turbonss/src/padding.zig

const std = @import("std");
const assert = std.debug.assert;

// rounds up an int to the nearest factor of nbits.
pub fn roundUp(comptime T: type, comptime nbits: u8, n: T) T {
    comptime assert(nbits < @bitSizeOf(T));
    const factor = comptime (1 << nbits) - 1;
    return ((n + factor) & ~@as(T, factor));
}

// rounds up an integer to the nearest factor of nbits and returns the
// difference (padding)
pub fn roundUpPadding(comptime T: type, comptime nbits: u8, n: T) T {
    return roundUp(T, nbits, n) - n;
}

const testing = std.testing;

test "padding" {
    try testing.expectEqual(roundUpPadding(u12, 2, 0), 0);
    try testing.expectEqual(roundUpPadding(u12, 2, 1), 3);
    try testing.expectEqual(roundUpPadding(u12, 2, 2), 2);
    try testing.expectEqual(roundUpPadding(u12, 2, 3), 1);
    try testing.expectEqual(roundUpPadding(u12, 2, 4), 0);
    try testing.expectEqual(roundUpPadding(u12, 2, 40), 0);
    try testing.expectEqual(roundUpPadding(u12, 2, 41), 3);
    try testing.expectEqual(roundUpPadding(u12, 2, 42), 2);
    try testing.expectEqual(roundUpPadding(u12, 2, 43), 1);
    try testing.expectEqual(roundUpPadding(u12, 2, 44), 0);
    try testing.expectEqual(roundUpPadding(u12, 2, 4091), 1);
    try testing.expectEqual(roundUpPadding(u12, 2, 4092), 0);
}