docs: improve functions reference

docs/misc/update_functions.rb

@@ -0,0 +1,77 @@
+#!/usr/bin/ruby
+## A script to update functions in nightly references
+## Usage: ruby misc/update_functions.rb [nightly | v0.x]
+# encoding: utf-8
+require 'net/http'
+require 'rdoc'
+
+scalar_functions = {
+  :name => "DataFusion Scalar Functions",
+  :url => "https://raw.githubusercontent.com/apache/datafusion/main/docs/source/user-guide/sql/scalar_functions.md"
+}
+agg_functions = {
+  :name => "DataFusion Aggregate Functions",
+  :url => "https://raw.githubusercontent.com/apache/datafusion/main/docs/source/user-guide/sql/aggregate_functions.md"
+}
+window_functions = {
+  :name => "DataFusion Window Functions",
+  :url => "https://raw.githubusercontent.com/apache/datafusion/main/docs/source/user-guide/sql/window_functions.md"
+}
+
+## Apache DataFusion functions
+datafusion_functions = [scalar_functions, agg_functions, window_functions]
+
+
+File.open("temp.md", "w") do |f|
+  f.puts("# DataFusion Functions")
+  f.puts("This page is generated from the Apache DataFusion project's documents:")
+
+  for doc in datafusion_functions
+    name = doc[:name]
+    url = doc[:url]
+    markdown = "  * [#{name}](#{url})"
+    f.puts markdown
+  end
+
+  f.puts
+
+  for doc in datafusion_functions
+    uri = URI(doc[:url])
+    markdown = Net::HTTP.get(uri).force_encoding(Encoding::UTF_8)
+
+    lines = markdown.split(/\n/)
+    lines.map! do |line|
+      ## Add a level of headlines
+      if line =~ /^\s*(#+)\s/
+        replacement = "#{$1}#"
+
+        if replacement.length >= 4
+          replacement += "#"
+        end
+        line.gsub! $1, replacement
+      end
+      ## Fix link: #a_b_c -> #a-b-c
+      if line =~ /(#[a-zA-Z0-9_]+)\)/
+        link = $1.to_s
+        replacement = link.gsub("_", "-")
+        line.gsub! link, replacement
+      end
+      ## Fix link: a_b_c.md -> #a-b-c
+      if line =~ /\]\(([a-zA-Z0-9_]+\.md)\)/
+        link = $1.to_s
+        replacement = "#" + link.gsub("_", "-").gsub("\.md", "")
+        line.gsub! link, replacement
+      end
+
+      line
+    end
+
+    f.puts lines.join("\n")
+  end
+end
+
+target = ARGV[0] || "nightly"
+
+File.rename "temp.md", "#{target}/en/reference/sql/df_functions.md"
+
+puts "#{target}/en/reference/sql/df_functions.md updated!"