net.php: add functions to encode and decode a typed cursor over an url

inc/functions/net.php

@@ -14,3 +14,63 @@ function is_connection_secure(bool $trust_headers): bool {
 	}
 	return false;
 }
+
+/**
+ * Encodes a string into a base64 variant without characters illegal in urls.
+ */
+function base64_url_encode(string $input): string {
+	return str_replace([ '+', '/', '=' ], [ '-', '_', '' ], base64_encode($input));
+}
+
+/**
+ * Decodes a string from a base64 variant without characters illegal in urls.
+ */
+function base64_url_decode(string $input): string {
+	return base64_decode(strtr($input, '-_', '+/'));
+}
+
+/**
+ * Encodes a typed cursor.
+ *
+ * @param string $type The type for the cursor. Only the first character is considered.
+ * @param array $map A map of key-value pairs to encode.
+ * @return string An encoded string that can be sent through urls. Empty if either parameter is empty.
+ */
+function encode_cursor(string $type, array $map): string {
+	if (empty($type) || empty($map)) {
+		return '';
+	}
+
+	$acc = $type[0];
+	foreach ($map as $key => $value) {
+		$acc .= "|$key#$value";
+	}
+	return base64_url_encode($acc);
+}
+
+/**
+ * Decodes a typed cursor.
+ *
+ * @param string $cursor A string emitted by `encode_cursor`.
+ * @return array An array with the type of the cursor and an array of key-value pairs. The type is null and the map
+ *               empty if either there are no key-value pairs or the encoding is incorrect.
+ */
+function decode_cursor(string $cursor): array {
+	$map = [];
+	$type = '';
+	$acc = base64_url_decode($cursor);
+	if ($acc === false || empty($acc)) {
+		return [ null, [] ];
+	}
+
+	$type = $acc[0];
+	foreach (explode('|', substr($acc, 2)) as $pair) {
+		$pair = explode('#', $pair);
+		if (count($pair) >= 2) {
+			$key = $pair[0];
+			$value = $pair[1];
+			$map[$key] = $value;
+		}
+	}
+	return [ $type, $map ];
+}