Is the conversion from bytes to Vec O(1) if the underlying memory is already contiguou

I tried to find where the docs for <a href="https://docs.rs/bytes/latest/bytes/struct.

Is the conversion from bytes to Vec<u8> O(1) about bytes HOT 5 OPEN

cBournhonesque commented on July 18, 2024

Is the conversion from bytes to Vec O(1)

from bytes.

Comments (5)

Darksonn commented on July 18, 2024

Beyond requiring that the refcount is 1. It is also required that the data is stored at the beginning of the underlying allocation. Otherwise, it will need to copy the data backwards.

bytes/src/bytes.rs

Lines 1215 to 1218 in fa1daac

 // Copy back buffer 

 ptr::copy(ptr, buf, len); 

 Vec::from_raw_parts(buf, len, cap)

if the underlying memory is already contiguous

The memory in a Bytes is always contiguous.

from bytes.

cBournhonesque commented on July 18, 2024

So in all cases there is a copy operation, but if the refcount is not 1 (or data not stored at the beginning of the underlying allocation) there is also an allocation?

Should I make a PR to update the docstring?

from bytes.

Darksonn commented on July 18, 2024

If the refcount is one, it's just a copy within the existing buffer. There is no allocation. Allocations only happen if the refcount is at least 2.

You're very welcome to send a PR.

from bytes.

cBournhonesque commented on July 18, 2024

I tried to find where the docs for https://docs.rs/bytes/latest/bytes/struct.Bytes.html#method.to_vec are defined but I couldn't find them

from bytes.

Darksonn commented on July 18, 2024

Oh, that method will certainly copy the bytes unconditionally. It's defined on the standard library slice type, and Bytes does not (and can't) override it.

The conversion that we provide is here:

bytes/src/bytes.rs

Lines 943 to 948 in fa1daac

 impl From<Bytes> for Vec<u8> { 

 fn from(bytes: Bytes) -> Vec<u8> { 

 let bytes = ManuallyDrop::new(bytes); 

 unsafe { (bytes.vtable.to_vec)(&bytes.data, bytes.ptr, bytes.len) } 

 } 

 }

It's called using the From trait rather than a method called to_vec.

from bytes.

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Is the conversion from bytes to Vec<u8> O(1) about bytes HOT 5 OPEN

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	// Copy back buffer
	ptr::copy(ptr, buf, len);

	Vec::from_raw_parts(buf, len, cap)

	impl From<Bytes> for Vec<u8> {
	fn from(bytes: Bytes) -> Vec<u8> {
	let bytes = ManuallyDrop::new(bytes);
	unsafe { (bytes.vtable.to_vec)(&bytes.data, bytes.ptr, bytes.len) }
	}
	}