levenshteinDistance does not support unicode characters

[dubzzz]

Details:

levenshteinDistance does not support unicode characters outside of the BMP range properly.
Step to reproduce: expect(levenshteinDistance('\u{1f431}', '')).toBe(1) // got 2

Fix

I can issue a PR with the fix dubzzz@856ba36

How did I find it?

Thanks to property based testing framework fast-check.
The property was the following:

import fc from 'fast-check';

fc.assert(
    fc.property(
      fc.fullUnicodeString(), fc.fullUnicodeString(),
      fc.fullUnicodeString(), fc.fullUnicodeString(),
      fc.fullUnicodeString(),
      (aBegin, aEnd, bBegin, bEnd, common) =>
        levenshteinDistance(aBegin + common + aEnd, bBegin + common + bEnd)
          <= Math.max([...aBegin].length, [...bBegin].length) + Math.max([...aEnd].length, [...bEnd].length)
    )
  )

Or:

for any a and b - strings
such that a = aBegin + common + aEnd and b = bBegin + common + bEnd
levenshteinDistance(a, b) is at most max(numChars(aBegin), numChars(bBegin)) + max(numChars(aEnd), numChars(bEnd))