json_parser.py

from typing import Any

class JsonParser:
    def __init__(self, input_text: str):
        self.input = input_text
        self.pos = 0

    @property
    def _current(self) -> str | None:
        if self.pos < len(self.input):
            return self.input[self.pos]
        
        return None

    def _move(self) -> None:
        self.pos += 1
    
    def _parse_number(self) -> int | float:
        extracted = ""
        digits = "0123456789"

        if self._current == "-":
            extracted += self._current
            self._move()

            if self._current is None or self._current not in digits:
                raise ValueError(f"Invalid character {self._current} after - sign only digits are allowed.")

        if self._current == "0":
            extracted += self._current
            self._move()

            if self._current is None:
                return int(extracted)
            
            if self._current in digits:
                raise ValueError("Numbers can't start with 0")
            
        seen_exp = False
        seen_sign = False
        seen_dot = False

        while self._current is not None:
            if self._current in digits:
                extracted += self._current
                self._move()

            elif self._current == ".":                
                if seen_dot:
                    raise ValueError("Already contains dot!")
                
                extracted += self._current
                self._move()
                seen_dot = True

                if self._current is None or self._current not in digits:
                    raise ValueError("After . at least 1 number ise expected.")

            elif self._current in "eE":
                if seen_exp:
                    raise ValueError("There is already exponent defined")
                
                extracted += self._current
                self._move()
                seen_exp = True

                if self._current is None:
                    raise ValueError("After exponent value is required.")
            
            elif self._current in "-+":
                if seen_sign:
                    raise ValueError(f"Sign {self._current} is already used.")
                
                extracted += self._current
                self._move()
                seen_sign = True

                if self._current is None:
                    raise ValueError("After +- sign value is required")
                
            else:
                break
            
        if "." in extracted or "e" in extracted or "E" in extracted:
            return float(extracted)
        
        else:
            return int(extracted)
        
    def _parse_string(self) -> str:
        self._move()
        extracted = ""
        
        while True:
            if self._current is None:
                raise ValueError("Expected strings to end with \"")
            
            if self._current == "\\":
                self._move() 

                if self._current == "\\":
                    extracted += self._current
                    self._move()

                elif self._current == "n":
                    extracted += "\n"
                    self._move()

                elif self._current == '"':
                    extracted += self._current
                    self._move()

                else:
                    raise ValueError(f"Invalid escape character: {self._current}")
            
            elif self._current == '"':
                self._move()
                break

            else:
                extracted += self._current
                self._move()

        return extracted

    def _parse_object(self) -> dict[str, Any]:
        self._move()
        self.ignore_whitespace()
        
        if self._current == "}":
            return {}
        
        obj = {}

        while True:
            self.ignore_whitespace()

            key = self._parse_string()

            if self._current != ":":
                raise ValueError("Missing : after parsing the key")

            self._move()
            self.ignore_whitespace()
            value = self.parse()
            obj[key] = value
            self.ignore_whitespace()
            
            if self._current == ",":
                self._move()
                continue

            if self._current == "}":
                self._move()
                break
        
        return obj
        

    def _parse_array(self) -> list[Any]:
        self._move()
        self.ignore_whitespace()
        
        if self._current == "]":
            return []
        
        arr = []
        while True:
            self.ignore_whitespace()

            if self._current is None:
                raise ValueError("Failed to parse array")
            
            value = self.parse()
            arr.append(value)
            self.ignore_whitespace()

            if self._current == ",":
                self._move()
                continue

            if self._current == "]":
                self._move()
                break

        return arr
        
    def parse(self) -> Any:
        self.ignore_whitespace()

        if self._current == "n" and self.pos + 3 < len(self.input) and self.input[self.pos:self.pos+4] == "null":
            self.pos += 4
            return None
        
        elif self._current == "t" and self.pos + 3 < len(self.input) and self.input[self.pos:self.pos+4] == "true":
            self.pos += 4
            return True
        
        elif self._current == "f" and self.pos + 4 < len(self.input) and self.input[self.pos:self.pos+5] == "false":
            self.pos += 5
            return False
        
        elif self._current == "-" or self._current in "0123456789":
            return self._parse_number()

        elif self._current == "{":
            return self._parse_object()
        
        elif self._current == "[":
            return self._parse_array()
        
        elif self._current == "\"":
            return self._parse_string()

        else:
            raise ValueError(f"Can't parse JSON at: {self.pos}")

    def ignore_whitespace(self) -> None:
        while self._current is not None and self.pos < len(self.input) and self.input[self.pos] in " \n":
            self._move()