Mount() adds a subrouter under a prefix pattern.

...though I'm already eyeing a better approach.

router.go

@@ -2,6 +2,9 @@ package gus
 
 import (
 	"context"
+	"crypto/tls"
+	"net/url"
+	"strings"
 
 	"tildegit.org/tjp/gus/internal"
 )
@@ -42,6 +45,16 @@ func (r Router) Handler(ctx context.Context, request *Request) *Response {
 		return nil
 	}
 
+	// as we may be a sub-router, check for existing stashed params
+	// and combine with that map if found.
+	priorParams := RouteParams(ctx)
+	for k, v := range priorParams {
+		if k == subrouterPathKey {
+			continue
+		}
+		params[k] = v
+	}
+
 	return handler(context.WithValue(ctx, routeParamsKey, params), request)
 }
 
@@ -54,6 +67,26 @@ func (r Router) Match(request *Request) (Handler, map[string]string) {
 	return *handler, params
 }
 
+// Mount attaches a sub-router to handle path suffixes after an initial prefix pattern.
+//
+// The prefix pattern may include segment :wildcards, but no *remainder segment. The
+// mounted sub-router should have patterns which only include the portion of the path
+// after whatever was matched by the prefix pattern.
+func (r Router) Mount(prefix string, subrouter *Router) {
+	prefix = strings.TrimSuffix(prefix, "/")
+	r.Route(prefix+"/*"+subrouterPathKey, func(ctx context.Context, request *Request) *Response {
+		r := cloneRequest(request)
+		r.Path = "/" + RouteParams(ctx)[subrouterPathKey]
+		return subrouter.Handler(ctx, r)
+	})
+
+	// TODO: better approach. the above works but it's a little hacky
+	//   - add a method to PathTree that returns all the registered patterns
+	//     and their associated handlers
+	//   - have Mount pull those out of the subrouter, prepend the prefix to
+	//     all its patterns, and re-add them to the parent router.
+}
+
 // RouteParams gathers captured path parameters from the request context.
 //
 // If the context doesn't contain a parameter map, it returns nil.
@@ -66,6 +99,23 @@ func RouteParams(ctx context.Context) map[string]string {
 	return nil
 }
 
+const subrouterPathKey = "subrouter_path"
+
 type routeParamsKeyType struct{}
 
 var routeParamsKey = routeParamsKeyType{}
+
+func cloneRequest(start *Request) *Request {
+	end := &Request{}
+	*end = *start
+
+	end.URL = &url.URL{}
+	*end.URL = *start.URL
+
+	if start.TLSState != nil {
+		end.TLSState = &tls.ConnectionState{}
+		*end.TLSState = *start.TLSState
+	}
+
+	return end
+}