Function message_to_digits

bitvm/src/signatures/utils.rs:47–72

Converts the given `message` into a vector of digits. ## Output format - sequence of `n_digits` many digits - each digit a `u32` value in range `0..2.pow(log2_base)` - message bytes are reversed (but not their nibbles!)

(n_digits: u32, log2_base: u32, message: &[u8])

/// - each digit a `u32` value in range `0..2.pow(log2_base)`
/// - message bytes are reversed (but not their nibbles!)
pub(crate) fn message_to_digits(n_digits: u32, log2_base: u32, message: &[u8]) -> Vec<u32> {
    debug_assert!((4..=8).contains(&log2_base));
    debug_assert!(
        message.len() as u32 * 8 <= n_digits * log2_base,
        "Message is too long to fit into the given number of digits"
    );

    let mut digits = vec![0u32; n_digits as usize]; // cast safety: 32-bit machine or higher
    let mut digit_idx: u32 = 0;
    let mut bit_idx: u32 = 0;

    for mut byte in message.iter().copied() {
        for _ in 0..8 {
            if bit_idx == log2_base {
                bit_idx = 0;
                digit_idx += 1;
            }
            digits[digit_idx as usize] |= ((byte & 1) as u32) << bit_idx; // cast safety: 32-bit machine or higher
            byte >>= 1;
            bit_idx += 1;
        }
    }

    digits.reverse();
    digits
}

/// Returns a Bitcoin script that converts a message into a number.
///