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18 JSON parser
// This is a coding exercise of implementing a parser. Some minor parsing capabilities are not implemented.
// The algorithm are mainly from Douglas Crockford's 'https://github.com/douglascrockford/JSON-js/blob/master/json_parse.js'
function json_parse(str) {
var i = 0
var ch = str[i]
// core function that recursively called to move forward scanning pointers and accept tokens
function next(c) {
if (c) {
if (str[i] == c) {
i++
ch = str[i]
return true
} else {
throw new Error("Error in parsing. '" + c + "' is expected at " + i + ".");
}
} else {
i++
ch = str[i]
}
}
// accepting functions for tokens in a single character
function colon() {
next(":")
}
function leftBrace() {
next("{")
}
function rightBrace() {
next("}")
}
function leftBracket() {
next("[")
}
function rightBracket() {
next("]")
}
function quote() {
next("\"")
}
function comma() {
next(",")
}
function dot() {
next(".")
}
// accepting functions for tokens that are formed by constant character sequence
function _null() {
next("n")
next("u")
next("l")
next("l")
}
function _true() {
next("t")
next("r")
next("u")
next("e")
}
function _false() {
next("f")
next("a")
next("l")
next("s")
next("e")
}
// accepting functions for tokens where multiple characters could appear on the same position
function sign() {
if (ch && ch == "+") {
next()
return 1
} else if (ch && ch == "-") {
next()
return -1
}
return 1
}
function expo() {
if (ch && (ch == "e" || ch == "E")) {
next()
return 1
}
return 0
}
function word() {
var parsed = ''
while (ch && ch != "\"") {
parsed += ch
next()
}
return parsed
}
function digitSequence() {
var seq = ''
while(ch && /[0-9]/.test(ch)) {
seq += ch
next()
}
return parseInt(seq)
}
function number() {
var parsed = 0
var s = sign()
var integer, fractional, exponential
integer = digitSequence()
if (ch && ch == ".") {
dot()
fractional = digitSequence()
}
var hasExpo = expo()
if (hasExpo) {
exponential = digitSequence()
} else {
exponential = 0
}
parsed = integer
if (fractional) {
parsed += fractional / Math.pow(10, fractional.toString().length)
}
if (exponential) {
parsed = parsed * Math.pow(10, exponential)
}
if (s == -1) {
parsed = -parsed
}
return parsed
}
function whitespace() {
while(ch && /\s/.test(ch)) {
next()
}
}
// accepting functions for some sub structures
function wordConstant() {
var result
if (ch && ch == 't') {
_true()
result = true
} else if (ch && ch == 'f') {
_false()
result = false
} else if (ch && ch == 'n') {
_null()
result = null
}
return result
}
function string() {
quote()
var parsed = word()
quote()
return parsed
}
function keyValue() {
var result = []
result[0] = string()
whitespace()
colon()
whitespace()
result[1] = element()
return result
}
function keyValues() {
var parsed = {}
while (ch && ch == '"') {
var kv = keyValue()
parsed[kv[0]] = kv[1]
whitespace()
if (ch && ch == ",") {
comma()
} else {
break
}
whitespace()
}
return parsed
}
function element() {
if (ch && ch == '"') {
return string()
} else if (ch && ch == "[") {
return array()
} else if (ch && ch == "{") {
return object()
} else if (ch && /[\+\-0-9]/.test(ch)) {
return number()
} else if (ch && /[tfn]/.test(ch)) {
return wordConstant()
} else {
if (ch) {
throw new Error("Error in parsing. '" + ch + "' is invalid as an element in array at " + i + ".")
}
}
}
function elements() {
var ele = []
while(ch && /[\"\[\{\+\-0-9tfn]/.test(ch)) {
var e = element()
ele.push(e)
whitespace()
if (ch && ch == ",") {
comma()
} else {
break
}
whitespace()
}
return ele
}
function array() {
leftBracket()
whitespace()
var ele = elements()
whitespace()
rightBracket()
return ele
}
function object() {
leftBrace()
whitespace()
var kvs = keyValues()
whitespace()
rightBrace()
return kvs
}
whitespace()
var result = element()
whitespace()
return result
}
@iShawnWang

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@iShawnWang iShawnWang commented Oct 12, 2018

Nice ~

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