"""
JSON parser
"""

def loads(s):
    let i = 0
    let value # okay dumb scoping thing, but we'll assign this to _value later...
    def peek():
        return s[i] if i < len(s) else None
    def advance():
        i++
    def whitespace():
        while peek() in ' \r\n\t': advance()
    def string():
        if peek() != '"': raise ValueError("String should start with \" but got " + peek())
        advance()
        let out = ""
        let c = peek()
        while c != '"':
            if c is None: raise ValueError("Unterminated string")
            if c == '\\':
                advance()
                c = peek()
                if c == '"': out += '"'
                elif c == '\\': out += '\\'
                elif c == '/': out += '/'
                elif c == 'b': out += '\x08' # Backspace
                elif c == 'f': out += '\x0c' # Formfeed
                elif c == 'n': out += '\n'
                elif c == 'r': out += '\r'
                elif c == 't': out += '\t'
                elif c == 'u':
                    advance()
                    let sub = [0,0,0,0]
                    sub[0] = peek()
                    advance()
                    sub[1] = peek()
                    advance()
                    sub[2] = peek()
                    advance()
                    sub[3] = peek()
                    if not all([x in '0123456789abcdefABCDEF' for x in sub]):
                        raise ValueError('Invalid codepoint in JSON string')
                    out += chr(int(sub,16))
                else:
                    raise ValueError('Invalid escape sequence in JSON string')
            else:
                out += c
            advance()
            c = peek()
        if peek() != '"':
            raise ValueError("That was not expected.")
        advance()
        return out
    def obj():
        if peek() != '{': raise ValueError("Object should start with {?")
        advance()
        let out = {}
        whitespace()
        if peek() == '}': return out
        while True:
            whitespace()
            let k = string()
            whitespace()
            if peek() != ':': raise ValueError("Expected : after key, got " + peek())
            advance()
            let v = value()
            out[k] = v
            if peek() == '}':
                advance()
                return out
            if peek() != ',':
                raise ValueError('Expected , after value, or } for end of object.')
            advance()
    def array():
        if peek() != '[': raise ValueError("Array should start with [?")
        advance()
        let out = []
        whitespace()
        if peek() == ']': return out
        while True:
            whitespace()
            out.append(value())
            if peek() == ']':
                advance()
                return out
            if peek() != ',':
                raise ValueError("Expected , after value, or ] for end of array.")
            advance()
    def number():
        let value = 0
        let sign = 1
        if peek() == '-':
            sign = -1
            advance()
        if peek() == '0':
            advance()
        elif peek() in '0123456789':
            value = int(peek())
            advance()
            while peek() in '0123456789':
                value = value * 10
                value = value + int(peek())
                advance()
        else:
            raise ValueError("Expected digit")
        if peek() == '.':
            advance()
            let mult = 0.1
            if peek() not in '0123456789':
                raise ValueError("Expected digit")
            while peek() in '0123456789':
                value = value + multiplier * int(peek())
                multiplier = multiplier * 0.1
                advance()
        if peek() in 'eE':
            raise NotImplementedError("Exponent values are not implemented.")
        return value
    def boolean():
        let inv = ValueError("Invalid literal while parsing bool")
        if peek() == 't':
            advance()
            if peek() != 'r': raise inv
            advance()
            if peek() != 'u': raise inv
            advance()
            if peek() != 'e': raise inv
            advance()
            return True
        elif peek() == 'f':
            advance()
            if peek() != 'a': raise inv
            advance()
            if peek() != 'l': raise inv
            advance()
            if peek() != 's': raise inv
            advance()
            if peek() != 'e': raise inv
            advance()
            return False
        raise inv
    def null():
        let inv = ValueError("Invalid literal while parsing null")
        if peek() != 'n': raise inv
        advance()
        if peek() != 'u': raise inv
        advance()
        if peek() != 'l': raise inv
        advance()
        if peek() != 'l': raise inv
        advance()
        return None
    def _value():
        whitespace()
        let c = peek()
        let out = None
        if c== '"':
            out = string()
        elif c == '{':
            out = obj()
        elif c == '[':
            out = array()
        elif c == '-' or c in '0123456789':
            out = number()
        elif c == 't' or c == 'f':
            out = boolean()
        elif c == 'n':
            out = null()
        else:
            raise ValueError("Unexpected character")
        whitespace()
        return out
    value = _value
    return value()