refactor: move filters out of parser

Filters are now separate structures to include a product or not based
on their own set of properties. For now, include and exclude filters
are supported. They take a regex as an argument and include a product
if the regex matches (or doesn't match) the product name. This commit
will allow us to create new filters on product like on a price range.

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

filter_exclude.go

@@ -0,0 +1,40 @@
+package main
+
+import (
+	"regexp"
+
+	log "github.com/sirupsen/logrus"
+)
+
+type ExcludeFilter struct {
+	regex *regexp.Regexp
+}
+
+func NewExcludeFilter(regex string) (*ExcludeFilter, error) {
+	var err error
+	var compiledRegex *regexp.Regexp
+
+	log.Debugf("compiling exclude filter regex")
+	if regex != "" {
+		compiledRegex, err = regexp.Compile(regex)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	return &ExcludeFilter{regex: compiledRegex}, nil
+}
+
+// Filter excludes product with name matching the regex
+// implements the Filter interface
+func (f *ExcludeFilter) Include(product *Product) bool {
+	if f.regex == nil {
+		return true
+	}
+	if f.regex.MatchString(product.Name) {
+		log.Debugf("product %s excluded because it matches the exclude regex", product.Name)
+		return false
+	}
+	log.Debugf("product %s included because it doesn't match the exclude regex", product.Name)
+	return true
+}