wt-status: show sparse checkout status as well

Some of the early feedback of folks trying out sparse-checkouts at
$dayjob is that sparse checkouts can sometimes be disorienting; users
can forget that they had a sparse-checkout and then wonder where files
went.  Add some output to 'git status' in the form of a simple line that
states:

    You are in a sparse checkout with 35% of files present.

where, obviously, the exact figure changes depending on what percentage
of files from the index do not have the SKIP_WORKTREE bit set.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

wt-status.c

@@ -1484,6 +1484,18 @@ static void show_bisect_in_progress(struct wt_status *s,
 	wt_longstatus_print_trailer(s);
 }
 
+static void show_sparse_checkout_in_use(struct wt_status *s,
+					const char *color)
+{
+	if (s->state.sparse_checkout_percentage == SPARSE_CHECKOUT_DISABLED)
+		return;
+
+	status_printf_ln(s, color,
+			 _("You are in a sparse checkout with %d%% of tracked files present."),
+			 s->state.sparse_checkout_percentage);
+	wt_longstatus_print_trailer(s);
+}
+
 /*
  * Extract branch information from rebase/bisect
  */
@@ -1623,6 +1635,31 @@ int wt_status_check_bisect(const struct worktree *wt,
 	return 0;
 }
 
+static void wt_status_check_sparse_checkout(struct repository *r,
+					    struct wt_status_state *state)
+{
+	int skip_worktree = 0;
+	int i;
+
+	if (!core_apply_sparse_checkout || r->index->cache_nr == 0) {
+		/*
+		 * Don't compute percentage of checked out files if we
+		 * aren't in a sparse checkout or would get division by 0.
+		 */
+		state->sparse_checkout_percentage = SPARSE_CHECKOUT_DISABLED;
+		return;
+	}
+
+	for (i = 0; i < r->index->cache_nr; i++) {
+		struct cache_entry *ce = r->index->cache[i];
+		if (ce_skip_worktree(ce))
+			skip_worktree++;
+	}
+
+	state->sparse_checkout_percentage =
+		100 - (100 * skip_worktree)/r->index->cache_nr;
+}
+
 void wt_status_get_state(struct repository *r,
 			 struct wt_status_state *state,
 			 int get_detached_from)
@@ -1658,6 +1695,7 @@ void wt_status_get_state(struct repository *r,
 	}
 	if (get_detached_from)
 		wt_status_get_detached_from(r, state);
+	wt_status_check_sparse_checkout(r, state);
 }
 
 static void wt_longstatus_print_state(struct wt_status *s)
@@ -1681,6 +1719,9 @@ static void wt_longstatus_print_state(struct wt_status *s)
 		show_revert_in_progress(s, state_color);
 	if (state->bisect_in_progress)
 		show_bisect_in_progress(s, state_color);
+
+	if (state->sparse_checkout_percentage != SPARSE_CHECKOUT_DISABLED)
+		show_sparse_checkout_in_use(s, state_color);
 }
 
 static void wt_longstatus_print(struct wt_status *s)

wt-status.h

@@ -79,6 +79,7 @@ enum wt_status_format {
 
 #define HEAD_DETACHED_AT _("HEAD detached at ")
 #define HEAD_DETACHED_FROM _("HEAD detached from ")
+#define SPARSE_CHECKOUT_DISABLED -1
 
 struct wt_status_state {
 	int merge_in_progress;
@@ -90,6 +91,7 @@ struct wt_status_state {
 	int bisect_in_progress;
 	int revert_in_progress;
 	int detached_at;
+	int sparse_checkout_percentage; /* SPARSE_CHECKOUT_DISABLED if not sparse */
 	char *branch;
 	char *onto;
 	char *detached_from;