library: math/big.Int (mini-impl for _cmptest/bigintdemo)

_cmptest/bigintdemo/fib.go

@@ -0,0 +1,25 @@
+package main
+
+import (
+	"fmt"
+	"math/big"
+)
+
+func main() {
+	// Initialize two big ints with the first two numbers in the sequence.
+	a := big.NewInt(0)
+	b := big.NewInt(1)
+
+	// Initialize limit as 10^99, the smallest integer with 100 digits.
+	var limit big.Int
+	limit.Exp(big.NewInt(10), big.NewInt(99), nil)
+
+	// Loop while a is smaller than 1e100.
+	for a.Cmp(&limit) < 0 {
+		// Compute the next Fibonacci number, storing it in a.
+		a.Add(a, b)
+		// Swap a and b so that b is the next number in the sequence.
+		a, b = b, a
+	}
+	fmt.Println(a) // 100-digit Fibonacci number
+}