partition

fn partition(a: usize, b: usize, pivot: *usize, context: anytype) bool {
    // move pivot to the first place
    context.swap(a, pivot.*);

    var i = a + 1;
    var j = b - 1;

    while (i <= j and context.lessThan(i, a)) i += 1;
    while (i <= j and !context.lessThan(j, a)) j -= 1;

    // check if items are already partitioned (no item to swap)
    if (i > j) {
        // put pivot back to the middle
        context.swap(j, a);
        pivot.* = j;
        return true;
    }

    context.swap(i, j);
    i += 1;
    j -= 1;

    const block_size = 64;
    var offsets_l: [block_size]u8 align(std.atomic.cache_line) = undefined;
    var offsets_r: [block_size]u8 align(std.atomic.cache_line) = undefined;

    var offsets_l_base = i;
    var offsets_r_base = j;
    var num_l: usize = 0;
    var num_r: usize = 0;
    var start_l: usize = 0;
    var start_r: usize = 0;

    while (i <= j) {
        const num_unknown = j + 1 - i;
        const left_split = if (num_l == 0)
            @min(block_size, if (num_r == 0) num_unknown / 2 else num_unknown)
        else
            0;
        const right_split = if (num_r == 0)
            @min(block_size, num_unknown - left_split)
        else
            0;

        for (0..left_split) |k| {
            offsets_l[num_l] = @intCast(k);
            num_l += @intFromBool(!context.lessThan(i + k, a));
        }
        i += left_split;

        for (0..right_split) |k| {
            offsets_r[num_r] = @intCast(k);
            num_r += @intFromBool(context.lessThan(j - k, a));
        }
        j -= right_split;

        const num = @min(num_l, num_r);
        for (0..num) |m| {
            context.swap(
                offsets_l_base + offsets_l[start_l + m],
                offsets_r_base - offsets_r[start_r + m],
            );
        }
        num_l -= num;
        num_r -= num;
        start_l += num;
        start_r += num;

        if (num_l == 0) {
            start_l = 0;
            offsets_l_base = i;
        }
        if (num_r == 0) {
            start_r = 0;
            offsets_r_base = j;
        }
    }

    if (num_l > 0) {
        while (num_l > 0) {
            num_l -= 1;
            context.swap(offsets_l_base + offsets_l[start_l + num_l], j);
            j -= 1;
        }
        i = j + 1;
    }
    if (num_r > 0) {
        while (num_r > 0) {
            num_r -= 1;
            context.swap(offsets_r_base - offsets_r[start_r + num_r], i);
            i += 1;
        }
        j = i - 1;
    }

    context.swap(j, a);
    pivot.* = j;
    return false;
}