calculator: add postfix parser

This commit is contained in:
Bruno BELANYI 2019-11-30 17:32:57 +01:00
parent 2490e1a5bd
commit 220e70ab33
4 changed files with 104 additions and 0 deletions

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from .postfix import parse_postfix

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from typing import List, Union
from calculator.ast import BinOp, Constant, Node, UnaryOp
from pydantic.dataclasses import dataclass
def begins_with_digit(d: str) -> bool:
return "0" <= d[0] <= "9"
@dataclass
class ParsedString:
input: str
def _is_done(self) -> bool:
return len(self.input) == 0
def _get_token(self) -> Union[int, str]:
ans = ""
self.input = self.input.strip() # Remove whitespace
while begins_with_digit(self.input):
ans += self.input[0]
self.input = self.input[1:]
if len(self.input) == 0 or not begins_with_digit(self.input):
break
if len(ans) != 0: # Was a number, return the converted int
return int(ans)
# Was not a number, return the (presumed) symbol
ans = self.input[0]
self.input = self.input[1:]
return ans
def tokenize(self) -> List[Union[int, str]]:
ans: List[str] = []
while not self._is_done():
ans.append(self._get_token())
return ans

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from __future__ import annotations
from typing import TYPE_CHECKING, List, Union, cast
from calculator.ast import BinOp, Constant, UnaryOp
from calculator.core import operations
from .parsed_string import ParsedString
if TYPE_CHECKING:
from calculator.ast import Node
def stack_to_tree(s: List[Union[int, str]]) -> Node:
top = s.pop()
if type(top) is int:
return Constant(top)
top = cast(str, top)
if top == "@":
rhs = stack_to_tree(s)
return UnaryOp(operations.negate, rhs)
rhs = stack_to_tree(s)
lhs = stack_to_tree(s)
return BinOp(operations.STR_TO_BIN[top], lhs, rhs)
def parse_postfix(input: str) -> Node:
"""
Parses the given string in postfix notation.
Negation is represented by the '@' sign.
"""
parsed = ParsedString(input).tokenize()
ans = stack_to_tree(parsed)
return ans

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from calculator.ast import BinOp, Constant, UnaryOp
from calculator.core import operations
from .postfix import parse_postfix
def test_parse_constant():
assert parse_postfix("42") == Constant(42)
def test_parse_negated_constant():
assert parse_postfix("42 @") == UnaryOp(operations.negate, Constant(42))
def test_parse_doubly_negated_constant():
assert parse_postfix("42@@") == UnaryOp(
operations.negate, UnaryOp(operations.negate, Constant(42))
)
def test_parse_binary_operation():
assert parse_postfix("12 27 +") == BinOp(
operations.plus, Constant(12), Constant(27)
)
def test_parse_complete_expression_tree():
assert parse_postfix("12 27 + 42 51 - *") == BinOp(
operations.times,
BinOp(operations.plus, Constant(12), Constant(27)),
BinOp(operations.minus, Constant(42), Constant(51)),
)