class HashParser
  def self.parse(s)
    self.new(s).send(:parse)
  end

private
  def initialize(s)
    @s, @i = s, 0
  end

  def parse
    @s[@i] == "{" or raise "Missing initial '{' in hash literal: #{@s.inspect}"
    @i += 1
    while @i < @s.size && @s[@i] == " " # Skip spaces before first key
      @i += 1
    end
    pairs = []
    while @i < @s.size && @s[@i] !~ /[,\}]/
      pairs << extract_key_value
      extract_element_separator
    end
    @s[@i] == "}" or raise "Missing terminating '}' in hash literal: #{@s.inspect}"
    Hash[*pairs.flatten]
  end

  def extract_element_separator
    while @i < @s.size && @s[@i] =~ /\s/
      @i += 1
    end
    if @i < @s.size && @s[@i] == ","
      @i += 1
      while @i < @s.size && @s[@i] =~ /\s/
        @i += 1
      end
    end
  end

  def extract_key_value
    key = extract_key
    extract_separator
    value = extract_value
    [key.to_sym, value]
  end

  def extract_key
    key = ""
    while @i < @s.size && @s[@i] =~ /\w/
      key += @s[@i]
      @i += 1
    end
    key
  end

  def extract_separator
    while @i < @s.size && @s[@i] =~ /[\s:]/
      @i += 1
    end
  end

  def extract_value
    if @s[@i] =~ /['"]/
      extract_quoted_value
    else
      extract_unquoted_value
    end
  end

  def extract_quoted_value
    quot = @s[@i]
    @i += 1

    litt = ""
    while @i < @s.size && @s[@i] != quot
      if @s[@i] == "\\"
        @i += 1
        @i < @s.size or raise "Syntax error in hash literal: #{@s.inspect}"
        case @s[@i]
          when "\\"; litt += "\\"
          when quot; litt += quot
        else
          litt += "\\" + @s[@i]
        end
      else
        litt += @s[@i]
      end
      @i += 1
    end

    @i < @s.size && @s[@i] == quot or raise "Unterminated hash literal: #{@s.inspect}"
    @i += 1
    return litt
  end

  def extract_unquoted_value
    j = @i + 1
    while j < @s.size && @s[j] !~ /[,\}]/
      j += 1
    end
    litt = @s[@i...j].strip
    @i = j
    value =
        case litt
          when "nil"; nil
          when "true"; true
          when "false"; false
          when /^\d+$/; litt.to_i
        else
          litt.strip
        end
    value
  end
end