tpl: Add truncate template function

This commit adds a truncate template function for safely truncating text without
breaking words. The truncate function is HTML aware, so if the input text is a
template.HTML it will be truncated without leaving broken or unclosed HTML tags.

    {{ "this is a very long text" | truncate 10 " ..." }}
    {{ "With [Markdown](/markdown) inside." | markdownify | truncate 10 }}

4 files changed, 243 insertions, 0 deletions

diff --git a/tpl/template_func_truncate.go b/tpl/template_func_truncate.go
new file mode 100644
index 000000000..b5886edae
--- /dev/null
+++ b/tpl/template_func_truncate.go
@@ -0,0 +1,156 @@
+// Copyright 2016 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 tpl
+
+import (
+	"errors"
+	"html"
+	"html/template"
+	"regexp"
+	"unicode"
+	"unicode/utf8"
+
+	"github.com/spf13/cast"
+)
+
+var (
+	tagRE        = regexp.MustCompile(`^<(/)?([^ ]+?)(?:(\s*/)| .*?)?>`)
+	htmlSinglets = map[string]bool{
+		"br": true, "col": true, "link": true,
+		"base": true, "img": true, "param": true,
+		"area": true, "hr": true, "input": true,
+	}
+)
+
+type htmlTag struct {
+	name    string
+	pos     int
+	openTag bool
+}
+
+func truncate(a interface{}, options ...interface{}) (template.HTML, error) {
+	length, err := cast.ToIntE(a)
+	if err != nil {
+		return "", err
+	}
+	var textParam interface{}
+	var ellipsis string
+
+	switch len(options) {
+	case 0:
+		return "", errors.New("truncate requires a length and a string")
+	case 1:
+		textParam = options[0]
+		ellipsis = " …"
+	case 2:
+		textParam = options[1]
+		ellipsis, err = cast.ToStringE(options[0])
+		if err != nil {
+			return "", errors.New("ellipsis must be a string")
+		}
+		if _, ok := options[0].(template.HTML); !ok {
+			ellipsis = html.EscapeString(ellipsis)
+		}
+	default:
+		return "", errors.New("too many arguments passed to truncate")
+	}
+	if err != nil {
+		return "", errors.New("text to truncate must be a string")
+	}
+	text, err := cast.ToStringE(textParam)
+	if err != nil {
+		return "", errors.New("text must be a string")
+	}
+
+	_, isHTML := textParam.(template.HTML)
+
+	if utf8.RuneCountInString(text) <= length {
+		if isHTML {
+			return template.HTML(text), nil
+		}
+		return template.HTML(html.EscapeString(text)), nil
+	}
+
+	tags := []htmlTag{}
+	var lastWordIndex, lastNonSpace, currentLen, endTextPos, nextTag int
+
+	for i, r := range text {
+		if i < nextTag {
+			continue
+		}
+
+		if isHTML {
+			// Make sure we keep tag of HTML tags
+			slice := text[i:]
+			m := tagRE.FindStringSubmatchIndex(slice)
+			if len(m) > 0 && m[0] == 0 {
+				nextTag = i + m[1]
+				tagname := slice[m[4]:m[5]]
+				lastWordIndex = lastNonSpace
+				_, singlet := htmlSinglets[tagname]
+				if !singlet && m[6] == -1 {
+					tags = append(tags, htmlTag{name: tagname, pos: i, openTag: m[2] == -1})
+				}
+
+				continue
+			}
+		}
+
+		currentLen++
+		if unicode.IsSpace(r) {
+			lastWordIndex = lastNonSpace
+		} else if unicode.In(r, unicode.Han, unicode.Hangul, unicode.Hiragana, unicode.Katakana) {
+			lastWordIndex = i
+		} else {
+			lastNonSpace = i + utf8.RuneLen(r)
+		}
+
+		if currentLen > length {
+			if lastWordIndex == 0 {
+				endTextPos = i
+			} else {
+				endTextPos = lastWordIndex
+			}
+			out := text[0:endTextPos]
+			if isHTML {
+				out += ellipsis
+				// Close out any open HTML tags
+				var currentTag *htmlTag
+				for i := len(tags) - 1; i >= 0; i-- {
+					tag := tags[i]
+					if tag.pos >= endTextPos || currentTag != nil {
+						if currentTag != nil && currentTag.name == tag.name {
+							currentTag = nil
+						}
+						continue
+					}
+
+					if tag.openTag {
+						out += ("</" + tag.name + ">")
+					} else {
+						currentTag = &tag
+					}
+				}
+
+				return template.HTML(out), nil
+			}
+			return template.HTML(html.EscapeString(out) + ellipsis), nil
+		}
+	}
+
+	if isHTML {
+		return template.HTML(text), nil
+	}
+	return template.HTML(html.EscapeString(text)), nil
+}
diff --git a/tpl/template_func_truncate_test.go b/tpl/template_func_truncate_test.go
new file mode 100644
index 000000000..95368f83e
--- /dev/null
+++ b/tpl/template_func_truncate_test.go
@@ -0,0 +1,82 @@
+// Copyright 2016 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 tpl
+
+import (
+	"html/template"
+	"reflect"
+	"strings"
+	"testing"
+)
+
+func TestTruncate(t *testing.T) {
+	var err error
+	cases := []struct {
+		v1    interface{}
+		v2    interface{}
+		v3    interface{}
+		want  interface{}
+		isErr bool
+	}{
+		{10, "I am a test sentence", nil, template.HTML("I am a …"), false},
+		{10, "", "I am a test sentence", template.HTML("I am a"), false},
+		{10, "", "a b c d e f g h i j k", template.HTML("a b c d e"), false},
+		{12, "", "<b>Should be escaped</b>", template.HTML("&lt;b&gt;Should be"), false},
+		{10, template.HTML(" <a href='#'>Read more</a>"), "I am a test sentence", template.HTML("I am a <a href='#'>Read more</a>"), false},
+		{20, template.HTML("I have a <a href='/markdown'>Markdown link</a> inside."), nil, template.HTML("I have a <a href='/markdown'>Markdown …</a>"), false},
+		{10, "IamanextremelylongwordthatjustgoesonandonandonjusttoannoyyoualmostasifIwaswritteninGermanActuallyIbettheresagermanwordforthis", nil, template.HTML("Iamanextre …"), false},
+		{10, template.HTML("<p>IamanextremelylongwordthatjustgoesonandonandonjusttoannoyyoualmostasifIwaswritteninGermanActuallyIbettheresagermanwordforthis</p>"), nil, template.HTML("<p>Iamanextre …</p>"), false},
+		{13, template.HTML("With <a href=\"/markdown\">Markdown</a> inside."), nil, template.HTML("With <a href=\"/markdown\">Markdown …</a>"), false},
+		{14, "Hello中国 Good 好的", nil, template.HTML("Hello中国 Good 好 …"), false},
+		{15, "", template.HTML("A <br> tag that's not closed"), template.HTML("A <br> tag that's"), false},
+		{14, template.HTML("<p>Hello中国 Good 好的</p>"), nil, template.HTML("<p>Hello中国 Good 好 …</p>"), false},
+		{2, template.HTML("<p>P1</p><p>P2</p>"), nil, template.HTML("<p>P1 …</p>"), false},
+		{3, template.HTML(strings.Repeat("<p>P</p>", 20)), nil, template.HTML("<p>P</p><p>P</p><p>P …</p>"), false},
+		{18, template.HTML("<p>test <b>hello</b> test something</p>"), nil, template.HTML("<p>test <b>hello</b> test …</p>"), false},
+		{4, template.HTML("<p>a<b><i>b</b>c d e</p>"), nil, template.HTML("<p>a<b><i>b</b>c …</p>"), false},
+		{10, nil, nil, template.HTML(""), true},
+		{nil, nil, nil, template.HTML(""), true},
+	}
+	for i, c := range cases {
+		var result template.HTML
+		if c.v2 == nil {
+			result, err = truncate(c.v1)
+		} else if c.v3 == nil {
+			result, err = truncate(c.v1, c.v2)
+		} else {
+			result, err = truncate(c.v1, c.v2, c.v3)
+		}
+
+		if c.isErr {
+			if err == nil {
+				t.Errorf("[%d] Slice didn't return an expected error", i)
+			}
+		} else {
+			if err != nil {
+				t.Errorf("[%d] failed: %s", i, err)
+				continue
+			}
+			if !reflect.DeepEqual(result, c.want) {
+				t.Errorf("[%d] got '%s' but expected '%s'", i, result, c.want)
+			}
+		}
+	}
+
+	// Too many arguments
+	_, err = truncate(10, " ...", "I am a test sentence", "wrong")
+	if err == nil {
+		t.Errorf("Should have errored")
+	}
+
+}
diff --git a/tpl/template_funcs.go b/tpl/template_funcs.go
index fd3337454..596902fcd 100644
--- a/tpl/template_funcs.go
+++ b/tpl/template_funcs.go
@@ -2188,6 +2188,7 @@ func initFuncMap() {
 		"title":        title,
 		"time":         asTime,
 		"trim":         trim,
+		"truncate":     truncate,
 		"upper":        upper,
 		"urlize":       helpers.CurrentPathSpec().URLize,
 		"where":        where,
diff --git a/tpl/template_funcs_test.go b/tpl/template_funcs_test.go
index 37f075a99..fd51e3a1a 100644
--- a/tpl/template_funcs_test.go
+++ b/tpl/template_funcs_test.go
@@ -157,6 +157,8 @@ substr: {{substr "BatMan" 3 3}}
 title: {{title "Bat man"}}
 time: {{ (time "2015-01-21").Year }}
 trim: {{ trim "++Batman--" "+-" }}
+truncate: {{ "this is a very long text" | truncate 10 " ..." }}
+truncate: {{ "With [Markdown](/markdown) inside." | markdownify | truncate 14 }}
 upper: {{upper "BatMan"}}
 urlize: {{ "Bat Man" | urlize }}
 `
@@ -228,6 +230,8 @@ substr: Man
 title: Bat Man
 time: 2015
 trim: Batman
+truncate: this is a ...
+truncate: With <a href="/markdown">Markdown …</a>
 upper: BATMAN
 urlize: bat-man
 `