namespace Cobra.Core

	class HashCodeUtils
		"""
		Provides convenient .combine methods for combining hash codes.
		
		The default implementation here is good enough for many applications, but if your
		application is heavily affected by hash codes then you will want to do your own hash code
		study and choose your own algorithm.
		
		to-do: should this be switched to the "Jenkins One at a time hash"?
			see: http://stackoverflow.com/questions/1079192/is-it-possible-to-combine-hash-codes-for-private-members-to-generate-a-new-hash
		"""

		test
			assert .combine(3, 4) == .combine(3, 4)
			assert .combine(3, 4) <> .combine(4, 3)
			assert .combine(6, 7) <> .combine(8, 9)

		shared

			pro defaultStart from var = 37
			
			pro defaultFactor from var = 23
			
			def combine(hashCodes as vari int) as int
				"""
				Accepts variable number of args such as:
					HashCodeUtils.combine(a.getHashCode, b.getHashCode, c.hashCode)
				"""
				return .combine(hashCodes, _defaultStart, _defaultFactor)

			def combine(hashCodes as int*) as int
				"""
				Accepts a stream of hash codes (int*, lists, arrays, IEnumerable<of int>,
				Iterable<of int>, etc.).
				"""
				return .combine(hashCodes, _defaultStart, _defaultFactor)
			
			def combine(hashCodes as int*, start as int, factor as int) as int
				hash = start
				for code in hashCodes
					hash = hash * factor + code
				return hash