wrapWords seems to ignore linebreaks when wrapping, leaving breaks in the wrong place

[maxgrenderjones]

I want to use wrapwords to ensure that some text is not more than x chars wide. I would expect that either:

1. Existing line breaks are elided to make sure that the lines are as long as they can be without being wider than x
2. Existing line breaks are honored, so any lines shorter than x are not changed

I get a third behaviour - lines are broken every x characters regardless of whether a newline has been seen in the meantime.

Example

import std/wordwrap

import unittest

const input = """
This is a long string. It is manually wrapped to 60
characters. I would not expect it to be changed by
wordwrap if wordwrap is set to wrap at 80 characters
"""

suite "Test Wrap":
    test "Rewrap":
        check(wrapWords(input)==input)

Current Output

Check failed: wrapWords(input) == input
    wrapWords(input) was This is a long string. It is manually wrapped to 60
characters. That means it
should not be changed by
wordwrap if wordwrap is set to wrap at 80 characters
    input was This is a long string. It is manually wrapped to 60
characters. That means it should not be changed by
wordwrap if wordwrap is set to wrap at 80 characters

  [FAILED] Rewrap

Expected Output

Test passes

Is this a bug or a feature? If a feature, it might help to document it. If a bug, what is the desired/expected behaviour?

> nim -v
Nim Compiler Version 1.3.5 [MacOSX: amd64]
Compiled at 2020-05-28
Copyright (c) 2006-2020 by Andreas Rumpf

active boot switches: -d:release

[maxgrenderjones]

Fwiw, python's answer is option 1

#!/usr/bin/python3
import textwrap

input = """
This is a long string. It is manually wrapped to 60
characters. I would not expect it to be changed by
wordwrap if wordwrap is set to wrap at 80 characters
""".strip()

rewrapped = """
This is a long string. It is manually wrapped to 60 characters. I
would not expect it to be changed by wordwrap if wordwrap is set to
wrap at 80 characters
""".strip()

def test_wrap():
    assert textwrap.fill(input) == rewrapped

[Araq]

We'll follow Python here too.