Python JSON Parser

json_parser.py

from __future__ import annotations

import re
from typing import Any, Never


class JSONParseError(Exception):
    pass


ESCAPE_CHARS = {
    '"': '"',
    "\\": "\\",
    "b": "\b",
    "f": "\f",
    "n": "\n",
    "r": "\r",
    "t": "\t",
}


class JSONParser:
    def __init__(self, text: str) -> None:
        self.text = text
        self.pos = 0

    def _expect(self, text: str) -> bool:
        if self.text[self.pos : self.pos + len(text)] == text:
            self.pos += len(text)
            return True
        return False

    def _expect_re(self, pattern: str) -> str | None:
        match = re.compile(pattern).match(self.text, self.pos)
        if match:
            self.pos = match.end()
            return match.group()
        return None

    def _parse_error(self, message: str = "", pos: int | None = None) -> Never:
        if pos is None:
            pos = self.pos
        if not message:
            message = f"Unexpected character '{self.text[pos]}'"
        context = f"{self.text[pos-10:pos+5]}\n{' ' * min(10, pos)}^\n"
        raise JSONParseError(f"at pos {pos}: {message}\n{context}")

    def _skip_whitespace(self) -> None:
        while self.pos < len(self.text) and self.text[self.pos].isspace():
            self.pos += 1

    def _parse_bool(self) -> bool:
        if self._expect("true"):
            return True
        if self._expect("false"):
            return False
        self._parse_error()

    def _parse_null(self) -> None:
        if self._expect("null"):
            return
        self._parse_error()

    def _parse_number(self) -> int | float:
        pattern = r"-?(0|[1-9][0-9]*)(\.[0-9]+)?([eE][+-]?[0-9]+)?"
        number = self._expect_re(pattern)
        if number is None:
            self._parse_error()
        return int(number) if number.isdigit() else float(number)

    def _parse_string(self) -> str:
        pos = self.pos
        text = self.text
        if text[pos] != '"':
            self._parse_error()
        pos += 1
        buffer = ""
        while pos < len(text):
            if text[pos] == "\\":
                pos += 1
                for escape_char in ESCAPE_CHARS:
                    if text[pos] == escape_char:
                        buffer += ESCAPE_CHARS[escape_char]
                        pos += 1
                        break
                else:
                    if text[pos] == "u":
                        pos += 1
                        if pos + 4 >= len(text):
                            self._parse_error(
                                f"Truncated unicode escape sequence {text[pos - 2:pos + 4]}",
                                pos,
                            )
                        else:
                            try:
                                buffer += chr(int(text[pos : pos + 4], 16))
                                pos += 4
                            except ValueError:
                                self._parse_error(
                                    f"Invalid unicode escape sequence {text[pos - 2:pos + 4]}",
                                    pos,
                                )
                    else:
                        self._parse_error("Invalid escape sequence", pos)
            elif text[pos] == '"':
                pos += 1
                break
            else:
                buffer += text[pos]
                pos += 1
        else:
            self._parse_error("Unterminated string", pos)
        self.pos = pos
        return buffer

    def _parse_array(self) -> list[Any]:
        if not self._expect("["):
            self._parse_error()
        elements: list[Any] = []
        last_comma = False
        while self.pos < len(self.text):
            self._skip_whitespace()
            if self._expect("]"):
                if last_comma:
                    self._parse_error(pos=self.pos - 1)
                return elements
            if elements and not last_comma:
                self._parse_error()
            elements.append(self.parse_value())
            self._skip_whitespace()
            last_comma = self._expect(",")

        self._parse_error("Unclosed array")

    def _parse_object(self) -> dict[str, Any]:
        if not self._expect("{"):
            self._parse_error()
        obj: dict[str, Any] = {}
        last_comma = False
        while self.pos < len(self.text):
            self._skip_whitespace()
            if self._expect("}"):
                if last_comma:
                    self._parse_error(pos=self.pos - 1)
                return obj
            if obj and not last_comma:
                self._parse_error()
            key = self._parse_string()
            self._skip_whitespace()
            if not self._expect(":"):
                self._parse_error()
            self._skip_whitespace()
            obj[key] = self.parse_value()
            self._skip_whitespace()
            last_comma = self._expect(",")

        self._parse_error("Unclosed object")

    def parse_value(self) -> Any:
        self._skip_whitespace()
        if self.pos >= len(self.text):
            self._parse_error("Unexpected end of input")
        c = self.text[self.pos]
        if c == '"':
            return self._parse_string()
        elif c == "[":
            return self._parse_array()
        elif c == "{":
            return self._parse_object()
        elif c in "tf":
            return self._parse_bool()
        elif c == "n":
            self._parse_null()
            return None
        else:
            return self._parse_number()


def loads(text: str) -> Any:
    return JSONParser(text).parse_value()