Parsing with Float

[teamon]

While migrating an app from integer to bigint db types I've noticed that Money does not handle big values properly:

iex(2)> Money.parse "$1.01"
{:ok, %Money{amount: 101, currency: :USD}}

iex(3)> Money.parse "$1,000.01"
{:ok, %Money{amount: 100001, currency: :USD}}

iex(4)> Money.parse "$1,000,000.01"
{:ok, %Money{amount: 100000001, currency: :USD}}

iex(5)> Money.parse "$1,000,000,000.01"
{:ok, %Money{amount: 100000000001, currency: :USD}}

iex(6)> Money.parse "$1,000,000,000,000.01"
{:ok, %Money{amount: 100000000000001, currency: :USD}}

iex(7)> Money.parse "$1,000,000,000,000,000.01"
{:ok, %Money{amount: 100000000000000000, currency: :USD}}             <= problem (1¢ lost)

iex(8)> Money.parse "$1,000,000,000,000,000,000.01"
{:ok, %Money{amount: 100000000000000000000, currency: :USD}}          <= problem (1¢ lost)

iex(9)> Money.parse "$1,000,000,000,000,000,000,000.01"
{:ok, %Money{amount: 99999999999999991611392, currency: :USD}}        <= problem

iex(10)> Money.parse "$1,000,000,000,000,000,000,000,000.01"
{:ok, %Money{amount: 100000000000000004764729344, currency: :USD}}    <= problem

The problem seems to be coming from the usage of Float.parse/1

iex(10)> {f,_}=Float.parse("1000.01"); round(f * 100)                    
100001
iex(11)> {f,_}=Float.parse("1000000.01"); round(f * 100)
100000001
iex(12)> {f,_}=Float.parse("1000000000.01"); round(f * 100)
100000000001
iex(13)> {f,_}=Float.parse("1000000000000.01"); round(f * 100)
100000000000001
iex(14)> {f,_}=Float.parse("1000000000000000.01"); round(f * 100)
100000000000000000
iex(15)> {f,_}=Float.parse("1000000000000000000.01"); round(f * 100)
100000000000000000000
iex(16)> {f,_}=Float.parse("1000000000000000000000.01"); round(f * 100)
99999999999999991611392
iex(17)> {f,_}=Float.parse("1000000000000000000000000.01"); round(f * 100)
100000000000000004764729344

Would it make sense to parse with Decimal.parse? Optionally if the decimal dependency is not available fallback to manual parsing of left & right sides of decimal point and combining using only integers.

[tensor-programming]

I've been running into similar problems when trying to use custom currencies with decimal values larger than 15. With Ethereum for example, the amount of precision seems to drastically drop when compared to a currency that requires fewer decimals. If I had to guess, like you mentioned it comes back to the Float.parse call because precision is really problematic when dealing with float to money conversions especially.

[joselo]

I have a similar issue, basically I need to get the percentage of an amount but the percentage is decimal, i.e

iex> 100 * 0.125
12.5

iex> Money.new(100, :USD) |> Money.multiply(0.125)
%Money{amount: 13, currency: :USD}

I'm not sure if it should return %Money{amount: 1250, currency: :USD} instead of %Money{amount: 13, currency: :USD}

[Nitrino]

To have higher precision, you need to add Decimal to the project dependencies. After that the library will use Decimal instead of Float