Function writeVarPackedInt [src]

Stores an integer to packed memory with provided bit_offset, bit_count, and signedness. If negative, the written value is sign-extended.

Prototype

 pub fn writeVarPackedInt(bytes: []u8, bit_offset: usize, bit_count: usize, value: anytype, endian: std.builtin.Endian) void

Parameters

bytes: []u8bit_offset: usizebit_count: usizeendian: std.builtin.Endian

Example

 test writeVarPackedInt {
    const T = packed struct(u16) { a: u3, b: u7, c: u6 };
    var st = T{ .a = 1, .b = 2, .c = 4 };
    const value: u64 = 0x7f;
    writeVarPackedInt(std.mem.asBytes(&st), @bitOffsetOf(T, "b"), 7, value, builtin.cpu.arch.endian());
    try testing.expectEqual(T{ .a = 1, .b = value, .c = 4 }, st);
}

Source

  pub fn writeVarPackedInt(bytes: []u8, bit_offset: usize, bit_count: usize, value: anytype, endian: std.builtin.Endian) void {
    const T = @TypeOf(value);
    const uN = std.meta.Int(.unsigned, @bitSizeOf(T));

    const bit_shift = @as(u3, @intCast(bit_offset % 8));
    const write_size = (bit_count + bit_shift + 7) / 8;
    const lowest_byte = switch (endian) {
        .big => bytes.len - (bit_offset / 8) - write_size,
        .little => bit_offset / 8,
    };
    const write_bytes = bytes[lowest_byte..][0..write_size];

    if (write_size == 0) {
        return;
    } else if (write_size == 1) {
        // Single byte writes are handled specially, since we need to mask bits
        // on both ends of the byte.
        const mask = (@as(u8, 0xff) >> @as(u3, @intCast(8 - bit_count)));
        const new_bits = @as(u8, @intCast(@as(uN, @bitCast(value)) & mask)) << bit_shift;
        write_bytes[0] = (write_bytes[0] & ~(mask << bit_shift)) | new_bits;
        return;
    }

    var remaining: T = value;

    // Iterate bytes forward for Little-endian, backward for Big-endian
    const delta: i2 = if (endian == .big) -1 else 1;
    const start = if (endian == .big) @as(isize, @intCast(write_bytes.len - 1)) else 0;

    var i: isize = start; // isize for signed index arithmetic

    // Write first byte, using a mask to protects bits preceding bit_offset
    const head_mask = @as(u8, 0xff) >> bit_shift;
    write_bytes[@intCast(i)] &= ~(head_mask << bit_shift);
    write_bytes[@intCast(i)] |= @as(u8, @intCast(@as(uN, @bitCast(remaining)) & head_mask)) << bit_shift;
    remaining = math.shr(T, remaining, @as(u4, 8) - bit_shift);
    i += delta;

    // Write bytes[1..bytes.len - 1]
    if (@bitSizeOf(T) > 8) {
        const loop_end = start + delta * (@as(isize, @intCast(write_size)) - 1);
        while (i != loop_end) : (i += delta) {
            write_bytes[@as(usize, @intCast(i))] = @as(u8, @truncate(@as(uN, @bitCast(remaining))));
            remaining >>= 8;
        }
    }

    // Write last byte, using a mask to protect bits following bit_offset + bit_count
    const following_bits = -%@as(u3, @truncate(bit_shift + bit_count));
    const tail_mask = (@as(u8, 0xff) << following_bits) >> following_bits;
    write_bytes[@as(usize, @intCast(i))] &= ~tail_mask;
    write_bytes[@as(usize, @intCast(i))] |= @as(u8, @intCast(@as(uN, @bitCast(remaining)) & tail_mask));
}  