rust-base64/README.md

[base64](https://crates.io/crates/base64)
===

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Made with CLion. Thanks to JetBrains for supporting open source!

It's base64. What more could anyone want?

This library's goals are to be *correct* and *fast*. It's thoroughly tested and widely used. It exposes functionality at multiple levels of abstraction so you can choose the level of convenience vs performance that you want, e.g. `decode_config_slice` decodes into an existing `&mut [u8]` and is pretty fast (2.6GiB/s for a 3 KiB input), whereas `decode_config` allocates a new `Vec<u8>` and returns it, which might be more convenient in some cases, but is slower (although still fast enough for almost any purpose) at 2.1 GiB/s.

Example
---

```rust
extern crate base64;

use base64::{encode, decode};

fn main() {
    let a = b"hello world";
    let b = "aGVsbG8gd29ybGQ=";

    assert_eq!(encode(a), b);
    assert_eq!(a, &decode(b).unwrap()[..]);
}
```

See the [docs](https://docs.rs/base64) for all the details.

Rust version compatibility
---

The minimum required Rust version is 1.34.0.

Developing
---

Benchmarks are in `benches/`. Running them requires nightly rust, but `rustup` makes it easy:

```bash
rustup run nightly cargo bench
```

Decoding is aided by some pre-calculated tables, which are generated by:

```bash
cargo run --example make_tables > src/tables.rs.tmp && mv src/tables.rs.tmp src/tables.rs
```

no_std
---

This crate supports no_std. By default the crate targets std via the `std` feature. You can deactivate the `default-features` to target core instead. In that case you lose out on all the functionality revolving around `std::io`, `std::error::Error` and heap allocations. There is an additional `alloc` feature that you can activate to bring back the support for heap allocations.

Profiling
---

On Linux, you can use [perf](https://perf.wiki.kernel.org/index.php/Main_Page) for profiling. Then compile the benchmarks with `rustup nightly run cargo bench --no-run`.

Run the benchmark binary with `perf` (shown here filtering to one particular benchmark, which will make the results easier to read). `perf` is only available to the root user on most systems as it fiddles with event counters in your CPU, so use `sudo`. We need to run the actual benchmark binary, hence the path into `target`. You can see the actual full path with `rustup run nightly cargo bench -v`; it will print out the commands it runs. If you use the exact path that `bench` outputs, make sure you get the one that's for the benchmarks, not the tests. You may also want to `cargo clean` so you have only one `benchmarks-` binary (they tend to accumulate).

```bash
sudo perf record target/release/deps/benchmarks-* --bench decode_10mib_reuse
```

Then analyze the results, again with perf:

```bash
sudo perf annotate -l
```

You'll see a bunch of interleaved rust source and assembly like this. The section with `lib.rs:327` is telling us that 4.02% of samples saw the `movzbl` aka bit shift as the active instruction. However, this percentage is not as exact as it seems due to a phenomenon called *skid*. Basically, a consequence of how fancy modern CPUs are is that this sort of instruction profiling is inherently inaccurate, especially in branch-heavy code.

```text
 lib.rs:322    0.70 :     10698:       mov    %rdi,%rax
    2.82 :        1069b:       shr    $0x38,%rax
         :                  if morsel == decode_tables::INVALID_VALUE {
         :                      bad_byte_index = input_index;
         :                      break;
         :                  };
         :                  accum = (morsel as u64) << 58;
 lib.rs:327    4.02 :     1069f:       movzbl (%r9,%rax,1),%r15d
         :              // fast loop of 8 bytes at a time
         :              while input_index < length_of_full_chunks {
         :                  let mut accum: u64;
         :
         :                  let input_chunk = BigEndian::read_u64(&input_bytes[input_index..(input_index + 8)]);
         :                  morsel = decode_table[(input_chunk >> 56) as usize];
 lib.rs:322    3.68 :     106a4:       cmp    $0xff,%r15
         :                  if morsel == decode_tables::INVALID_VALUE {
    0.00 :        106ab:       je     1090e <base64::decode_config_buf::hbf68a45fefa299c1+0x46e>
```


Fuzzing
---

This uses [cargo-fuzz](https://github.com/rust-fuzz/cargo-fuzz). See `fuzz/fuzzers` for the available fuzzing scripts. To run, use an invocation like these:

```bash
cargo +nightly fuzz run roundtrip
cargo +nightly fuzz run roundtrip_no_pad
cargo +nightly fuzz run roundtrip_random_config -- -max_len=10240
cargo +nightly fuzz run decode_random
```


License
---

This project is dual-licensed under MIT and Apache 2.0.