1 files changed, 65 insertions, 0 deletions
diff --git a/psi/parsers.py b/psi/parsers.py
new file mode 100644
index 0000000..db3e52c
--- /dev/null
+++ b/psi/parsers.py
@@ -0,0 +1,65 @@
+from psi.lexer import Lexer, Token
+
+
+__all__ = ['Parser']
+
+class Parser:
+    def __init__(self, input):
+        self.lexer = Lexer(input)
+        self.tokens = iter(self.lexer)
+        self.current_token = next(self.tokens)
+
+    def parse(self):
+        return self.parse_expr()
+
+    def parse_expr(self):
+        token = self.current_token
+        if token.value == '?':
+            self.eat('?')
+
+            condition = self.parse_condition()
+
+            self.eat(':')
+
+            if condition:
+                result = self.parse_reply()
+            else:
+                result = None
+
+            return result
+
+    def parse_condition(self):
+        variable = self.parse_variable()
+        self.eat('==')
+        value = self.parse_value()
+
+        return variable == value
+
+    def parse_variable(self):
+        token = self.current_token
+        self.eat('IDENTIFIER')
+        return token.value
+
+    def parse_value(self):
+        token = self.current_token
+        if token.type == 'INTEGER':
+            self.eat('INTEGER')
+            return token.value
+        else:
+            raise Exception(f'Invalid value: {token.value}')
+
+    def parse_reply(self):
+        self.eat('reply')
+        self.eat(':')
+
+        token = self.current_token
+        if token.type != 'SEPARATOR':
+            raise Exception(f'Invalid reply: {token.value}')
+
+        return token.value
+
+    def eat(self, expected_type):
+        if self.current_token.type == expected_type:
+            self.current_token = next(self.tokens)
+        else:
+            raise Exception(f'Unexpected token: {self.current_token.value}')