bf7c721815
Refactor the `PostQuery::AST#simplify` method to split it into three methods: `#trim` to eliminate redundant AND and OR clauses, `#simplify` to expand deeply nested subexpressions, and `#sort` to sort the query into alphabetical order. This is so we can normalize queries written by users by parsing and rewriting them, but without expanding out nested subexpressions, which can substantially alter the way the query is written.
349 lines
10 KiB
Ruby
349 lines
10 KiB
Ruby
# frozen_string_literal: true
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# A PostQuery::AST is an abstract syntax tree representing a search parsed by
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# `PostQuery::Parser#parse`. It has methods for printing, manipulating, and
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# simpifying ASTs returned by the parser.
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#
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# There are nine AST node types:
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#
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# * :all (representing the search that returns everything, aka the empty search)
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# * :none (representing the search that returns nothing)
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# * :tag (a single tag)
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# * :metatag (a metatag with a name and value)
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# * :wildcard (a wildcard tag, e.g. `blue_*`)
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# * :and (an n-ary AND clause)
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# * :or (an n-nary OR clause)
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# * :not (a unary NOT clause)
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# * :opt (the unary `~`, or 'optional' operator)
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#
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# The AST returned by the parser is normally rewritten with `#to_cnf` before
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# it's used. This is for several reasons:
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#
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# * To replace the `~` operator with `or` clauses.
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# * To remove redundant `and` and `or` nodes.
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# * To transform the AST to conjunctive normal form.
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# * To sort the AST into alphabetical order.
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#
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# @example
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#
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# PostQuery::AST.new(:or, [PostQuery::AST.new(:tag, "1girl"), PostQuery::AST.new(:metatag, "rating", "s")]).to_sexp
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# => "(or 1girl rating:s)"
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#
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# PostQuery::Parser.parse("cat_girl or (cat_ears tail)").to_sexp
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# => "(or (and cat_girl) (and (and cat_ears tail)))"
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#
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# PostQuery::Parser.parse("cat_girl or (cat_ears tail)").to_cnf.to_sexp
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# => "(and (or cat_ears cat_girl) (or cat_girl tail))"
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class PostQuery
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class AST
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extend Memoist
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include Comparable
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include Enumerable
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attr_reader :type, :args
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delegate :all?, :none?, :and?, :or?, :not?, :opt?, :tag?, :metatag?, :wildcard?, to: :inquirer
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# Create an AST node.
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#
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# @param type [Symbol] The type of the AST node.
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# @param args [Array] The arguments for the node (either a list of child nodes for
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# AND/OR/NOT/OPT nodes, or the name and/or value for tag, metatag, or wildcard nodes).
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def initialize(type, args)
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@type = type
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@args = args
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end
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# Create an AST node.
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def node(type, *args)
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AST.new(type, args)
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end
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concerning :SimplificationMethods do
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# Convert the AST to conjunctive normal form, that is, product-of-sums
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# form, or an AND of ORs. The result is a single top-level AND clause,
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# containing a series of tags, metatags, and OR clauses, with no deeply
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# nested subexpressions.
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#
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# @return [AST] A new AST in conjunctive normal form.
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def to_cnf
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rewrite_opts.simplify.sort
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end
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# Rewrite the `~` operator to `or` clauses.
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#
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# @return [AST] A new AST with `:opt` nodes replaced with `:or` nodes.
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def rewrite_opts
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rewrite do |ast|
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# ... ~A ~B ... = ... (or A B) ...
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# ... ~A ... = ... (or A) ... = ... A ...
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if ast.children.any?(&:opt?)
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opts, non_opts = ast.children.partition(&:opt?)
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or_node = node(:or, *opts.flat_map(&:children))
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node(ast.type, or_node, *non_opts)
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else
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ast
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end
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end
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end
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# Simplify the AST by eliminating unnecessary AND and OR nodes, and by
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# expanding out deeply nested subexpressions. The result is an AST in
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# conjunctive normal form.
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#
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# @return [AST] A new AST in conjunctive normal form.
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def simplify
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repeat_until_unchanged do |ast|
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ast.trim_once.simplify_once
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end
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end
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# Simplify the AST once in a single top-down pass by applying the double
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# negation law, DeMorgan's law, and the distributive law. This expands
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# out deeply nested subexpressions.
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#
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# @return [AST] A new simplified AST
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def simplify_once
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rewrite do |ast|
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case ast
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# Double negation: -(-A) = A
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in [:not, [:not, a]]
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a
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# DeMorgan's law: -(A and B) = -A or -B
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in [:not, [:and, *children]]
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node(:or, *children.map { node(:not, _1) })
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# DeMorgan's law: -(A or B) = -A and -B
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in [:not, [:or, *children]]
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node(:and, *children.map { node(:not, _1) })
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# Distributive law: A or (B and C) = (A or B) and (A or C)
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# (or A (and B C ...) ... = (and (or A B ...) (or A C ...) ...
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in [:or, *children] if children.any?(&:and?)
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ands, non_ands = children.partition(&:and?)
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first_and, rest = ands.first, ands[1..] + non_ands
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node(:and, *first_and.children.map { node(:or, _1, *rest) })
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else
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ast
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end
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end
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end
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# Trim the AST by eliminating redundant AND and OR clauses.
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def trim
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repeat_until_unchanged(&:trim_once)
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end
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def trim_once
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rewrite do |ast|
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case ast
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# (and A) = A; (or A) = A
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in :and | :or, a
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a
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# Associative law: (and (and A B) C) = (and A B C)
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in :and, *children
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node(:and, *children.flat_map { _1.and? ? _1.children : _1 })
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# Associative law: (or (or A B) C) = (or A B C)
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in :or, *children
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node(:or, *children.flat_map { _1.or? ? _1.children : _1 })
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else
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ast
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end
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end
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end
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# Sort the AST into alphabetical order.
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def sort
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if children.present?
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node(type, *children.map(&:sort).sort)
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else
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self
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end
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end
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end
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concerning :OutputMethods do
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def inspect
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to_sexp
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end
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# Display the AST as an S-expression.
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def to_sexp
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case self
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in [:all]
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"all"
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in [:none]
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"none"
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in [:tag, name]
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name
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in [:metatag, name, value]
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"#{name}:#{quoted_value}"
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in [:wildcard, name]
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"(wildcard #{name})"
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in [type, *args]
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"(#{type} #{args.map(&:to_sexp).join(" ")})"
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end
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end
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# Display the AST in infix notation.
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def to_infix
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case self
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in [:all]
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""
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in [:none]
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"none"
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in [:wildcard, name]
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name
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in [:tag, name]
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name
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in [:metatag, name, value]
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"#{name}:#{quoted_value}"
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in :not, child
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child.term? ? "-#{child.to_infix}" : "-(#{child.to_infix})"
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in :opt, child
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child.term? ? "~#{child.to_infix}" : "~(#{child.to_infix})"
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in :and, *children
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children.map { _1.children.many? ? "(#{_1.to_infix})" : _1.to_infix }.join(" ")
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in :or, *children
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children.map { _1.children.many? ? "(#{_1.to_infix})" : _1.to_infix }.join(" or ")
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end
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end
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# Convert the AST to a series of nested arrays.
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def to_tree
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if term?
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[type, *args]
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else
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[type, *args.map(&:to_tree)]
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end
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end
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end
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concerning :TraversalMethods do
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# Traverse the AST in depth-first left-to-right order, calling the block on each
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# node and passing it the current node and the results from visiting each subtree.
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def visit(&block)
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return enum_for(:visit) unless block_given?
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results = children.map { _1.visit(&block) }
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yield self, *results
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end
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# Traverse the AST in depth-first left-to-right order, calling the block on each node.
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def each(&block)
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return enum_for(:each) unless block_given?
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visit { |node| yield node }
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self
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end
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# Rewrite the AST by calling the block on each node and replacing the node with the result.
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def rewrite(&block)
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ast = yield self
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if ast.children.any?
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node(ast.type, *ast.children.map { _1.rewrite(&block) } )
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else
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ast
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end
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end
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# Call the block on the AST repeatedly until the output stops changing.
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#
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# `ast.repeat_until_unchanged(&:trim)` is like doing `ast.trim.trim.trim...`
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# until the AST can't be trimmed any more.
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def repeat_until_unchanged(&block)
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old = nil
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new = self
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until new == old
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old = new
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new = yield old
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end
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new
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end
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end
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concerning :UtilityMethods do
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# @return [Array<AST>] A flat list of all the nodes in the AST, in depth-first left-to-right order.
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def nodes
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each.map
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end
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# @return [Array<AST>] A list of all unique tag nodes in the AST.
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def tags
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nodes.select(&:tag?).uniq.sort
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end
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# @return [Array<AST>] A list of all unique metatag nodes in the AST.
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def metatags
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nodes.select(&:metatag?).uniq.sort
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end
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# @return [Array<AST>] A list of all unique wildcard nodes in the AST.
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def wildcards
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nodes.select(&:wildcard?).uniq.sort
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end
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# @return [Array<String>] The names of all unique tags in the AST.
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def tag_names
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tags.map(&:name)
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end
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# True if the AST is a simple node, that is a leaf node with no child nodes.
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def term?
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type.in?(%i[tag metatag wildcard all none])
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end
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# @return [String, nil] The name of the tag, metatag, or wildcard, if one of these nodes.
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def name
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args.first if tag? || metatag? || wildcard?
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end
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# @return [String, nil] The value of the metatag, if a metatag node.
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def value
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args.second if metatag?
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end
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# @return [String, nil] The value of the metatag as a quoted string, if a metatag node.
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def quoted_value
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return nil unless metatag?
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if value.include?(" ") || value.starts_with?('"') || value.empty?
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%Q{"#{value.gsub(/"/, '\\"')}"}
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else
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value
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end
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end
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# @return [Array<AST>] The child nodes, if the node has children.
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def children
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term? ? [] : args
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end
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def <=>(other)
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return nil unless other.is_a?(AST)
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deconstruct <=> other.deconstruct
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end
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# Deconstruct the node into an array (used for pattern matching).
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def deconstruct
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[type, *args]
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end
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def inquirer
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ActiveSupport::StringInquirer.new(type.to_s)
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end
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end
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memoize :to_cnf, :simplify, :simplify_once, :rewrite_opts, :trim, :trim_once, :sort, :inquirer, :deconstruct, :inspect, :to_sexp, :to_infix, :to_tree, :nodes, :tags, :metatags, :tag_names
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end
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end
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