Fold and reduce for NSArray. Mostly an academic exercise.

NSArray+DHAccumulate.m

@import Foundation;

@implementation NSArray (DHAccumulate)

- (id)dh_objectByReducingWithOptions:(NSEnumerationOptions)options usingBlock:(id (^)(id object1, id object2))reductionBlock
{
	if (options & NSEnumerationConcurrent) {
		[NSException raise:NSInvalidArgumentException format:@"Bad things will happen if you try to reduce an array concurrently."];
	}
	
	__block id accumulatedValue;
	
	[self enumerateObjectsWithOptions:options usingBlock:^(id object, NSUInteger idx, BOOL *stop) {
		if (accumulatedValue == nil) {
			accumulatedValue = object;
			return;
		}
		accumulatedValue = reductionBlock(accumulatedValue, object);
		
		if (accumulatedValue == nil) {
			[NSException raise:NSInvalidArgumentException format:@"@s - reductionBlock must not return nil."];
		}
	}];
	
	if (accumulatedValue == nil) {
		[NSException raise:NSInvalidArgumentException format:@"%s must not be called on an empty array.", __PRETTY_FUNCTION__];
	}
	
	return accumulatedValue;
}

- (id)dh_valueByFoldingWithInitialValue:(id)initialValue options:(NSEnumerationOptions)options usingBlock:(id (^)(id accumulatedValue, id object))foldBlock
{
	if (options & NSEnumerationConcurrent) {
		[NSException raise:NSInvalidArgumentException format:@"Bad things will happen if you try to fold an array concurrently."];
	}
	
	__block id accumulatedValue = initialValue;
	
	[self enumerateObjectsWithOptions:options usingBlock:^(id object, NSUInteger idx, BOOL *stop) {
		accumulatedValue = foldBlock(accumulatedValue, object);
	}];
	
	return accumulatedValue;
}

- (NSInteger)dh_integerByFoldingWithInitialValue:(NSInteger)initialValue options:(NSEnumerationOptions)options usingBlock:(NSInteger (^)(NSInteger accumulatedValue, id object))foldBlock
{
	if (options & NSEnumerationConcurrent) {
		[NSException raise:NSInvalidArgumentException format:@"Bad things will happen if you try to fold an array concurrently."];
	}
	
	__block NSInteger accumulatedValue = initialValue;
	
	[self enumerateObjectsWithOptions:options usingBlock:^(id object, NSUInteger idx, BOOL *stop) {
		accumulatedValue = foldBlock(accumulatedValue, object);
	}];
	
	return accumulatedValue;
}

- (BOOL)dh_booleanByFoldingWithInitialValue:(BOOL)initialValue options:(NSEnumerationOptions)options usingBlock:(BOOL (^)(BOOL accumulatedValue, id object))foldBlock
{
	if (options & NSEnumerationConcurrent) {
		[NSException raise:NSInvalidArgumentException format:@"Bad things will happen if you try to fold an array concurrently."];
	}
	
	__block BOOL accumulatedValue = initialValue;
	
	[self enumerateObjectsWithOptions:options usingBlock:^(id object, NSUInteger idx, BOOL *stop) {
		accumulatedValue = foldBlock(accumulatedValue, object);
	}];
	
	return accumulatedValue;
}

@end