import "fmt"

type ErrNegativeSqrt float64

func (e ErrNegativeSqrt) Error() string {
    return fmt.Sprintf("cannot Sqrt negative number: %g\n", float64(e))
}

// Sqrt calculates the square root of a number.
// If given negative number it returns an error.
func Sqrt(x float64) (float64, error) {
    if x < 0 {
        return 0, ErrNegativeSqrt(x)
    } else {
        z := float64(x)
        for i := 0; i < 100; i++ {
            z -= (z*z - x) / (2 * z)
        }
        return z, nil
    }
}

import (
    "math"
    "testing"
)

func TestSqrt(t *testing.T) {
    var tests = map[float64]float64{
        3:  math.Sqrt(3),
        2:  math.Sqrt(2),
        1:  math.Sqrt(1),
        0:  math.Sqrt(0),
        4:  math.Sqrt(4),
        5:  math.Sqrt(5),
        -5: 0,
        -1: 0,
    }
    precision := 0.00000001

    for key, expectedVal := range tests {
        val, _ := Sqrt(key)
        if val < expectedVal-precision || val > expectedVal+precision {
            t.Error(
                "For", key,
                "expected", expectedVal,
                "got", val,
            )
        }
    }
}