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mtbl_merger - merge multiple MTBL data sources into a single output

#include <mtbl.h>
Merger objects:
struct mtbl_merger *
mtbl_merger_init(const struct mtbl_merger_options *mopt);
mtbl_merger_destroy(struct mtbl_merger **m);
mtbl_merger_add_source(struct mtbl_merger *m, const struct mtbl_source *s);
const struct mtbl_source *
mtbl_merger_source(struct mtbl_merger *m);
Merger options:
struct mtbl_merger_options *
mtbl_merger_options_destroy(struct mtbl_merger_options **mopt);
        struct mtbl_merger_options *mopt,
        mtbl_merge_func fp,
        void *clos);
typedef void
(*mtbl_merge_func)(void *clos,
        const uint8_t *key, size_t len_key,
        const uint8_t *val0, size_t len_val0,
        const uint8_t *val1, size_t len_val1,
        uint8_t **merged_val, size_t *len_merged_val);

Multiple MTBL data sources may be merged together using the mtbl_merger interface, which reads key-value entries from one or more sources and provides these entries in sorted order. The sorted entries may be consumed via the mtbl_source(3) and mtbl_iter(3) interfaces.
Because the MTBL format does not allow duplicate keys, the caller must provide a function which will accept a key and two conflicting values for that key and return a replacement value. This function may be called multiple times for the same key if more than two sources are being merged.
mtbl_merger objects are created with the mtbl_merger_init() function, which requires a non-NULL mopt argument which has been configured with a merge function fp.
One or more mtbl_reader objects must be provided as input to the mtbl_merger object by calling mtbl_merger_add_source(). After the desired sources have been configured, mtbl_merger_source() should be called in order to consume the merged output via the mtbl_source(3) interface.

This option specifies a merge function callback, consisting of a function pointer fp and a pointer to user data clos which will be passed as the first argument to fp. The merge function callback will be used during iteration over the mtbl_merger object to merge entries with duplicate keys in the input sources.
The remaining arguments to the merge function are:
key — pointer to the key for which there exist duplicate values.
len_key — length of the key.
val0 — pointer to the first value.
len_val_0 — length of the first value.
val1 — pointer to the second value.
len_val_1 — length of the second value.
merged_val — pointer to where the callee should place its merged value.
len_merged_val — pointer to where the callee should place the length of its merged value.
merged_val must be allocated with the system allocator, and the mtbl_merger interface takes responsibility for free()ing the value once it is no longer needed.
The callee may provide an empty value as the merged value, in which case merged_val must still contain an allocated, non-NULL value and len_merged_val must contain the value 0.
The callee may indicate an error by returning NULL in the merged_val argument, which will abort iteration over the mtbl_merger object.

If the merge function callback is unable to provide a merged value (that is, it fails to return a non-NULL value in its merged_val argument), the merge process will be aborted, and any iterators over the mtbl_merger object (via the mtbl_source(3) interface) will return mtbl_res_failure.

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