First LL parser

ll.py

@@ -0,0 +1,79 @@
+#!/usr/bin/env python
+
+TS_A = 'a'
+TS_B = 'b'
+TS_PLUS = '+'
+TS_MULT = '*'
+SYM_END = object()
+NTS_E = 1
+NTS_T = 2
+NTS_F = 3
+
+# 1. E -> E + T
+# 2. E -> T
+# 3. T -> T * F
+# 4. T -> F
+# 5. F -> 'a'
+
+# 1. E -> T F
+# 2. F -> '+' T F
+# 3. F ->
+# 4. T -> 'b'
+# 5. T -> 'a'
+
+table = {
+    NTS_E : {
+        TS_A: 1,
+        TS_B: 1,
+    },
+    NTS_F : {
+        TS_PLUS: 2,
+        SYM_END: 3
+    },
+    NTS_T : {
+        TS_A: 4,
+        TS_B: 5
+    }
+}
+
+def tparse(text):
+    stack = []
+    stack.append(NTS_E);
+    while len(stack) > 0:
+        if len(text) == 0:
+            c = SYM_END
+        else:
+            c = text[0]
+        if c == stack[-1]:
+            text = text[1:]
+            stack.pop()
+        else:
+            try:
+                rule = table[stack[-1]][c]
+            except KeyError:
+                print("Unexpected %s" % c)
+                return
+            if rule == 1:
+                stack.pop()
+                stack.append(NTS_F)
+                stack.append(NTS_T)
+            elif rule == 2:
+                stack.pop()
+                stack.append(NTS_F)
+                stack.append(NTS_T)
+                stack.append(TS_PLUS)
+            elif rule == 3:
+                stack.pop()
+            elif rule == 4:
+                stack.pop()
+                stack.append(TS_A)
+            elif rule == 5:
+                stack.pop()
+                stack.append(TS_B)
+            else:
+                print("Unexpected %s" % (text[0]))
+                return
+            print("Applied rule %d" % rule)
+
+if __name__ == '__main__':
+    tparse("b+a+b+b+a")