Long multiplication

# title: Long multiplication
# source: https://rosettacode.org/wiki/Long_multiplication
# category: Rosetta Code
# difficulty: Intro
# summary: Break a multiplication problem into the partial products used by long multiplication.
# tags: popular, math, formatting, digits
# vibe: 0.2

def digit_value(char)
  "0123456789".index(char)
end

def power_of_ten(exponent)
  value = 1
  count = 0

  while count < exponent
    value = value * 10
    count = count + 1
  end

  value
end

def partial_products(multiplicand, multiplier)
  digits = "" + multiplier
  products = []
  shift = 0
  index = digits.length - 1

  while index >= 0
    products = products.push(multiplicand * digit_value(digits.slice(index)) * power_of_ten(shift))
    shift = shift + 1
    index = index - 1
  end

  products
end

def run
  multiplicand = 123
  multiplier = 45
  {
    multiplicand: multiplicand,
    multiplier: multiplier,
    partials: partial_products(multiplicand, multiplier),
    product: multiplicand * multiplier
  }
end