private static CursorPagingRange SliceRange(
CursorPagingArguments arguments,
int maxElementCount)
{
// [SPEC] if after is set then remove all elements of edges before and including
// afterEdge.
//
// The cursor is increased by one so that the index points to the element after
var startIndex = arguments.After is { } a
? IndexEdge <TEntity> .DeserializeCursor(a) + 1
: 0;
// [SPEC] if before is set then remove all elements of edges before and including
// beforeEdge.
var before = arguments.Before is { } b
? IndexEdge <TEntity> .DeserializeCursor(b)
: maxElementCount;
// if after is negative we have know how much of the offset was in the negative range.
// The amount of positions that are in the negative range, have to be subtracted from
// the take or we will fetch too many items.
var startOffsetCorrection = 0;
if (startIndex < 0)
{
startOffsetCorrection = Math.Abs(startIndex);
startIndex = 0;
}
CursorPagingRange range = new(startIndex, before);
//[SPEC] If first is less than 0 throw an error
ValidateFirst(arguments, out var first);
if (first is not null)
{
first -= startOffsetCorrection;
if (first < 0)
{
first = 0;
}
}
//[SPEC] Slice edges to be of length first by removing edges from the end of edges.
range.Take(first);
//[SPEC] if last is less than 0 throw an error
ValidateLast(arguments, out var last);
//[SPEC] Slice edges to be of length last by removing edges from the start of edges.
range.TakeLast(last);
return(range);
}
public static ICursorPageSlice <T> SliceAsCursorPage <T>(this IEnumerable <T> items, string?after, int?first, string?before, int?last)
where T : class
{
//Do nothing if there are no results...
if (!items.Any())
{
return(new CursorPageSlice <T>(Enumerable.Empty <ICursorResult <T> >(), 0, false, false));
}
var afterIndex = after != null
? IndexEdge <string> .DeserializeCursor(after)
: 0;
var beforeIndex = before != null
? IndexEdge <string> .DeserializeCursor(before)
: 0;
//FIRST log the index of all items in the list BEFORE slicing, as these indexes are
// the Cursor Indexes for paging up/down the entire list, & ICursorResult is the Decorator
// around the Entity Models.
//NOTE: We MUST materialize this after applying index values to prevent ongoing increments...
int index = 0;
IEnumerable <ICursorResult <T> > slice = items
.Select(c => new CursorResult <T>(c, ++index))
.ToList();
int totalCount = slice.Count();
//If After specified, remove all before After (or skip past After)
if (afterIndex > 0 && slice.Last().CursorIndex > afterIndex)
{
slice = slice.Skip(afterIndex);
}
//If Before is specified, remove all after Before (Skip Until Before is reached)
if (beforeIndex > 0 && slice.Last().CursorIndex > beforeIndex)
{
slice = slice.SkipWhile(c => c.CursorIndex < beforeIndex);
}
//If First is specified, then take the first/top rows from the current Slice!
if (first.HasValue && first > 0 && slice.Count() > first)
{
slice = slice.Take(first.Value);
}
//If First is specified, then take the first/top rows from the current Slice!
if (last.HasValue && last > 0 && slice.Count() > last)
{
slice = slice.TakeLast(last.Value);
}
//Wrap all results into a PagedCursor Slice result wit Total Count...
//NOTE: to ensure our pagination is complete, we materialize the Results!
var results = slice.ToList();
var firstCursor = results.FirstOrDefault();
var lastCursor = results.LastOrDefault();
var cursorPageSlice = new CursorPageSlice <T>(
results,
totalCount,
hasPreviousPage: firstCursor?.CursorIndex > 1,
hasNextPage: lastCursor?.CursorIndex < totalCount
);
return(cursorPageSlice);
}
