We will open source our CF-based recommendation engine in the next days. We use the code in production (on a site with a few hundred million views a month) with up to 80GB datasets for item<->item CF. The engine generates thousands of recommendations per second per core. HTTP round-trip-time to fetch a recommendation from localhost is well below one millisecond.
I don't have the full engine source code or benchmarks to share right now (hopefully in the next two weeks as it is approved by our legal department), but I can tell you all the hard lifting is done by a library called "libsmatrix", which implements a fast, memory-efficient sparse matrix data structure that is used at the core of the CF algo. libsmatrix is 100% threadsafe and persists to disk (this way you can also work with datasets larger than the available main memory). this library is already open sourced:
https://github.com/paulasmuth/libsmatrix
Using libsmatrix, you can build the rest of the "engine" in a matter of a hundred lines of C or so...
#include "smatrix.h"
smatrix_t* my_smatrix;
// libsmatrix example: simple CF based recommendation engine
int main(int argc, char **argv) {
my_smatrix = smatrix_open(NULL);
// one preference set = list of items in one session
// e.g. list of viewed items by the same user
// e.g. list of bought items in the same checkout
uint32_t input_ids[5] = {12,52,63,76,43};
import_preference_set(input_ids, 5);
// generate recommendations (similar items) for item #76
void neighbors_for_item(76);
smatrix_close(my_smatrix);
return 0;
}
// train / add a preference set (list of items in one session)
void import_preference_set(uint32_t* ids, uint32_t num_ids) {
uint32_t i, n;
for (n = 0; n < num_ids; n++) {
smatrix_incr(my_smatrix, ids[n], 0, 1);
for (i = 0; i < pset->len; i++) {
if (i != n) {
smatrix_incr(my_smatrix, ids[n], ids[i], 1);
}
}
}
}
// get recommendations for item with id "item_id"
void neighbors_for_item(uint32_t item_id)
uint32_t neighbors, *row, total;
total = smatrix_get(my_smatrix, item_id, 0);
neighbors = smatrix_getrow(my_smatrix, item_id, row, 8192);
for (pos = 0; pos < neighbors; pos++) {
uint32_t cur_id = row[pos * 2];
printf("found neighbor for item %u: item %u with distance %f\n",
item_id, cf_cosine(smatrix, cur_id, row[pos * 2 + 1], total));
}
free(row);
}
// calculates the cosine vector distance between two items
double cf_cosine(smatrix_t* smatrix, uint32_t b_id, uint32_t cc_count, uint32_t a_total) {
uint32_t b_total;
double num, den;
b_total = smatrix_get(smatrix, b_id, 0);
if (b_total == 0)
b_total = 1;
num = cc_count;
den = sqrt((double) a_total) * sqrt((double) b_total);
if (den == 0.0)
return 0.0;
if (num > den)
return 0.0;
return (num / den);
}