[Currency support] Map currency from bid request

[benjaminch]

This CL aims to take into account `request.Cur. It's yet another step forward the full currency support CF #280.

Please let me know what you think about this one and whether I forgot anything.

Cheers :)

[hhhjort]

Should we worry that the typical case is bidResponse.Currency == bidReqCur, or is the processing to get out the conversion rate 1 light enough that the performance hit isn't worth an extra check at this point to skip GetRate() or the loop entirely?

[benjaminch]

Hey !
Well, for sure the processing is more than a plain inline if here, but I think it worth passing via the method rather than inline for following:

• The method does also check currency iso code existence (bad currencies names would end up being discarded using the rates method where it won't be unless adding more logic inline doing pretty much the same with the same perfs hit)
• Having the logic / responsibility in one place (also easier to test :))
• Finally, for the perfs hit, I guess it won't be much is both currencies are the same, costing 2 currencies parsing and string comparison: https://github.com/prebid/prebid-server/blob/master/currencies/rates.go#L50

// GetRate returns the conversion rate between two currencies
// returns an error in case the conversion rate between the two given currencies is not in the currencies rates map
func (r *Rates) GetRate(from string, to string) (float64, error) {
	var err error
	fromUnit, err := currency.ParseISO(from)
	if err != nil {
		return 0, err
	}
	toUnit, err := currency.ParseISO(to)
	if err != nil {
		return 0, err
	}
	if fromUnit.String() == toUnit.String() {
		return 1, nil
	}
	if r.Conversions != nil {
		if conversion, present := r.Conversions[fromUnit.String()][toUnit.String()]; present == true {
			return conversion, err
		}

		return 0, fmt.Errorf("Currency conversion rate not found: '%s' => '%s'", fromUnit.String(), toUnit.String())
	}
	return 0, errors.New("rates are nil")
}

What do you guys think?

[guscarreon]

Passes test cases, changes are minimal, and defensive. Makes total sense.

[benjaminch]

Thanks @hhhjort ;)