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This is based on Martin Koegler's idea to keep the object that was successfully used as the base of the delta when it is about to fall off the edge of the window. Instead of doing so only for the objects at the edge of the window, this makes the window a lru eviction mechanism. If an entry is used as a base, it is moved to the last of the queue to be evicted. This is a quick-and-dirty implementation, as it keeps the original implementation of the data structure used for the window. This originally was done as an array, not as an array of pointers, because it was meant to be used as a cyclic FIFO buffer and a plain array avoids an extra pointer indirection, while its FIFOness eant that we are not "moving" the entries like this patch does. The runtime from three versions were comparable. It seems to make the resulting chain even shorter, which can only be good. (stock "master") 15782196 bytes chain length = 1: 2972 objects chain length = 2: 2651 objects chain length = 3: 2369 objects chain length = 4: 2121 objects chain length = 5: 1877 objects ... chain length = 46: 490 objects chain length = 47: 515 objects chain length = 48: 527 objects chain length = 49: 570 objects chain length = 50: 408 objects (with your patch) 15745736 bytes (0.23% smaller) chain length = 1: 3137 objects chain length = 2: 2688 objects chain length = 3: 2322 objects chain length = 4: 2146 objects chain length = 5: 1824 objects ... chain length = 46: 503 objects chain length = 47: 509 objects chain length = 48: 536 objects chain length = 49: 588 objects chain length = 50: 357 objects (with this patch) 15612086 bytes (1.08% smaller) chain length = 1: 4831 objects chain length = 2: 3811 objects chain length = 3: 2964 objects chain length = 4: 2352 objects chain length = 5: 1944 objects ... chain length = 46: 327 objects chain length = 47: 353 objects chain length = 48: 304 objects chain length = 49: 298 objects chain length = 50: 135 objects [jc: this is with code simplification follow-up from Nico] Signed-off-by: Junio C Hamano <gitster@pobox.com>
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