1114 Print in Order.py

Author: rajeevranjancom

1114 Print in Order.py

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+#!/usr/bin/python3
+"""
+Suppose we have a class:
+
+public class Foo {
+  public void first() { print("first"); }
+  public void second() { print("second"); }
+  public void third() { print("third"); }
+}
+The same instance of Foo will be passed to three different threads. Thread A
+will call first(), thread B will call second(), and thread C will call third().
+Design a mechanism and modify the program to ensure that second() is executed
+after first(), and third() is executed after second().
+
+
+
+Example 1:
+
+Input: [1,2,3]
+Output: "firstsecondthird"
+Explanation: There are three threads being fired asynchronously. The input
+[1,2,3] means thread A calls first(), thread B calls second(), and thread C
+calls third(). "firstsecondthird" is the correct output.
+Example 2:
+
+Input: [1,3,2]
+Output: "firstsecondthird"
+Explanation: The input [1,3,2] means thread A calls first(), thread B calls
+third(), and thread C calls second(). "firstsecondthird" is the correct output.
+"""
+from typing import Callable
+from threading import Lock
+
+
+class Foo:
+    def __init__(self):
+        """
+        Two locks
+        """
+        self.locks = [Lock(), Lock()]
+        self.locks[0].acquire()
+        self.locks[1].acquire()
+
+
+    def first(self, printFirst: Callable[[], None]) -> None:
+        # printFirst() outputs "first". Do not change or remove this line.
+        printFirst()
+        self.locks[0].release()
+
+
+
+    def second(self, printSecond: Callable[[], None]) -> None:
+        with self.locks[0]:
+            # printSecond() outputs "second". Do not change or remove this line.
+            printSecond()
+            self.locks[1].release()
+
+
+    def third(self, printThird: Callable[[], None]) -> None:
+        with self.locks[1]:
+            # printThird() outputs "third". Do not change or remove this line.
+            printThird()
+
+
+class FooError:
+    def __init__(self):
+        """
+        Have a counter, and only the corresponding method can change update the
+        counter.
+
+        Error, will miss an input.
+        """
+        self._value = 1
+        self._lock = Lock()
+
+
+    def first(self, printFirst: 'Callable[[], None]') -> None:
+        with self._lock:
+            if self._value == 1:
+                # printFirst() outputs "first". Do not change or remove this line.
+                self._value += 1
+                printFirst()
+
+
+    def second(self, printSecond: 'Callable[[], None]') -> None:
+        with self._lock:
+            if self._value == 2:
+                # printSecond() outputs "second". Do not change or remove this line.
+                self._value += 1
+                printSecond()
+
+
+    def third(self, printThird: 'Callable[[], None]') -> None:
+        with self._lock:
+            if self._value == 3:
+                # printThird() outputs "third". Do not change or remove this line.
+                self._value += 1
+                printThird()