Assessment

main.rb

slips = {
  "slip_23" =>
    {
      transactions: [123, 456],
      shop: 1
    },
  "slip_42" =>
    {
      transactions: [789],
      shop: 2
    }
}

transactions = [
  {
    id: 123,
    amount: 10.01,
    payout: false
  },
  {
    id: 456,
    amount: 5.01,
    payout: true
  },
  {
    id: 789,
    amount: 20.10,
    payout: false
  }
]

shops = [
  [1, 'Zalando.de'],
  [2, 'Amazon.com']
]

################################
# Main code

def calculate_transaction_total(transaction_ids, transactions)
  total = 0
  
  transaction_ids
    .map do |trx_id|
    # find transaction hash
      transactions.find { |trx| trx[:id] == trx_id } || {}
    end
    .each do |trx|
      # sum up transaction amounts, payout amt will be deducted from total
      total += trx[:amount] * (trx[:payout] ? -1 : 1)
    end

  total
end

def find_shop(shop_id, shops)
  # find shop data with id, then take the second item in array
  _shop_id, shop_name = shops.find { |shop| shop[0] == shop_id }

  shop_name
end

SLIP_FORMAT = "slip_".freeze
def get_slip_number(slip_id)
  slip_id.sub(SLIP_FORMAT, '').to_i
end

result = Hash.new

slips.each do |slip_id, slip|
  slip_number = get_slip_number(slip_id)

  result[slip_number] = {
    number_transactions: slip[:transactions].length,
    total_amount: calculate_transaction_total(slip[:transactions], transactions),
    shop: find_shop(slip[:shop], shops)
  }
end

################################
# Test
expected_result = {
  23 => {
    number_transactions: 2, # no of transactions per slip
    total_amount: 5,        # total amount of transactions (a payout must be subtracted instead of added!)
    shop: 'Zalando.de'      # Shop Title
  },
  42 => {
    number_transactions: 1,
    total_amount: 20.10,
    shop: 'Amazon.com'
  }
}

p result
puts "Expected result == result => #{expected_result == result}"

short.rb

slips = {
  "slip_23" =>
    {
      transactions: [123, 456],
      shop: 1
    },
  "slip_42" =>
    {
      transactions: [789],
      shop: 2
    }
}

transactions = [
  {
    id: 123,
    amount: 10.01,
    payout: false
  },
  {
    id: 456,
    amount: 5.01,
    payout: true
  },
  {
    id: 789,
    amount: 20.10,
    payout: false
  }
]

shops = [
  [1, 'Zalando.de'],
  [2, 'Amazon.com']
]

################################
# Main code

result = Hash.new

slips.each do |slip_id, slip|
  result[
    slip_id[5..-1].to_i
  ] = {
    number_transactions: slip[:transactions].length,
    total_amount: slip[:transactions].reduce(0) { |total, trx_id|
      trx = transactions.find { |trx| trx[:id] == trx_id}
      total + trx[:amount] * (trx[:payout] ? -1 : 1)
    }.round(2),
    shop: shops.find { |shop| shop[0] == slip[:shop] }.last
  }
end

################################
# Test
expected_result = {
  23 => {
    number_transactions: 2, # no of transactions per slip
    total_amount: 5,        # total amount of transactions (a payout must be subtracted instead of added!)
    shop: 'Zalando.de'      # Shop Title
  },
  42 => {
    number_transactions: 1,
    total_amount: 20.10,
    shop: 'Amazon.com'
  }
}

p result
puts "Expected result == result => #{expected_result == result}"