[SPARK-10382] Make example code in user guide testable

A POC code for making example code in user guide testable.

mengxr We still need to talk about the labels in code.

Author: Xusen Yin <yinxusen@gmail.com>

Closes #9109 from yinxusen/SPARK-10382.

docs/_plugins/include_example.rb

+#
+# Licensed to the Apache Software Foundation (ASF) under one or more
+# contributor license agreements.  See the NOTICE file distributed with
+# this work for additional information regarding copyright ownership.
+# The ASF licenses this file to You 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.
+#
+
+require 'liquid'
+require 'pygments'
+
+module Jekyll
+  class IncludeExampleTag < Liquid::Tag
+    
+    def initialize(tag_name, markup, tokens)
+      @markup = markup
+      super
+    end
+ 
+    def render(context)
+      site = context.registers[:site]
+      config_dir = (site.config['code_dir'] || '../examples/src/main').sub(/^\//,'')
+      @code_dir = File.join(site.source, config_dir)
+
+      clean_markup = @markup.strip
+      @file = File.join(@code_dir, clean_markup)
+      @lang = clean_markup.split('.').last
+
+      code = File.open(@file).read.encode("UTF-8")
+      code = select_lines(code)
+ 
+      Pygments.highlight(code, :lexer => @lang)
+    end
+ 
+    # Trim the code block so as to have the same indention, regardless of their positions in the
+    # code file.
+    def trim_codeblock(lines)
+      # Select the minimum indention of the current code block.
+      min_start_spaces = lines
+        .select { |l| l.strip.size !=0 }
+        .map { |l| l[/\A */].size }
+        .min
+
+      lines.map { |l| l[min_start_spaces .. -1] }
+    end
+
+    # Select lines according to labels in code. Currently we use "$example on$" and "$example off$"
+    # as labels. Note that code blocks identified by the labels should not overlap.
+    def select_lines(code)
+      lines = code.each_line.to_a
+
+      # Select the array of start labels from code.
+      startIndices = lines
+        .each_with_index
+        .select { |l, i| l.include? "$example on$" }
+        .map { |l, i| i }
+
+      # Select the array of end labels from code.
+      endIndices = lines
+        .each_with_index
+        .select { |l, i| l.include? "$example off$" }
+        .map { |l, i| i }
+
+      raise "Start indices amount is not equal to end indices amount, please check the code." \
+        unless startIndices.size == endIndices.size
+
+      raise "No code is selected by include_example, please check the code." \
+        if startIndices.size == 0
+
+      # Select and join code blocks together, with a space line between each of two continuous
+      # blocks.
+      lastIndex = -1
+      result = ""
+      startIndices.zip(endIndices).each do |start, endline|
+        raise "Overlapping between two example code blocks are not allowed." if start <= lastIndex
+        raise "$example on$ should not be in the same line with $example off$." if start == endline
+        lastIndex = endline
+        range = Range.new(start + 1, endline - 1)
+        result += trim_codeblock(lines[range]).join
+        result += "\n"
+      end
+      result
+    end
+  end
+end
+
+Liquid::Template.register_tag('include_example', Jekyll::IncludeExampleTag)