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parse.rb

require 'pp'

str = 'a+b*c*(d+e)+f+g'

$idx = 0

def term(str)
  ops = []
  while $idx < str.length
    if str[$idx] == '('
      $idx += 1
      ops.push(parse(str))
      $idx += 1
    else
      ops.push(str[$idx])
      $idx += 1
    end
    break if str[$idx] != '*'
    $idx += 1
  end
  if ops.length == 1
    return ops[0]
  else
    return ['*'] + ops
  end
end

def parse(str)
  ops = []
  while $idx < str.length
    if str[$idx] == '('
      $idx += 1
      ops.push(parse(str))
      $idx += 1
    else
      ops.push(term(str))
    end
    break if str[$idx] != '+'
    $idx += 1
  end
  if ops.length == 1
    return ops[0]
  else
    return ['+'] + ops
  end
end

pp parse('a+b*c*(d+e)+f+g')

def calc(m, tree)
  if tree.class != Array
    return m[tree]
  end
  nums = []
  tree.slice(1, tree.length).each do |a|
    nums.push(calc(m, a))
  end
  if tree[0] == '+'
    return nums.reduce{|i, j| i + j}
  else
    return nums.reduce{|i, j| i * j}
  end
end

$idx = 0
t = parse('a+b*c')

pp t
pp calc({'a' => 10, 'b' => 20, 'c' => 3}, t)