libs/json: add proper support for decoding unicode escapes

libs/json/luasrc/json.lua

@@ -39,6 +39,7 @@ Encoder:
 		luci.ltn12.pump.all(encoder:source(), luci.ltn12.sink.file(io.open("someFile", w)))
 ]]--
 
+local nixio     = require "nixio"
 local util      = require "luci.util"
 local table     = require "table"
 local string    = require "string"
@@ -54,6 +55,11 @@ local ipairs    = ipairs
 local next      = next
 local pcall		= pcall
 
+local band      = nixio.bit.band
+local bor       = nixio.bit.bor
+local rshift    = nixio.bit.rshift
+local char      = string.char
+
 local getmetatable = getmetatable
 
 --- LuCI JSON-Library
@@ -412,6 +418,35 @@ function Decoder.parse_string(self, chunk)
 end
 
 
+function Decoder.utf8_encode(self, s1, s2)
+	local n = s1 * 256 + s2
+
+	if n >= 0 and n <= 0x7F then
+		return char(n)
+	elseif n >= 0 and n <= 0x7FF then
+		return char(
+			bor(band(rshift(n,  6), 0x1F), 0xC0),
+			bor(band(n,             0x3F), 0x80)
+		)
+	elseif n >= 0 and n <= 0xFFFF then
+		return char(
+			bor(band(rshift(n, 12), 0x0F), 0xE0),
+			bor(band(rshift(n,  6), 0x3F), 0x80),
+			bor(band(n,             0x3F), 0x80)
+		)
+	elseif n >= 0 and n <= 0x10FFFF then
+		return char(
+			bor(band(rshift(n, 18), 0x07), 0xF0),
+			bor(band(rshift(n, 12), 0x3F), 0x80),
+			bor(band(rshift(n,  6), 0x3F), 0x80),
+			bor(band(n,             0x3F), 0x80)
+		)
+	else
+		return "?"
+	end
+end
+
+
 function Decoder.parse_escape(self, chunk)
 	local str = ""
 	chunk = self:fetch_atleast(chunk:sub(2), 1)
@@ -428,8 +463,7 @@ function Decoder.parse_escape(self, chunk)
 		s1, s2 = tonumber(s1, 16), tonumber(s2, 16)
 		assert(s1 and s2, "Invalid Unicode character")
 
-		-- ToDo: Unicode support
-		return chunk:sub(5), s1 == 0 and string.char(s2) or ""
+		return chunk:sub(5), self:utf8_encode(s1, s2)
 	elseif char == "/" then
 		return chunk, "/"
 	elseif char == "b" then