get feed URL and use that to resolve URL if link is empty

Sources/ObjC/RSRSSParser.m

@@ -300,7 +300,18 @@ - (NSString *)urlString:(NSString *)s {
 	}
 
 	if (!self.link) {
-		//TODO: get feed URL and use that to resolve URL.*/
+        NSURL *feedURL = [NSURL URLWithString:self.urlString];
+        if (!feedURL) {
+            return s;
+        }
+        NSURL *baseURL = [NSURL URLWithString:[NSString stringWithFormat:@"%@://%@", feedURL.scheme, feedURL.host]];
+        if (!baseURL) {
+            return s;
+        }
+        NSURL *resolvedURL = [NSURL URLWithString:s relativeToURL:baseURL];
+        if (resolvedURL.absoluteString) {
+            return resolvedURL.absoluteString;
+        }
 		return s;
 	}
 

Tests/RSParserTests/RSSParserTests.swift

@@ -185,4 +185,13 @@ class RSSParserTests: XCTestCase {
 //			XCTAssertNotNil(article.contentHTML)
 //		}
 //	}
+
+    func testFeedWithHomeEmpty() {
+        let d = parserData("fuli51", "html", "https://www.fuli51.net/feed")
+        let parsedFeed = try? FeedParser.parse(d)
+
+        for article in parsedFeed?.items ?? [] {
+            XCTAssertTrue(article.url?.starts(with: "https://www.fuli51.net") ?? false )
+        }
+    }
 }