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const LEFT_SHIFT: [u8; 8] = [ 0b00000001, 0b00000011, 0b00000111, 0b00001111, 0b00011111, 0b00111111, 0b01111111, 0b11111111, ]; const RIGHT_SHIFT: [u8; 8] = [ 0b10000000, 0b11000000, 0b11100000, 0b11110000, 0b11111000, 0b11111100, 0b11111110, 0b11111111, ]; #[inline] pub fn ceil(value: usize, divisor: usize) -> usize { let (quot, rem) = (value / divisor, value % divisor); if rem > 0 && divisor > 0 { quot + 1 } else { quot } } pub extern fn slice(mut arr: Vec<u8>, old_offset: usize, offset: usize, len: usize) -> Vec<u8> { let mut arr: &mut [u8] = arr.as_mut(); let offset = old_offset + offset; // shift by whole bytes let pop_by = offset / 8; let offset_rem = offset % 8; let offset_diff = 8 - offset_rem; arr = &mut arr[pop_by..]; let shift_by = ceil(offset_rem + len, 8); arr = &mut arr[..shift_by]; let mut left = 0u8; // determine the length of the output buffer let total_rem = (len + offset_rem) % 8; // remove trailing bits at end of bytes if total_rem > 0 { if let Some(v) = arr.iter_mut().last() { *v &= LEFT_SHIFT[total_rem - 1]; } } // shorter route if offset is div by 8 if offset_rem == 0 { return arr[0..ceil(len, 8)].to_vec(); } else { // reverse array to enable carrying over arr.reverse(); if let Some(v) = arr.iter_mut().last() { *v &= RIGHT_SHIFT[offset_diff - 1]; } } // replace bits from each byte in reverse order arr.iter_mut().for_each(|byte| { // assign the bits that we'll copy let new_left = *byte & LEFT_SHIFT[offset_rem - 1]; // shift bytes to remove offset *byte >>= offset_rem; // add the bits copied from the previous byte *byte |= left; // shift the bytes we're copying to the next byte left = new_left << offset_diff; }); arr.reverse(); arr[0..ceil(len, 8)].to_vec() }
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