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>

currency_converter.go

@@ -0,0 +1,96 @@
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"io/ioutil"
+	"net/http"
+	"strings"
+
+	log "github.com/sirupsen/logrus"
+)
+
+// CurrencyConverter to cache rates of different currency pairs
+type CurrencyConverter struct {
+	rates map[string]float64
+}
+
+// NewCurrencyConverter to create a CurrencyConverter
+func NewCurrencyConverter() *CurrencyConverter {
+	return &CurrencyConverter{
+		rates: make(map[string]float64),
+	}
+}
+
+// Convert an amount in a given currency to another currency
+// Eventually fetch rate from a remote API then cache the result
+func (c *CurrencyConverter) Convert(amount float64, fromCurrency string, toCurrency string) (float64, error) {
+	var err error
+
+	if fromCurrency == toCurrency {
+		return amount, nil
+	}
+
+	// exclude invalid currencies
+	if fromCurrency == "" || toCurrency == "" {
+		return 0.0, fmt.Errorf("invalid currency pair used for convertion (from='%s', to='%s')", fromCurrency, toCurrency)
+	}
+
+	// searching currency pair rate in cache
+	pair := fmt.Sprintf("%s%s", strings.ToLower(fromCurrency), strings.ToLower(toCurrency))
+	rate, exists := c.rates[pair]
+	if !exists {
+		// fetching rate from api
+		rate, err = c.getRate(fromCurrency, toCurrency)
+		if err != nil {
+			return 0.0, err
+		}
+		// store rate in cache
+		c.rates[pair] = rate
+	}
+
+	return rate * amount, nil
+}
+
+// CurrencyResponse to unmarshall JSON response from API
+type CurrencyResponse struct {
+	Date string  `json:"date"`
+	Rate float64 `json:"rate"`
+}
+
+// getRate retreives rate from a remote API
+func (c *CurrencyConverter) getRate(fromCurrency string, toCurrency string) (float64, error) {
+	if fromCurrency == "" || toCurrency == "" {
+		return 0.0, fmt.Errorf("invalid currency pair used for convertion (from=%s, to=%s)", fromCurrency, toCurrency)
+	}
+
+	// lowering currency names to match the API route
+	fromCurrency = strings.ToLower(fromCurrency)
+	toCurrency = strings.ToLower(toCurrency)
+
+	log.Debugf("fetching %s%s rate from currency api", fromCurrency, toCurrency)
+	resp, err := http.Get("https://cdn.jsdelivr.net/gh/fawazahmed0/currency-api@1/latest/currencies/" + fromCurrency + "/" + toCurrency + ".json")
+	if err != nil {
+		return 0.0, fmt.Errorf("could not retreive currency rates for pair %s%s: %s", fromCurrency, toCurrency, err)
+	}
+	defer resp.Body.Close()
+
+	body, err := ioutil.ReadAll(resp.Body)
+	if err != nil {
+		return 0.0, fmt.Errorf("could not parse currency rates response for pair %s%s: %s", fromCurrency, toCurrency, err)
+	}
+
+	// response has a dynamic name for the rate
+	//   -> {"date": "2021-05-22", "usd": 1.218125}
+	// making it predictable
+	//   -> {"date": "2021-05-22", "rate": 1.218125}}
+	bodyParsed := strings.Replace(string(body), toCurrency, "rate", 1)
+
+	var response CurrencyResponse
+	err = json.Unmarshal([]byte(bodyParsed), &response)
+	if err != nil {
+		return 0.0, err
+	}
+
+	return response.Rate, nil
+}