[PATCH] libs6rc/s6rc_graph_closure.c: add comments explaining behavior

[Adam Joseph <adam@westernsemico.com>]

While reviewing the algorithm implemented by s6-rc-update I found it
difficult to determine the meaning of some of the arguments to the
functions in libs6rc/s6rc_graph_closure.c.  This commit adds
comments to that file documenting those arguments and functions
where their behavior was not immediately obvious to me.

Signed-off-by: Adam Joseph <adam@westernsemico.com>
---
 src/libs6rc/s6rc_graph_closure.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/src/libs6rc/s6rc_graph_closure.c b/src/libs6rc/s6rc_graph_closure.c
index 7c3a8f9..50af30b 100644
--- a/src/libs6rc/s6rc_graph_closure.c
+++ b/src/libs6rc/s6rc_graph_closure.c
@@ -11,11 +11,19 @@ struct recinfo_s
   s6rc_db_t const *db ;
   unsigned int n ;
   unsigned char *bits ;
+  /* bitarray in which the i^th bit is set iff the closure routine
+     has already visited the i^th graph node */
   unsigned char *mark ;
   unsigned char mask ;
   unsigned char h : 1 ;
 } ;
 
+/*
+  If the i^th graph node has *any* bit of `recinfo->mask` set then
+  this function will be called on each of the i^th node's
+  dependencies.  This function will then set *every* `mask` bit on
+  that node and call itself recursively on that node.
+*/
 static void s6rc_graph_closure_rec (recinfo_t *recinfo, unsigned int i)
 {
   if (!bitarray_peek(recinfo->mark, i))
@@ -27,6 +35,11 @@ static void s6rc_graph_closure_rec (recinfo_t *recinfo, unsigned int i)
   }
 }
 
+/*
+  `h` indicates whether you want a forward or reverse search.
+  - If `h==0` then we close over everything that the marked node *depends upon*
+  - If `h==1` then we close over everything that *depends upon* the marked node
+*/
 void s6rc_graph_closure (s6rc_db_t const *db, unsigned char *bits, unsigned int bitno, int h)
 {
   unsigned int n = db->nshort + db->nlong ;
-- 
2.41.0