Add XPath selector cache feature.

Author: zhengchun

.travis.yml

@@ -9,6 +9,7 @@ go:
 install:
   - go get github.com/antchfx/xpath
   - go get github.com/mattn/goveralls
-
+  - go get github.com/golang/groupcache
+  
 script:
   - $HOME/gopath/bin/goveralls -service=travis-ci

README.md

@@ -8,14 +8,15 @@ jsonquery
 Overview
 ===
 
-jsonquery is an XPath query package for JSON document, lets you extract data from JSON documents through an XPath expression.
+jsonquery is an XPath query package for JSON document, lets you extract data from JSON documents through an XPath expression. Built-in XPath expression cache avoid re-compile XPath expression each query.
 
 Getting Started
 ===
 
 ### Install Package
-
-> $ go get github.com/antchfx/jsonquery
+```
+go get github.com/antchfx/jsonquery
+```
 
 #### Load JSON document from URL.
 

cache.go

@@ -0,0 +1,40 @@
+package jsonquery
+
+import (
+	"sync"
+
+	"github.com/golang/groupcache/lru"
+
+	"github.com/antchfx/xpath"
+)
+
+// DisableSelectorCache will disable caching for the query selector if value is true.
+var DisableSelectorCache = false
+
+// SelectorCacheMaxEntries allows how many selector object can be caching. Default is 50.
+// Will disable caching if SelectorCacheMaxEntries <= 0.
+var SelectorCacheMaxEntries = 50
+
+var (
+	cacheOnce sync.Once
+	cache     *lru.Cache
+)
+
+func getQuery(expr string) (*xpath.Expr, error) {
+	if DisableSelectorCache || SelectorCacheMaxEntries <= 0 {
+		return xpath.Compile(expr)
+	}
+	cacheOnce.Do(func() {
+		cache = lru.New(50)
+	})
+	if v, ok := cache.Get(expr); ok {
+		return v.(*xpath.Expr), nil
+	}
+	v, err := xpath.Compile(expr)
+	if err != nil {
+		return nil, err
+	}
+	cache.Add(expr, v)
+	return v, nil
+
+}

query.go

@@ -34,7 +34,7 @@ func FindOne(top *Node, expr string) *Node {
 // QueryAll searches the Node that matches by the specified XPath expr.
 // Return an error if the expression `expr` cannot be parsed.
 func QueryAll(top *Node, expr string) ([]*Node, error) {
-	exp, err := xpath.Compile(expr)
+	exp, err := getQuery(expr)
 	if err != nil {
 		return nil, err
 	}
@@ -44,7 +44,7 @@ func QueryAll(top *Node, expr string) ([]*Node, error) {
 // Query searches the Node that matches by the specified XPath expr,
 // and returns first element of matched.
 func Query(top *Node, expr string) (*Node, error) {
-	exp, err := xpath.Compile(expr)
+	exp, err := getQuery(expr)
 	if err != nil {
 		return nil, err
 	}

query_test.go

@@ -7,6 +7,20 @@ import (
 	"github.com/antchfx/xpath"
 )
 
+func BenchmarkSelectorCache(b *testing.B) {
+	DisableSelectorCache = false
+	for i := 0; i < b.N; i++ {
+		getQuery("/AAA/BBB/DDD/CCC/EEE/ancestor::*")
+	}
+}
+
+func BenchmarkDisableSelectorCache(b *testing.B) {
+	DisableSelectorCache = true
+	for i := 0; i < b.N; i++ {
+		getQuery("/AAA/BBB/DDD/CCC/EEE/ancestor::*")
+	}
+}
+
 func TestNavigator(t *testing.T) {
 	s := `{
 		"name":"John",