feat: add price range filter (#26)

To avoid scalpers' price, the bot now understand filters on prices
using a minimum and maximum value, in a currency and a pattern to
detect the model.

Example:
```
"price_ranges": [
  {"model": "3090", "max": 3000, "currency": "EUR"},
  {"model": "3080", "max": 1600, "currency": "EUR"},
  {"model": "3070", "max": 1200, "currency": "EUR"}
],
```

More details in README.md.

Signed-off-by: Julien Riou <julien@riou.xyz>

filter_range.go

@@ -6,15 +6,20 @@ import (
 	log "github.com/sirupsen/logrus"
 )
 
+// DefaultCurrency to fallback when no currency is provided
+const DefaultCurrency = "USD"
+
 // RangeFilter to store the pattern to match product model and price limits
 type RangeFilter struct {
-	model *regexp.Regexp
-	min   float64
-	max   float64
+	model     *regexp.Regexp
+	min       float64
+	max       float64
+	currency  string
+	converter *CurrencyConverter
 }
 
 // NewRangeFilter to create a RangeFilter
-func NewRangeFilter(regex string, min float64, max float64) (*RangeFilter, error) {
+func NewRangeFilter(regex string, min float64, max float64, currency string, converter *CurrencyConverter) (*RangeFilter, error) {
 	var err error
 	var compiledRegex *regexp.Regexp
 
@@ -26,23 +31,56 @@ func NewRangeFilter(regex string, min float64, max float64) (*RangeFilter, error
 		}
 	}
 
+	var detectedCurrency string
+	if currency != "" {
+		detectedCurrency = currency
+	} else {
+		detectedCurrency = DefaultCurrency
+	}
+
 	return &RangeFilter{
-		model: compiledRegex,
-		min:   min,
-		max:   max,
+		model:     compiledRegex,
+		min:       min,
+		max:       max,
+		converter: converter,
+		currency:  detectedCurrency,
 	}, nil
 }
 
 // Include returns false when a product name matches the model regex and price is outside of the range
 // implements the Filter interface
 func (f *RangeFilter) Include(product *Product) bool {
+	// include products with a missing model regex
 	if f.model == nil {
+		log.Debugf("product %s included because range filter model is missing", product.Name)
 		return true
 	}
-	if f.model.MatchString(product.Name) && product.Price < f.min || product.Price > f.max {
-		log.Debugf("product %s excluded because price for the model is outside of the range [%.2f-%.2f]", product.Name, f.min, f.max)
-		return false
+
+	// include products with a different model
+	if !f.model.MatchString(product.Name) {
+		log.Debugf("product %s included because range filter model hasn't been detected in the product name", product.Name)
+		return true
 	}
-	log.Debugf("product %s included because price range filter is not applicable", product.Name)
-	return true
+
+	// convert price to the filter currency
+	convertedPrice, err := f.converter.Convert(product.Price, product.PriceCurrency, f.currency)
+	if err != nil {
+		log.Warnf("could not convert price %.2f %s to %s for range filter: %s", product.Price, product.PriceCurrency, f.currency, err)
+		return true
+	}
+
+	// include prices with unlimited maximum if min is respected
+	if f.max == 0 && convertedPrice > f.max && f.min <= convertedPrice {
+		log.Debugf("product %s included because max value is unlimited and converted price of %.2f%s is higher than lower limit of %.2f%s", product.Name, convertedPrice, f.currency, f.min, f.currency)
+		return true
+	}
+
+	// include prices inside the range
+	if f.min <= convertedPrice && convertedPrice <= f.max {
+		log.Debugf("product %s included because range filter model matches and converted price is inside of the range", product.Name)
+		return true
+	}
+
+	log.Debugf("product %s excluded because range filter model matches and converted price is outside of the range", product.Name)
+	return false
 }