20

cmd/20/main.go

@@ -0,0 +1,22 @@
+package main
+
+import(
+	"fmt"
+	ds "euler/pkg/datastructures"
+)
+
+func main() {
+	bi := ds.NewBigInt(1)
+	for i := 1; i <= 100; i += 1 {
+		bi.Multiply(i)
+	}
+
+	fmt.Printf("%s\n", bi.Stringify())
+
+	var sum uint64 = 0
+	for _, digit := range bi {
+		sum += uint64(digit)
+	}
+
+	fmt.Printf("sum: %d\n", sum)
+}

pkg/datastructures/bigint.go

@@ -0,0 +1,41 @@
+package datastructures
+
+import "fmt"
+
+type BigInt []int
+
+func NewBigInt(i int) BigInt {
+	num := i
+	bi := []int{}
+	for num > 0 {
+		bi = append(bi, num % 10)
+		num = num / 10
+	}
+
+	return bi
+}
+
+func (bi *BigInt) Multiply(i int) BigInt {
+	var carry int = 0
+	for index, digit := range *bi {
+		product := (digit * i) + carry
+		carry = product / 10
+		(*bi)[index] = product % 10
+	}
+
+	for carry > 0 {
+		*bi = append(*bi, carry % 10)
+		carry = carry / 10
+	}
+
+	return *bi
+}
+
+func (bi BigInt) Stringify() string {
+	out := ""
+	for i := len(bi)-1; i >= 0; i -= 1 {
+		out += fmt.Sprintf("%d", bi[i])
+	}
+
+	return out
+}