Rework KeywordMute interface to use a matcher object; spec out matcher. #164.

A matcher object that builds a match from KeywordMute data and runs it
over text is, in my view, one of the easier ways to write examples for
this sort of thing.

app/models/keyword_mute.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 # == Schema Information
 #
 # Table name: keyword_mutes
@@ -10,6 +11,34 @@
 #
 
 class KeywordMute < ApplicationRecord
-  def self.matches?(text)
+  belongs_to :account, required: true
+
+  validates_presence_of :keyword
+
+  def self.matcher_for(account)
+    Rails.cache.fetch("keyword_mutes:matcher:#{account}") { Matcher.new(account) }
+  end
+
+  class Matcher
+    attr_reader :regex
+
+    def initialize(account)
+      re = String.new.tap do |str|
+        scoped = KeywordMute.where(account: account)
+        keywords = scoped.select(:id, :keyword)
+        count = scoped.count
+
+        keywords.find_each.with_index do |kw, index|
+          str << Regexp.escape(kw.keyword.strip)
+          str << '|' if index < count - 1
+        end
+      end
+
+      @regex = /\b(?:#{re})\b/i unless re.empty?
+    end
+
+    def =~(str)
+      @regex ? @regex =~ str : false
+    end
   end
 end