Reuse alias tree for discovery and help

internal/alias/resolver.go

@@ -182,3 +182,50 @@ func (resolver *Resolver) ResolveWithProvenance(path commandpath.Path) (commandp
 		origins = nextOrigins
 	}
 }
+
+// ProperSourcePrefixes returns distinct nonempty proper prefixes of source paths.
+// Returned paths are owned by the caller.
+func (resolver *Resolver) ProperSourcePrefixes() []commandpath.Path {
+	if resolver == nil {
+		return nil
+	}
+	var paths []commandpath.Path
+	for index := 1; index < len(resolver.trie); index++ {
+		node := &resolver.trie[index]
+		if len(node.children) != 0 {
+			paths = append(paths, node.path.Clone())
+		}
+	}
+	return paths
+}
+
+// Descendants returns owned mappings whose sources are strictly below prefix.
+func (resolver *Resolver) Descendants(prefix commandpath.Path) []Mapping {
+	if resolver == nil {
+		return nil
+	}
+	index := 0
+	for _, component := range prefix {
+		next, ok := resolver.trie[index].children[component]
+		if !ok {
+			return nil
+		}
+		index = next
+	}
+	var mappings []Mapping
+	var visit func(int)
+	visit = func(index int) {
+		node := &resolver.trie[index]
+		if node.mapping >= 0 {
+			mapping := resolver.mappings[node.mapping]
+			mappings = append(mappings, Mapping{Source: mapping.Source.Clone(), Target: mapping.Target.Clone()})
+		}
+		for _, child := range node.children {
+			visit(child)
+		}
+	}
+	for _, child := range resolver.trie[index].children {
+		visit(child)
+	}
+	return mappings
+}