There is no tests. There is just a single test. Which only tests the decoding of a single known value. No encoding test. Go has infrastructure for benchmarkin

There is no tests. There is just a single test. Which only tests the decoding of a single known value. No encoding test. Go has infrastructure for benchmarking and fuzzing. Use it! Also, you took code from https://github.com/oklog/ulid/blob/main/ulid.go https://github.com/oklog/ulid/blob/main/ulid.go which has "Copyright 2016 The Oklog Authors" but this is not mentionned in your base32.go.