landed enough rewriters and filters to begin building configurations for the HTML checker

htmlcheck/filter.go

@@ -0,0 +1,69 @@
+/*
+ * Amsterdam Web Communities System
+ * Copyright (c) 2025 Erbosoft Metaverse Design Solutions, All Rights Reserved
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+// The htmlcheck package contains the HTML Checker.
+package htmlcheck
+
+import "strings"
+
+// outputFilter is the interface for an HTML checker output filter.
+type outputFilter interface {
+	tryOutputCharacter(strings.Builder, byte) bool
+	matchCharacter(byte) bool
+	lengthNoMatch(string) int
+}
+
+// outputFilterRegistry contains a listing of all defined output filters.
+var outputFilterRegistry = make(map[string]outputFilter)
+
+// init registers all known filters.
+func init() {
+	f := htmlEncodingFilter{}
+	outputFilterRegistry["html"] = &f
+}
+
+// htmlEncodingFilter is a filter that escapes certain characters in HTML.
+type htmlEncodingFilter struct{}
+
+// htmlEscapedChars is a list of HTML characters that are escaped.
+const htmlEscapedChars = "<>&"
+
+// tryOutputCharacter outputs a character that needs to be escaped.
+func (f *htmlEncodingFilter) tryOutputCharacter(buf strings.Builder, ch byte) bool {
+	switch ch {
+	case '<':
+		buf.WriteString("&lt;")
+	case '>':
+		buf.WriteString("&gt;")
+	case '&':
+		buf.WriteString("&amp;")
+	default:
+		return false
+	}
+	return true
+}
+
+// matchCharacter returns true if this character needs to be escaped.
+func (f *htmlEncodingFilter) matchCharacter(ch byte) bool {
+	return strings.IndexByte(htmlEscapedChars, ch) >= 0
+}
+
+// lengthNoMatch returns the maximum length of unmatched characters at the start of the string.
+func (f *htmlEncodingFilter) lengthNoMatch(s string) int {
+	rc := len(s)
+	for _, c := range []byte(htmlEscapedChars) {
+		tmp := strings.IndexByte(s, c)
+		if tmp >= 0 && tmp < rc {
+			rc = tmp
+			if rc == 0 {
+				return 0
+			}
+		}
+	}
+	return rc
+}