Python sliding window: longest span without repeated symbols

Operation contract

The scanner finds the length of the longest span with distinct ASCII symbols. Each symbol stores its most recent position. A repeat inside the current window moves the left boundary past that earlier position; a repeat outside it must not move the boundary backward. The program rejects non-ASCII input because this exercise defines symbols as protocol bytes represented in text.

Failure and ownership boundary

This result is a length, not a returned substring. Tie handling would need a separate contract if start and end positions were returned. For Unicode user-visible text, code points do not necessarily correspond to grapheme clusters. Python strings and bytes: reject decoding errors before parsing records and Python prefix-function search: overlapping matches without rescanning use other scanning invariants.

Working program

def distinct_span(sequence):
    if not isinstance(sequence, str) or not sequence.isascii() or len(sequence) > 256:
        raise ValueError("bounded ASCII sequence required")
    left = 0
    best = 0
    latest = {}
    for right, symbol in enumerate(sequence):
        left = max(left, latest.get(symbol, -1) + 1)
        latest[symbol] = right
        best = max(best, right - left + 1)
    return best

print(distinct_span("ABCADEAF"))
print(distinct_span(""))
try:
    distinct_span("DEL\u00e9")
except ValueError:
    print("wire alphabet rejected")

Output

5
0
wire alphabet rejected

Costs and limits

For n symbols, expected time is O(n) with suitable dictionary behavior. Stored last positions use O(min(n,128)) entries for this ASCII contract. The max operation prevents a stale repeat from reopening an invalid earlier window.

Common Mistakes

• Never move the left boundary backward after an old repeat.
• A different symbol definition needs different preprocessing.

Connected lessons

Python dictionaries: insertion order and duplicate-key replacement, Python prefix-function search: overlapping matches without rescanning, Python strings and bytes: reject decoding errors before parsing records.

Follow the related contract

Python window exercise: verify a fixed-width maximum against direct slices.

Check the next state boundary

Python longest palindrome: expand around odd and even centers.

Follow the integrity boundary

Python sliding-window maximum: retain only useful deque indexes.