feat: basic bloom filter impl

.clang-format

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+BasedOnStyle: LLVM
+IndentWidth: 4
+TabWidth: 4
+UseTab: Never
+ContinuationIndentWidth: 4

.gitignore

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+a.out

DEV.md

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+# Dev Docs
+
+I've scaffolded but it I feel it makes more sense to just have a one file impl first in some .cpp file
+and take it from there. So off we go to top level main.cpp
+
+There is an original paper but I'm reading off from wiki for now https://en.wikipedia.org/wiki/Bloom_filter
+
+

main.cpp

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+#include <cstdio>
+#include <string>
+#include <set>
+
+/*
+ * Bloom filters
+ *   - n bit array
+ *   - k hash functions
+ *   - add_elem:
+ *       - get the set of {h_i(elem) mod n} and mark them 1
+ *   - check elem:
+ *       - x is elem
+ *       - generate same positions set, if any are false this does not belong
+ * for sure
+ *       - no guarantees on presence
+ */
+
+const int N = 1024;
+const int K = 5; 
+std::vector<bool> bf(N);
+
+int hash_i(int i, int x) {
+    switch (i) {
+    case 1:
+        return x; // h1(i) = i is perfectly ok, std::hash will do the same
+    case 2:
+        // here we need something better
+        // the | 1 makes it co-rime to N always
+        return std::hash<std::string>{}(std::to_string(x)) | 1;
+    default:
+        return hash_i(1, x) + i * hash_i(2, x);
+    }
+}
+
+void add(int elem){
+    for(int i = 1;i<=K;i++){
+        int hash = hash_i(i, elem) % N;
+        bf[hash] = 1;
+    }
+}
+
+bool missing(int elem){
+    for(int i = 1;i<=K;i++){
+        int hash = hash_i(i, elem) % N;
+        if(!bf[hash]) return true; // actually missing
+    }
+    return false; // may be present
+}
+
+int main() {
+    srand(time(nullptr));
+    std::set<int> actually_present{};
+    const int LIM = 500;
+    for(int i = 1; i< LIM;i++){
+        if(rand()&1){
+            actually_present.insert(i);
+            add(i);
+        } 
+    }
+
+    int wrong = 0;
+    for(int i=1;i<LIM;i++){
+        bool true_missing = !actually_present.count(i);
+        bool bloom_missing = missing(i);
+        if( true_missing != bloom_missing ) wrong++;
+    }
+
+    std::printf("%d/%d incorrect\n", wrong, actually_present.size());
+}