Encode and Decode Strings

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Problem

Network protocols and log pipelines need to pack a list of strings into a single payload and recover them exactly on the other end. Any fixed delimiter you choose can appear inside the strings - the encoding must round-trip every input exactly, including empty strings, newlines, and arbitrary unicode.

Level 1 - Basic round-trip

Implement encode and decode. Empty list, empty strings, and arbitrary ASCII characters must all round-trip.

Example:

codec.encode(["hi", "world"]) == "2:hi5:world"
 
codec.decode("2:hi5:world") == ["hi", "world"]

Level 2 - Adversarial inputs

The strings may contain the delimiter character, newlines, embedded <digits>: prefixes, or multi-byte unicode. The same encoding must round-trip all of them.

Example:

codec.encode([":weird:", "5:fake", "中文"])
# => "7::weird:6:5:fake2:中文"
 
codec.decode("7::weird:6:5:fake2:中文") == [":weird:", "5:fake", "中文"]

Level 3 - Streaming decode

Add decode_stream(s: str) -> Iterator[str] that yields one string at a time without materializing the full list. Same framing.

Example:

encoded = codec.encode(["hi", "world", "foo"])
# => "2:hi5:world3:foo"
 
for s in codec.decode_stream(encoded):
    print(s)
# prints: hi, world, foo  (one at a time, no list materialized)

Mastery checklist

Check	Why it matters
Use length-prefix framing, never a delimiter scheme	Any delimiter you pick can occur inside the data; escape rules add bugs without benefit
Read exactly `n` characters by index, not by scanning	The whole point of the length prefix is to bypass content scanning
Empty list, empty string, and `["", "", ""]` all round-trip	Three different zero-length cases; one usually breaks
`str` slicing is character-based in Python	Multi-byte unicode falls out for free when you stay in `str`; switch to `bytes` only if asked

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