# Arrays - Get Products of all other elements in Ruby

# Algorithm challenge description:
# Given an array of integers, return a new array such that
# each element at index `i` of the new array is the product of
# all the numbers in the original array except the one at `i`.

#
# 1. Brute force solution
#
def calculate_products_of_all_other_elements(nums)
  product_of_other_elements = Array.new(nums.length, 1)

  nums.each_with_index do |_num1, i|
    nums.each_with_index do |num2, j|
      product_of_other_elements[i] = product_of_other_elements[i] * num2 if i != j
    end
  end

  product_of_other_elements
end

puts(calculate_products_of_all_other_elements([1, 2, 3]))

#
# 2. O(n) time and space
# Arrays - Get Products of all other elements in Ruby
#

# Generates prefix products
def build_prefix_products(nums)
  prefix_products = []

  nums.each do |num|
    prefix_products << if prefix_products.count > 0
                         (prefix_products.last * num)
                       else
                         num
                       end
  end

  prefix_products
end

# Generates suffix products
def build_suffix_products(nums)
  suffix_products = []

  nums.reverse.each do |num|
    suffix_products << if suffix_products.count > 0
                         (suffix_products.last * num)
                       else
                         num
                       end
  end

  suffix_products
end

# Builds output
def output(prefix_products, suffix_products, nums)
  result = []

  nums.reverse.each_with_index do |_num, index|
    result << if index == 0
                suffix_products[index + 1]
              elsif index == nums.length - 1
                prefix_products[index - 1]
              else
                (prefix_products[index - 1] * suffix_products[index + 1])
              end
  end

  result
end

# Generate result from the product of prefixes and suffixes
def products(nums)
  prefix_products = build_prefix_products(nums)
  suffix_products = build_suffix_products(nums)
  suffix_products = suffix_products.reverse

  output(prefix_products, suffix_products, nums)
end

puts(products([1, 2, 3]))
# => [6, 3, 2]