feature: set cjson.array_mt on decoded JSON arrays.

this can be turned on via cjson.decode_array_with_array_mt(true). off by
default.

Signed-off-by: Yichun Zhang (agentzh) <agentzh@gmail.com>

Author: thibaultcha

README.md

@@ -14,6 +14,7 @@ Table of Contents
     * [array_mt](#array_mt)
     * [empty_array_mt](#empty_array_mt)
     * [encode_number_precision](#encode_number_precision)
+    * [decode_array_with_array_mt](#decode_array_with_array_mt)
 
 Description
 ===========
@@ -156,3 +157,39 @@ encode_number_precision
 This fork allows encoding of numbers with a `precision` up to 16 decimals (vs. 14 in mpx/lua-cjson).
 
 [Back to TOC](#table-of-contents)
+
+decode_array_with_array_mt
+--------------------------
+**syntax:** `cjson.decode_array_with_array_mt(enabled)`
+
+**default:** false
+
+If enabled, JSON Arrays decoded by `cjson.decode` will result in Lua
+tables with the [`array_mt`](#array_mt) metatable. This can ensure a 1-to-1
+relationship between arrays upon multiple encoding/decoding of your
+JSON data with this module.
+
+If disabled, JSON Arrays will be decoded to plain Lua tables, without
+the `array_mt` metatable.
+
+The `enabled` argument is a boolean.
+
+Example:
+
+```lua
+local cjson = require "cjson"
+
+-- default behavior
+local my_json = [[{"my_array":[]}]]
+local t = cjson.decode(my_json)
+cjson.encode(t) -- {"my_array":{}} back to an object
+
+-- now, if this behavior is enabled
+cjson.decode_array_with_array_mt(true)
+
+local my_json = [[{"my_array":[]}]]
+local t = cjson.decode(my_json)
+cjson.encode(t) -- {"my_array":[]} properly re-encoded as an array
+```
+
+[Back to TOC](#table-of-contents)

lua_cjson.c

@@ -79,6 +79,7 @@
 #define DEFAULT_ENCODE_KEEP_BUFFER 1
 #define DEFAULT_ENCODE_NUMBER_PRECISION 14
 #define DEFAULT_ENCODE_EMPTY_TABLE_AS_OBJECT 1
+#define DEFAULT_DECODE_ARRAY_WITH_ARRAY_MT 0
 
 #ifdef DISABLE_INVALID_NUMBERS
 #undef DEFAULT_DECODE_INVALID_NUMBERS
@@ -148,6 +149,7 @@ typedef struct {
 
     int decode_invalid_numbers;
     int decode_max_depth;
+    int decode_array_with_array_mt;
 } json_config_t;
 
 typedef struct {
@@ -329,6 +331,16 @@ static int json_cfg_encode_empty_table_as_object(lua_State *l)
     return json_enum_option(l, 1, &cfg->encode_empty_table_as_object, NULL, 1);
 }
 
+/* Configures how to decode arrays */
+static int json_cfg_decode_array_with_array_mt(lua_State *l)
+{
+    json_config_t *cfg = json_arg_init(l, 1);
+
+    json_enum_option(l, 1, &cfg->decode_array_with_array_mt, NULL, 1);
+
+    return 1;
+}
+
 /* Configures JSON encoding buffer persistence */
 static int json_cfg_encode_keep_buffer(lua_State *l)
 {
@@ -420,6 +432,7 @@ static void json_create_config(lua_State *l)
     cfg->encode_keep_buffer = DEFAULT_ENCODE_KEEP_BUFFER;
     cfg->encode_number_precision = DEFAULT_ENCODE_NUMBER_PRECISION;
     cfg->encode_empty_table_as_object = DEFAULT_ENCODE_EMPTY_TABLE_AS_OBJECT;
+    cfg->decode_array_with_array_mt = DEFAULT_DECODE_ARRAY_WITH_ARRAY_MT;
 
 #if DEFAULT_ENCODE_KEEP_BUFFER > 0
     strbuf_init(&cfg->encode_buf, 0);
@@ -1261,6 +1274,13 @@ static void json_parse_array_context(lua_State *l, json_parse_t *json)
 
     lua_newtable(l);
 
+    /* set array_mt on the table at the top of the stack */
+    if (json->cfg->decode_array_with_array_mt) {
+        lua_pushlightuserdata(l, &json_array);
+        lua_rawget(l, LUA_REGISTRYINDEX);
+        lua_setmetatable(l, -2);
+    }
+
     json_next_token(json, &token);
 
     /* Handle empty arrays */
@@ -1415,6 +1435,7 @@ static int lua_cjson_new(lua_State *l)
         { "encode", json_encode },
         { "decode", json_decode },
         { "encode_empty_table_as_object", json_cfg_encode_empty_table_as_object },
+        { "decode_array_with_array_mt", json_cfg_decode_array_with_array_mt },
         { "encode_sparse_array", json_cfg_encode_sparse_array },
         { "encode_max_depth", json_cfg_encode_max_depth },
         { "decode_max_depth", json_cfg_decode_max_depth },

tests/agentzh.t

@@ -153,7 +153,56 @@ print(cjson.encode(data))
 
 
 
-=== TEST 12: array_mt on tables with hash part
+=== TEST 12: decode() by default does not set array_mt on empty arrays
+--- lua
+local cjson = require "cjson"
+local json = [[{"my_array":[]}]]
+local t = cjson.decode(json)
+local has_metatable = getmetatable(t.my_array) == cjson.array_mt
+print("decoded JSON array has metatable: " .. tostring(has_metatable))
+print(cjson.encode(t))
+--- out
+decoded JSON array has metatable: false
+{"my_array":{}}
+
+
+
+=== TEST 13: decode() sets array_mt on non-empty arrays if enabled
+--- lua
+local cjson = require "cjson"
+cjson.decode_array_with_array_mt(true)
+local json = [[{"my_array":["hello","world"]}]]
+local t = cjson.decode(json)
+t.my_array.hash_value = "adding a hash value"
+-- emptying the array part
+t.my_array[1] = nil
+t.my_array[2] = nil
+local has_metatable = getmetatable(t.my_array) == cjson.array_mt
+print("decoded JSON array has metatable: " .. tostring(has_metatable))
+print(cjson.encode(t))
+--- out
+decoded JSON array has metatable: true
+{"my_array":[]}
+
+
+
+=== TEST 14: cfg can enable/disable setting array_mt
+--- lua
+local cjson = require "cjson"
+cjson.decode_array_with_array_mt(true)
+cjson.decode_array_with_array_mt(false)
+local json = [[{"my_array":[]}]]
+local t = cjson.decode(json)
+local has_metatable = getmetatable(t.my_array) == cjson.array_mt
+print("decoded JSON array has metatable: " .. tostring(has_metatable))
+print(cjson.encode(t))
+--- out
+decoded JSON array has metatable: false
+{"my_array":{}}
+
+
+
+=== TEST 15: array_mt on tables with hash part
 --- lua
 local cjson = require "cjson"
 local data
@@ -175,7 +224,7 @@ print(cjson.encode(data))
 
 
 
-=== TEST 13: multiple calls to lua_cjson_new (1/3)
+=== TEST 16: multiple calls to lua_cjson_new (1/3)
 --- lua
 local cjson = require "cjson"
 package.loaded["cjson"] = nil
@@ -187,7 +236,7 @@ print(cjson.encode(arr))
 
 
 
-=== TEST 14: multiple calls to lua_cjson_new (2/3)
+=== TEST 17: multiple calls to lua_cjson_new (2/3)
 --- lua
 local cjson = require "cjson"
 package.loaded["cjson"] = nil
@@ -199,7 +248,7 @@ print(cjson.encode(arr))
 
 
 
-=== TEST 15: multiple calls to lua_cjson_new (3/3)
+=== TEST 18: multiple calls to lua_cjson_new (3/3)
 --- lua
 local cjson = require "cjson.safe"
 -- load another cjson instance (not in package.loaded)
@@ -211,7 +260,7 @@ print(cjson.encode(arr))
 
 
 
-=== TEST 16: & in JSON
+=== TEST 19: & in JSON
 --- lua
 local cjson = require "cjson"
 local a="[\"a=1&b=2\"]"
@@ -222,7 +271,7 @@ print(cjson.encode(b))
 
 
 
-=== TEST 17: default and max precision
+=== TEST 20: default and max precision
 --- lua
 local math = require "math"
 local cjson = require "cjson"