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https://issues.apache.org/jira/browse/ARROW-2398?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16426809#comment-16426809
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ASF GitHub Bot commented on ARROW-2398:
---------------------------------------
xhochy commented on a change in pull request #1838: ARROW-2398: [Rust] Create
Builder<T> for building buffers directly in aligned memory
URL: https://github.com/apache/arrow/pull/1838#discussion_r179441418
##########
File path: rust/src/buffer.rs
##########
@@ -18,10 +18,74 @@
use bytes::Bytes;
use libc;
use std::mem;
+use std::ptr;
use std::slice;
use super::memory::*;
+/// Buffer builder with zero-copy build method
+pub struct Builder<T> {
+ data: *mut T,
+ len: usize,
+ capacity: usize,
+}
+
+impl<T> Builder<T> {
+ /// Creates a builder with a default capacity
+ pub fn new() -> Self {
+ Builder::with_capacity(64)
+ }
+
+ /// Creates a builder with a fixed capacity
+ pub fn with_capacity(capacity: usize) -> Self {
+ let sz = mem::size_of::<T>();
+ let buffer = allocate_aligned((capacity * sz) as i64).unwrap();
+ Builder {
+ len: 0,
+ capacity,
+ data: unsafe { mem::transmute::<*const u8, *mut T>(buffer) },
+ }
+ }
+
+ /// Push a value into the builder, growing the internal buffer as needed
+ pub fn push(&mut self, v: T) {
+ assert!(!self.data.is_null());
+ if self.len == self.capacity {
+ let sz = mem::size_of::<T>();
+ let new_capacity = self.capacity * 2;
+ unsafe {
+ let old_buffer = self.data;
+ let new_buffer = allocate_aligned((new_capacity * sz) as
i64).unwrap();
+ libc::memcpy(
+ mem::transmute::<*const u8, *mut libc::c_void>(new_buffer),
+ mem::transmute::<*const T, *const
libc::c_void>(old_buffer),
+ self.len * sz,
+ );
+ self.capacity = new_capacity;
+ self.data = mem::transmute::<*const u8, *mut T>(new_buffer);
+ mem::drop(old_buffer);
+ }
+ }
+ assert!(self.len < self.capacity);
+ unsafe {
+ *self.data.offset(self.len as isize) = v;
+ }
+ self.len += 1;
+ }
+
+ /// Build a Buffer from the existing memory
+ pub fn build(&mut self) -> Buffer<T> {
Review comment:
Is there a reason that this is called `build`? In C++ we call this `Finish`
and it would be nice to have them consistent. If `build` is a typical name used
in Rust, it's probably better to keep it as is.
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> [Rust] Provide a zero-copy builder for type-safe Buffer<T>
> ----------------------------------------------------------
>
> Key: ARROW-2398
> URL: https://issues.apache.org/jira/browse/ARROW-2398
> Project: Apache Arrow
> Issue Type: New Feature
> Components: Rust
> Reporter: Andy Grove
> Assignee: Andy Grove
> Priority: Major
> Labels: pull-request-available
> Fix For: 0.10.0
>
>
> This PR implements a builder so that buffers can be populated directly in
> aligned memory (as opposed to being created from Vec<T>).
>
> https://github.com/apache/arrow/pull/1838
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