problem8.go

// Copyright 2014 Peter Mrekaj. All rights reserved.
// Use of this source code is governed by a MIT-style
// license that can be found in the LICENSE.txt file.

package euler

import "fmt"

// Problem8 is solution for finding the thirteen adjacent digits
// in the 1000-digit number that have the greatest product.
func Problem8() (int, error) {
	const cnt = 13
	const num = "73167176531330624919225119674426574742355349194934" +
		"96983520312774506326239578318016984801869478851843" +
		"85861560789112949495459501737958331952853208805511" +
		"12540698747158523863050715693290963295227443043557" +
		"66896648950445244523161731856403098711121722383113" +
		"62229893423380308135336276614282806444486645238749" +
		"30358907296290491560440772390713810515859307960866" +
		"70172427121883998797908792274921901699720888093776" +
		"65727333001053367881220235421809751254540594752243" +
		"52584907711670556013604839586446706324415722155397" +
		"53697817977846174064955149290862569321978468622482" +
		"83972241375657056057490261407972968652414535100474" +
		"82166370484403199890008895243450658541227588666881" +
		"16427171479924442928230863465674813919123162824586" +
		"17866458359124566529476545682848912883142607690042" +
		"24219022671055626321111109370544217506941658960408" +
		"07198403850962455444362981230987879927244284909188" +
		"84580156166097919133875499200524063689912560717606" +
		"05886116467109405077541002256983155200055935729725" +
		"71636269561882670428252483600823257530420752963450"

	gp := 0
	ad := make([]int, cnt)
	for i, d := range num {
		ad[i%cnt] = int(d - '0') // i%cnt is a ring buffer.
		gpc, ok := MulInts(ad)
		if !ok {
			return 0, fmt.Errorf("multiplication overflows on %d\n", ad)
		}
		if gpc > gp {
			gp = gpc
		}
	}
	return gp, nil
}