explanation.pyth

L?-b=dsm?.&.<1db3 4hlbydb@mXdH1mydSQ0

L					create lambda function named y with param b
	?					ternary if
		-					minus used for expressing "if d != b"
			b					lambda param
			=					assign
				d					d =
				s					sum
					m					map
						?					if
							.&					bitwise and
							 	.<				bitshift
									1			1 literal
									d			map lambda param
								b				input
											then
						3						3
											else
						4						4
			
					 	hlb					2nd param of map - [0..log2(b)]

						ternary then
		yd					return y(d)
						ternary else
		b					return b
		
@					grab zeroth element from array
	m					map
		X					B[A] += C
			d					the map lambda param
			H					the default pyth H={}
			1					increment by 1
		m					map
			yd					y(d)
			SQ					[1..input]
	0					"zeroth element"