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readPackedIntBig

mem.readPackedIntBig

fn readPackedIntBig(comptime T: type, bytes: []const u8, bit_offset: usize) T

File

lib/std/mem.zig:1961

Code

fn readPackedIntBig(comptime T: type, bytes: []const u8, bit_offset: usize) T {
    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 = @divCeil(@as(usize, bit_shift) + bit_count, 8);

    const load_size = @divCeil(bit_count, 8);
    const load_tail_bits = @as(u3, @intCast((load_size * 8) - bit_count));
    const LoadInt = @Int(.unsigned, load_size * 8);

    if (bit_count == 0)
        return 0;

    // Read by loading a LoadInt, and then follow it up with a 1-byte read
    // of the tail if bit_offset pushed us over a byte boundary.
    const end = bytes.len - (bit_offset / 8);
    const read_bytes = bytes[(end - byte_count)..end];
    const val = @as(uN, @truncate(readInt(LoadInt, bytes[(end - load_size)..end][0..load_size], .big) >> bit_shift));
    if (bit_shift > load_tail_bits) {
        const tail_bits = @as(Log2N, @intCast(bit_shift - load_tail_bits));
        const tail_byte = if (bit_count < 8) @as(uN, @truncate(read_bytes[0])) else @as(uN, read_bytes[0]);
        return @bitCast(val | (tail_byte << (@as(Log2N, @truncate(bit_count)) -% tail_bits)));
    } else {
        return @bitCast(val);
    }
}