perf: Optimize strings slices (#17996)

crates/polars-arrow/src/array/binview/mod.rs

@@ -232,6 +232,29 @@ impl<T: ViewType + ?Sized> BinaryViewArrayGeneric<T> {
         )
     }
 
+    /// Apply a function over the views. This can be used to update views in operations like slicing.
+    ///
+    /// # Safety
+    /// Update the views. All invariants of the views apply.
+    pub unsafe fn apply_views<F: FnMut(View, &T) -> View>(&self, mut update_view: F) -> Self {
+        let arr = self.clone();
+        let (views, buffers, validity, total_bytes_len, total_buffer_len) = arr.into_inner();
+
+        let mut views = views.make_mut();
+        for v in views.iter_mut() {
+            let str_slice = T::from_bytes_unchecked(v.get_slice_unchecked(&buffers));
+            *v = update_view(*v, str_slice);
+        }
+        Self::new_unchecked(
+            self.data_type.clone(),
+            views.into(),
+            buffers,
+            validity,
+            total_bytes_len,
+            total_buffer_len,
+        )
+    }
+
     pub fn try_new(
         data_type: ArrowDataType,
         views: Buffer<View>,

crates/polars-core/src/chunked_array/ops/apply.rs

@@ -573,3 +573,15 @@ where
         });
     }
 }
+
+impl StringChunked {
+    /// # Safety
+    /// Update the views. All invariants of the views apply.
+    pub unsafe fn apply_views<F: FnMut(View, &str) -> View + Copy>(&self, update_view: F) -> Self {
+        let mut out = self.clone();
+        for arr in out.downcast_iter_mut() {
+            *arr = arr.apply_views(update_view);
+        }
+        out
+    }
+}

crates/polars-ops/src/chunked_array/strings/namespace.rs

@@ -622,7 +622,7 @@ pub trait StringNameSpaceImpl: AsString {
         let ca = self.as_string();
         let n = n.strict_cast(&DataType::Int64)?;
 
-        Ok(substring::head(ca, n.i64()?))
+        substring::head(ca, n.i64()?)
     }
 
     /// Slice the last `n` values of the string.
@@ -633,7 +633,7 @@ pub trait StringNameSpaceImpl: AsString {
         let ca = self.as_string();
         let n = n.strict_cast(&DataType::Int64)?;
 
-        Ok(substring::tail(ca, n.i64()?))
+        substring::tail(ca, n.i64()?)
     }
 }