Recursive Excel-like evaluator

excel_eval.rb

require 'awesome_print'

require_relative 'table_evaluator.rb'

def print_test(label, expectation)
  evaluator = TableEvaluator.new
  actual    = yield(evaluator)

  if actual == expectation
    puts "#{label}: PASS"
  else
    puts "#{label}: FAIL (actual value was #{actual.inspect})"
  end
end

print_test('testing [1]cost = 180', 180) { |te| te.cost_of(1) }

# console output

# expanding [2]quantity + [3]value
# expanding 3
# expanding 7
# => 3 + 7
# expanding [2]cost
# expanding 3
# expanding 6
# => 18
# testing [1]cost = 180: PASS

table_evaluator.rb

class TableEvaluator
  def initialize
  end

  def cost_of(row_id)
    quantity_of(row_id) * value_of(row_id)
  end

  def quantity_of(row_id)
    eval(expand_references((rows[row_id][:quantity])))
  end

  def value_of(row_id)
    eval(expand_references(rows[row_id][:value]))
  end

  def rows
    @rows ||= ROWS.each_with_object({}) do |row, memo|
      memo[row[:id]] = row
    end
  end

  private

  def eval(expr)
    ::Kernel.eval(expr.to_s)
  end

  def expand_references(expr)
    puts "expanding #{expr}"
    return expr unless expr.is_a?(String)

    res = expr.gsub(REFERENCE_REGEX) do |reference|
      match = reference.match(REFERENCE_REGEX)
      get_value(match[:row_id].to_i, match[:column].to_sym)
    end
    puts "=> #{res}"
    res
  end

  def get_value(row_id, column)
    case column
    when :quantity
      quantity_of(row_id)
    when :value
      value_of(row_id)
    when :cost
      cost_of(row_id)
    else
      fail "unknown column: #{column.inspect}"
    end
  end

  REFERENCE_REGEX = /\[(?<row_id>\d+)\](?<column>quantity|value|cost)/

  ROWS = [
    {
      id:       1,
      name:     'Regulating',
      quantity: '[2]quantity + [3]value',
      value:    '[2]cost',
    },
    {
      id:       2,
      name:     'Kerbs',
      quantity: 3,
      value:    6,
    },
    {
      id:       3,
      name:     'Bricks',
      quantity: 9,
      value:    7,
    },
    {
      id:       4,
      name:     'Sausages',
      quantity: '[3]cost',
      value:    '3',
    },
    {
      id:       5,
      name:     'Bamboo',
      quantity: '[4]quantity',
      value:    '[7]cost',
    },
    {
      id:       6,
      name:     'Clams',
      quantity: '[4]quantity',
      value:    nil,
    },
    {
      id:       7,
      name:     'Hardcore',
      quantity: '[3]quantity * 0.5',
      value:    12,
    },
    {
      id:       8,
      name:     'Beetles',
      quantity: '[6]quantity * [4]value',
      value:    '[2]value',
    },
  ]
end