Interview Constraint Patterns — Write-the-Constraint Drills

Overview

Every verification interview has a round where the interviewer says “write a constraint so that…” — and the value is constrained to be even, or a power of two, or all array elements are unique. These drills look trivial but discriminate sharply: a candidate who thinks declaratively (relations the solver must satisfy) answers in seconds; a candidate who thinks procedurally (assignments executed in order) writes code that does not compile or silently over-constrains.

This topic is a pure drill set. Each sub-lesson states problems exactly as an interviewer would, walks the solution space, gives the full working constraint, the variation that comes next, and the classic wrong answers — so you recognize the trap before you write it on the whiteboard.

Sub-topics

1. Even/Odd, Alignment & Divisibility — bit slices, modulo, shifts, alignment masks.

2. Ranges With & Without inside — bounds, disjoint sets, exclusions, rand-on-rand bounds.

3. Powers of 2, One-Hot & Bit Games — $onehot, $countones, gray codes, palindromes.

4. Unique Values & Ordering — unique, pairwise foreach, sorted arrays, permutations.

5. Sum, Checksum & Distribution Drills — sum-to-N, length fields, dist weight math.

6. Dependent-Field Puzzles — implication ranges, capacity products, wraparound, FSM pairs.

7. Sequence & Timing Patterns — gaps, jitter, bursts, fairness bounds.

8. Constraint Anti-Patterns Review — every interviewer-bait wrong answer in one place.

Legend: [DRILL] [TRAP]

  THE WRITE-THE-CONSTRAINT INTERVIEW MAP

  "Make x even"            ──► bit slice / modulo / shift     [DRILL 1]
  "x between a and b"      ──► relational pair / inside       [DRILL 2]
  "x is a power of 2"      ──► 1<<k helper / $onehot          [DRILL 3]
  "all elements unique"    ──► unique / pairwise foreach      [DRILL 4]
  "array sums to N"        ──► sum() with width cast          [DRILL 5]
  "addr depends on kind"   ──► implication / if-else          [DRILL 6]
  "requests with min gap"  ──► element-wise timing arrays     [DRILL 7]

  Behind every drill, the same trap: [TRAP]
  procedural thinking  →  "x = x & ~1;"      (assignment — illegal)
  declarative thinking →  "x[0] == 1'b0;"    (relation — correct)

Key takeaways

• Constraints are relations the solver satisfies simultaneously — never sequenced assignments.

• Most drills have 2-3 valid forms; know them all and say which you prefer and why.

• The follow-up variation is where seniority shows — anticipate it in your first answer.