math/fast_mod

src/math/fast_mod.h

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+#pragma once
+#include "../template.h"
+
+using ull = uint64_t;
+
+// Compute a % b faster, where b is constant,
+// but not known at compile time.
+// Returns value in range [0,2b).
+//! Source: https://github.com/kth-competitive-programming/kactl/blob/main/content/various/FastMod.h
+struct FastMod {
+	ull b, m;
+	FastMod(ull a) : b(a), m(-1ULL / a) {}
+	ull operator()(ull a) { // a % b + (0 or b)
+		return a - ull((__uint128_t(m)*a)>>64) * b;
+	}
+};

tests/own/math/fast_mod.cpp

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+#include "../../../src/math/fast_mod.h"
+#include "../base_test.hpp"
+
+void deterministic() {
+}
+
+void fuzz() {
+	rep(i, 0, 20'000'000) {
+		ull a = randRange<ull>(0, 1e18);
+		ull b = randRange<ull>(1, 1e18);
+		ull got = FastMod(b)(a);
+		assert(got == a%b || got == a%b+b);
+	}
+}
+
+void benchmark() {
+}