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+#ifndef PACK_REVINDEX_H
+#define PACK_REVINDEX_H
+
+/**
+ * A revindex allows converting efficiently between three properties
+ * of an object within a pack:
+ *
+ * - index position: the numeric position within the list of sorted object ids
+ *   found in the .idx file
+ *
+ * - pack position: the numeric position within the list of objects in their
+ *   order within the actual .pack file (i.e., 0 is the first object in the
+ *   .pack, 1 is the second, and so on)
+ *
+ * - offset: the byte offset within the .pack file at which the object contents
+ *   can be found
+ *
+ * The revindex can also be used with a multi-pack index (MIDX). In this
+ * setting:
+ *
+ *   - index position refers to an object's numeric position within the MIDX
+ *
+ *   - pack position refers to an object's position within a non-existent pack
+ *     described by the MIDX. The pack structure is described in
+ *     gitformat-pack(5).
+ *
+ *     It is effectively a concatanation of all packs in the MIDX (ordered by
+ *     their numeric ID within the MIDX) in their original order within each
+ *     pack), removing duplicates, and placing the preferred pack (if any)
+ *     first.
+ */
+
+
+#define RIDX_SIGNATURE 0x52494458 /* "RIDX" */
+#define RIDX_VERSION 1
+
+#define GIT_TEST_NO_WRITE_REV_INDEX "GIT_TEST_NO_WRITE_REV_INDEX"
+#define GIT_TEST_REV_INDEX_DIE_IN_MEMORY "GIT_TEST_REV_INDEX_DIE_IN_MEMORY"
+#define GIT_TEST_REV_INDEX_DIE_ON_DISK "GIT_TEST_REV_INDEX_DIE_ON_DISK"
+
+struct packed_git;
+struct multi_pack_index;
+struct repository;
+
+/*
+ * load_pack_revindex populates the revindex's internal data-structures for the
+ * given pack, returning zero on success and a negative value otherwise.
+ *
+ * If a '.rev' file is present it is mmap'd, and pointers are assigned into it
+ * (instead of using the in-memory variant).
+ */
+int load_pack_revindex(struct repository *r, struct packed_git *p);
+
+/*
+ * Specifically load a pack revindex from disk.
+ *
+ * Returns 0 on success, 1 on "no .rev file", and -1 when there is an
+ * error parsing the .rev file.
+ */
+int load_pack_revindex_from_disk(struct packed_git *p);
+
+/*
+ * verify_pack_revindex verifies that the on-disk rev-index for the given
+ * pack-file is the same that would be created if written from scratch.
+ *
+ * A negative number is returned on error.
+ */
+int verify_pack_revindex(struct packed_git *p);
+
+/*
+ * load_midx_revindex loads the '.rev' file corresponding to the given
+ * multi-pack index by mmap-ing it and assigning pointers in the
+ * multi_pack_index to point at it.
+ *
+ * A negative number is returned on error.
+ */
+int load_midx_revindex(struct multi_pack_index *m);
+
+/*
+ * Frees resources associated with a multi-pack reverse index.
+ *
+ * A negative number is returned on error.
+ */
+int close_midx_revindex(struct multi_pack_index *m);
+
+/*
+ * offset_to_pack_pos converts an object offset to a pack position. This
+ * function returns zero on success, and a negative number otherwise. The
+ * parameter 'pos' is usable only on success.
+ *
+ * If the reverse index has not yet been loaded, this function loads it lazily,
+ * and returns an negative number if an error was encountered.
+ *
+ * This function runs in time O(log N) with the number of objects in the pack.
+ */
+int offset_to_pack_pos(struct packed_git *p, off_t ofs, uint32_t *pos);
+
+/*
+ * pack_pos_to_index converts the given pack-relative position 'pos' by
+ * returning an index-relative position.
+ *
+ * If the reverse index has not yet been loaded, or the position is out of
+ * bounds, this function aborts.
+ *
+ * This function runs in constant time.
+ */
+uint32_t pack_pos_to_index(struct packed_git *p, uint32_t pos);
+
+/*
+ * pack_pos_to_offset converts the given pack-relative position 'pos' into a
+ * pack offset. For a pack with 'N' objects, asking for position 'N' will return
+ * the total size (in bytes) of the pack.
+ *
+ * If the reverse index has not yet been loaded, or the position is out of
+ * bounds, this function aborts.
+ *
+ * This function runs in constant time under both in-memory and on-disk reverse
+ * indexes, but an additional step is taken to consult the corresponding .idx
+ * file when using the on-disk format.
+ */
+off_t pack_pos_to_offset(struct packed_git *p, uint32_t pos);
+
+/*
+ * pack_pos_to_midx converts the object at position "pos" within the MIDX
+ * pseudo-pack into a MIDX position.
+ *
+ * If the reverse index has not yet been loaded, or the position is out of
+ * bounds, this function aborts.
+ *
+ * This function runs in constant time.
+ */
+uint32_t pack_pos_to_midx(struct multi_pack_index *m, uint32_t pos);
+
+/*
+ * midx_to_pack_pos converts from the MIDX-relative position at "at" to the
+ * corresponding pack position.
+ *
+ * If the reverse index has not yet been loaded, or the position is out of
+ * bounds, this function aborts.
+ *
+ * This function runs in time O(log N) with the number of objects in the MIDX.
+ */
+int midx_to_pack_pos(struct multi_pack_index *midx, uint32_t at, uint32_t *pos);
+
+#endif