ファイル検索用のインタプリタを作成する過程から、 interpreter パターンを学ぶ

** すべてのファイルを検索する/名前でファイルを検索する/大きなファイル/書き込み可能ファイル

require 'find'

class Expression
end

# すべてのファイルを検索する
class All < Expression
 def evaluate(dir)
   results= []
   Find.find(dir) do |p|
     next unless File.file?(p)
     results << p
   end
   results
 end
end

expr_all = All.new
files = expr_all.evaluate('test_dir')

files # => ["test_dir/small2.txt", "test_dir/subdir/small.jpg", "test_dir/subdir/other.mp3", "test_dir/small1.txt", "test_dir/big.mp3", "test_dir/small.mp3", "test_dir/big2.mp3"]

# 名前でファイルを検索する
class FileName < Expression
 def initialize(pattern)
   @pattern = pattern
 end

 def evaluate(dir)
   results= []
   Find.find(dir) do |p|
     next unless File.file?(p)
     name = File.basename(p)
     results << p if File.fnmatch(@pattern, name)
   end
   results
 end
end

expr_mp3 = FileName.new('*.mp3')
mp3s = expr_mp3.evaluate('test_dir')
mp3s  # => ["test_dir/subdir/other.mp3", "test_dir/big.mp3", "test_dir/small.mp3", "test_dir/big2.mp3"]

# 大きなファイル
class Bigger < Expression
 def initialize(size)
   @size = size
 end

 def evaluate(dir)
   results = []
   Find.find(dir) do |p| 
     next unless File.file?(p)
     results << p if( File.size(p) > @size)
   end
   results
 end
end

expr_big = Bigger.new(80)
bigs = expr_big.evaluate('test_dir')
bigs  # => ["test_dir/subdir/other.mp3", "test_dir/big.mp3", "test_dir/big2.mp3"]

#書き込み可能ファイル
class Writable < Expression
 def evaluate(dir)
   results = []
   Find.find(dir) do |p| 
     next unless File.file?(p)
     results << p if( File.writable?(p) )
   end
   results
 end
end

expr_writable = Writable.new
writables = expr_writable.evaluate('test_dir')
writables # => ["test_dir/small2.txt", "test_dir/subdir/small.jpg", "test_dir/subdir/other.mp3", "test_dir/small1.txt", "test_dir/big.mp3", "test_dir/small.mp3", "test_dir/big2.mp3"]

** Not,And,Or を使ったより複雑な検索

class Not < Expression
 def initialize(expression)
   @expression = expression
 end

 def evaluate(dir)
   All.new.evaluate(dir) - @expression.evaluate(dir)
 end
end

expr_not_writable = Not.new(Writable.new)
readonly_files = expr_not_writable.evaluate('test_dir')
readonly_files    # => ["test_dir/small1.txt"]

small_expr = Not.new(Bigger.new(1024))
small_files = small_expr.evaluate('test_dir')
small_files    # => ["test_dir/small2.txt", "test_dir/subdir/small.jpg", "test_dir/small1.txt", "test_dir/small.mp3"]

not_mp3_expr = Not.new(FileName.new('*.mp3'))
not_mp3_files = not_mp3_expr.evaluate('test_dir')
not_mp3_files  # => ["test_dir/small2.txt", "test_dir/subdir/small.jpg", "test_dir/small1.txt"]

class Or < Expression
 def initialize(expression1, expression2)
   @expression1 = expression1
   @expression2 = expression2
 end
 
 def evaluate(dir)
   result1 = @expression1.evaluate(dir)
   result2 = @expression2.evaluate(dir)
   (result1 + result2).sort.uniq
 end
end

# MP3ファイルまたは1KBより大きいファイルを一度に検索する

big_or_mp3_expr = Or.new(Bigger.new(1024), FileName.new('*.mp3'))
big_or_mp3s = big_or_mp3_expr.evaluate('test_dir')
big_or_mp3s   # => ["test_dir/big.mp3", "test_dir/big2.mp3", "test_dir/small.mp3", "test_dir/subdir/other.mp3"]

#このような複雑なASTを作ると、別のコンテキストでそれを繰り返し利用できる。
# big_or_mp3_expr.evaluate('test_dir')
# big_or_mp3_expr.evaluate('/home/me/tmp')

class And < Expression
 def initialize(expression1, expression2)
   @expression1 = expression1
   @expression2 = expression2
 end

 def evaluate(dir)
   result1 = @expression1.evaluate(dir)
   result2 = @expression2.evaluate(dir)
   (result1 & result2)
 end
end

** パーサを使わないインタープリタにすることにした/便利なショートカットを導入(DSL)

上位クラス(Expression) にAnd,Or のシンタックスシュガーを作成して書きやすくする

class Expression
  def |(other)
    Or.new(self, other)
  end

  def &(other)
    And.new(self, other)
  end
end

expr = Or.new(
         And.new(Bigger.new(1024), Not.new(Writable.new )),
         FileName.new('*.mp3'))
files = expr.evaluate('test_dir')
files              # => ["test_dir/big.mp3", "test_dir/big2.mp3", "test_dir/small.mp3", "test_dir/subdir/other.mp3"]

expr = (Bigger.new(1024) & Not.new(Writable.new)) | FileName.new('*.mp3') #これは短かくて読みやすい!
files = expr.evaluate('test_dir')
files              # => ["test_dir/big.mp3", "test_dir/big2.mp3", "test_dir/small.mp3", "test_dir/subdir/other.mp3"]