Python exercise: encode consecutive states without merging distant runs

Operation contract

The receipt-status stream contains two queued runs separated by a sent state. The encoder keeps those queued runs separate; a Counter would merge their totals and lose order. It rejects non-ASCII, nonalphabetic states and caps input length before scanning. The empty input has an empty encoding, which the decoder can distinguish from a missing field only if the caller declares that policy.

Failure and ownership boundary

This is a teaching encoder, not a compression format with framing, escaping or integrity checks. Long runs and attacker-controlled counts need limits in any decoder. Python Counter and deque: counts, queues and bounded history solves a different question; Python struct: a fixed-endian receipt record with exact byte length explains why wire compression needs more than this loop.

Working program

def encode_states(states):
    if type(states) is not str or len(states) > 100 or not states.isascii() or not states.isalpha():
        if states != "":
            raise ValueError("ASCII status sequence")
    if not states:
        return []
    encoded = []
    current, count = states[0], 1
    for state in states[1:]:
        if state == current:
            count += 1
        else:
            encoded.append((current, count))
            current, count = state, 1
    encoded.append((current, count))
    return encoded

print(encode_states("QQSQQ"))
print(encode_states(""))
try:
    encode_states("Q１")
except ValueError:
    print("non-ASCII state rejected")

Output

[('Q', 2), ('S', 1), ('Q', 2)]
[]
non-ASCII state rejected

Costs and limits

The scan costs O(n) time and O(r) output for r runs. A sequence with no repeated adjacent states can make the encoded representation larger than the input; compression is not guaranteed.

Common Mistakes

• A Counter loses run boundaries and order.
• A decoder must cap declared counts before allocating output.

Connected lessons

Python Counter and deque: counts, queues and bounded history, Python struct: a fixed-endian receipt record with exact byte length, Python strings and bytes: reject decoding errors before parsing records.