From 9a4110f3c205107a3a48a0e48b760abf3cd1f3bc Mon Sep 17 00:00:00 2001
From: David Oberhollenzer <david.oberhollenzer@sigma-star.at>
Date: Fri, 9 Jun 2023 13:43:50 +0200
Subject: libio: eliminate direct access of the interal buffer

Instead, go through helper functions, which in a next step can be
moved inside the implementation.

Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>
---
 include/io/istream.h | 63 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 63 insertions(+)

(limited to 'include/io')

diff --git a/include/io/istream.h b/include/io/istream.h
index 9a8257f..06e8a3b 100644
--- a/include/io/istream.h
+++ b/include/io/istream.h
@@ -115,6 +115,69 @@ SQFS_INLINE const char *istream_get_filename(istream_t *strm)
 	return strm->get_filename(strm);
 }
 
+/**
+ * @brief Peek into the data buffered in an istream
+ *
+ * @memberof istream_t
+ *
+ * If the internal buffer is empty, the function tries to fetch more, which can
+ * block, and returns a positive return code if there is no more data to be
+ * read. Since this and other functions can alter the buffer pointer and
+ * contents, do not store the pointers returned here across function calls.
+ *
+ * Higher level functions like @ref istream_read, providing a Unix read() style
+ * API are built on top of this primitive.
+ *
+ * @param strm A pointer to an istream_t implementation.
+ * @param out Returns a pointer into an internal buffer on success.
+ * @param size Returns the number of bytes available in the buffer.
+ *
+ * @return Zero on success, a negative error code on failure,
+ *         a postive number on EOF.
+ */
+SQFS_INLINE int istream_get_buffered_data(istream_t *strm, const sqfs_u8 **out,
+					  size_t *size)
+{
+	if (strm->buffer_used == 0) {
+		int ret = strm->precache(strm);
+		if (ret)
+			return ret;
+		if (strm->buffer_used == 0)
+			return 1;
+	}
+
+	*out = strm->buffer;
+	*size = strm->buffer_used;
+	return 0;
+}
+
+/**
+ * @brief Mark a section of the internal buffer of an istream as used
+ *
+ * @memberof istream_t
+ *
+ * This marks the first `count` bytes of the internal buffer as used,
+ * forcing @ref istream_get_buffered_data to return data afterwards
+ * and potentially to load more data.
+ *
+ * @param strm A pointer to an istream_t implementation.
+ * @param count The number of bytes used up.
+ *
+ * @return Zero on success, a negative error code on failure.
+ */
+SQFS_INLINE int istream_advance_buffer(istream_t *strm, size_t count)
+{
+	if (count >= strm->buffer_used) {
+		strm->buffer += strm->buffer_used;
+		strm->buffer_used = 0;
+		return strm->precache(strm);
+	}
+
+	strm->buffer += count;
+	strm->buffer_used -= count;
+	return 0;
+}
+
 /**
  * @brief Skip over a number of bytes in an input stream.
  *
-- 
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