Move internal packages to an internal folder

For reference: https://go.dev/doc/go1.4#internalpackages

internal/reader/sanitizer/srcset.go

@@ -0,0 +1,81 @@
+// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package sanitizer
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+)
+
+type ImageCandidate struct {
+	ImageURL   string
+	Descriptor string
+}
+
+type ImageCandidates []*ImageCandidate
+
+func (c ImageCandidates) String() string {
+	var htmlCandidates []string
+
+	for _, imageCandidate := range c {
+		var htmlCandidate string
+		if imageCandidate.Descriptor != "" {
+			htmlCandidate = fmt.Sprintf(`%s %s`, imageCandidate.ImageURL, imageCandidate.Descriptor)
+		} else {
+			htmlCandidate = imageCandidate.ImageURL
+		}
+
+		htmlCandidates = append(htmlCandidates, htmlCandidate)
+	}
+
+	return strings.Join(htmlCandidates, ", ")
+}
+
+// ParseSrcSetAttribute returns the list of image candidates from the set.
+// https://html.spec.whatwg.org/#parse-a-srcset-attribute
+func ParseSrcSetAttribute(attributeValue string) (imageCandidates ImageCandidates) {
+	unparsedCandidates := strings.Split(attributeValue, ", ")
+
+	for _, unparsedCandidate := range unparsedCandidates {
+		if candidate, err := parseImageCandidate(unparsedCandidate); err == nil {
+			imageCandidates = append(imageCandidates, candidate)
+		}
+	}
+
+	return imageCandidates
+}
+
+func parseImageCandidate(input string) (*ImageCandidate, error) {
+	input = strings.TrimSpace(input)
+	parts := strings.Split(strings.TrimSpace(input), " ")
+	nbParts := len(parts)
+
+	if nbParts > 2 || nbParts == 0 {
+		return nil, fmt.Errorf(`srcset: invalid number of descriptors`)
+	}
+
+	if nbParts == 2 {
+		if !isValidWidthOrDensityDescriptor(parts[1]) {
+			return nil, fmt.Errorf(`srcset: invalid descriptor`)
+		}
+		return &ImageCandidate{ImageURL: parts[0], Descriptor: parts[1]}, nil
+	}
+
+	return &ImageCandidate{ImageURL: parts[0]}, nil
+}
+
+func isValidWidthOrDensityDescriptor(value string) bool {
+	if value == "" {
+		return false
+	}
+
+	lastChar := value[len(value)-1:]
+	if lastChar != "w" && lastChar != "x" {
+		return false
+	}
+
+	_, err := strconv.ParseFloat(value[0:len(value)-1], 32)
+	return err == nil
+}