diff --git a/src/lyb.h b/src/lyb.h
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+/**
+ * @file lyb.h
+ * @author Michal Vasko <mvasko@cesnet.cz>
+ * @brief Header for LYB format printer & parser
+ *
+ * Copyright (c) 2020 CESNET, z.s.p.o.
+ *
+ * This source code is licensed under BSD 3-Clause License (the "License").
+ * You may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://opensource.org/licenses/BSD-3-Clause
+ */
+
+#ifndef LY_LYB_H_
+#define LY_LYB_H_
+
+#include <stddef.h>
+#include <stdint.h>
+
+#include "set.h"
+#include "tree.h"
+
+struct hash_table;
+struct ly_ctx;
+struct lyd_node;
+struct lysc_node;
+
+/**
+ * @brief Internal structure for LYB parser/printer.
+ */
+struct lyd_lyb_ctx {
+    struct lyd_lyb_subtree {
+        size_t written;
+        size_t position;
+        uint8_t inner_chunks;
+    } *subtrees;
+    LY_ARRAY_SIZE_TYPE subtree_size;
+
+    size_t byte_count;  /**< printed/parsed bytes */
+    const struct ly_ctx *ctx;
+    int options;
+
+    /* LYB parser only */
+    const char *data;
+    int int_opts;
+    const struct lys_module **models;
+    struct ly_set unres_node_type;
+    struct ly_set unres_meta_type;
+    struct ly_set when_check;
+    struct lyd_node *op_ntf;
+
+    /* LYB printer only */
+    struct lyd_lyb_sib_ht {
+        struct lysc_node *first_sibling;
+        struct hash_table *ht;
+    } *sib_hts;
+};
+
+/**
+ * LYB format
+ *
+ * Unlike XML or JSON, it is binary format so most data are represented in similar way but in binary.
+ * Some notable differences:
+ *
+ * - schema nodes are identified based on their hash instead of their string name. In case of collisions
+ * an array of hashes is created with each next hash one bit shorter until a unique sequence of all these
+ * hashes is found and then all of them are stored.
+ *
+ * - tree structure is represented as individual strictly bounded subtrees. Each subtree begins
+ * with its metadata, which consist of 1) the whole subtree length in bytes and 2) number
+ * of included metadata chunks of nested subtrees.
+ *
+ * - since length of a subtree is not known before it is printed, holes are first written and
+ * after the subtree is printed, they are filled with actual valid metadata. As a consequence,
+ * LYB data cannot be directly printed into streams!
+ *
+ * - data are preceded with information about all the used modules. It is needed because of
+ * possible augments and deviations which must be known beforehand, otherwise schema hashes
+ * could be matched to the wrong nodes.
+ */
+
+/* just a shortcut */
+#define LYB_LAST_SUBTREE(lybctx) lybctx->subtrees[LY_ARRAY_SIZE(lybctx->subtrees) - 1]
+
+/* struct lyd_lyb_subtree allocation step */
+#define LYB_SUBTREE_STEP 4
+
+/* current LYB format version */
+#define LYB_VERSION_NUM 0x10
+
+/* LYB format version mask of the header byte */
+#define LYB_VERSION_MASK 0x10
+
+/**
+ * LYB schema hash constants
+ *
+ * Hash is divided to collision ID and hash itself.
+ *
+ * First bits are collision ID until 1 is found. The rest is truncated 32b hash.
+ * 1xxx xxxx - collision ID 0 (no collisions)
+ * 01xx xxxx - collision ID 1 (collision ID 0 hash collided)
+ * 001x xxxx - collision ID 2 ...
+ */
+
+/* Number of bits the whole hash will take (including hash collision ID) */
+#define LYB_HASH_BITS 8
+
+/* Masking 32b hash (collision ID 0) */
+#define LYB_HASH_MASK 0x7f
+
+/* Type for storing the whole hash (used only internally, publicly defined directly) */
+#define LYB_HASH uint8_t
+
+/* Need to move this first >> collision number (from 0) to get collision ID hash part */
+#define LYB_HASH_COLLISION_ID 0x80
+
+/* How many bytes are reserved for one data chunk SIZE (8B is maximum) */
+#define LYB_SIZE_BYTES 1
+
+/* Maximum size that will be written into LYB_SIZE_BYTES (must be large enough) */
+#define LYB_SIZE_MAX UINT8_MAX
+
+/* How many bytes are reserved for one data chunk inner chunk count */
+#define LYB_INCHUNK_BYTES 1
+
+/* Maximum size that will be written into LYB_INCHUNK_BYTES (must be large enough) */
+#define LYB_INCHUNK_MAX UINT8_MAX
+
+/* Just a helper macro */
+#define LYB_META_BYTES (LYB_INCHUNK_BYTES + LYB_SIZE_BYTES)
+
+/* Type large enough for all meta data */
+#define LYB_META uint16_t
+
+#endif /* LY_LYB_H_ */