Const Writer

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Documentation

Compile time verified byte writer for slice and vector.

Install

toml [dependencies] const-writer = "0.1.0"

Usage example

```rust use constwriter::ConstWrite; fn main() { let mut vec = vec![]; let writer = vec.constwriter::<10>() // reserve 10 bytes in vec .writeu32le(12) // no runtime checks .writeu32le(34); // no runtime checks

assert_eq!(writer.remaining(), 2);
assert_eq!(vec.len(), 8);
assert_eq!(&vec[0..8], &[12, 0, 0, 0, 34, 0, 0, 0]);

} ```

rust use const_writer::ConstWrite; fn main() { let mut buff = [0u8; 1024]; buff.as_mut().const_writer::<10>() .write_slice(&[1, 2, 3, 4, 5, 6]) .write_u64_le(111); // compile error. }

```rust use constwriter::{ConstWriterAdapter, ConstWriter, ConstWrite}; fn main() { // write 10 bytes fn writestruct(writer: ConstWriter) { writer .writeu16le(34) .writeu16le(2) .writeu16le(3) .writeu16le(4) .writeu16le(5); }

let mut buff = [0u8; 16]; writestruct(buff.asmut().constwriter()); asserteq!(buff, [34, 0, 2, 0, 3, 0, 4, 0, 5, 0, 0, 0, 0, 0, 0, 0]); } ```

Assembly

It generates super optimal assembly with pretty rust syntax

Rust:

```rust // we return modified reference so compiler won't optimise away reference manipulation pub fn write(mut refbuff: &mut [u8]) -> &mut [u8] { use crate::ConstWrite; refbuff.constwriter::<31>() .writeu8le(0x01) .writeu16le(0x0203) .writeu32le(0x04050607) .writeu64le(0x08090A0B0C0D0E0F) .writeu128_le(0x101112131415161718191A1B1C1D1E1F);

ref_buff

} ```

Assembly:

Assembly consists only one comparison asm write: subq $88, %rsp movq %rsi, %rdx cmpq $30, %rsi jbe .LBB2_1 # panic movq %rdi, %rax movb $1, (%rdi) movw $515, 1(%rdi) # imm = 0x203 movl $67438087, 3(%rdi) # imm = 0x4050607 movabsq $579005069656919567, %rcx # imm = 0x8090A0B0C0D0E0F movq %rcx, 7(%rdi) movabsq $1157726452361532951, %rcx # imm = 0x1011121314151617 movq %rcx, 23(%rdi) movabsq $1736447835066146335, %rcx # imm = 0x18191A1B1C1D1E1F movq %rcx, 15(%rdi) addq $31, %rax addq $-31, %rdx addq $88, %rsp retq And sometimes rustc can generate really pretty assembly :heart_eyes:

Rust:

rust pub fn write_struct<T: ConstWriterAdapter>(writer: ConstWriter<T, 12>) -> ConstWriter<T, 0> { writer.write_u32_le(34).write_u16_le(2).write_u16_le(3).write_u16_le(4).write_u16_le(5) }

Assembly:

asm write_struct: movq %rdi, %rax movl $34, (%rsi) movabsq $1407392063619074, %rcx # imm = 0x5000400030002 <- OMG movq %rcx, 4(%rsi) leaq 12(%rsi), %rdx retq

Features