Optimize some string handling (#170)

* Don't use fmt.Sprintf on simple strings
* Use bytes.Buffer in encodeTomlString

name                old time/op    new time/op    delta
Lexer-8                162µs ± 0%     161µs ± 0%   -0.12%
TreeToTomlString-8    19.7µs ± 0%     7.5µs ± 0%  -61.80%

name                old alloc/op   new alloc/op   delta
TreeToTomlString-8    9.75kB ± 0%    4.96kB ± 0%  -49.12%

name                old allocs/op  new allocs/op  delta
TreeToTomlString-8       485 ± 0%        78 ± 0%  -83.92%

tomltree_write.go

@@ -13,33 +13,34 @@ import (
 
 // encodes a string to a TOML-compliant string value
 func encodeTomlString(value string) string {
-	result := ""
+	var b bytes.Buffer
+
 	for _, rr := range value {
 		switch rr {
 		case '\b':
-			result += "\\b"
+			b.WriteString(`\b`)
 		case '\t':
-			result += "\\t"
+			b.WriteString(`\t`)
 		case '\n':
-			result += "\\n"
+			b.WriteString(`\n`)
 		case '\f':
-			result += "\\f"
+			b.WriteString(`\f`)
 		case '\r':
-			result += "\\r"
+			b.WriteString(`\r`)
 		case '"':
-			result += "\\\""
+			b.WriteString(`\"`)
 		case '\\':
-			result += "\\\\"
+			b.WriteString(`\\`)
 		default:
 			intRr := uint16(rr)
 			if intRr < 0x001F {
-				result += fmt.Sprintf("\\u%0.4X", intRr)
+				b.WriteString(fmt.Sprintf("\\u%0.4X", intRr))
 			} else {
-				result += string(rr)
+				b.WriteRune(rr)
 			}
 		}
 	}
-	return result
+	return b.String()
 }
 
 func tomlValueStringRepresentation(v interface{}) (string, error) {
@@ -111,7 +112,7 @@ func (t *Tree) writeTo(w io.Writer, indent, keyspace string, bytesCount int64) (
 			return bytesCount, err
 		}
 
-		kvRepr := fmt.Sprintf("%s%s = %s\n", indent, k, repr)
+		kvRepr := indent + k + " = " + repr + "\n"
 		writtenBytesCount, err := w.Write([]byte(kvRepr))
 		bytesCount += int64(writtenBytesCount)
 		if err != nil {
@@ -130,7 +131,7 @@ func (t *Tree) writeTo(w io.Writer, indent, keyspace string, bytesCount int64) (
 		switch node := v.(type) {
 		// node has to be of those two types given how keys are sorted above
 		case *Tree:
-			tableName := fmt.Sprintf("\n%s[%s]\n", indent, combinedKey)
+			tableName := "\n" + indent + "[" + combinedKey + "]\n"
 			writtenBytesCount, err := w.Write([]byte(tableName))
 			bytesCount += int64(writtenBytesCount)
 			if err != nil {
@@ -142,7 +143,7 @@ func (t *Tree) writeTo(w io.Writer, indent, keyspace string, bytesCount int64) (
 			}
 		case []*Tree:
 			for _, subTree := range node {
-				tableArrayName := fmt.Sprintf("\n%s[[%s]]\n", indent, combinedKey)
+				tableArrayName := "\n" + indent + "[[" + combinedKey + "]]\n"
 				writtenBytesCount, err := w.Write([]byte(tableArrayName))
 				bytesCount += int64(writtenBytesCount)
 				if err != nil {

tomltree_write_test.go

@@ -293,3 +293,51 @@ func TestTreeWriteToMapWithArrayOfInlineTables(t *testing.T) {
 	treeMap := tree.ToMap()
 	testMaps(t, treeMap, expected)
 }
+
+func BenchmarkTreeToTomlString(b *testing.B) {
+	toml, err := Load(sampleHard)
+	if err != nil {
+		b.Fatal("Unexpected error:", err)
+	}
+
+	for i := 0; i < b.N; i++ {
+		_, err := toml.ToTomlString()
+		if err != nil {
+			b.Fatal(err)
+		}
+	}
+}
+
+var sampleHard = `# Test file for TOML
+# Only this one tries to emulate a TOML file written by a user of the kind of parser writers probably hate
+# This part you'll really hate
+
+[the]
+test_string = "You'll hate me after this - #"          # " Annoying, isn't it?
+
+    [the.hard]
+    test_array = [ "] ", " # "]      # ] There you go, parse this!
+    test_array2 = [ "Test #11 ]proved that", "Experiment #9 was a success" ]
+    # You didn't think it'd as easy as chucking out the last #, did you?
+    another_test_string = " Same thing, but with a string #"
+    harder_test_string = " And when \"'s are in the string, along with # \""   # "and comments are there too"
+    # Things will get harder
+
+        [the.hard."bit#"]
+        "what?" = "You don't think some user won't do that?"
+        multi_line_array = [
+            "]",
+            # ] Oh yes I did
+            ]
+
+# Each of the following keygroups/key value pairs should produce an error. Uncomment to them to test
+
+#[error]   if you didn't catch this, your parser is broken
+#string = "Anything other than tabs, spaces and newline after a keygroup or key value pair has ended should produce an error unless it is a comment"   like this
+#array = [
+#         "This might most likely happen in multiline arrays",
+#         Like here,
+#         "or here,
+#         and here"
+#         ]     End of array comment, forgot the #
+#number = 3.14  pi <--again forgot the #         `