1279. Traffic Light Controlled Intersection.py

# There is an intersection of two roads. 
# First road is road A where cars travel from North to South in direction 1 
# and from South to North in direction 2. 
# Second road is road B where cars travel from West to East in direction 3 
# and from East to West in direction 4.
# There is a traffic light located on each road before the intersection. 
# A traffic light can either be green or red.
# Green means cars can cross the intersection in both directions of the road.
# Red means cars in both directions cannot cross the intersection 
# and must wait until the light turns green.
# The traffic lights cannot be green on both roads at the same time. 
# That means when the light is green on road A, 
# it is red on road B and when the light is green on road B, it is red on road A.
# Initially, the traffic light is green on road A and red on road B. 
# When the light is green on one road, 
# all cars can cross the intersection in both directions 
# until the light becomes green on the other road. 
# No two cars traveling on different roads should cross at the same time.

# Design a deadlock-free traffic light controlled system at this intersection.
# Implement the function void carArrived(carId, roadId, direction, turnGreen, crossCar) where:
# carId is the id of the car that arrived.
# roadId is the id of the road that the car travels on.
# direction is the direction of the car.
# turnGreen is a function you can call to turn the traffic light to green on the current road.
# crossCar is a function you can call to let the current car cross the intersection.
# Your answer is considered correct if it avoids cars deadlock in the intersection. 
# Turning the light green on a road when it was already green is considered a wrong answer.

# Example 1:
# Input: cars = [1,3,5,2,4], directions = [2,1,2,4,3], arrivalTimes = [10,20,30,40,50]
# Output: [
# "Car 1 Has Passed Road A In Direction 2",    // Traffic light on road A is green, car 1 can cross the intersection.
# "Car 3 Has Passed Road A In Direction 1",    // Car 3 crosses the intersection as the light is still green.
# "Car 5 Has Passed Road A In Direction 2",    // Car 5 crosses the intersection as the light is still green.
# "Traffic Light On Road B Is Green",          // Car 2 requests green light for road B.
# "Car 2 Has Passed Road B In Direction 4",    // Car 2 crosses as the light is green on road B now.
# "Car 4 Has Passed Road B In Direction 3"     // Car 4 crosses the intersection as the light is still green.
# ]

# Example 2:
# Input: cars = [1,2,3,4,5], directions = [2,4,3,3,1], arrivalTimes = [10,20,30,40,40]
# Output: [
# "Car 1 Has Passed Road A In Direction 2",    // Traffic light on road A is green, car 1 can cross the intersection.
# "Traffic Light On Road B Is Green",          // Car 2 requests green light for road B.
# "Car 2 Has Passed Road B In Direction 4",    // Car 2 crosses as the light is green on road B now.
# "Car 3 Has Passed Road B In Direction 3",    // Car 3 crosses as the light is green on road B now.
# "Traffic Light On Road A Is Green",          // Car 5 requests green light for road A.
# "Car 5 Has Passed Road A In Direction 1",    // Car 5 crosses as the light is green on road A now.
# "Traffic Light On Road B Is Green",          // Car 4 requests green light for road B. Car 4 blocked until car 5 crosses and then traffic light is green on road B.
# "Car 4 Has Passed Road B In Direction 3"     // Car 4 crosses as the light is green on road B now.
# ]
# Explanation: This is a dead-lock free scenario. Note that the scenario when car 4 crosses before turning light into green on road A and allowing car 5 to pass is also correct and Accepted scenario.
 

# Constraints:
# 1 <= cars.length <= 20
# cars.length = directions.length
# cars.length = arrivalTimes.length
# All values of cars are unique
# 1 <= directions[i] <= 4
# arrivalTimes is non-decreasing

from threading import Lock
class TrafficLight:
    def __init__(self):
        self.lock = Lock()
        self.state = 1

    def carArrived(
        self,
        carId: int,                      # ID of the car
        roadId: int,                     # ID of the road the car travels on. Can be 1 (road A) or 2 (road B)
        direction: int,                  # Direction of the car
        turnGreen: 'Callable[[], None]', # Use turnGreen() to turn light to green on current road
        crossCar: 'Callable[[], None]'   # Use crossCar() to make car cross the intersection
    ) -> None:
        with self.lock:
            if self.state != roadId:
                self.state = roadId
                turnGreen()
            crossCar()

from threading import BoundedSemaphore
class TrafficLight:
    def __init__(self):
        self.sem = BoundedSemaphore(1)
        self.state  = 1

    def carArrived(
        self,
        carId: int,                      # ID of the car
        roadId: int,                     # ID of the road the car travels on. Can be 1 (road A) or 2 (road B)
        direction: int,                  # Direction of the car
        turnGreen: 'Callable[[], None]', # Use turnGreen() to turn light to green on current road
        crossCar: 'Callable[[], None]'   # Use crossCar() to make car cross the intersection
    ) -> None:
        with self.sem:
            if self.state != roadId:
                self.state = roadId
                turnGreen()
            crossCar()