From f0e1c1b1bc9709be729f911f9f650b82b5e2e30b Mon Sep 17 00:00:00 2001
From: Amaury Denoyelle <adenoyelle@haproxy.com>
Date: Thu, 16 Oct 2025 15:05:50 +0200
Subject: [PATCH] MINOR: ncbmbuf: implement advance operation

---
 include/haproxy/ncbmbuf.h |  3 ++-
 src/ncbmbuf.c             | 46 ++++++++++++++++++++++++++++++++++++++-
 2 files changed, 47 insertions(+), 2 deletions(-)

diff --git a/include/haproxy/ncbmbuf.h b/include/haproxy/ncbmbuf.h
index f06807264..4ca5ea728 100644
--- a/include/haproxy/ncbmbuf.h
+++ b/include/haproxy/ncbmbuf.h
@@ -42,6 +42,7 @@ static inline ncb_sz_t ncbmb_size(const struct ncbmbuf *buf)
 ncb_sz_t ncbmb_data(const struct ncbmbuf *buf, ncb_sz_t offset);
 
 enum ncb_ret ncbmb_add(struct ncbmbuf *buf, ncb_sz_t off,
-                      const char *data, ncb_sz_t len, enum ncb_add_mode mode);
+                       const char *data, ncb_sz_t len, enum ncb_add_mode mode);
+enum ncb_ret ncbmb_advance(struct ncbmbuf *buf, ncb_sz_t adv);
 
 #endif /* _HAPROXY_NCBMBUF_H */
diff --git a/src/ncbmbuf.c b/src/ncbmbuf.c
index cc586b582..9ca28b84b 100644
--- a/src/ncbmbuf.c
+++ b/src/ncbmbuf.c
@@ -142,6 +142,13 @@ static struct itbmap itbmap_next(const struct ncbmbuf *buf, const struct itbmap
 	return next;
 }
 
+/* ******** bit set/unset utilities ******** */
+
+static void bit_unset(unsigned char *value, char i)
+{
+	*value &= ~(1 << (8 - i));
+}
+
 /* ******** public API ******** */
 
 /* Construct a ncbmbuf with all its parameters. */
@@ -250,8 +257,45 @@ enum ncb_ret ncbmb_add(struct ncbmbuf *buf, ncb_sz_t off,
 	return NCB_RET_OK;
 }
 
+/* Advance the head of <buf> to the offset <adv>. Data at the start of buffer
+ * will be lost while some space will be formed at the end to be able to insert
+ * new data.
+ *
+ * Always returns NCB_RET_OK as this operation cannot fail.
+ */
 enum ncb_ret ncbmb_advance(struct ncbmbuf *buf, ncb_sz_t adv)
 {
-	/* TODO */
+	struct itbmap it;
+
+	BUG_ON_HOT(adv > ncbmb_size(buf));
+
+	while (adv) {
+		it = itbmap_get(buf, 0);
+		if (it.bitcount <= adv) {
+			adv -= it.bitcount;
+			*it.b &= ~it.mask;
+			buf->head += it.bitcount;
+		}
+		else {
+			unsigned char mask = 0xff;
+			int i = 1;
+
+			while (!(it.mask & 0x80)) {
+				it.mask <<= 1;
+				++i;
+			}
+
+			while (adv && (it.mask & 0x80)) {
+				bit_unset(&mask, i);
+				--adv;
+				++i;
+				++buf->head;
+			}
+
+			BUG_ON(adv);
+			*it.b &= mask;
+		}
+	}
+
 	return NCB_RET_OK;
 }
-- 
2.47.3