This is ex/vi descended from the original Bill Joy source as it was in
4.4BSD Unix.

Current BSD vi is actually nvi, a clean implementation derived from the
original Elvis code.

The original ex/vi code, even though it was written at Berkely, belonged
to AT&T at the time of the 4.4BSDlite release. It has subsequently been
released as open source at the time the Solaris source code was made
available.

The binary program derived from these sources is roughly half the size
of nvi and several orders of magnitude smaller than vim (don't get me
started on Emacs), while still being a tremendously useful and efficient
editor. It is still capable of being your daily driver, and should feel
immediately familiar to anyone who has spent time in nvi or vim.