An inefficient but minimalist pure lua json parser

json.lua

-- An inefficient but minimalist pure lua json parser

local escape_table = {
	['"'] = '"',
	['/'] = '/',
	['\\'] = '\\',
	['b'] = '\b',
	['f'] = '\f',
	['n'] = '\n',
	['r'] = '\r',
	['t'] = '\t',
}
for k,v in pairs(escape_table) do
	escape_table[k] = "\\u" .. ("%04x"):format(v:byte())
end

local function hextochar(hex)
	return string.char(tonumber(hex, 16))
end

local function decode_json(s)
	local const_table = {}
	local function tokenize(s)
		s = s:gsub('\\(["/\bfnrt])', escape_table)	-- unify escape
		local n = 0
		s = s:gsub('"([^"]*)"', function (str) 
			str = str:gsub("\\u(....)", hextochar)
			n = n + 1
			const_table[n] = str
			return "@" .. n
		end)
		return s
	end
	s = tokenize(s)
	s = s:gsub("[\r\n\t ]+", "")	-- strip white space
	local nest_table = { s }
	local function nest(s)
		local n = #nest_table
		local function append_object(obj)
			n = n + 1
			nest_table[n] = obj:sub(2,-2)
			return "#" .. n
		end
		local function append_array(obj)
			n = n + 1
			nest_table[n] = obj:sub(2,-2)
			return "$" .. n
		end
		s = s:gsub("%b{}", append_object)
		s = s:gsub("%b[]", append_array)
		return s
	end
	local from = 1; repeat
		local n = #nest_table
		for i = from, n do
			nest_table[i] = nest(nest_table[i])
		end
		from = n + 1
	until #nest_table == n
	
	local parse	; -- function decl
	
	local function parse_object(s)
		local ret = {}
		local function parse_kv(k,v)
			k = const_table[tonumber(k)]
			v = parse(v)
			ret[k] = v
		end
		s:gsub("@(%d+):([^,]+)", parse_kv)
		return ret
	end
	
	local function parse_array(s)
		local ret = {}
		local n = 0
		local function parse_item(v)
			n = n + 1
			ret[n] = parse(v)
		end
		s:gsub("[^,]+", parse_item)
		return ret
	end
	
	local converter = {
		["@"] = { assert, const_table },
		["#"] = { parse_object, nest_table },
		["$"] = { parse_array, nest_table },
		["true"] = true,
		["false"] = true,
		["null"] = true,
	}
	local const = {
		["true"] = true,
		["false"] = false,
		["null"] = nil,
	}
	
	function parse(s)
		local c = converter[s:sub(1,1)]
		if c then
			return c[1](c[2][tonumber(s:sub(2))])
		else
			return tonumber(s) or (converter[s] or error ("Invalid constant " .. s) and const[s])
		end
	end
	
	return parse(nest_table[1])
end

return decode_json

test.lua

-- testcase from https://github.com/DaveGamble/cJSON/
local test = {

[[
{
    "glossary": {
        "title": "example glossary",
		"GlossDiv": {
            "title": "S",
			"GlossList": {
                "GlossEntry": {
                    "ID": "SGML",
					"SortAs": "SGML",
					"GlossTerm": "Standard Generalized Markup Language",
					"Acronym": "SGML",
					"Abbrev": "ISO 8879:1986",
					"GlossDef": {
                        "para": "A meta-markup language, used to create markup languages such as DocBook.",
						"GlossSeeAlso": ["GML", "XML"]
                    },
					"GlossSee": "markup"
                }
            }
        }
    }
}
]],
[[
{"menu": {
  "id": "file",
  "value": "File",
  "popup": {
    "menuitem": [
      {"value": "New", "onclick": "CreateNewDoc()"},
      {"value": "Open", "onclick": "OpenDoc()"},
      {"value": "Close", "onclick": "CloseDoc()"}
    ]
  }
}}
]],
[[
{"widget": {
    "debug": "on",
    "window": {
        "title": "Sample Konfabulator Widget",
        "name": "main_window",
        "width": 500,
        "height": 500
    },
    "image": {
        "src": "Images/Sun.png",
        "name": "sun1",
        "hOffset": 250,
        "vOffset": 250,
        "alignment": "center"
    },
    "text": {
        "data": "Click Here",
        "size": 36,
        "style": "bold",
        "name": "text1",
        "hOffset": 250,
        "vOffset": 100,
        "alignment": "center",
        "onMouseUp": "sun1.opacity = (sun1.opacity / 100) * 90;"
    }
}}
]],
[=[
{"web-app": {
  "servlet": [  
    {
      "servlet-name": "cofaxCDS",
      "servlet-class": "org.cofax.cds.CDSServlet",
      "init-param": {
        "configGlossary:installationAt": "Philadelphia, PA",
        "configGlossary:adminEmail": "[email protected]",
        "configGlossary:poweredBy": "Cofax",
        "configGlossary:poweredByIcon": "/images/cofax.gif",
        "configGlossary:staticPath": "/content/static",
        "templateProcessorClass": "org.cofax.WysiwygTemplate",
        "templateLoaderClass": "org.cofax.FilesTemplateLoader",
        "templatePath": "templates",
        "templateOverridePath": "",
        "defaultListTemplate": "listTemplate.htm",
        "defaultFileTemplate": "articleTemplate.htm",
        "useJSP": false,
        "jspListTemplate": "listTemplate.jsp",
        "jspFileTemplate": "articleTemplate.jsp",
        "cachePackageTagsTrack": 200,
        "cachePackageTagsStore": 200,
        "cachePackageTagsRefresh": 60,
        "cacheTemplatesTrack": 100,
        "cacheTemplatesStore": 50,
        "cacheTemplatesRefresh": 15,
        "cachePagesTrack": 200,
        "cachePagesStore": 100,
        "cachePagesRefresh": 10,
        "cachePagesDirtyRead": 10,
        "searchEngineListTemplate": "forSearchEnginesList.htm",
        "searchEngineFileTemplate": "forSearchEngines.htm",
        "searchEngineRobotsDb": "WEB-INF/robots.db",
        "useDataStore": true,
        "dataStoreClass": "org.cofax.SqlDataStore",
        "redirectionClass": "org.cofax.SqlRedirection",
        "dataStoreName": "cofax",
        "dataStoreDriver": "com.microsoft.jdbc.sqlserver.SQLServerDriver",
        "dataStoreUrl": "jdbc:microsoft:sqlserver://LOCALHOST:1433;DatabaseName=goon",
        "dataStoreUser": "sa",
        "dataStorePassword": "dataStoreTestQuery",
        "dataStoreTestQuery": "SET NOCOUNT ON;select test='test';",
        "dataStoreLogFile": "/usr/local/tomcat/logs/datastore.log",
        "dataStoreInitConns": 10,
        "dataStoreMaxConns": 100,
        "dataStoreConnUsageLimit": 100,
        "dataStoreLogLevel": "debug",
        "maxUrlLength": 500}},
    {
      "servlet-name": "cofaxEmail",
      "servlet-class": "org.cofax.cds.EmailServlet",
      "init-param": {
      "mailHost": "mail1",
      "mailHostOverride": "mail2"}},
    {
      "servlet-name": "cofaxAdmin",
      "servlet-class": "org.cofax.cds.AdminServlet"},

    {
      "servlet-name": "fileServlet",
      "servlet-class": "org.cofax.cds.FileServlet"},
    {
      "servlet-name": "cofaxTools",
      "servlet-class": "org.cofax.cms.CofaxToolsServlet",
      "init-param": {
        "templatePath": "toolstemplates/",
        "log": 1,
        "logLocation": "/usr/local/tomcat/logs/CofaxTools.log",
        "logMaxSize": "",
        "dataLog": 1,
        "dataLogLocation": "/usr/local/tomcat/logs/dataLog.log",
        "dataLogMaxSize": "",
        "removePageCache": "/content/admin/remove?cache=pages&id=",
        "removeTemplateCache": "/content/admin/remove?cache=templates&id=",
        "fileTransferFolder": "/usr/local/tomcat/webapps/content/fileTransferFolder",
        "lookInContext": 1,
        "adminGroupID": 4,
        "betaServer": true}}],
  "servlet-mapping": {
    "cofaxCDS": "/",
    "cofaxEmail": "/cofaxutil/aemail/*",
    "cofaxAdmin": "/admin/*",
    "fileServlet": "/static/*",
    "cofaxTools": "/tools/*"},

  "taglib": {
    "taglib-uri": "cofax.tld",
    "taglib-location": "/WEB-INF/tlds/cofax.tld"}}}
]=],
[[
{"menu": {
    "header": "SVG Viewer",
    "items": [
        {"id": "Open"},
        {"id": "OpenNew", "label": "Open New"},
        null,
        {"id": "ZoomIn", "label": "Zoom In"},
        {"id": "ZoomOut", "label": "Zoom Out"},
        {"id": "OriginalView", "label": "Original View"},
        null,
        {"id": "Quality"},
        {"id": "Pause"},
        {"id": "Mute"},
        null,
        {"id": "Find", "label": "Find..."},
        {"id": "FindAgain", "label": "Find Again"},
        {"id": "Copy"},
        {"id": "CopyAgain", "label": "Copy Again"},
        {"id": "CopySVG", "label": "Copy SVG"},
        {"id": "ViewSVG", "label": "View SVG"},
        {"id": "ViewSource", "label": "View Source"},
        {"id": "SaveAs", "label": "Save As"},
        null,
        {"id": "Help"},
        {"id": "About", "label": "About Adobe CVG Viewer..."}
    ]
}}
]],
[[
[
	 {
	 "precision": "zip",
	 "Latitude":  37.7668,
	 "Longitude": -122.3959,
	 "Address":   "",
	 "City":      "SAN FRANCISCO",
	 "State":     "CA",
	 "Zip":       "94107",
	 "Country":   "US"
	 },
	 {
	 "precision": "zip",
	 "Latitude":  37.371991,
	 "Longitude": -122.026020,
	 "Address":   "",
	 "City":      "SUNNYVALE",
	 "State":     "CA",
	 "Zip":       "94085",
	 "Country":   "US"
	 }
	 ]
]],
[[
{
		"Image": {
			"Width":  800,
			"Height": 600,
			"Title":  "View from 15th Floor",
			"Thumbnail": {
				"Url":    "http:/*www.example.com/image/481989943",
				"Height": 125,
				"Width":  "100"
			},
			"IDs": [116, 943, 234, 38793]
		}
	}
]],
[[
[
    [0, -1, 0],
    [1, 0, 0],
    [0, 0, 1]
	]
]],
[[
["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"]
]],
[[
{
"name": "Jack (\"Bee\") Nimble",
"format": {"type":       "rect",
"width":      1920,
"height":     1080,
"interlace":  false,"frame rate": 24
}
}
]],
}
local decode_json = require "json"
local print_r = require "print_r"

for _, v in ipairs(test) do
	local t = decode_json(v)
	print_r(t)
end