ffiUtil: Make orderedPairs behave with mixed key types (#1517)

Usually involves sorting numbers with strings, but could theoretically be more esoteric ;).

Still assumes that matching types supports comparisons, which may not always be true for custom types without an overloaded operator...

Should be fine for our current needs, though.

If need be, se the wiki page referenced in the function's description, as it features some more complex solutions.

ffi/util.lua

@@ -655,12 +655,20 @@ end
 -- pairs(), but with *keys* sorted alphabetically.
 -- c.f., http://lua-users.org/wiki/SortedIteration
 -- See also http://lua-users.org/wiki/SortedIterationSimple
-local function __genOrderedIndex( t )
+local function __genOrderedIndex(t)
     local orderedIndex = {}
     for key in pairs(t) do
-        table.insert( orderedIndex, key )
+        table.insert(orderedIndex, key)
     end
-    table.sort( orderedIndex )
+    table.sort(orderedIndex, function(v1, v2)
+        if type(v1) == type(v2) then
+            -- Assumes said type supports the < comparison operator
+            return v1 < v2
+        else
+            -- Handle type mismatches by squashing to string
+            return tostring(v1) < tostring(v2)
+        end
+    end)
     return orderedIndex
 end
 
@@ -669,10 +677,10 @@ local function orderedNext(t, state)
     -- We use a temporary ordered key table that is stored in the table being iterated.
 
     local key = nil
-    --print("orderedNext: state = "..tostring(state) )
+    -- print("orderedNext: state = "..tostring(state))
     if state == nil then
         -- the first time, generate the index
-        t.__orderedIndex = __genOrderedIndex( t )
+        t.__orderedIndex = __genOrderedIndex(t)
         key = t.__orderedIndex[1]
     else
         -- fetch the next value