AT Chart Rule Set

There has been considerable confusion as to how the rules work, so I've decided to create this document to specify exactly how AT Chart functions.

Eligibility
Any kind of music is eligible to enter or re-enter as long as it has been either (1a) released or (1b) given a new official video within 3 months of the date of the previous chart and is (2) not recurrent. For example, to be eligible to enter in the chart dated 26/8/2018, music must have either been released or given a new video no earlier than 19/5/2018.

There exists a "lower table" or "bubbling under" comprising positions 31 through 40. Prior to 2017, this was visible and could vary in size such that, on some occasions, positions upwards of 50 were tracked. It still has a special property in that it "extends" eligibility -- Everything not affected by recurrency is always eligible for as long as it remains in the top 40.

Starting in mid-2016 and continuing for part of 2017 was a rule that no song could re-enter more than once -- after re-entering, songs which fell off a second time were deleted. However, subsequent changes to the rules made re-entries relatively rare and so this rule was lifted.

Recurrency
Starting in mid-2017, songs become recurrent and are deleted permanently if, after being on for more than 10 weeks, they are both (1) outside the top 13 and (2) would have otherwise fallen. Anything in the top 13 is always safe, and something below the top 13 can be safe is it either rises or does not move. If a song old enough to be recurrent leaves the top 30 entirely, meaning that it still would have left without the recurrency rule, it is marked recurrent anyway and is deleted rather than being placed in the lower table. Finally, because the weeks counter is not incremented outside the top 30, anything in the lower table is safe from recurrency.

Votes
For votes with a specified rank order, which form the majority of votes, 10 votes per person can be counted. If the vote contains ineligible songs, either through being too old, recurrent, or not real songs, these will be disregarded and the rest of the songs in the vote will be shifted up by one position, unless the user specifies something else to replace ineligible songs with. In cases where more than 10 songs are given, the surplus songs can be given the now-empty lowest votes when ineligible songs are removed to bring the total back up to 10. If somebody attempts to vote for 11 or more songs, but one of the top 10 is ineligible, then the #11 song, if it is eligible, will take the #10 vote.

Normal Points
The "raw normal points" granted by a vote are given by the following formula:

$$\frac{400}{X+3}$$

Where X is the position of the vote. This gives the following (approximate) values:

Votes will be assumed to be ranked even if no numbers are given unless there is clear evidence that they're not meant to be ranked, for instance, if they're listed in the order of where they appear on the chart. In this case, all songs will be given an equal weight, and they will be given an illusory "position" such that the sum of all points will be weaker than they would be if the votes were ranked.

Normal points determine a minimum of 85% of the total chart. Normal points are applied for 4 weeks, and decay linearly -- for instance, a #2 vote will contribute 80 normal points in the first week it applies, 60 points in the second week, 40 points in the third week and 20 points in the fourth.

Bonus Points
Up to 15% of the points can be "bonus points". Songs are eligible for bonus points if they have never reached the top 30, or in special cases if a song falls off but is later given a very significant new video (an example of this is Sad- XXXTentacion). Their role is make it easier for new songs to enter -- it is very difficult for songs to enter or re-enter based on normal points alone because established songs have several past weeks which also contribute normal points. Re-entries with no bonus points do occasionally occur regardless (one instance of that was You Can Cry- Marshmello). These points are given in addition to normal points and are determined by the same formula.

Bonus points decay differently to normal points. They are only applied for 2 weeks, and, unlike normal points, the previous week's points are not simply included as part of the total bonus points. Instead, it works like this:


 * The raw normal points are all scaled such that they sum to 85%.
 * If the total raw new bonus points, excluding bonus points from the previous week, is 450 or more, they are scaled such that they sum to 10%.
 * Otherwise, they will sum to less than 10%, such that the points correspond to the same percentage that they would had the total raw new bonus points been 450. For instance, if the total raw new bonus points for a given week are 360, then they will be scaled such that they only sum to 8%.
 * The previous week's bonus points are then scaled by the same procedure such that they sum to no more than 5%.
 * Then, all of the above are added together to give a final point value.

This is the reason why, in 2016-era videos in which points were visible, they were expressed as a percentage.

Finally, if a song receives bonus points for two consecutive weeks, then the bonus points from the first week are wasted and are not included in the song's total.