require "./parcom/*"

module Parcom
  VERSION = "0.1.0"

  # A ParserFail exception should be raised by `Parser#parse` when
  # a parse attempt is unsuccessful.
  # Raising this exception in the `#parse` method of a Parser "Foo"
  # usually follows this pattern to allow for error tracing:
  #
  # ```
  # class Foo(T, V) < Parser(T, V)
  #   def parse(tokens : Tokens(T)) : Result(T, V)
  #     helper.parse(tokens)
  #   rescue ex : ParserFail
  #     raise ParserFail.new("Foo: #{ex.message}")
  #   end
  # ```
  class ParserFail < Exception
  end

  # `Tokens` is an `Array` wrapper struct to store the input
  # stream of one or more `Parser` objects.
  # A `Tokens` can be created from any `Iterable`, along with
  # `String` objects using a special constructor.
  struct Tokens(T)
    getter tokens

    # Constructs a `Tokens(Char)` from a `String`.
    def self.from_string(s : String) : Tokens(Char)
      Tokens.new(s.chars)
    end

    # Constructs a `Tokens` from an `Iterable`.
    def initialize(ts : Iterable(T))
      if ts.responds_to?(:to_a)
        @tokens = ts.to_a
      else
        @tokens = [] of T
        ts.each { |t| @tokens << t }
      end
    end

    # Exposes `Array#[](Int)`.
    def [](index : Int) : T
      @tokens[index]
    end

    # Exposes `Array#[](Int, Int)`, but wraps the returned array in a new `Tokens`.
    def [](start : Int, count : Int) : Tokens(T)
      Tokens.new(@tokens[start, count])
    end

    # Exposes `Array#[](Range)`, but wraps the returned array in a new `Tokens`.
    def [](range : Range) : Tokens(T)
      Tokens.new(@tokens[range])
    end

    # Like `#[]`, but returns `nil` instead of raising an `IndexError`.
    def []?(*args)
      self.[](*args)
    rescue IndexError
      nil
    end

    # Exposes `Array#empty?`.
    def empty? : Bool
      @tokens.empty?
    end
  end

  # A `Result` stores a `Tokens` object and a parsed value,
  # and is effectively used to store the state of a parser chain.
  # This is used instead of a `Tuple` or `NamedTuple` because:
  #   1. This is more idiomatic than a `Tuple`.
  #   2. Crystal does not support generic named tuples.
  struct Result(T, V)
    getter tokens, value

    def initialize(@tokens : Tokens(T), @value : V)
    end
  end

  class Between(T, V) < Parser(T, Array(V))
    @p : Map(T, {Array(V), Array(V)}, Array(V))

    def initialize(i : Int, j : Int, p : Parser(T, V))
      lower = i < j ? i : j
      upper = (i - j).abs
      @p = (Exactly.new(lower, p) + AtMost.new(upper, p)).map do |tup|
        tup[0] + tup[1]
      end
    end

    def parse(tokens : Tokens(T)) : Result(T, Array(V))
      @p.parse(tokens)
    rescue ex : ParserFail
      raise ParserFail.new("Between: #{ex.message}")
    end
  end

  class FirstOf(T, V) < Parser(T, V)
    @p : Parser(T, V)

    def initialize(ps : Iterable(Parser(T, V)))
      ps_iter = ps.each
      p = ps_iter.next

      if p.is_a?(Iterator::Stop)
        raise ArgumentError.new("FirstOf requires atleast one parser, got none")
      end

      @p = p
      p = ps_iter.next

      until p.is_a?(Iterator::Stop)
        @p = @p | p
        p = ps_iter.next
      end
    end

    def parse(tokens : Tokens(T)) : Result(T, V)
      @p.parse(tokens)
    rescue ex : ParserFail
      raise ParserFail.new("FirstOf: #{ex.message}")
    end
  end

  class SepBy(T, V, U) < Parser(T, Array(V))
    @p : Map(T, {V, Array(V)}, Array(V))

    def initialize(elem : Parser(T, V), sep : Parser(T, U))
      @p = (elem + Many(T, U).new(sep >> elem)).map do |tup|
        [tup[0]] + tup[1]
      end
    end

    def parse(tokens : Tokens(T)) : Result(T, Array(V))
      @p.parse(tokens)
    rescue ex : ParserFail
      raise ParserFail.new("SepBy: #{ex.message}")
    end
  end
end