first commit

.gitignore

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+/target

Cargo.toml

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+[package]
+name = "fastdiv10"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+
+[dev-dependencies]
+rayon = "1.10.0"

src/lib.rs

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+/// It is not actually necissary or even recommended to use this code.
+/// The compiler should always optimize division with constant denominators for you.
+/// This code is just to help understand how the compilers optimization works.
+pub fn div10(num: u32) -> (u32, u32) {
+    // Calculate num / 10
+    // Note: 0.10 = 3435973837 / (1<<35)
+    let quot: u32 = ((3435973837_u64 * num as u64) >> 35) as u32;
+
+    // Calculate quot * 10
+    // Note: 10x = 2x + 4(2x)
+    let quot2 = quot << 1;
+    let quot8 = quot2 << 3;
+    let quot10 = quot2 + quot8;
+
+    // Calculate the remainder
+    // Note: rem = num - (quotient * denominator)
+    let rem = num - quot10;
+
+    return (quot, rem);
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use rayon::prelude::*;
+
+    #[test]
+    /// This will take a REALLY long time
+    fn brute_check() {
+        (0..=u32::MAX).into_par_iter().for_each(|i| {
+            let ans = (i / 10, i % 10);
+            assert_eq!(ans, div10(i));
+        });
+    }
+}