25: 1000-digit fibonacci number

25_1000_digit_fibonacci_number/main.go

@@ -0,0 +1,40 @@
+package main
+
+import (
+	"fmt"
+	"math/big"
+)
+
+func main() {
+	n := 11
+	for {
+		fb := fibonacci(n)
+		fmt.Println("index =", n, "len =", len(fb.String()))
+		if numberDigits(fb) >= 1000 {
+			break
+		}
+		n++
+	}
+
+	fmt.Println("n is", n)
+}
+
+func fibonacci(index int) *big.Int {
+	if index < 3 {
+		ret := (&big.Int{}).SetUint64(1)
+		return ret
+	}
+	fn := &big.Int{}
+	fn1 := (&big.Int{}).SetInt64(1)
+	fn2 := (&big.Int{}).SetInt64(1)
+	for i := 3; i <= index; i++ {
+		fn.Set(fn.Add(fn1, fn2))
+		fn2.Set(fn1)
+		fn1.Set(fn)
+	}
+	return fn
+}
+
+func numberDigits(num *big.Int) int {
+	return len(num.String())
+}