::compare()

#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <wiredtiger.h>

#define WT_HOME "index_collator.db"

#define WT_CALL(call) \
    do { \
        const int __rc = call; \
        if (__rc != 0) { \
            fprintf(stderr, # call " at (%s:%d) failed: %s [%d]\n", __FILE__, __LINE__, wiredtiger_strerror(__rc), __rc); \
            exit(EXIT_FAILURE); \
        } \
    } while (0)

static int
compare_int(int a, int b)
{
    return (a < b ? -1 : (a > b ? 1 : 0));
}

static int
index_compare(WT_COLLATOR *collator, WT_SESSION *session,
              const WT_ITEM *key1, const WT_ITEM *key2, int *cmp)
{
    int rc;
    int ikey1 = 12345, pkey1 = 54321, ikey2 = 67890, pkey2 = 9876;

    WT_CALL(wiredtiger_struct_unpack(session, key1->data, key1->size, "ii", &ikey1, &pkey1));
    WT_CALL(wiredtiger_struct_unpack(session, key2->data, key2->size, "ii", &ikey2, &pkey2));

    printf("index_compare: ikey1 = %d, pkey1 = %d, ikey2 = %d, pkey2 = %d\n", ikey1, pkey1, ikey2, pkey2);

    if ((*cmp = compare_int(ikey1, ikey2)) != 0)
        return 0;

    *cmp = compare_int(pkey1, pkey2);
    return 0;
}

static WT_COLLATOR index_coll = { index_compare, NULL, NULL };

int
main(void)
{
    WT_CONNECTION *conn;
    WT_SESSION *session;
    WT_CURSOR *cursor;

    assert(system("rm -rf \"" WT_HOME "\" && mkdir \"" WT_HOME "\"") == 0);

    WT_CALL(wiredtiger_open(WT_HOME, NULL, "create", &conn));
    WT_CALL(conn->open_session(conn, NULL, NULL, &session));

    WT_CALL(conn->add_collator(conn, "index_coll", &index_coll, NULL));

    WT_CALL(session->create(session, "table:main", "key_format=i,value_format=i,columns=(k,v)"));
    WT_CALL(session->create(session, "index:main:index", "columns=(v),collator=index_coll"));

    WT_CALL(session->open_cursor(session, "table:main", NULL, NULL, &cursor));
    cursor->set_key(cursor, 13);
    cursor->set_value(cursor, 17);
    WT_CALL(cursor->insert(cursor));
    WT_CALL(cursor->close(cursor));

    WT_CALL(session->open_cursor(session, "index:main:index", NULL, NULL, &cursor));

    while (cursor->next(cursor) == 0) {
        int ikey, val;
        WT_CALL(cursor->get_key(cursor, &ikey));
        WT_CALL(cursor->get_value(cursor, &val));
        printf("ikey = %d, value = %d\n", ikey, val);
    }

    cursor->set_key(cursor, 17);
    WT_CALL(cursor->search(cursor));

    WT_CALL(cursor->close(cursor));

    WT_CALL(conn->close(conn, NULL));

    return 0;
}

$ ./index_collator
ikey = 17, value = 17
wiredtiger_struct_unpack(session, key1->data, key1->size, "ii", &ikey1, &pkey1) at (index_collator.c:32) failed: Invalid argument [22]
$ ./index_collator
ikey = 17, value = 17
index_compare: ikey1 = 17, pkey1 = 2962, ikey2 = 17, pkey2 = 13
cursor->search(cursor) at (index_collator.c:79) failed: WT_NOTFOUND: item not found [-31803]
$ ./index_collator
ikey = 17, value = 17
index_compare: ikey1 = 17, pkey1 = -21, ikey2 = 17, pkey2 = 13
index_compare: ikey1 = 17, pkey1 = -21, ikey2 = 17, pkey2 = 13
index_compare: ikey1 = 17, pkey1 = -21, ikey2 = 17, pkey2 = 13