subtree(users/wpcarro): docking briefcase at '24f5a642'

git-subtree-dir: users/wpcarro
git-subtree-mainline: 464bbcb15c
git-subtree-split: 24f5a642af
Change-Id: I6105b3762b79126b3488359c95978cadb3efa789

users/wpcarro/scratch/facebook/language.py

@@ -0,0 +1,70 @@
+import random
+
+# Write an evaluator for a small language:
+#   - operators: '+', '*'
+#   - operands:  Integers
+#
+# E.g. evaluate("2+14*90+5*16")
+
+def tokenize(xs):
+    result = []
+    i = 0
+    while i < len(xs):
+        current = xs[i]
+        if current in {'*', '+'}:
+            result.append(current)
+            i += 1
+            continue
+        elif current == ' ':
+            i += 1
+            continue
+        else:
+            i += 1
+            while i < len(xs) and xs[i] in {str(x) for x in range(10)}:
+                current += xs[i]
+                i += 1
+            result.append(int(current))
+    return result
+
+def ast(tokens):
+    result = []
+    series = []
+    for token in tokens:
+        if token == '+':
+            result.append(series)
+            series = []
+        elif token == '*':
+            continue
+        else:
+            series.append(token)
+    if series:
+        result.append(series)
+    return result
+
+def product(xs):
+    result = 1
+    for x in xs:
+        result *= x
+    return result
+
+def evaluate(x):
+    tokens = tokenize(x)
+    tree = ast(tokens)
+    return sum([product(xs) for xs in tree])
+
+n = 7
+operands = [random.randint(0, 100) for _ in range(n)]
+operators = [random.choice(['+','*']) for _ in range(n - 1)]
+expr = []
+for i in range(n - 1):
+    expr.append(operands[i])
+    expr.append(operators[i])
+expr.append(operands[-1])
+
+expr = ' '.join([str(x) for x in expr])
+print("Expression: {}".format(expr))
+print("Tokens: {}".format(tokenize(expr)))
+print("AST: {}".format(ast(tokenize(expr))))
+print("Answer: {}".format(evaluate(expr)))
+assert evaluate(expr) == eval(expr)
+print("Success!")