Logarithm of 2 (Algorithm)

Logarithm.go

package main

import "fmt"
import "math"

func log2(f float64) (float64) {
	frac, exp := math.Frexp(f)
	exp--
	frac*=2
	log := float64(0)
	part := float64(1)
	for i := 0 ; i<32; i++ {
		/* frac >= 2.0 */
		if frac>1.999999999999 {
			log += part
			frac /= 2
		}
		
		frac *= frac
		part /= 2
	}
	
	return float64(exp)+log
}

func main(){
	for i := 0; i<11; i++ {
		f := float64(i)/10.0
		e := math.Exp2(f)
		fmt.Printf("%f\t%f\t%f\n",f,math.Log2(e),log2(e))
	}
}

More.go

package main

import "fmt"
import "math"
import "math/bits"

func log2(f float64) (float64) {
	frac, exp := math.Frexp(f)
	exp--
	frac*=2
	log := float64(0)
	part := float64(1)
	for i := 0 ; i<32 ; i++ {
		/* frac >= 2.0 */
		if frac>1.999999999999 {
			log += part
			frac /= 2
		}
		
		frac *= frac
		part /= 2
	}
	
	return float64(exp)+log
}

func main2(){
	for i := 0; i<11; i++ {
		f := float64(i)/10.0
		e := math.Exp2(f)
		fmt.Printf("%f\t%f\t%f\n",f,math.Log2(e),log2(e))
	}
}

func lshift(i uint64,s int) uint64 {
	if s<0 { return i >> uint(-s) }
	return i << uint(s)
}
func ilog2(i int) (float64) {
	const TWO = 1<<31
	exp := bits.Len(uint(i))
	exp--
	frac := lshift(uint64(i),30-exp)
	log := float64(exp)
	part := float64(1)
	for i := 0 ; i<30 ; i++ {
		if frac>=TWO {
			frac >>= 1
			log += part
		} else {
		}
		
		part /= 2
		frac = (frac*frac)>>30
	}
	return log
}

func ilog2n(i,n int) (int) {
	const TWO = 1<<31
	exp := bits.Len(uint(i))
	exp--
	frac := lshift(uint64(i),30-exp)
	log := exp*n
	part := n
	for i := 0 ; i<30 ; i++ {
		if frac>=TWO {
			frac >>= 1
			log += part
		} else {
		}
		
		part /= 2
		frac = (frac*frac)>>30
	}
	return log
}

func main(){
	for i := 1; i<10; i++ {
		f := float64(i)
		//fmt.Printf("%f\t%f\t%f\n",f,math.Log2(f),ilog2(i))
		fmt.Printf("%f\t%f\t%d\n",math.Log2(f),ilog2(i),ilog2n(i,100)/33)
		//fmt.Printf("%f\t%f\t%f\t%f\n",f,math.Log2(f),log2(f),ilog2(i))
	}
}