// Package crc32combine provides functionality to calculate the combined
// CRC-32 hash of two given CRC-32 hash values.
package crc32combine

// Ported from C to Go in 2016 by Justin Ruggles, with minimal alteration.
// Used uint for unsigned long. Used uint32 for input arguments in order to match
// the Go hash/crc32 package. zlib CRC32 combine (https://github.com/madler/zlib)

func gf2MatrixTimes(mat []uint, vec uint) uint {
	var sum uint

	for vec != 0 {
		if vec&1 != 0 {
			sum ^= mat[0]
		}
		vec >>= 1
		mat = mat[1:]
	}
	return sum
}

func gf2MatrixSquare(square, mat []uint) {
	for n := 0; n < 32; n++ {
		square[n] = gf2MatrixTimes(mat, mat[n])
	}
}

// CRC32Combine returns the combined CRC-32 hash value of the two passed CRC-32
// hash values crc1 and crc2. poly represents the generator polynomial
// and len2 specifies the byte length that the crc2 hash covers.
func CRC32Combine(poly uint32, crc1, crc2 uint32, len2 int64) uint32 {
	// degenerate case (also disallow negative lengths)
	if len2 <= 0 {
		return crc1
	}

	even := make([]uint, 32) // even-power-of-two zeros operator
	odd := make([]uint, 32)  // odd-power-of-two zeros operator

	// put operator for one zero bit in odd
	odd[0] = uint(poly) // CRC-32 polynomial
	row := uint(1)
	for n := 1; n < 32; n++ {
		odd[n] = row
		row <<= 1
	}

	// put operator for two zero bits in even
	gf2MatrixSquare(even, odd)

	// put operator for four zero bits in odd
	gf2MatrixSquare(odd, even)

	// apply len2 zeros to crc1 (first square will put the operator for one
	// zero byte, eight zero bits, in even)
	crc1n := uint(crc1)
	for {
		// apply zeros operator for this bit of len2
		gf2MatrixSquare(even, odd)
		if len2&1 != 0 {
			crc1n = gf2MatrixTimes(even, crc1n)
		}
		len2 >>= 1

		// if no more bits set, then done
		if len2 == 0 {
			break
		}

		// another iteration of the loop with odd and even swapped
		gf2MatrixSquare(odd, even)
		if len2&1 != 0 {
			crc1n = gf2MatrixTimes(odd, crc1n)
		}
		len2 >>= 1

		// if no more bits set, then done
		if len2 == 0 {
			break
		}
	}

	// return combined crc
	crc1n ^= uint(crc2)
	return uint32(crc1n)
}