version: derive from git tags instead of a hand-bumped constant

VERSION was a constant in the Makefile that had to be bumped by hand and
could silently disagree with the git tag a release was cut from. Derive it
instead, with a committed ./VERSION as the floor and no-git fallback:

  on a tag        v0.2.0            -> 0.2.0
  past a tag      v0.2.0-5-gabc123  -> 0.2.0-5-gabc123   (sorts after 0.2.0)
  before any tag  0.1.0-197-g2d1815 -> ./VERSION + commit count + short id
  no .git at all  0.1.0             -> ./VERSION verbatim (cgit snapshot ->
                                       Homebrew source build)
A dirty tree appends -dirty. The commit count keeps 'brew upgrade' ordering
monotonic before the first tag exists.

version.h now bakes just VERSION (dropping the redundant +GITDESC, which the
derived string already carries). contrib/brew-release reads the base from
./VERSION rather than grepping the Makefile's VERSION line, which is now a
$(shell ...) expression it could not parse.

3 files changed, 33 insertions, 6 deletions

diff --git a/Makefile b/Makefile
index 759b1ac..b7cbee7 100644
--- a/Makefile
+++ b/Makefile
@@ -14,11 +14,32 @@ VT_INC   := $(VT_OUT)/include
 # header rebuilds what actually uses it. Without this a header edit rebuilt
 # nothing and the only safe move was `make clean`, which is how a 0.1s build
 # turns into a habit of throwing the whole thing away.
+#
 # The version is baked in, because the first question when a session is
 # behaving oddly is "which build is this?" -- and a session keeps the binary it
 # started with, so the answer is not "whatever is in build/ now".
-VERSION  := 0.1.0
-GITDESC  := $(shell git describe --always --dirty 2>/dev/null || echo unknown)
+#
+# It is derived from git tags (vX.Y.Z), so a tag is the single source of truth
+# and nothing here is bumped by hand:
+#   on a tag        v0.2.0            -> 0.2.0
+#   past a tag      v0.2.0-5-gabc123  -> 0.2.0-5-gabc123   (sorts after 0.2.0)
+#   before any tag  0.1.0-197-g2d1815 -> ./VERSION + commit count + short id
+#   no .git at all  0.1.0             -> ./VERSION verbatim (a cgit snapshot
+#                                        tarball, which is how Homebrew builds)
+# A dirty tree appends -dirty. ./VERSION is the committed floor and the only
+# value a source tarball without git can show; contrib/brew-release reads it too.
+# The commit count is in there so `brew upgrade` sees a later build as later:
+# without it 0.1.0-gabc123 vs 0.1.0-gdef456 would sort alphabetically, which
+# has nothing to do with which came first.
+VERSION := $(shell \
+  if git rev-parse --git-dir >/dev/null 2>&1; then \
+    d=$$(git describe --tags --match 'v[0-9]*' --dirty 2>/dev/null); \
+    if [ -n "$$d" ]; then printf '%s\n' "$${d#v}"; \
+    else printf '%s-%s-g%s%s\n' "$$(cat VERSION 2>/dev/null || echo 0.0.0)" \
+      "$$(git rev-list --count HEAD 2>/dev/null || echo 0)" \
+      "$$(git rev-parse --short=8 HEAD 2>/dev/null || echo unknown)" \
+      "$$(git diff --quiet 2>/dev/null || echo -dirty)"; fi; \
+  else cat VERSION 2>/dev/null || echo 0.0.0; fi)
 
 CFLAGS   := -std=c23 -O1 -g -Wall -Wextra -Wno-unused-parameter -MMD -MP \
             -I$(VT_INC) -Iinclude -Ibuild
@@ -90,8 +111,8 @@ build/logo.h: logo.txt | build
 	  printf '};\n'; } > $@
 
 build/version.h: FORCE | build
-	@printf '/* generated by the Makefile */\n#define SLOSH_VERSION "%s+%s"\n' \
-	  '$(VERSION)' '$(GITDESC)' > $@.tmp
+	@printf '/* generated by the Makefile */\n#define SLOSH_VERSION "%s"\n' \
+	  '$(VERSION)' > $@.tmp
 	@cmp -s $@.tmp $@ || mv $@.tmp $@
 	@rm -f $@.tmp
 
diff --git a/VERSION b/VERSION
new file mode 100644
index 0000000..6e8bf73
--- /dev/null
+++ b/VERSION
@@ -0,0 +1 @@
+0.1.0
diff --git a/contrib/brew-release b/contrib/brew-release
index d3eadda..dbaf526 100755
--- a/contrib/brew-release
+++ b/contrib/brew-release
@@ -84,8 +84,13 @@ esac
 # A tag must look like a version (digits with a dot); "1368b7f" starts with a
 # digit but is a commit, and calling it version 1368b7f would be a lie brew
 # could never climb down from.
-mkver="$(sed -n 's/^VERSION *:*= *//p' "$root/Makefile" | head -1)"
-[ -n "$mkver" ] || fail "no VERSION in $root/Makefile"
+# The base/floor version. It lives in ./VERSION (one line, e.g. 0.1.0), which
+# is also what the Makefile bakes into a source build that has no git and the
+# floor it builds tag-less versions on top of. It used to be grepped out of the
+# Makefile's `VERSION :=` line; that line is now a $(shell ...) derivation, so
+# read the file both of them trust instead.
+mkver="$(sed -n '1s/[[:space:]]*$//p' "$root/VERSION" 2>/dev/null | head -1)"
+[ -n "$mkver" ] || fail "no base version in $root/VERSION"
 short="$(printf '%s' "$full" | cut -c1-7)"
 # The count must be a real count. Falling back to 0 when the commit is not in
 # this clone would mint "0.1.0-0-<short>", which brew reads as older than every