chore: checkpoint before Python removal

vendor/num-bigint-dig/src/algorithms/shl.rs

@@ -0,0 +1,37 @@
+use alloc::borrow::Cow;
+use core::iter::repeat;
+
+use smallvec::SmallVec;
+
+use crate::big_digit::BITS;
+use crate::BigUint;
+
+#[inline]
+pub fn biguint_shl(n: Cow<BigUint>, bits: usize) -> BigUint {
+    let n_unit = bits / BITS;
+    let mut data = match n_unit {
+        0 => n.into_owned().data,
+        _ => {
+            let len = n_unit + n.data.len() + 1;
+            let mut data = SmallVec::with_capacity(len);
+            data.extend(repeat(0).take(n_unit));
+            data.extend(n.data.iter().cloned());
+            data
+        }
+    };
+
+    let n_bits = bits % BITS;
+    if n_bits > 0 {
+        let mut carry = 0;
+        for elem in data[n_unit..].iter_mut() {
+            let new_carry = *elem >> (BITS - n_bits);
+            *elem = (*elem << n_bits) | carry;
+            carry = new_carry;
+        }
+        if carry != 0 {
+            data.push(carry);
+        }
+    }
+
+    BigUint::new_native(data)
+}