metrics: Detect partialCached candidates

This commit adds a "cache potential" column when running `hugo --templateMetrics --templateMetricsHints`.

This is only calculated when `--templateMetricsHints` is set, as these calculations has an negative effect on the other timings.

This gives a value for partials only, and is a number between 0-100 that indicates if `partial` can be replaced with `partialCached`.

100 means that all execution of the same partial resulted in the same output.

You should do  some manual research before going "all cache".

metrics/metrics_test.go

@@ -0,0 +1,50 @@
+// Copyright 2017 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 metrics
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestSimilarPercentage(t *testing.T) {
+	assert := require.New(t)
+
+	sentence := "this is some words about nothing, Hugo!"
+	words := strings.Fields(sentence)
+	for i, j := 0, len(words)-1; i < j; i, j = i+1, j-1 {
+		words[i], words[j] = words[j], words[i]
+	}
+	sentenceReversed := strings.Join(words, " ")
+
+	assert.Equal(100, howSimilar("Hugo Rules", "Hugo Rules"))
+	assert.Equal(50, howSimilar("Hugo Rules", "Hugo Rocks"))
+	assert.Equal(66, howSimilar("The Hugo Rules", "The Hugo Rocks"))
+	assert.Equal(66, howSimilar("The Hugo Rules", "The Hugo"))
+	assert.Equal(66, howSimilar("The Hugo", "The Hugo Rules"))
+	assert.Equal(0, howSimilar("Totally different", "Not Same"))
+	assert.Equal(14, howSimilar(sentence, sentenceReversed))
+
+}
+
+func BenchmarkHowSimilar(b *testing.B) {
+	s1 := "Hugo is cool and " + strings.Repeat("fun ", 10) + "!"
+	s2 := "Hugo is cool and " + strings.Repeat("cool ", 10) + "!"
+
+	for i := 0; i < b.N; i++ {
+		howSimilar(s1, s2)
+	}
+}