Q235FreeSystemVerilog

Handle wrapping 8-bit sequence numbers

Interview prompt

Question

Adapt window membership and advancement to 8-bit sequence numbers that wrap from 255 to 0.

Starting point

function bit in_window(byte unsigned seq);
function void advance_base();

Reviewed example

Input

8-bit sequence stream near rollover: 254, 255, 0, 1

Expected output

All four values are classified as consecutive; next expected sequence is 2.

Modulo-256 distance treats 0 as the successor of 255 rather than as an old packet.

What to cover

Compute forward distance modulo 256.
Treat seq equal to base as distance zero.
Restrict the window to at most 128 entries so old and future values are unambiguous.
Wrap base modulo 256 after retirement.

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