Q273FreeSystemVerilog

Return the first unique value in a stream

Interview prompt

Question

Process a stream of integers and return the earliest value whose current occurrence count is exactly one.

Starting point

class FirstUnique;
  function void push(int x);
  function bit first_unique(output int x);
endclass

Reviewed example

Input

push sequence=[2,3,2,4,3]

Expected output

first unique after each push=[2,2,3,3,4]

Duplicated values remain in the order queue until they reach its head, where lazy cleanup exposes the earliest surviving singleton.

What to cover

Increment a count for every push.
Append a value to the order queue only on its first occurrence.
Lazily remove duplicated values from the queue head.
first_unique(x) returns 0 when no unique value remains and leaves x unspecified in that case.

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