feat(byte_array): add at and index to ByteSpan

[giladchase]

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[giladchase]

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• feat(byte_array): add at and index to ByteSpan #8427 👈 (View in Graphite)
• feat(byte_array): add slice #8417
• feat(byte_array): add ByteSpan::to_byte_array #8416
• refactor(byte_array): extract ByteArray::append logic #8486
• feat(byte_array): add ByteArray::span and Span::len #8329
• refactor(test): remove fixed-size bytearray testutils #8343
• refactor(test): remove compare_spans util #8342
• refactor(test): remove compare_byte_array util and rework tests #8336
• main

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---

corelib/src/byte_array.cairo line 703 at r4 (raw file):

    fn at(self: @ByteSpan, index: usize) -> Option<u8> {
        let actual_index = index.checked_add(upcast(self.first_char_start_offset))?;
        let (word_index, index_in_word) = DivRem::div_rem(actual_index, BYTES_IN_BYTES31_NONZERO);

use bounded_int::div_rem and get index_in_word in [0,30] range.
you can use bounded_int::constrain later to decide if it is accessing the lower or upper word.

Code quote:

let (word_index, index_in_word) = DivRem::div_rem(actual_index, BYTES_IN_BYTES31_NONZERO);

---

corelib/src/byte_array.cairo line 713 at r4 (raw file):

                if word_index == self.data.len() && index_in_word < upcast(self.remainder_len) {
                    // index_in_word is from MSB, we need index from LSB.
                    let index_in_remainder = upcast(self.remainder_len) - 1 - index_in_word;

bounded_int::sub as well.

Code quote:

 let index_in_remainder = upcast(self.remainder_len) - 1 - index_in_word;