Bringing the sliding window metaphor to RACStreams (RACSequences, RACSignals, etc).

RACStream+SlidingWindow.m

//
//  RACStream+SlidingWindow.m
//  Bringing the sliding window stream metaphor to RACStreams (RACSequences, RACSignals, etc).
//  Copyright © 2015 Talus Baddley. MIT License.
//
//  This is heavily UNTESTED. Please contact me if you find bugs.
//

#import <ReactiveCocoa/ReactiveCocoa.h>

@interface RACStream (SlidingWindow)

//
// Takes a sliding window of the receiver, with as many as <size> elements,
// but no fewer than <minCount> elements. Each element in the stream is an NSArray.
//
// For instance, the sequence
//     --(1)--(2)----(3)------(4)------(5)------
// with size 3 and minimum 2, produces arrays
//     -------[1,2]--[1,2,3]--[2,3,4]--[3,4,5]--
//
- (instancetype)slidingWindowSized:(NSUInteger)size minimumCount:(NSUInteger)minCount;

//
// Takes a sliding window as above, with no minimum.
- (instancetype)slidingWindowSized:(NSUInteger)size;

//
// Takes a sliding window of the receiver at exactly <size> elements wide. Each element in
// the stream is a RACTuple. If there are fewer than <size> elements in the window, nils pad
// the head of the tuple. (If there are fewer than <minCount> elements, no tuple is produced.)
//
// For instance, the sequence
//     --(1)----------(2)--------(3)------(4)------(5)------
// with size 3 and minimum 1, produces tuples
//     --[nil,nil,1]--[nil,1,2]--[1,2,3]--[2,3,4]--[3,4,5]--
//
- (instancetype)nilPaddedSlidingWindowSized:(NSUInteger)size minimumCount:(NSUInteger)minCount;

@end


@implementation RACStream (SlidingWindow)

- (instancetype)slidingWindowSized:(NSUInteger)size {
    return [self slidingWindowSized:size minimumCount:0];
}

- (instancetype)slidingWindowSized:(NSUInteger)size minimumCount:(NSUInteger)minCount {
    // Based on https://github.com/baconjs/bacon.js/blob/master/src/slidingwindow.coffee

    return [[self
             scanWithStart:@[] reduce:^id(NSArray *running, id next) {
                 NSRange sliceRange = ([running count] >= size ?
                                       NSMakeRange(1, size) :
                                       NSMakeRange(0, [running count] + 1)
                                       );
                 
                 return [[running arrayByAddingObject:next] subarrayWithRange:sliceRange];
                 
             }]
            filter:^BOOL(NSArray *window) {
                return [window count] >= minCount;
            }];
}

- (instancetype)nilPaddedSlidingWindowSized:(NSUInteger)size minimumCount:(NSUInteger)minCount {
    return [[self slidingWindowSized:size minimumCount:minCount]
            map:^id(NSArray *flexWindow) {
                
                NSInteger sizeDiff = size - [flexWindow count];
                NSMutableArray *buf = [NSMutableArray arrayWithCapacity:size];
                for (NSUInteger fuckingManualIteration = 0; fuckingManualIteration < sizeDiff; fuckingManualIteration++) {
                    [buf addObject:[RACTupleNil tupleNil]];
                }
                // Or, as you'd say in Ruby, Array.new(sizeDiff, nil)
                
                [buf addObjectsFromArray:flexWindow];
                return [RACTuple tupleWithObjectsFromArray:buf];
                
            }];
}

@end