NSArray.partition()

NSArray+Underscore.m

@@ -4,6 +4,16 @@
 
 @implementation NSArray (Underscore)
 
+- (NSArray *(^)(id o))partition {
+    return ^NSArray *(BOOL(^block)(id o)) {
+        id aa = [NSMutableArray new];
+        id bb = [NSMutableArray new];
+        for (id o in self)
+            [block(o) ? aa : bb addObject:o];
+        return @[aa, bb];
+    };
+}
+
 - (NSArray *(^)(NSString *))pluck {
     return ^NSArray *(NSString *key) {
         if (!key.length)

README.markdown

@@ -461,6 +461,17 @@ code.
 
 `has` was chosen over `contains` or `includes` because it is short and clear.
 
+###NSArray.partition()
+```objc
+id rv = @[@"A", @"B", @"AA"].partition(^(NSString *s){
+    return [s hasPrefix:@"A"];
+});
+//rv => @[@[@"A", @"AA"], @[@"B"]]
+```
+
+Partitions an array into two arrays based on the boolean return value of your
+block.
+
 ###NSDictionary.map()
 ```objc
 id rv = @{@1: @2, @2: @4, @3: @9}.map(^(id key, id obj){

YOLO.h

@@ -27,6 +27,7 @@
 @end
 
 @interface NSArray (Underscore)
+- (NSArray *(^)(id blockReturningBool))partition;
 - (NSArray *(^)(NSString *keypath))pluck;
 - (NSArray *(^)(id arrayOrSetOrObject))without;
 @end

tests

@@ -491,6 +491,20 @@ int main() {
     STAssertEqualObjects(aa, bb, @"");
 }
 
+- (void)testPartition {
+    id aa = @[@1, @2, @3, @4].partition(^(NSNumber *n){
+        return n.intValue % 2 == 0;
+    });
+    id bb = @[@[@2, @4], @[@1, @3]];
+    STAssertEqualObjects(aa, bb, @"");
+    
+    aa = @[@"A", @"B", @"AA"].partition(^(NSString *s){
+        return [s hasPrefix:@"A"];
+    });
+    bb = @[@[@"A", @"AA"], @[@"B"]];
+    STAssertEqualObjects(aa, bb, @"");
+}
+
 @end