Here’s a version that does it by calculating the offset and building up the string in a single pass.
def annotate(str) do
length = byte_size(str)
offset = rem(length, 3)
{acc, rest} = String.split_at(str, offset)
do_annotate(acc, rest)
end
defp do_annotate(acc, ""), do: acc
defp do_annotate("", <<next::binary-3, rest::binary>>), do: do_annotate(next, rest)
defp do_annotate(acc, <<next::binary-3, rest::binary>>),
do: do_annotate(<<acc::binary, ",", next::binary>>, rest)
In my benchmarks it’s about 3x faster than the formatter implementation for 7-digit strings (8x for 1kb strings but that’s probably not a realistic use-case). A fun little challenge but I know people aren’t a fan of the bitstring syntax 
Name ips average deviation median 99th %
adamu 5.46 M 183.02 ns ±8688.03% 125 ns 250 ns
formatter 1.97 M 508.32 ns ±3909.15% 334 ns 542 ns
Comparison:
adamu 5.46 M
formatter 1.97 M - 2.78x slower +325.30 ns
Memory usage statistics:
Name Memory usage
adamu 0.40 KB
formatter 1.89 KB - 4.75x memory usage +1.49 KB
Operating System: macOS
CPU Information: Apple M1 Pro
Number of Available Cores: 10
Available memory: 16 GB
Elixir 1.15.4
Erlang 26.1
Benchmark suite executing with the following configuration:
warmup: 2 s
time: 5 s
memory time: 500 ms
reduction time: 0 ns
parallel: 1
inputs: none specified
Estimated total run time: 15 s
