add value and type constructors for variable-length buffers and sha256 ctx8#372
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apoelstra merged 5 commits intoJul 21, 2026
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The core::array::from_fn has been stable since Rust 1.63. We can use this to build an array, rather than constructing a vector, converting it to an arary, and unwrapping the conversion. This saves an allocation, eliminates a panic path, and (IMHO) makes the code easier to read. I need to duplicate this logic for the buffer type in the next commit, so I figured I should first clean it up.
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On c5f250c successfully ran local tests |
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| /// # Panics | ||
| /// | ||
| /// Panics if you request a number `n` greater than or equal to the length | ||
| /// of [`Tmr::BUFFER8_TWO_N_PLUS_1`]. |
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| /// of [`Tmr::BUFFER8_TWO_N_PLUS_1`]. | |
| /// of [`Tmr::BUFFER8_TWO_N_PLUS_ONE`]. |
| } | ||
| } | ||
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| /// Slice exceeded the maximum capacity of the buffer type in [`Value::buffer8_two_n`]. |
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| /// Slice exceeded the maximum capacity of the buffer type in [`Value::buffer8_two_n`]. | |
| /// Slice exceeded the maximum capacity of the buffer type in [`Value::buffer8_two_n_plus_one`]. |
| } | ||
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| fn initialize_buffers(write: &mut Option<[Arc<Final>; N_BUFFERS]>) { | ||
| // (TWO^8)^<1 = S(TWO^8) |
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| // (TWO^8)^<1 = S(TWO^8) | |
| // (TWO^8)^<2 = S(TWO^8) |
| pub fn two_two_n(n: usize) -> Arc<Self> { | ||
| super::precomputed::nth_power_of_2(n) | ||
| #[inline] | ||
| pub fn two_two_n(n: usize) -> Result<Arc<Self>, TypeTooLargeError> { |
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Maybe a changelog note about this public signature change?
This function panics if given an n which is out of range. This is arguably fine but we might as well return a Result, which gives the user the choice of whether or not to panic. Because the overwhelming number of calls in this codebase provide a fixed compile-time constant, also add a Final::two_two_n_fixed method which takes a complie-time constant and fails to compile if it's out of range.
Add precomputation tables for both `Tmr` and `Final`. Write a unit test which does a direct computation of the type and verifies all the precomp vectors. Also check against the Ctx8 output type of the sha256 init jet. Don't provide a Final::buffer8_two_n_plus_one_fixed function that avoids the Result by doing a compile-time check. It seems unlikely that this will be called with compile-time fixed values, except for the value 5 which is used in the Ctx8 type. But in the next commit we'll special-case Ctx8.
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Addressed all nits. |
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On 9281768 successfully ran local tests |
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Can you also take a look at #371 and BlockstreamResearch/simplicity#346 ? Both are small and simple. |
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Hey @apoelstra sure, have just reviewed and tested those |
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This PR lays the groundwork for constructing and manipulating SHA256 contexts in an ergonomic way. This lets you construct variable-length buffers, and by producting those with a couple fixed-size words you can create a context. It also provides precomputed types for various sizes of buffers and
Ctx8itself.A later PR will implement conversions between these objects and Rust
sha256::Engineobjects, but this may require an update to bitcoin-hashes 1.0 so I am still working out how to do that.