Window Validation Logic

Validation logic decides whether the current window is legal: frequency caps, sorted order, XOR constraints, or “all vowels present.” Keep validation incremental so each move is O(1) or O(log n).

Why this shows up in the real world

Password strength meters validate sliding character windows (no keyboard walks). Fraud rules validate bursts of transactions in rolling minutes.

Core idea (explained for students)

Encode constraints into counters or deques. Example: “at most two duplicates” → track counts per value; invalid when any count > 2.

Try this in Python

def valid_window_counts(counts: dict[str, int], max_repeat: int) -> bool:
    return all(c <= max_repeat for c in counts.values())


# Example: maintain counts externally while sliding
from collections import Counter

c = Counter("aabb")
print(valid_window_counts(c, 2))

Common mistakes

• Re-scanning the whole window each move.
• Partial updates when duplicate characters leave—decrement carefully.

Key takeaways

• Validation should be a pure function of your small state struct.
• Unit-test state updates separately from pointer loops.