Rsync checksum5/9/2023 It was looking at rsync, in fact, that led me to see how rsync compares two files using a rolling checksum. This is important for tools like diff and rsync where they don’t want to just know what matched, they want to understand why things don’t match. While it is easy to tell where the string matches and, therefore, where it doesn’t match, it is hard to figure out if there has been just a small insertion or deletion when it doesn’t match. When you discover that the current text has 199 of the 200 characters you are looking for, it is going to be disappointing. For the word “the” that isn’t a big deal because it is short, but what if you were sifting through large search strings? (I don’t know - DNA genome data or something.) You’d spend a lot of time chasing down dead ends. But it also has “there,” “then,” and “other” all of which contain our target word. For example, War and Peace probably has many occurrences of the word “the” within it. Let’s assume that the text contains many strings that are almost what we are looking for, but not quite. Can we do better? Better Algorithms Basic searchĪgain, it really depends on your definition of better. That works and, actually, with some modern hardware you could write some fast code for that. But, in other words, just compare the text with the search string, character by character until you find a match. That’s actually hard to describe in English. If not the same, reset the current letter of “convict” and also go back to the original current letter of the text and then move to the next letter, going back to step 2. If it is the same, keep repeating this step until there are no more letters in “convict” (at which point you have a match). If the current letters are the same, move to the next letter of convict and, without forgetting the current letter of the text, compare it to the next letter.If the current letter isn’t the same as the current letter of “convict” then move to the next letter, reset the current letter in “convict” and go back to step 2 until there are no more letters.Start with the first letter of War and Peace.Let’s say we are looking for the word “convict” in the text of War and Peace: If you are just looking for a string, you could, of course, do the brute force algorithm. Best can mean many things and will depend on the kind of data you are dealing with and even the type of machine you are using. So here’s a question: You have a large body of text and you want to search for it. No matter how fast you can add, you aren’t likely to beat someone who knows that algorithm. So you can easily find that there are 50 pairs that add up to 101 and know the answer is 5,050. While the other students laboriously added each number, Gauss realized that 100+1 is 101 and 99 + 2 is also 101. There is the old story about the mathematician Gauss who, when in school, was given busy work to add the integers from 1 to 100. We are often struck by how often we spend time trying to optimize something when we would be better off just picking a better algorithm.
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