Merge pull request #32 from bliutech/12-parser-write-an-recursive-des…

…cent-parser

feat:Added parser code (that does not build tree)

parser/parse.py

@@ -0,0 +1,76 @@
+import re
+import sys
+
+
+class Parser:
+    """Parser object for analyzing tokens for errors"""
+
+    def __init__(self) -> None:
+        """Initializes the parser with attributes to be used"""
+        self.pos: int = -1
+
+    def error(msg: str, pos: int):
+        print(f"Parse error: {msg} at position {pos}", file=sys.stderr)
+        exit(1)
+
+    def parse(self, tokens: list[str]) -> None:
+        """Parses given tokens"""
+        self.tokens: list[str] = tokens
+
+        # Initializes the first token
+        self.advance()
+
+        rv = self.expr()
+        self.assert_end()
+        return rv
+
+    def assert_end(self) -> None:
+        if self.next_token != "<EOF>":
+            Parser.error(f"Expected end '<EOF>' but found {self.next_token}", self.pos)
+
+    def eat(self, expected: str) -> None:
+        """Skips a token"""
+        if self.next_token == expected:
+            self.advance()
+        else:
+            Parser.error(
+                f"Expected '{expected}' but found '{self.next_token}'", self.pos
+            )
+
+    def advance(self) -> None:
+        """Moves to the next token"""
+        self.pos += 1
+        self.next_token: str = self.tokens[self.pos]
+
+    def expr(self) -> None:
+        """Parses an expression"""
+        if re.match("[A-Z]+", self.next_token):
+            self.var()
+            self.expr_prime()
+        elif self.next_token == "!":
+            self.eat("!")
+            self.expr()
+        elif self.next_token == "(":
+            self.eat("(")
+            self.expr()
+            self.eat(")")
+        else:
+            Parser.error(
+                f"Expected [var, !, (] but found '{self.next_token}'", self.pos
+            )
+
+    def expr_prime(self) -> None:
+        """Parses an expression prime (explain what this is later)"""
+        if self.next_token == "&":
+            self.eat("&")
+            self.expr()
+        elif self.next_token == "|":
+            self.eat("|")
+            self.expr()
+
+    def var(self) -> None:
+        """Parses a variable that represents a boolean expression"""
+        if not re.match("[A-Z]+", self.next_token):
+            Parser.error(f"Expected [A-Z]+ but found '{self.next_token}'", self.pos)
+        else:
+            self.eat(self.next_token)

tests/test_parse.py

@@ -0,0 +1,5 @@
+from parser.parse import Parser
+
+p: Parser = Parser()
+tree = p.parse(["!", "(", "A", "&", "!", "B", "|", "C", ")", "<EOF>"])
+print(tree)