diff --git a/go.mod b/go.mod
index fd11da7..3be8798 100644
--- a/go.mod
+++ b/go.mod
@@ -9,6 +9,7 @@ require (
 
 require (
 	git.sr.ht/~sbinet/gg v0.5.0 // indirect
+	github.com/Knetic/govaluate v3.0.0+incompatible // indirect
 	github.com/ajstarks/svgo v0.0.0-20211024235047-1546f124cd8b // indirect
 	github.com/campoy/embedmd v1.0.0 // indirect
 	github.com/ebitengine/purego v0.5.0 // indirect
diff --git a/go.sum b/go.sum
index 3acb0f1..90ff35c 100644
--- a/go.sum
+++ b/go.sum
@@ -3,6 +3,8 @@ git.sr.ht/~sbinet/cmpimg v0.1.0/go.mod h1:FU12psLbF4TfNXkKH2ZZQ29crIqoiqTZmeQ7dk
 git.sr.ht/~sbinet/gg v0.5.0 h1:6V43j30HM623V329xA9Ntq+WJrMjDxRjuAB1LFWF5m8=
 git.sr.ht/~sbinet/gg v0.5.0/go.mod h1:G2C0eRESqlKhS7ErsNey6HHrqU1PwsnCQlekFi9Q2Oo=
 github.com/BurntSushi/toml v0.3.1/go.mod h1:xHWCNGjB5oqiDr8zfno3MHue2Ht5sIBksp03qcyfWMU=
+github.com/Knetic/govaluate v3.0.0+incompatible h1:7o6+MAPhYTCF0+fdvoz1xDedhRb4f6s9Tn1Tt7/WTEg=
+github.com/Knetic/govaluate v3.0.0+incompatible/go.mod h1:r7JcOSlj0wfOMncg0iLm8Leh48TZaKVeNIfJntJ2wa0=
 github.com/ajstarks/deck v0.0.0-20200831202436-30c9fc6549a9/go.mod h1:JynElWSGnm/4RlzPXRlREEwqTHAN3T56Bv2ITsFT3gY=
 github.com/ajstarks/deck/generate v0.0.0-20210309230005-c3f852c02e19/go.mod h1:T13YZdzov6OU0A1+RfKZiZN9ca6VeKdBdyDV+BY97Tk=
 github.com/ajstarks/svgo v0.0.0-20211024235047-1546f124cd8b h1:slYM766cy2nI3BwyRiyQj/Ud48djTMtMebDqepE95rw=
diff --git a/main.go b/main.go
index e39985b..1e9fb1f 100644
--- a/main.go
+++ b/main.go
@@ -1,28 +1,57 @@
 package main
 
 import (
+	"fmt"
 	"image"
 	"log"
+	"math"
 	"time"
 
 	"github.com/hajimehoshi/ebiten/v2"
 )
 
-func f(x float64) float64  { return x*x + 5*x - 3 }
-func df(x float64) float64 { return 2*x + 5 }
+func f(x float64) float64 { return math.Cos(x*x + 3*x + 97) } // FUCTION CONTROL
+
+// TODO: use govaluate to get function as user input
+// i can get string from user, parse it to function, but then what?
+
+func derivative(f func(float64) float64) func(float64) float64 {
+	return func(x float64) float64 {
+		dx := 0.00000000005          // dx -> 0
+		return (f(x+dx) - f(x)) / dx //literally todays math lesson.
+	}
+}
+func GradientDescent(f func(float64) float64, x0, alpha float64) float64 {
+	x := x0
+	var stop bool
+	for !stop {
+		x -= alpha * derivative(f)(x)
+		if derivative(f)(x) == 0 || math.Abs(derivative(f)(x)) < 0.00001 {
+			stop = true
+		}
+	}
+	return x
+}
 
 func main() {
 	ebiten.SetWindowSize(640, 480)
 	ebiten.SetWindowTitle("Gradient descent")
 
+	fmt.Println(GradientDescent(f, -math.Pi/6, 0.01)) //CONTROL X0 AND ALPHA HERE FOR PRINTLN OUTPUT
+
 	img := make(chan *image.RGBA, 1)
 	go func() {
-		p := Plot(-5, 0, 0.1, f)
-		x := 0.0
+		p := Plot(-math.Pi, math.Pi/3, 0.01, f) //CONTROL RANGE AND STEP HERE
+		x := -math.Pi / 6                       //CONTROL X0 HERE FOR GRAPHICS
 		img <- p(x)
-		for i := 0; i < 50; i++ {
+		var stop bool
+		for !stop {
 			time.Sleep(30 * time.Millisecond)
-			x -= df(x) * 0.1
+			x -= 0.1 * derivative(f)(x) // CONTROL ALPHA HERE FOR GRAPHICS
+			// if you put alpha = 0.9, you wil get badly animated film about snake :)
+			if derivative(f)(x) == 0 || math.Abs(derivative(f)(x)) < 0.00001 {
+				stop = true
+			}
 			img <- p(x)
 		}
 	}()
diff --git a/main_test.go b/main_test.go
new file mode 100644
index 0000000..2ec80b5
--- /dev/null
+++ b/main_test.go
@@ -0,0 +1,62 @@
+package main
+
+import (
+	"math"
+	"testing"
+)
+
+func TestDerivative(t *testing.T) {
+	for _, tc := range []struct {
+		name string
+		f    func(float64) float64
+		x    float64
+		want float64
+	}{
+		{name: "constant", f: func(x float64) float64 { return 5 }, x: 0.0, want: 0.0},
+		{name: "linear", f: func(x float64) float64 { return 2*x + 5 }, x: 0.0, want: 2.0},
+		{name: "simple square", f: func(x float64) float64 { return x * x }, x: 2.0, want: 4.0},
+		{name: "not so simple square", f: func(x float64) float64 { return x*x + 5*x - 3 }, x: 1.0, want: 7.0},
+		{name: "hyperbola", f: func(x float64) float64 { return 1 / x }, x: 2.0, want: -0.25},
+		{name: "diff.135 lpp. N18 a)", f: func(x float64) float64 { return 2*x*x - 3*x }, x: 3, want: 9},
+		{name: "diff.135 lpp. N18 b)", f: func(x float64) float64 { return x*x - x + 2 }, x: 0, want: -1},
+		{name: "diff.135 lpp. N18 c)", f: func(x float64) float64 { return 4 - 5*x - x*x }, x: 1, want: -7},
+		{"diff.135 lpp. N18 d)", func(x float64) float64 { return -3 / x }, 3, 0.3333333}, //1/3
+		{"diff.135 lpp. N18 e)", func(x float64) float64 { return 4 / x * x }, -2, 0},
+		{"diff.135 lpp. N18 f)", func(x float64) float64 { return 4/x + 1 }, -2, -1},
+		{"diff.135 lpp. N18 g)", func(x float64) float64 { return 6/2 - x }, -1, -1},
+		{"diff.135 lpp. N18 h)", func(x float64) float64 { return 1 / math.Sqrt(x) }, 4, -0.062499},
+		{"diff.135 lpp. N18 i)", func(x float64) float64 { return 2 * math.Sqrt(1-x) }, 0, -1},
+		{"diff.135 lpp. N18 j)", func(x float64) float64 { return math.Sqrt(1 + 2*x) }, 4, 0.3333333},
+	} {
+		t.Run("", func(t *testing.T) {
+			d := derivative(tc.f)
+			got := d(tc.x)
+			if math.Abs(got-tc.want) > 0.0001 {
+				t.Errorf("Expected derivative(%f) to be %f, but got %f", tc.x, tc.want, got)
+			}
+		})
+	}
+}
+
+func TestGradientDescend(t *testing.T) {
+	for _, tc := range []struct {
+		name  string
+		f     func(float64) float64
+		x0    float64
+		alpha float64
+		want  float64
+	}{
+		{name: "constant", f: func(x float64) float64 { return 5 }, x0: 0.0, alpha: 0.01, want: 0.0},
+		{name: "simple square", f: func(x float64) float64 { return x * x }, x0: 2.0, alpha: 0.01, want: 0.0},
+		{name: "not so simple square", f: func(x float64) float64 { return x*x + 5*x - 3 }, x0: 1.0, alpha: 0.01, want: -2.5},
+		{name: "diff.135 lpp. N18 a)", f: func(x float64) float64 { return 2*x*x - 3*x }, x0: 3, alpha: 0.01, want: 0.75},
+		{name: "diff.135 lpp. N18 b)", f: func(x float64) float64 { return x*x - x + 2 }, x0: 0, alpha: 0.01, want: 0.5},
+	} {
+		t.Run("", func(t *testing.T) {
+			got := GradientDescent(tc.f, tc.x0, tc.alpha)
+			if math.Abs(got-tc.want) > 0.0001 {
+				t.Errorf("Expected GradientDescent(%f, %f) to be %f, but got %f", tc.x0, tc.alpha, tc.want, got)
+			}
+		})
+	}
+}