fix: bouncing ball exercises

18-project-bouncing-ball/exercises/03-previous-positions/main.go

@@ -0,0 +1,131 @@
+// 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 column := range board {
+		board[column] = make([]bool, height)
+	}
+
+	// drawing buffer length
+	// *2 for extra spaces
+	// +1 for newlines
+	bufLen := (width*2 + 1) * height
+
+	// create a drawing buffer
+	buf := make([]rune, 0, bufLen)
+
+	// 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)
+	}
+}