Improve performance of manif_core.get_versions

To get all installed versions of a package using deps_mode,
instead of deeply merging all manifests of corresponding trees,
shallowly merge only tables related to that package.

This affects speed of manifest manipulation and dependency resolution.
Minimal performance testing suggests 3x-4x speed-up on reinstallation
of a package using `luarocks make`.

1 files changed, 10 insertions, 11 deletions

diff --git a/src/luarocks/manif_core.lua b/src/luarocks/manif_core.lua
index 5c8928d4..82e7ea4d 100644
--- a/src/luarocks/manif_core.lua
+++ b/src/luarocks/manif_core.lua
@@ -88,20 +88,19 @@ end
 function manif_core.get_versions(name, deps_mode)
    assert(type(name) == "string")
    assert(type(deps_mode) == "string")
-   
-   local manifest = {}
+
+   local version_set = {}
    path.map_trees(deps_mode, function(tree)
-      local loaded = manif_core.load_local_manifest(path.rocks_dir(tree))
-      if loaded then
-         util.deep_merge(manifest, loaded)
+      local manifest = manif_core.load_local_manifest(path.rocks_dir(tree))
+
+      if manifest and manifest.repository[name] then
+         for version in pairs(manifest.repository[name]) do
+            version_set[version] = true
+         end
       end
    end)
-   
-   local item = next(manifest) and manifest.repository[name]
-   if item then
-      return util.keys(item)
-   end
-   return {}
+
+   return util.keys(version_set)
 end
 
 return manif_core