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diff --git a/include/sqfs/block_writer.h b/include/sqfs/block_writer.h
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+/* SPDX-License-Identifier: LGPL-3.0-or-later */
+/*
+ * block_writer.h - This file is part of libsquashfs
+ *
+ * Copyright (C) 2019 David Oberhollenzer <goliath@infraroot.at>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published
+ * by the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+#ifndef SQFS_BLOCK_WRITER_H
+#define SQFS_BLOCK_WRITER_H
+
+#include "sqfs/predef.h"
+
+/**
+ * @file block_writer.h
+ *
+ * @brief Contains declarations for the @ref sqfs_block_writer_t structure.
+ */
+
+/**
+ * @struct sqfs_block_writer_t
+ *
+ * @brief Abstracts writing and deduplicating of data and fragment blocks.
+ */
+
+/**
+ * @struct sqfs_block_hooks_t
+ *
+ * @brief A set of hooks for tapping into the data writer.
+ *
+ * This structure can be registered with an @ref sqfs_block_writer_t and
+ * contains function pointers that will be called during various stages
+ * when writing data to disk.
+ *
+ * The callbacks can not only be used for accounting but may also write extra
+ * data to the output file or make modifications to the blocks before they are
+ * writtien.
+ *
+ * The callbacks can be individually set to NULL to disable them.
+ */
+struct sqfs_block_hooks_t {
+	/**
+	 * @brief Set this to the size of the struct.
+	 *
+	 * This is required for future expandabillity while maintaining ABI
+	 * compatibillity. At the current time, the implementation of
+	 * @ref sqfs_block_writer_set_hooks rejects any hook struct where
+	 * this isn't the exact size. If new hooks are added in the future,
+	 * the struct grows and the future implementation can tell by the size
+	 * whether the application uses the new version or the old one.
+	 */
+	size_t size;
+
+	/**
+	 * @brief Gets called before writing a block to disk.
+	 *
+	 * If this is not NULL, it gets called before a block is written to
+	 * disk. If the block has the @ref SQFS_BLK_ALIGN flag set, the
+	 * function is called before padding the file.
+	 *
+	 * The function may modify the block itself or write data to the file.
+	 * which is taken into account when padding the file.
+	 *
+	 * @param user A user pointer.
+	 * @param block The block that is about to be written.
+	 * @param file The file that the block will be written to.
+	 */
+	void (*pre_block_write)(void *user, sqfs_block_t *block,
+				sqfs_file_t *file);
+
+	/**
+	 * @brief Gets called after writing a block to disk.
+	 *
+	 * If this is not NULL, it gets called after a block is written to
+	 * disk. If the block has the @ref SQFS_BLK_ALIGN flag set, the
+	 * function is called before padding the file.
+	 *
+	 * Modifying the block is rather pointless, but the function may
+	 * write data to the file which is taken into account when padding
+	 * the file.
+	 *
+	 * @param user A user pointer.
+	 * @param block The block that is about to be written.
+	 * @param file The file that the block was written to.
+	 */
+	void (*post_block_write)(void *user, const sqfs_block_t *block,
+				 sqfs_file_t *file);
+
+	/**
+	 * @brief Gets called before storing a fragment in a fragment block.
+	 *
+	 * The function can modify the block before it is stored.
+	 *
+	 * @param user A user pointer.
+	 * @param block The data chunk that is about to be merged into the
+	 *              fragment block.
+	 */
+	void (*pre_fragment_store)(void *user, sqfs_block_t *block);
+
+	/**
+	 * @brief Gets called if block deduplication managed to get
+	 *        rid of the data blocks of a file.
+	 *
+	 * @param user A user pointer.
+	 * @param count The number of blocks that have been erased.
+	 * @param bytes The number of bytes that have been erased. Includes
+	 *              potential padding before and after the end.
+	 */
+	void (*notify_blocks_erased)(void *user, size_t count,
+				     sqfs_u64 bytes);
+
+	/**
+	 * @brief Gets called before throwing away a fragment that turned out
+	 *        to be a duplicate.
+	 *
+	 * @param user A user pointer.
+	 * @param block The data chunk that is about to be merged into the
+	 *              fragment block.
+	 */
+	void (*notify_fragment_discard)(void *user, const sqfs_block_t *block);
+
+	/**
+	 * @brief Gets called before writing a block of padding bytes to disk.
+	 *
+	 * @param user A user pointer.
+	 * @param block The padding bytes that are about to be written.
+	 * @param count The number of padding bytes in the block.
+	 */
+	void (*prepare_padding)(void *user, sqfs_u8 *block, size_t count);
+};
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+SQFS_API sqfs_block_writer_t *sqfs_block_writer_create(sqfs_file_t *file,
+						       size_t devblksz,
+						       sqfs_u32 flags);
+
+SQFS_API int sqfs_block_writer_set_hooks(sqfs_block_writer_t *wr,
+					 void *user_ptr,
+					 const sqfs_block_hooks_t *hooks);
+
+SQFS_API void sqfs_block_writer_destroy(sqfs_block_writer_t *wr);
+
+SQFS_API int sqfs_block_writer_write(sqfs_block_writer_t *wr,
+				     sqfs_block_t *block,
+				     sqfs_u64 *location);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* SQFS_BLOCK_WRITER_H */