defmodule Money.Financial do
@moduledoc """
A set of financial functions, primarily related to discounted cash flows.
Some of the algorithms are from [finance formulas](http://www.financeformulas.net)
"""
alias Cldr.Math
@doc """
Calculates the future value for a present value, an interest rate
and a number of periods.
* `present_value` is a %Money{} representation of the present value
* `interest_rate` is a float representation of an interest rate. For
example, 12% would be represented as `0.12`
* `periods` in an integer number of periods
## Examples
iex> Money.Financial.future_value Money.new(:USD, 10000), 0.08, 1
Money.new(:USD, "10800.00")
iex> Money.Financial.future_value Money.new(:USD, 10000), 0.04, 2
Money.new(:USD, "10816.0000")
iex> Money.Financial.future_value Money.new(:USD, 10000), 0.02, 4
Money.new(:USD, "10824.32160000")
"""
@spec future_value(Money.t(), number, number) :: Money.t()
@one Decimal.new(1)
def future_value(%Money{amount: amount} = money, interest_rate, periods)
when is_number(interest_rate) and is_number(periods) do
fv =
interest_rate
|> Decimal.from_float()
|> Decimal.add(@one)
|> Math.power(periods)
|> Decimal.mult(amount)
%{money | amount: fv}
end
@doc """
Calculates the future value for a list of cash flows and an interest rate.
* `flows` is a list of tuples representing a cash flow. Each flow is
represented as a tuple of the form `{period, %Money{}}`
* `interest_rate` is a float representation of an interest rate. For
example, 12% would be represented as `0.12`
## Example
iex> Money.Financial.future_value([{4, Money.new(:USD, 10000)}, {5, Money.new(:USD, 10000)}, {6, Money.new(:USD, 10000)}], 0.13)
Money.new(:USD, "34068.99999999999999999999999")
iex> Money.Financial.future_value [{0, Money.new(:USD, 5000)},{1, Money.new(:USD, 2000)}], 0.12
Money.new(:USD, "7600.000000000000000000000000")
"""
@spec future_value(list({number, Money.t()}), number) :: Money.t()
def future_value(flows, interest_rate)
def future_value([{period, %Money{}} | _other_flows] = flows, interest_rate)
when is_integer(period) and is_number(interest_rate) do
{max_period, _} = Enum.max(flows)
present_value(flows, interest_rate)
|> future_value(interest_rate, max_period)
end
@doc """
Calculates the present value for future value, an interest rate
and a number of periods
* `future_value` is a %Money{} representation of the future value
* `interest_rate` is a float representation of an interest rate. For
example, 12% would be represented as `0.12`
* `periods` in an integer number of periods
## Examples
iex> Money.Financial.present_value Money.new(:USD, 100), 0.08, 2
Money.new(:USD, "85.73388203017832647462277092")
iex> Money.Financial.present_value Money.new(:USD, 1000), 0.10, 20
Money.new(:USD, "148.6436280241436864020760472")
"""
@spec present_value(Money.t(), number, number) :: Money.t()
def present_value(%Money{amount: amount} = money, interest_rate, periods)
when is_number(interest_rate) and is_number(periods) and periods >= 0 do
pv_1 =
interest_rate
|> Decimal.from_float()
|> Decimal.add(@one)
|> Math.power(periods)
%{money | amount: Decimal.div(amount, pv_1)}
end
@doc """
Calculates the present value for a list of cash flows and an interest rate.
* `flows` is a list of tuples representing a cash flow. Each flow is
represented as a tuple of the form `{period, %Money{}}`
* `interest_rate` is a float representation of an interest rate. For
example, 12% would be represented as `0.12`
## Example
iex> Money.Financial.present_value([{4, Money.new(:USD, 10000)}, {5, Money.new(:USD, 10000)}, {6, Money.new(:USD, 10000)}], 0.13)
Money.new(:USD, "16363.97191111964880256655144")
iex> Money.Financial.present_value [{0, Money.new(:USD, -1000)},{1, Money.new(:USD, -4000)}], 0.1
Money.new(:USD, "-4636.363636363636363636363636")
"""
@spec present_value(list({integer, Money.t()}), number) :: Money.t()
def present_value(flows, interest_rate)
def present_value([{period, %Money{}} | _other_flows] = flows, interest_rate)
when is_integer(period) and is_number(interest_rate) do
validate_same_currency!(flows)
do_present_value(flows, interest_rate)
end
defp do_present_value({period, %Money{} = flow}, interest_rate)
when is_integer(period) and is_number(interest_rate) do
present_value(flow, interest_rate, period)
end
defp do_present_value([{period, %Money{}} = flow | []], interest_rate)
when is_integer(period) and is_number(interest_rate) do
do_present_value(flow, interest_rate)
end
defp do_present_value([{period, %Money{}} = flow | other_flows], interest_rate)
when is_integer(period) and is_number(interest_rate) do
do_present_value(flow, interest_rate)
|> Money.add!(do_present_value(other_flows, interest_rate))
end
@doc """
Calculates the net present value of an initial investment, a list of
cash flows and an interest rate.
* `flows` is a list of tuples representing a cash flow. Each flow is
represented as a tuple of the form `{period, %Money{}}`
* `interest_rate` is a float representation of an interest rate. For
example, 12% would be represented as `0.12`
* `investment` is a %Money{} struct representing the initial investment
## Example
iex> flows = [{0, Money.new(:USD, 5000)},{1, Money.new(:USD, 2000)},{2, Money.new(:USD, 500)},{3, Money.new(:USD,10_000)}]
iex> Money.Financial.net_present_value flows, 0.08, Money.new(:USD, 100)
Money.new(:USD, "15118.84367220444038002337042")
iex> Money.Financial.net_present_value flows, 0.08
Money.new(:USD, "15218.84367220444038002337042")
"""
@spec net_present_value(list({integer, Money.t()}), number) :: Money.t()
def net_present_value([{period, %Money{currency: currency}} | _] = flows, interest_rate)
when is_integer(period) and is_number(interest_rate) do
net_present_value(flows, interest_rate, Money.zero(currency))
end
@spec net_present_value(list({integer, Money.t()}), number, Money.t()) :: Money.t()
def net_present_value([{period, %Money{}} | _] = flows, interest_rate, %Money{} = investment)
when is_integer(period) and is_number(interest_rate) do
validate_same_currency!(investment, flows)
present_value(flows, interest_rate)
|> Money.sub!(investment)
end
@doc """
Calculates the net present value of an initial investment, a recurring
payment, an interest rate and a number of periods
* `investment` is a %Money{} struct representing the initial investment
* `future_value` is a %Money{} representation of the future value
* `interest_rate` is a float representation of an interest rate. For
example, 12% would be represented as `0.12`
* `periods` in an integer number of a period
## Example
iex> Money.Financial.net_present_value Money.new(:USD, 10000), 0.13, 2
Money.new(:USD, "7831.466833737959119743127888")
iex> Money.Financial.net_present_value Money.new(:USD, 10000), 0.13, 2, Money.new(:USD, 100)
Money.new(:USD, "7731.466833737959119743127888")
"""
@spec net_present_value(Money.t(), float, number) :: Money.t()
def net_present_value(%Money{currency: currency} = future_value, interest_rate, periods) do
net_present_value(future_value, interest_rate, periods, Money.new(currency, 0))
end
@spec net_present_value(Money.t(), float, number, Money.t()) :: Money.t()
def net_present_value(%Money{} = future_value, interest_rate, periods, %Money{} = investment) do
present_value(future_value, interest_rate, periods)
|> Money.sub!(investment)
end
@doc """
Calculates the interal rate of return for a given list of cash flows.
* `flows` is a list of tuples representing a cash flow. Each flow is
represented as a tuple of the form `{period, %Money{}}`
"""
@spec internal_rate_of_return(list({integer, Money.t()})) :: float()
def internal_rate_of_return([{_period, %Money{}} | _other_flows] = flows) do
# estimate_m = sum_of_inflows(flows)
# |> Kernel./(abs(Math.to_float(amount)))
# |> :math.pow(2 / (number_of_flows(flows) + 1))
# |> Kernel.-(1)
# estimate_n = :math.pow(1 + estimate_m, )
estimate_n = 0.2
estimate_m = 0.1
do_internal_rate_of_return(flows, estimate_m, estimate_n)
end
@irr_precision 0.000001
defp do_internal_rate_of_return(flows, estimate_m, estimate_n) do
npv_n = net_present_value(flows, estimate_n).amount |> Math.to_float()
npv_m = net_present_value(flows, estimate_m).amount |> Math.to_float()
if abs(npv_n - npv_m) > @irr_precision do
estimate_o = estimate_n - (estimate_n - estimate_m) / (npv_n - npv_m) * npv_n
do_internal_rate_of_return(flows, estimate_n, estimate_o)
else
estimate_n
end
end
@doc """
Calculates the effective interest rate for a given present value,
a future value and a number of periods.
* `present_value` is a %Money{} representation of the present value
* `future_value` is a %Money{} representation of the future value
* `periods` is an integer number of a period
## Examples
iex> Money.Financial.interest_rate Money.new(:USD, 10000), Money.new(:USD, 10816), 2
Decimal.new("0.04")
iex> Money.Financial.interest_rate Money.new(:USD, 10000), Money.new(:USD, "10824.3216"), 4
Decimal.new("0.02")
"""
@spec interest_rate(Money.t(), Money.t(), number) :: Decimal.t()
def interest_rate(
%Money{currency: pv_currency, amount: pv_amount} = _present_value,
%Money{currency: fv_currency, amount: fv_amount} = _future_value,
periods
)
when pv_currency == fv_currency and is_integer(periods) and periods > 0 do
fv_amount
|> Decimal.div(pv_amount)
|> Math.root(periods)
|> Decimal.sub(@one)
end
@doc """
Calculates the number of periods between a present value and
a future value with a given interest rate.
* `present_value` is a %Money{} representation of the present value
* `future_value` is a %Money{} representation of the future value
* `interest_rate` is a float representation of an interest rate. For
example, 12% would be represented as `0.12`
## Example
iex> Money.Financial.periods Money.new(:USD, 1500), Money.new(:USD, 2000), 0.005
Decimal.new("57.68013595323872502502238648")
"""
@spec periods(Money.t(), Money.t(), float) :: Decimal.t()
def periods(
%Money{currency: pv_currency, amount: pv_amount} = _present_value,
%Money{currency: fv_currency, amount: fv_amount} = _future_value,
interest_rate
)
when pv_currency == fv_currency and is_float(interest_rate) and interest_rate > 0 do
Decimal.div(
Math.log(Decimal.div(fv_amount, pv_amount)),
Math.log(Decimal.add(@one, Decimal.from_float(interest_rate)))
)
end
@doc """
Calculates the payment for a given loan or annuity given a
present value, an interest rate and a number of periods.
* `present_value` is a %Money{} representation of the present value
* `interest_rate` is a float representation of an interest rate. For
example, 12% would be represented as `0.12`
* `periods` is an integer number of periods
## Example
iex> Money.Financial.payment Money.new(:USD, 100), 0.12, 20
Money.new(:USD, "13.38787800396606622792492299")
"""
@spec payment(Money.t(), float, number) :: Money.t()
def payment(
%Money{amount: pv_amount} = present_value,
interest_rate,
periods
)
when is_float(interest_rate) and interest_rate > 0 and is_number(periods) and periods > 0 do
interest_rate = Decimal.from_float(interest_rate)
p1 = Decimal.mult(pv_amount, interest_rate)
p2 = Decimal.sub(@one, Decimal.add(@one, interest_rate) |> Math.power(-periods))
%{present_value | amount: Decimal.div(p1, p2)}
end
defp validate_same_currency!(%Money{} = flow, flows) do
validate_same_currency!([{0, flow} | flows])
end
defp validate_same_currency!(flows) do
number_of_currencies =
flows
|> Enum.map(fn {_period, %Money{currency: currency}} -> currency end)
|> Enum.uniq()
|> Enum.count()
if number_of_currencies > 1 do
raise ArgumentError,
message:
"More than one currency found in cash flows; " <>
"implicit currency conversion is not supported. Cash flows: " <> inspect(flows)
end
end
end
