Fix resolving of absolute URLs

ansi/image.go

@@ -25,7 +25,7 @@ func (e *ImageElement) Render(w io.Writer, ctx RenderContext) error {
 	}
 	if len(e.URL) > 0 {
 		el := &BaseElement{
-			Token:  resolveRelativeURL(e.BaseURL, e.URL),
+			Token:  resolveURL(e.BaseURL, e.URL),
 			Prefix: " ",
 			Style:  ctx.options.Styles.Image,
 		}

ansi/link.go

@@ -64,7 +64,7 @@ func (e *LinkElement) Render(w io.Writer, ctx RenderContext) error {
 		}
 
 		el := &BaseElement{
-			Token:  resolveRelativeURL(e.BaseURL, e.URL),
+			Token:  resolveURL(e.BaseURL, e.URL),
 			Prefix: pre,
 			Style:  style,
 		}

ansi/renderer.go

@@ -3,7 +3,6 @@ package ansi
 import (
 	"io"
 	"net/url"
-	"strings"
 
 	"github.com/muesli/termenv"
 	east "github.com/yuin/goldmark-emoji/ast"
@@ -150,15 +149,14 @@ func isChild(node ast.Node) bool {
 	return false
 }
 
-func resolveRelativeURL(baseURL string, rel string) string {
+func resolveURL(baseURL string, rel string) string {
 	u, err := url.Parse(rel)
 	if err != nil {
 		return rel
 	}
 	if u.IsAbs() {
 		return rel
 	}
-	u.Path = strings.TrimPrefix(u.Path, "/")
 
 	base, err := url.Parse(baseURL)
 	if err != nil {

ansi/renderer_test.go

@@ -168,3 +168,65 @@ func TestRendererIssues(t *testing.T) {
 		}
 	}
 }
+
+func TestUrlResolver(t *testing.T) {
+	// resolveURL should be compliant with https://tools.ietf.org/html/rfc3986#section-5.2.2
+
+	host := "https://example.com"
+	host2 := "https://elpmaxe.moc"
+
+	assertEqual(t, resolveURL("a/b", "/c/d"), "/c/d")
+	assertEqual(t, resolveURL("a/b", "./c/d"), "/a/c/d")
+	assertEqual(t, resolveURL("a/b", "../c/d"), "/c/d")
+	assertEqual(t, resolveURL("a/b/", "./c/d"), "/a/b/c/d")
+	assertEqual(t, resolveURL("a/b/", "c/d"), "/a/b/c/d")
+	assertEqual(t, resolveURL("a/b/", host2+"/c/d"), host2+"/c/d")
+
+	assertEqual(t, resolveURL("/a/b", "/c/d"), "/c/d")
+	assertEqual(t, resolveURL("/a/b", "./c/d"), "/a/c/d")
+	assertEqual(t, resolveURL("/a/b", "../c/d"), "/c/d")
+	assertEqual(t, resolveURL("/a/b/", "./c/d"), "/a/b/c/d")
+	assertEqual(t, resolveURL("/a/b/", "c/d"), "/a/b/c/d")
+	assertEqual(t, resolveURL("/a/b/", host2+"/c/d"), host2+"/c/d")
+
+	assertEqual(t, resolveURL(host+"/a/b", "/c/d"), host+"/c/d")
+	assertEqual(t, resolveURL(host+"/a/b", "./c/d"), host+"/a/c/d")
+	assertEqual(t, resolveURL(host+"/a/b", "../c/d"), host+"/c/d")
+	assertEqual(t, resolveURL(host+"/a/b/", "./c/d"), host+"/a/b/c/d")
+	assertEqual(t, resolveURL(host+"/a/b/", "c/d"), host+"/a/b/c/d")
+	assertEqual(t, resolveURL(host+"/a/b/", host2+"/c/d"), host2+"/c/d")
+}
+
+func TestUrlResolverForLocalFiles(t *testing.T) {
+	base := "/home/foobar/project/"
+	assertEqual(t,
+		resolveURL(base, "/assets/logo.png"),
+		"/home/foobar/project/assets/logo.png")
+}
+
+func TestUrlResolverForGitforgeUsage(t *testing.T) {
+	// in git-forges like github & gitea, URLs are often written relative,
+	// where the repo-url (`gitea.com/gitea/tea`) treated as reference.
+	// The base URL needs to be treated with a trailing slash, acting as a "directory",
+	// requiring preprocessing of the URLs by applications (appending trailing slash to base URL).
+	base := "https://gitea.com/gitea/tea/"
+	assertEqual(t,
+		resolveURL(base, "src/branch/master/modules/print/markdown.go"),
+		"https://gitea.com/gitea/tea/src/branch/master/modules/print/markdown.go")
+
+	base = "https://raw.githubusercontent.com/foo/bar/master/"
+	assertEqual(t,
+		resolveURL(base, "/assets/logo.png"),
+		"https://raw.githubusercontent.com/foo/bar/master/assets/logo.png")
+
+	base = "https://raw.githubusercontent.com/foo/bar/master/some/dir/"
+	assertEqual(t,
+		resolveURL(base, "/assets/logo.png"),
+		"https://raw.githubusercontent.com/foo/bar/master/assets/logo.png")
+}
+
+func assertEqual(t *testing.T, value interface{}, expected interface{}) {
+	if value != expected {
+		t.Errorf("Expected '%v', but got '%v'", expected, value)
+	}
+}