1 files changed, 76 insertions, 0 deletions
diff --git a/projects/hackc/expressions.py b/projects/hackc/expressions.py
new file mode 100644
index 0000000..de58528
--- /dev/null
+++ b/projects/hackc/expressions.py
@@ -0,0 +1,76 @@
+from .tokens import Token
+
+OPS = "+-*/&|<>="
+UNARY_OPS = "-~"
+CONSTANTS = ["true", "false", "null", "this"]
+
+
+class Term:
+    def __init__(self):
+        pass
+
+    @classmethod
+    def from_tokens(cls, tokens: list) -> tuple:
+        if not tokens:
+            return (None, 0)
+        if tokens[0].type in ["integer", "string"] or tokens[0] in CONSTANTS:
+            return (ConstantTerm(tokens[0]), 1)
+        if tokens[0] in UNARY_OPS:
+            term, dt = Term.from_tokens(tokens[1:])
+            return (UnaryTerm(tokens[0], term), dt + 1)
+
+
+class ConstantTerm:
+    def __init__(self, term: Token):
+        self.term = term
+
+
+class VarTerm:
+    def __init__(self, var: Token, subscript=None):
+        self.var = var
+        self.subscript = subscript
+
+
+class UnaryTerm:
+    def __init__(self, op: Token, term: Term):
+        self.op = op
+        self.term = term
+
+
+class SubroutineCall:
+    def __init__(self, name: Token, exprs: list):
+        self.name = name
+        self.exprs = exprs
+
+    @classmethod
+    def from_tokens(cls, tokens: list) -> tuple:
+        pass
+
+
+class Expression:
+    def __init__(self, lhs: Term, op=None, rhs=None):
+        self.lhs = lhs
+        self.op = op
+        self.rhs = rhs
+
+    @classmethod
+    def from_tokens(cls, tokens: list) -> tuple:
+        """Construct expression.
+
+        Format:
+        <term> (<op> <term>)?
+        """
+        lhs, dt = Term.from_tokens(tokens)
+        t = dt
+
+        op = tokens[t]
+        if op.token not in OPS:
+            return (Expression(lhs), t)
+
+        t += 1
+        rhs, dt = Term.from_tokens(tokens[t])
+        if rhs is None:
+            raise JSE(f"Expected other term, got `{rhs}` instead", rhs)
+        t += dt
+
+        return (Expression(lhs, op, rhs), t)