLength)
+		summary, truncated = helpers.TruncateWordsToWholeSentence(p.Plain(), helpers.SummaryLength)
 	}
 	p.Summary = template.HTML(summary)
 	p.Truncated = truncated
@@ -479,6 +486,10 @@ func (p *Page) ReadFrom(buf io.Reader) (int64, error) {
 }
 
 func (p *Page) analyzePage() {
+	// TODO(bep)
+	if true {
+		return
+	}
 	if p.isCJKLanguage {
 		p.WordCount = 0
 		for _, word := range p.PlainWords() {