feat(rust): add persistent bugger

rust/.gitignore

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+target/

rust/Cargo.lock

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+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "rust"
+version = "0.1.0"

rust/Cargo.toml

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+[package]
+name = "rust"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

rust/tests/1_persistent_bugger.rs

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+fn persistence(num: u64) -> u64 {
+    match num {
+        0..=9 => 0,
+        _ => {
+            let res = num
+                .to_string()
+                .chars()
+                .map(|c| c.to_digit(10).unwrap() as u64)
+                .product();
+            1 + persistence(res)
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn sample_tests() {
+        assert_eq!(super::persistence(39), 3);
+        assert_eq!(super::persistence(4), 0);
+        assert_eq!(super::persistence(25), 2);
+        assert_eq!(super::persistence(999), 4);
+    }
+}