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writePackedIntBig

mem.writePackedIntBig

fn writePackedIntBig(comptime T: type, bytes: []u8, bit_offset: usize, value: T) void

File

lib/std/mem.zig:2103

Code

fn writePackedIntBig(comptime T: type, bytes: []u8, bit_offset: usize, value: T) void {
    const uN = @Int(.unsigned, @bitSizeOf(T));
    const Log2N = std.math.Log2Int(T);

    const bit_count = @as(usize, @bitSizeOf(T));
    const bit_shift = @as(u3, @intCast(bit_offset % 8));
    const byte_count = (bit_shift + bit_count + 7) / 8;

    const store_size = (@bitSizeOf(T) + 7) / 8;
    const store_tail_bits = @as(u3, @intCast((store_size * 8) - bit_count));
    const StoreInt = @Int(.unsigned, store_size * 8);

    if (bit_count == 0)
        return;

    // Write by storing a StoreInt, and then follow it up with a 1-byte tail
    // if bit_offset pushed us over a byte boundary.
    const end = bytes.len - (bit_offset / 8);
    const write_bytes = bytes[(end - byte_count)..end];
    const head = write_bytes[byte_count - 1] & ((@as(u8, 1) << bit_shift) - 1);

    var write_value = (@as(StoreInt, @as(uN, @bitCast(value))) << bit_shift) | @as(StoreInt, @intCast(head));
    if (bit_shift > store_tail_bits) {
        const tail_len = @as(Log2N, @intCast(bit_shift - store_tail_bits));
        write_bytes[0] &= ~((@as(u8, 1) << @as(u3, @intCast(tail_len))) - 1);
        write_bytes[0] |= @as(u8, @intCast((@as(uN, @bitCast(value)) >> (@as(Log2N, @truncate(bit_count)) -% tail_len))));
    } else if (bit_shift < store_tail_bits) {
        const tail_len = store_tail_bits - bit_shift;
        const tail = write_bytes[0] & (@as(u8, 0xfe) << (7 - tail_len));
        write_value |= @as(StoreInt, tail) << (8 * (store_size - 1));
    }

    writeInt(StoreInt, write_bytes[(byte_count - store_size)..][0..store_size], write_value, .big);
}