9. The system of claim 7 , wherein each hash character corresponds to a bit string.
7. A system comprising: one or more computers; and a computer-readable storage device having stored thereon instructions that, when executed by the one or more computers, cause the one or more computers to perform operations comprising: receiving a search query; identifying a lookup table corresponding to the search query; for each image in a collection of images: generating an image hash for the image based on one or more features extracted from the image, wherein the image hash comprises a plurality of hash characters, and computing a score for the image hash using the lookup table, wherein computing the score for each image hash comprises summing lookup table weights for each hash character of the plurality of hash characters, and wherein the lookup table includes a first index that corresponds to the lookup table value of each hash character, and a second index that corresponds to a position of each hash character in the image hash; ordering the images by the score of each image hash; and providing a group of the ordered images as search results responsive to the search query.