Implement Expression.fromstring

[linpy.git] / pypol / linear.py

diff --git a/pypol/linear.py b/pypol/linear.py
index 3939c71..8a61744 100644 (file)
--- a/pypol/linear.py
+++ b/pypol/linear.py
@@ -1,5 +1,7 @@
+import ast
 import functools
 import numbers
+import re
 
 from fractions import Fraction, gcd
 
@@ -86,9 +88,40 @@ class Expression:
         self._dimension = len(self._symbols)
         return self
 
+    @classmethod
+    def _fromast(cls, node):
+        if isinstance(node, ast.Module):
+            assert len(node.body) == 1
+            return cls._fromast(node.body[0])
+        elif isinstance(node, ast.Expr):
+            return cls._fromast(node.value)
+        elif isinstance(node, ast.Name):
+            return Symbol(node.id)
+        elif isinstance(node, ast.Num):
+            return Constant(node.n)
+        elif isinstance(node, ast.UnaryOp):
+            if isinstance(node.op, ast.USub):
+                return -cls._fromast(node.operand)
+        elif isinstance(node, ast.BinOp):
+            left = cls._fromast(node.left)
+            right = cls._fromast(node.right)
+            if isinstance(node.op, ast.Add):
+                return left + right
+            elif isinstance(node.op, ast.Sub):
+                return left - right
+            elif isinstance(node.op, ast.Mult):
+                return left * right
+            elif isinstance(node.op, ast.Div):
+                return left / right
+        raise SyntaxError('invalid syntax')
+
     @classmethod
     def fromstring(cls, string):
-        raise NotImplementedError
+        string = re.sub(r'(\d+|\))\s*([^\W\d_]\w*|\()',
+                lambda m: '{}*{}'.format(m.group(1), m.group(2)),
+                string)
+        tree = ast.parse(string, 'eval')
+        return cls._fromast(tree)
 
     @property
     def symbols(self):
@@ -587,7 +620,7 @@ class Polyhedron:
         return bset
 
     @classmethod
-    def _fromisl(cls, bset):
+    def _fromisl(cls, bset, symbols):
         raise NotImplementedError
         equalities = ...
         inequalities = ...
@@ -602,9 +635,9 @@ Empty = eq(0,1)
 Universe = Polyhedron()
 
 if __name__ == '__main__':
-    e1 = Expression(coefficients={'a': 2, 'b': 2}, constant= 1)
+    e1 = Expression('2a + 2b + 1')
     p1 = Polyhedron(equalities=[e1]) # empty
-    e2 = Expression(coefficients={'x': 3, 'y': 2}, constant= 3)
+    e2 = Expression('3x + 2y + 3')
     p2 = Polyhedron(equalities=[e2]) # not empty
     print(p1._toisl())
     print(p2._toisl())