197 * 8000 = 1576000. If you only have 1.5e6 data points why are you trying to match them against 4**21 possible combinations when all bar an infinitessimal fraction of those won't be present?

Just compare your 1.5e6 against themselves and mark everything else as zero. No matrices required.

For fun, see The 10**21 Problem (Part I) and the subsequent parts.

(Edit: BrowserUk managed to construct a fuller and therefore better response in the time it took me to deduce and type this. Still, great minds and all that ;-)