Seqlock Read Retry

Problem

Count read retries in a seqlock with a pending writer.

operations contains "begin_write" (values[i] is the new value and the sequence becomes odd), "end_write" (publish the value and make the sequence even), or "read". A read that overlaps an in-progress write must retry once before observing the last committed value.

Return [last_read_value, retry_count].

Hint

An odd sequence number means a writer is mid-update, so the reader retries.

Test cases

case 1

in: [["begin_write","read","end_write","read"],[7,0,0,0]]

expected: [7,1]

case 2

in: [["begin_write","end_write","read"],[5,0,0]]

expected: [5,0]

case 3

in: [["read"],[0]]

expected: [null,0]

case 4

in: [["begin_write","read","read","end_write","read"],[1,0,0,0,0]]

expected: [1,2]

Starter codePython

def seqlock_read_retries(operations, values):
    # Your code here
    pass

4 test cases · run your code against them in the editor