Split a Chunked Text Stream into Line-Balanced Parts

A text file arrives through an iterator that yields arbitrary string chunks. Split its content into n ordered parts while keeping every line intact and making the numbers of lines in any two parts differ by at most one.

Implement split_balanced_lines(chunks: string[], parts: int) -> string[]. The array is a portable fixture for the source iterator; chunk boundaries may occur in the middle of a line.

Constraints & Assumptions

The source does not state the unit of balance. This practice version balances line counts, not bytes: arbitrary line lengths can make a one-byte balance impossible while keeping lines intact.

• 1 <= parts <= 10000; at most 100000 chunks and 1000000 characters in total. Chunks may be empty.
• A newline \n terminates a line and remains attached to that line. A final nonempty fragment without a newline is also a line. A final newline does not create an extra empty line after it. An empty file has zero lines.
• Other characters, including \r, are ordinary content; do not normalize line endings.
• Preserve exact content and line order. Every output part is a concatenation of consecutive whole lines.
• For L lines, each part contains either floor(L/parts) or ceil(L/parts) lines, and the total is L. Any placement of the larger parts is valid; there is no required earliest-extra-lines policy.
• Always return exactly parts strings; parts with zero lines are empty.

Examples of Valid Outputs

chunks = ["a\nb", "\nc\n", "d"], parts = 3
one valid result = ["a\nb\n","c\n","d"]

chunks = ["\n", "\n"], parts = 3
one valid result = ["\n","\n",""]

Explain time and memory complexity. For a real one-pass iterator with unknown total line count, discuss why exact final balance may require buffering/spooling or a preliminary counting pass; do not claim that already-emitted part boundaries can always be fixed with constant memory.

Clarifying Questions

Does balance refer to line count or byte count? Must output order preserve the file? Can the iterator be rewound? Is total line count known in advance?