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71 lines
1.5 KiB
Python
71 lines
1.5 KiB
Python
from __future__ import annotations
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from dataclasses import dataclass
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from typing import List, Union
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# A tiny placeholder AST for expressions like: "a AND NOT b"
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@dataclass
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class Atom:
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name: str
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@dataclass
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class Not:
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expr: "Expr"
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@dataclass
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class BinOp:
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op: str
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left: "Expr"
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right: "Expr"
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Expr = Union[Atom, Not, BinOp]
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def tokenize(s: str) -> List[str]:
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return s.replace("(", " ( ").replace(")", " ) ").split()
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def parse(tokens: List[str]) -> Expr:
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"""
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Very small, permissive parser:
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grammar ~> expr := term (("AND"|"OR") term)*
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term := "NOT" term | atom | "(" expr ")"
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atom := /[A-Za-z_][A-Za-z0-9_]*/
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"""
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pos = 0
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def peek() -> str | None:
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return tokens[pos] if pos < len(tokens) else None
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def eat() -> str:
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nonlocal pos
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tok = tokens[pos]
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pos += 1
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return tok
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def parse_term() -> Expr:
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t = peek()
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if t is None:
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raise ValueError("unexpected end")
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if t == "NOT":
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eat()
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return Not(parse_term())
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if t == "(":
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eat()
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node = parse_expr()
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if eat() != ")":
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raise ValueError("expected ')'")
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return node
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# atom
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return Atom(eat())
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def parse_expr() -> Expr:
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left = parse_term()
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while peek() in ("AND", "OR"):
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op = eat()
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right = parse_term()
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left = BinOp(op, left, right)
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return left
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return parse_expr()
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