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DescriptionQ878

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Q878DVASIC interview problem

Model credit-based flow control

TechniquesPythonCreditsFlow controlInvariant

DifficultyMedium

TopicReference Models

LanguagePython

Requirements4 checkpoints

Problem

Implement a small reference model for a transmitter that may send only when at least one credit is available. Sending consumes one credit and asynchronous returns replenish credits.

Example input and output

Use this case to check your interpretation

Input

capacity=2, initial=2; send(), send(), send(), return_credits(1)

Output

credits: 1,0; third send raises underflow and stays 0; return restores credits to 1

Explanation

The rejected send never makes the count negative, and every successful transition preserves 0 <= credits <= 2.

Requirements (4)

Reject a send at zero credits without allowing the count to become negative.
Reject negative credit returns and define whether returns may exceed the configured capacity.
Expose the current count for scoreboard diagnostics.
Maintain the invariant 0 <= credits <= capacity after every successful operation.