Round amount to Currency's Precision

[lasuk]

Background

CashCtrl stores a transactions' currencyRate instead of the base_currency_amount, while pyledger and most large accounting systems choose the more flexible approach and store a transactions' base_currency_amount.

When mapping to/from CashCtrl, we compute base_currency_amount = amount * currencyRate. We currently round this amount to eight digits after the comma. The eight digits are arbitrarily chosen and hard coded.

Task

1. Implement CashCtrlLedger.precision(currency: str) -> float to fetch the smallest increment for amounts in the specified currency from CashCtrl.
2. Overload LedgerEngine.round_to_precision with a vectorized version.

   # existing
   LedgerEngine.round_to_precision(amount: float, currency: str) -> float
   # new
   LedgerEngine.round_to_precision(amount: pd.series, currency: pd.series) -> pd.series

3. Use LedgerEngine.round_to_precision instead of round(..., 8) to round the base_currency_amount. See TODOs in cashctrl_ledger.

[AlexTheWizardL]

Possible understanding

CashCtrlLedger.precision(currency: str) -> float.

• Precision is the quantity of digits after the comma. Simple method examples of calculation:

  1.02 return -> 0.01
  1.0002 return -> 0.0001
  1.000002 return -> 0.000001

• Or: The smallest increment for amounts in a specified currency is the "smallest unit of currency" or "currency subunit." This increment represents the smallest denomination in which a currency can be divided and is typically the value of the smallest coin or note in circulation. Example:

  United States Dollar (USD): The smallest unit is the cent, with 100 cents making up one dollar.
  Euro (EUR): The smallest unit is the cent, with 100 cents making up one euro.

Following question

• What is the the smallest increment for amounts in the specified currency and where to look for it?
• Is that correct that we need to loop through all transaction amount filed and find a value that has the most digits after the comma?
• If the previous question is correct, CashCtrl always returns 2 digits after the comma even if more are passed

[AlexTheWizardL]

Correct understanding

precision(currency: str) -> float and precision is the smallest increment for amounts in a specified currency is the "smallest unit of currency" or "currency subunit." Example:

USD -> smallest is CENT -> 0.01
Mauritania -> Khoums -> 0.00005
Madagascar -> Iraimbilanja -> 0.00005

• Define a _settings field that will store dict for currencies and precisions. By default empty. Define set() and get() for it to make it possible for user to provide own precisions.
• precision(currency: str) -> float will look up for a currency there, if user haven`t stored something there, return default 0.01