add: bouncing ball exercise #2

18-bouncing-ball-project/README.md

@@ -2,13 +2,11 @@
 
 Use the following tips only when you get stuck. This document isn't in a particular order, please do not follow it like so.
 
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+## CALCULATING THE VELOCITY
-
-## Ball position and velocity
 
 You can use velocity to change the ball's speed and position. In my example, the speed is constant, so I always use unit value: 1.
 
-On each loop step: Add velocities to ball's position. This will make the ball move.
+* On each loop step: Add velocities to ball's position. This will make the ball move.
 
 * **Velocity means: Speed and Direction**
 
@@ -18,36 +16,26 @@ On each loop step: Add velocities to ball's position. This will make the ball mo
   * Y velocity = -1 -> _ball moves up_
 
 * **For more information on graphics and velocity:**
-  * Youtube: Crash Course: 2D Graphics
-    -> https://www.youtube.com/watch?v=7Jr0SFMQ4Rs&t=529
 
-  * Youtube: Crash Course: Velocity
+  * [Youtube: Crash Course: 2D Graphics](https://www.youtube.com/watch?v=7Jr0SFMQ4Rs&t=529)
-    -> https://www.youtube.com/watch?v=ZM8ECpBuQYE
 
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+  * [Youtube: Crash Course: Velocity](https://www.youtube.com/watch?v=ZM8ECpBuQYE)
 
-## CREATE THE BOARD
+## CREATING THE BOARD
 
 I use `[][]bool` for the board but you can use anything you like. For example, you can directly use `[][]rune` or `[]rune`. Experiment with them and decide which one is the best for you.
 
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+## CLEARING THE SCREEN
 
-## CLEAR THE SCREEN
+* Before the loop, clear the screen once by using my [screen package](https://github.com/inancgumus/screen), click on the link. You can find its [documentation here](https://godoc.org/github.com/inancgumus/screen).
-
-* Before the loop, clear the screen once by using my screen package.
 
 * After each loop step, move the cursor to the top-left position by using the screen package. So that you can draw the animation frame all over again in the same position.
 
-* It's [here](https://github.com/inancgumus/screen).
+* You can find more information about the screen package and screen clearing in the [Retro Clock project section lectures](https://github.com/inancgumus/learngo/tree/master/15-project-retro-led-clock).
-* Its documentation is [here](https://godoc.org/github.com/inancgumus/screen).
-* You can find more information about it in the Retro Clock project section.
 
-➖➖➖➖➖➖➖➖➖➖➖➖➖➖➖➖➖➖➖➖➖➖➖➖➖➖➖➖➖➖➖➖
+## DRAWING THE BOARD
 
-## DRAW THE BOARD
+Instead of drawing the board and the ball to the screen everytime, you will fill a buffer, and when you complete, you can draw the board and the ball once by printing the buffer. I use a `[]rune` slice as a buffer because `rune` can store an emoji character.
-Instead of drawing the board and the ball to the screen everytime, you will fill a buffer, and when you complete, you can draw the board and the ball once by printing the buffer.
-
-I use a `[]rune` buffer because `rune` can store an emoji character, and I use a slice of runes because I want to draw empty cells and a ball in the end.
 
 * Make a large enough rune slice named `buf` using the `make` function.
 
@@ -65,10 +53,10 @@ var buffer []rune
 str := string(buffer)
 ```
 
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+## SLOWING DOWN THE SPEED
-
-## SLOW DOWN THE SPEED
 
 Call the `time.Sleep` function to slow down the speed of the loop a little bit, so you can see the ball :)
 
-  `time.Sleep(time.Second / 20)`
+```go
+time.Sleep(time.Second / 20)
+```

18-bouncing-ball-project/exercises/01-width-and-height/main.go

@@ -20,6 +20,9 @@
 //
 //  1. Go here: https://godoc.org/golang.org/x/crypto/ssh/terminal
 //
+//     Download the package:
+//     go get -u golang.org/x/crypto/ssh/terminal
+//
 //  2. Find the function that gives you the width and height
 //     of the terminal.
 //
@@ -47,7 +50,7 @@
 //     columns (or cells). Ordinary characters have a
 //     single column.
 //
-//     1. Get the width of the ball emoji using function
+//     1. Get the width of the ball emoji using a function
 //        from the following package:
 //
 //        go get -u github.com/mattn/go-runewidth

18-bouncing-ball-project/exercises/02-previous-positions/main.go

@@ -0,0 +1,126 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import (
+	"fmt"
+	"time"
+
+	"github.com/inancgumus/screen"
+	"github.com/mattn/go-runewidth"
+)
+
+// ---------------------------------------------------------
+// EXERCISE: Previous positions
+//
+//  Let's optimize the program once more. This time you're
+//  going to optimize the clearing off the previous positions.
+//
+//  1. Find the code below marked as "remove the previous ball"
+//
+//  2. Instead of clearing every position on the board to false,
+//     only set the previous position to false. So, don't use
+//     a loop, remove it.
+//
+//  3. Change the velocity of the ball like so:
+//
+//     vx, vy = 5, 2
+//
+//  4. Run the program and solve the problem
+//
+//
+// HINT
+//
+//  Don't forget saving the previous position.
+//
+// ---------------------------------------------------------
+
+func main() {
+	const (
+		cellEmpty = ' '
+		cellBall  = '⚾'
+
+		maxFrames = 1200
+		speed     = time.Second / 20
+	)
+
+	var (
+		px, py int    // ball position
+		vx, vy = 1, 1 // velocities
+
+		cell rune // current cell (for caching)
+	)
+
+	// you can get the width and height using the screen package easily:
+	width, height := screen.Size()
+
+	// get the rune width of the ball emoji
+	ballWidth := runewidth.RuneWidth(cellBall)
+
+	// adjust the width and height
+	width /= ballWidth
+	height-- // there is a 1 pixel border in my terminal
+
+	// create the board
+	board := make([][]bool, width)
+	for row := range board {
+		board[row] = make([]bool, height)
+	}
+
+	// create a drawing buffer
+	buf := make([]rune, 0, width*height)
+
+	// clear the screen once
+	screen.Clear()
+
+	for i := 0; i < maxFrames; i++ {
+		// calculate the next ball position
+		px += vx
+		py += vy
+
+		// when the ball hits a border reverse its direction
+		if px <= 0 || px >= width-1 {
+			vx *= -1
+		}
+		if py <= 0 || py >= height-1 {
+			vy *= -1
+		}
+
+		// remove the previous ball
+		for y := range board[0] {
+			for x := range board {
+				board[x][y] = false
+			}
+		}
+
+		// put the new ball
+		board[px][py] = true
+
+		// rewind the buffer (allow appending from the beginning)
+		buf = buf[:0]
+
+		// draw the board into the buffer
+		for y := range board[0] {
+			for x := range board {
+				cell = cellEmpty
+				if board[x][y] {
+					cell = cellBall
+				}
+				buf = append(buf, cell, ' ')
+			}
+			buf = append(buf, '\n')
+		}
+
+		// print the buffer
+		screen.MoveTopLeft()
+		fmt.Print(string(buf))
+
+		// slow down the animation
+		time.Sleep(speed)
+	}
+}

18-bouncing-ball-project/exercises/02-previous-positions/solution/main.go

@@ -0,0 +1,101 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import (
+	"fmt"
+	"time"
+
+	"github.com/inancgumus/screen"
+	"github.com/mattn/go-runewidth"
+)
+
+func main() {
+	const (
+		cellEmpty = ' '
+		cellBall  = '⚾'
+
+		maxFrames = 1200
+		speed     = time.Second / 20
+	)
+
+	var (
+		px, py   int    // ball position
+		ppx, ppy int    // previous ball position
+		vx, vy   = 5, 2 // velocities
+
+		cell rune // current cell (for caching)
+	)
+
+	// you can get the width and height using the screen package easily:
+	width, height := screen.Size()
+
+	// get the rune width of the ball emoji
+	ballWidth := runewidth.RuneWidth(cellBall)
+
+	// adjust the width and height
+	width /= ballWidth
+	height-- // there is a 1 pixel border in my terminal
+
+	// create the board
+	board := make([][]bool, width)
+	for row := range board {
+		board[row] = make([]bool, height)
+	}
+
+	// create a drawing buffer
+	buf := make([]rune, 0, width*height)
+
+	// clear the screen once
+	screen.Clear()
+
+	for i := 0; i < maxFrames; i++ {
+		// calculate the next ball position
+		px += vx
+		py += vy
+
+		// when the ball hits a border reverse its direction
+		if px <= 0 || px >= width-1 {
+			vx *= -1
+		}
+		if py <= 0 || py >= height-1 {
+			vy *= -1
+		}
+
+		// check whether the ball goes beyond the borders
+		if px < width && py < height {
+			// remove the previous ball and put the new ball
+			board[px][py], board[ppx][ppy] = true, false
+
+			// save the previous positions
+			ppx, ppy = px, py
+		}
+
+		// rewind the buffer (allow appending from the beginning)
+		buf = buf[:0]
+
+		// draw the board into the buffer
+		for y := range board[0] {
+			for x := range board {
+				cell = cellEmpty
+				if board[x][y] {
+					cell = cellBall
+				}
+				buf = append(buf, cell, ' ')
+			}
+			buf = append(buf, '\n')
+		}
+
+		// print the buffer
+		screen.MoveTopLeft()
+		fmt.Print(string(buf))
+
+		// slow down the animation
+		time.Sleep(speed)
+	}
+}

18-bouncing-ball-project/exercises/README.md

@@ -4,3 +4,7 @@
 
     In this exercise, your goal is getting the width and height of the terminal screen from your operating system (instead of setting the width and height manually).
 
+2. **[Previous positions](https://github.com/inancgumus/learngo/tree/master/18-bouncing-ball-project/exercises/02-previous-positions)**
+
+    Let's optimize the program once more. This time you're going to optimize the clearing off the previous positions.
+