Update json.lua from pending pull request

rxi/json.lua#51

src/resources/pandoc/datadir/_json.lua

@@ -5,6 +5,7 @@
 -- https://github.com/rxi/json.lua
 --
 -- includes unreleased upstream changes: https://github.com/rxi/json.lua/blob/dbf4b2dd2eb7c23be2773c89eb059dadd6436f94/json.lua
+-- includes unmerged upstream pull request: https://github.com/rxi/json.lua/pull/51
 --
 -- Permission is hereby granted, free of charge, to any person obtaining a copy of
 -- this software and associated documentation files (the "Software"), to deal in
@@ -27,6 +28,21 @@
 
 local json = { _version = "0.1.2-quarto" }
 
+-- taken from https://www.lua.org/pil/19.3.html
+function pairsByKeys (t, f)
+  local a = {}
+  for n in pairs(t) do table.insert(a, n) end
+  table.sort(a, f)
+  local i = 0      -- iterator variable
+  local iter = function ()   -- iterator function
+    i = i + 1
+    if a[i] == nil then return nil
+    else return a[i], t[a[i]]
+    end
+  end
+  return iter
+end
+
 -------------------------------------------------------------------------------
 -- Encode
 -------------------------------------------------------------------------------
@@ -89,7 +105,7 @@ local function encode_table(val, stack)
 
   else
     -- Treat as an object
-    for k, v in pairs(val) do
+    for k, v in pairsByKeys(val) do
       if type(k) ~= "string" then
         error("invalid table: mixed or invalid key types")
       end
@@ -388,4 +404,4 @@ function json.decode(str)
 end
 
 
-return json
+return json