miden-precompiles 0.29.2

//! Shared chunk ↔ byte codec for hash precompiles. Hash evaluation consumes framework-owned chunk
//! list children, derives the expected chunk count from the assertion's declared byte length, and
//! uses this codec to validate and strip trailing zero padding back to `n_bytes`.

use alloc::vec::Vec;
use core::num::NonZeroU32;

use miden_core::deferred::{DataChunk, Node, PrecompileError};

/// Bytes packed per 8-felt chunk: each felt carries a u32 (4 bytes) little-endian limb.
pub const BYTES_PER_CHUNK: u32 = Node::PACKED_BYTES_PER_CHUNK as u32;

/// Number of 8-felt chunks needed to encode `n_bytes` of u32-packed input.
///
/// Empty input still needs one chunk: deferred data payloads are non-empty, so a 0-byte hash
/// preimage is encoded as a single zero chunk. The count is therefore clamped to at least 1.
pub fn n_chunks(n_bytes: u32) -> NonZeroU32 {
    NonZeroU32::new(n_bytes.div_ceil(BYTES_PER_CHUNK).max(1)).expect("clamped to at least 1")
}

/// Unpack a slice of u32-packed-LE chunks back to a `n_bytes`-length byte vector, returning
/// `PrecompileError::InvalidNode` if any felt holds a value larger than `u32::MAX`.
///
/// The caller-supplied `n_bytes` may be shorter than `chunks.len() * BYTES_PER_CHUNK as usize`;
/// the trailing bytes are zero-pad and are stripped from the output after validating they are
/// zero.
pub fn chunks_to_bytes(chunks: &[DataChunk], n_bytes: usize) -> Result<Vec<u8>, PrecompileError> {
    let chunk_bytes = BYTES_PER_CHUNK as usize;
    if n_bytes > chunks.len() * chunk_bytes {
        return Err(PrecompileError::InvalidNode);
    }
    let mut bytes = Vec::with_capacity(chunks.len() * chunk_bytes);
    for chunk in chunks {
        for felt in chunk {
            let limb =
                u32::try_from(felt.as_canonical_u64()).map_err(|_| PrecompileError::InvalidNode)?;
            bytes.extend_from_slice(&limb.to_le_bytes());
        }
    }
    if bytes[n_bytes..].iter().any(|&b| b != 0) {
        return Err(PrecompileError::InvalidNode);
    }
    bytes.truncate(n_bytes);
    Ok(bytes)
}

/// Unpack exactly `expected_chunks` u32-packed-LE chunks back to a `n_bytes`-length byte vector.
///
/// Returns [`PrecompileError::InvalidNode`] if the chunk count is not exact, if `n_bytes` is too
/// large for the supplied chunks, if any felt is not a canonical `u32`, or if stripped padding
/// bytes are non-zero.
pub fn chunks_to_bytes_exact(
    chunks: &[DataChunk],
    expected_chunks: usize,
    n_bytes: usize,
) -> Result<Vec<u8>, PrecompileError> {
    if chunks.len() != expected_chunks {
        return Err(PrecompileError::InvalidNode);
    }
    chunks_to_bytes(chunks, n_bytes)
}