calculator: add prefix parser

calculator/calculator/parse/__init__.py

@@ -1 +1,2 @@
 from .postfix import parse_postfix
+from .prefix import parse_prefix

calculator/calculator/parse/prefix.py

@@ -0,0 +1,34 @@
+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 queue_to_tree(q: List[Union[int, str]]) -> Node:
+    top = q.pop(0)
+    if type(top) is int:
+        return Constant(top)
+    top = cast(str, top)
+    if top == "@":
+        rhs = queue_to_tree(q)
+        return UnaryOp(operations.negate, rhs)
+    lhs = queue_to_tree(q)
+    rhs = queue_to_tree(q)
+    return BinOp(operations.STR_TO_BIN[top], lhs, rhs)
+
+
+def parse_prefix(input: str) -> Node:
+    """
+    Parses the given string in prefix notation.
+    Negation is represented by the '@' sign.
+    """
+    parsed = ParsedString(input).tokenize()
+    ans = queue_to_tree(parsed)
+    return ans

calculator/calculator/parse/test_prefix.py

@@ -0,0 +1,30 @@
+from calculator.ast import BinOp, Constant, UnaryOp
+from calculator.core import operations
+
+from .prefix import parse_prefix
+
+
+def test_parse_constant():
+    assert parse_prefix("42") == Constant(42)
+
+
+def test_parse_negated_constant():
+    assert parse_prefix("@ 42") == UnaryOp(operations.negate, Constant(42))
+
+
+def test_parse_doubly_negated_constant():
+    assert parse_prefix("@@42") == UnaryOp(
+        operations.negate, UnaryOp(operations.negate, Constant(42))
+    )
+
+
+def test_parse_binary_operation():
+    assert parse_prefix("+ 12 27") == BinOp(operations.plus, Constant(12), Constant(27))
+
+
+def test_parse_complete_expression_tree():
+    assert parse_prefix("*+12 27-42 51") == BinOp(
+        operations.times,
+        BinOp(operations.plus, Constant(12), Constant(27)),
+        BinOp(operations.minus, Constant(42), Constant(51)),
+    )