path: keep order of existing entries in PATH

Implements suggestion by @FSMaxB:

> Add an additional flag to util.cleanup_path that specifies if the cleanup
> happens from the right or from the left. If append is true, clean up from the
> left, otherwise clean up from the right.

Fixes #763.

2 files changed, 18 insertions, 7 deletions

diff --git a/src/luarocks/cmd/path.lua b/src/luarocks/cmd/path.lua
index 9046cd2e..bb383ad9 100644
--- a/src/luarocks/cmd/path.lua
+++ b/src/luarocks/cmd/path.lua
@@ -60,10 +60,10 @@ function path_cmd.command(flags)
    
    local lpath_var, lcpath_var = util.lua_path_variables()
 
-   util.printout(fs.export_cmd(lpath_var, util.cleanup_path(lr_path, ';', cfg.lua_version)))
-   util.printout(fs.export_cmd(lcpath_var, util.cleanup_path(lr_cpath, ';', cfg.lua_version)))
+   util.printout(fs.export_cmd(lpath_var, util.cleanup_path(lr_path, ';', cfg.lua_version, flags["append"])))
+   util.printout(fs.export_cmd(lcpath_var, util.cleanup_path(lr_cpath, ';', cfg.lua_version, flags["append"])))
    if not flags["no-bin"] then
-      util.printout(fs.export_cmd("PATH", util.cleanup_path(lr_bin, path_sep)))
+      util.printout(fs.export_cmd("PATH", util.cleanup_path(lr_bin, path_sep, nil, flags["append"])))
    end
    return true
 end
diff --git a/src/luarocks/core/util.lua b/src/luarocks/core/util.lua
index 1499dc62..6a306026 100644
--- a/src/luarocks/core/util.lua
+++ b/src/luarocks/core/util.lua
@@ -165,13 +165,23 @@ end
 -- @param list string: A path string (from $PATH or package.path)
 -- @param sep string: The separator
 -- @param lua_version (optional) string: The Lua version to use.
-function util.cleanup_path(list, sep, lua_version)
+-- @param keep_last (optional) if true, keep last occurrence in case
+-- of duplicates; otherwise keep first occurrence. The default is false.
+function util.cleanup_path(list, sep, lua_version, keep_last)
    assert(type(list) == "string")
    assert(type(sep) == "string")
    local parts = util.split_string(list, sep)
    local final, entries = {}, {}
-   for _, part in ipairs(parts) do
-      part = part:gsub("//", "/")
+   local start, stop, step
+
+   if keep_last then
+      start, stop, step = 1, #parts, 1
+   else
+      start, stop, step = #parts, 1, -1
+   end
+
+   for i = start, stop, step do
+      local part = parts[i]:gsub("//", "/")
       if lua_version then
          part = part:gsub("/lua/([%d.]+)/", function(part_version)
             if part_version:sub(1, #lua_version) ~= lua_version then            
@@ -180,7 +190,8 @@ function util.cleanup_path(list, sep, lua_version)
          end)
       end
       if not entries[part] then
-         table.insert(final, part)
+         local at = keep_last and #final+1 or 1
+         table.insert(final, at, part)
          entries[part] = true
       end
    end