From 992128d7fe0cb7b55bb0ada73a0edc579fcb2f1b Mon Sep 17 00:00:00 2001
From: Orson Peters <orsonpeters@gmail.com>
Date: Fri, 1 Nov 2024 08:23:55 +0100
Subject: [PATCH] refactor(rust): Removed unnecessary flatten function (#19551)

---
 crates/polars-io/src/file_cache/entry.rs   | 43 +++++++++-------------
 crates/polars-utils/src/functions.rs       | 11 ------
 crates/polars-utils/src/python_function.rs | 15 +-------
 3 files changed, 20 insertions(+), 49 deletions(-)

diff --git a/crates/polars-io/src/file_cache/entry.rs b/crates/polars-io/src/file_cache/entry.rs
index ce9998ecaae4..e18ca63cbec5 100644
--- a/crates/polars-io/src/file_cache/entry.rs
+++ b/crates/polars-io/src/file_cache/entry.rs
@@ -6,7 +6,6 @@ use std::sync::{Arc, Mutex};
 use fs4::fs_std::FileExt;
 use polars_core::config;
 use polars_error::{polars_bail, to_compute_err, PolarsError, PolarsResult};
-use polars_utils::flatten;
 
 use super::cache_lock::{self, GLOBAL_FILE_CACHE_LOCK};
 use super::file_fetcher::{FileFetcher, RemoteMetadata};
@@ -370,32 +369,26 @@ fn get_data_file_path(
     remote_version: &FileVersion,
 ) -> PathBuf {
     let owned;
-    let path = flatten(
-        &[
-            path_prefix,
-            &[b'/', DATA_PREFIX, b'/'],
-            uri_hash,
-            match remote_version {
-                FileVersion::Timestamp(v) => {
-                    owned = Some(format!("{:013x}", v));
-                    owned.as_deref().unwrap()
-                },
-                FileVersion::ETag(v) => v.as_str(),
-                FileVersion::Uninitialized => panic!("impl error: version not initialized"),
-            }
-            .as_bytes(),
-        ],
-        None,
-    );
-    PathBuf::from(std::str::from_utf8(&path).unwrap())
+    let path = [
+        path_prefix,
+        &[b'/', DATA_PREFIX, b'/'],
+        uri_hash,
+        match remote_version {
+            FileVersion::Timestamp(v) => {
+                owned = Some(format!("{:013x}", v));
+                owned.as_deref().unwrap()
+            },
+            FileVersion::ETag(v) => v.as_str(),
+            FileVersion::Uninitialized => panic!("impl error: version not initialized"),
+        }
+        .as_bytes(),
+    ]
+    .concat();
+    PathBuf::from(String::from_utf8(path).unwrap())
 }
 
 /// `[prefix]/m/[uri hash]`
 fn get_metadata_file_path(path_prefix: &[u8], uri_hash: &[u8]) -> PathBuf {
-    let bytes = flatten(
-        &[path_prefix, &[b'/', METADATA_PREFIX, b'/'], uri_hash],
-        None,
-    );
-    let s = std::str::from_utf8(bytes.as_slice()).unwrap();
-    PathBuf::from(s)
+    let bytes = [path_prefix, &[b'/', METADATA_PREFIX, b'/'], uri_hash].concat();
+    PathBuf::from(String::from_utf8(bytes).unwrap())
 }
diff --git a/crates/polars-utils/src/functions.rs b/crates/polars-utils/src/functions.rs
index 528bae5ed291..f91e8611e3c3 100644
--- a/crates/polars-utils/src/functions.rs
+++ b/crates/polars-utils/src/functions.rs
@@ -15,17 +15,6 @@ pub fn range_portion(i: usize, k: usize, r: Range<usize>) -> Range<usize> {
     r.start + num_before..r.start + num_before + our_size
 }
 
-// Faster than collecting from a flattened iterator.
-pub fn flatten<T: Clone, R: AsRef<[T]>>(bufs: &[R], len: Option<usize>) -> Vec<T> {
-    let len = len.unwrap_or_else(|| bufs.iter().map(|b| b.as_ref().len()).sum());
-
-    let mut out = Vec::with_capacity(len);
-    for b in bufs {
-        out.extend_from_slice(b.as_ref());
-    }
-    out
-}
-
 pub fn arc_map<T: Clone, F: FnMut(T) -> T>(mut arc: Arc<T>, mut f: F) -> Arc<T> {
     unsafe {
         // Make the Arc unique (cloning if necessary).
diff --git a/crates/polars-utils/src/python_function.rs b/crates/polars-utils/src/python_function.rs
index 178f89aa2ca1..81f4575fff17 100644
--- a/crates/polars-utils/src/python_function.rs
+++ b/crates/polars-utils/src/python_function.rs
@@ -8,8 +8,6 @@ pub use serde_wrap::{
     SERDE_MAGIC_BYTE_MARK as PYTHON_SERDE_MAGIC_BYTE_MARK,
 };
 
-use crate::flatten;
-
 #[derive(Clone, Debug)]
 pub struct PythonFunction(pub PyObject);
 
@@ -96,10 +94,7 @@ pub fn serialize_pyobject_with_cloudpickle_fallback(py_object: &PyObject) -> Pol
 
         let py_bytes = dumped.extract::<PyBackedBytes>().map_err(from_pyerr)?;
 
-        Ok(flatten(
-            &[&[used_cloudpickle as u8, b'C'][..], py_bytes.as_ref()],
-            None,
-        ))
+        Ok([&[used_cloudpickle as u8, b'C'][..], py_bytes.as_ref()].concat())
     })
 }
 
@@ -129,8 +124,6 @@ mod serde_wrap {
     use once_cell::sync::Lazy;
     use polars_error::PolarsResult;
 
-    use crate::flatten;
-
     pub const SERDE_MAGIC_BYTE_MARK: &[u8] = "PLPYFN".as_bytes();
     /// [minor, micro]
     pub static PYTHON3_VERSION: Lazy<[u8; 2]> = Lazy::new(super::get_python3_version);
@@ -158,11 +151,7 @@ mod serde_wrap {
                 .map_err(|e| S::Error::custom(e.to_string()))?;
 
             serializer.serialize_bytes(
-                flatten(
-                    &[SERDE_MAGIC_BYTE_MARK, &*PYTHON3_VERSION, dumped.as_slice()],
-                    None,
-                )
-                .as_slice(),
+                &[SERDE_MAGIC_BYTE_MARK, &*PYTHON3_VERSION, dumped.as_slice()].concat(),
             )
         }
     }