Custom paging algorithm to calculate pages to display
I'm working on a custom data pager for a custom google maps control. The control needs to work out what range of pages to display. For example, if the user is on page 6 then the control must display pages 1 through to 10. If the user is on page 37, then the control must display pages 30 throught to 40.
The variables I have available are:
X - Total results (points on the map)
Y - The current pag开发者_StackOverflow社区e size. i.e. the amount of points per page. Z - The current page being displayed Q - The number of page numbers to display (a const of 10)
I have come up with:
Starting Index = Z - (Z % Q)
Ending Index = Z - (Z % Q) + Q
This, however, doesn't work for when the current page is less than 10. It also doesn't figure out whether there is a max page reached, i.e. we always display a full range of 10. However, if we display the range 30-40 the final page could actually be 38.
If anyone can come up with a more elegant algorithm it would be appreciated.
It might be easier if you think in terms of chapters.
Say each set of pages is a chapter, chapter are numbered starting from 0, 1, 2,...
Then the rth chapter has pages in the range
Qr + 1 <= page <= Q(r+1)
Now consider floor(page/Q). This is r if page is not a multiple of Q, otherwise it is r+1.
Given an r, you can find out the pages of the chapter as Lower = Qr + 1 and higher = min(max, Q(r+1)).
So you can do this.
if (Z < 1 || Z > max_page) { error;}
if (Z % Q == 0) {
r = Z/Q - 1; // integer division, gives floor.
}
else {
r = Z/Q; // floor.
}
Begin = Q*r + 1;
End = Min (Q*(r+1), max_page);
To get rid of the if, you can now replace it with
if (Z < 1 || Z > max_page) { error;}
r = (Z-1)/Q;
Begin = Q*r + 1;
End = Min (Q*(r+1), max_page);
This works because:
Qr + 1 <= Z <= Q(r+1) if and only if
Qr <= Z-1 <= Qr + (Q-1).
Thus floor((Z-1)/Q) = r.
Here we go:
def lower(Z):
return (Z - 1) // Q * Q + 1
def upper(Z):
return min(int(ceil(X / Y)), ((Z - 1) // Q + 1) * Q)
//
is integer division.
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