FOLD_MSG = "NOTE: '\\' line wrapping per RFC 8792".freeze
MIN_FOLD_COLUMNS = FOLD_MSG.size
FOLD_COLUMNS = 69
RE_IDENT = /\A[A-Za-z0-9_]\z/

def fold8792_1(s, columns = FOLD_COLUMNS, left = false, dry = false)
  if s.index("\t")
    warn "*** HT (\"TAB\") in text to be folded. Giving up."
    return s
  end
  if columns < MIN_FOLD_COLUMNS
    columns =
      if columns == 0
        FOLD_COLUMNS
      else
        warn "*** folding to #{MIN_FOLD_COLUMNS}, not #{columns}"
        MIN_FOLD_COLUMNS
      end
  end

  lines = s.lines.map(&:chomp)
  did_fold = false
  ix = 0
  while li = lines[ix]
    col = columns
    if li[col].nil?
      if li[-1] == "\\"
        lines[ix..ix] = [li << "\\", ""]
        ix += 1
      end
      ix += 1
    else
      did_fold = true
      min_indent = left || 0
      col -= 1                  # space for "\\"
      while li[col] == " "      # can't start new line with " "
        col -= 1
      end
      if col <= min_indent
        warn "*** Cannot RFC8792-fold1 to #{columns} cols #{"with indent #{left}" if left}  |#{li.inspect}|"
      else
        if RE_IDENT === li[col] # Don't split IDs
          col2 = col
          while col2 > min_indent && RE_IDENT === li[col2-1]
            col2 -= 1
          end
          if col2 > min_indent
            col = col2
          end
        end
        rest = li[col..-1]
        indent = left || columns - rest.size
        if !left && li[-1] == "\\"
          indent -= 1           # leave space for next round
        end
        if indent > 0
         rest = " " * indent + rest
        end
        lines[ix..ix] = [li[0...col] << "\\", rest]
      end
      ix += 1
    end
  end

  if did_fold
    msg = FOLD_MSG.dup
    if !dry && columns >= msg.size + 4
      delta = columns - msg.size - 2 # 2 spaces
      half = delta/2
      msg = "#{"=" * half} #{msg} #{"=" * (delta - half)}"
    end
    lines[0...0] = [msg, ""]
    lines.map{|x| x << "\n"}.join
  else
    s
  end
end