Toy CLI progress bar (thread safe) - experiment in using bits as placeholders for the indicator so we can do bitwise operations to shift the spinner left and right instead of mucking with string magic.

progress_bar.go

////////////////////////////////////////////////////////////////////////////////
// The MIT License (MIT)
//
// Copyright (c) 2018 Mark LaPerriere
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.
////////////////////////////////////////////////////////////////////////////////

// Package main is a thread-safe toy CLI progress bar experiment in using bits to store the state
// of the spinner so we can do bitwise operations to move it left and right instead of a glut of
// string operations.
// BONUS: it only depends on the `sync` package (other dependencies are for the example only)
package main

import (
	"fmt"
	"sync"
	"time"
)

const (
	indicatorLeft  = 0x8000 // 1000 0000 0000 0000
	indicatorRight = 0x1    // 0000 0000 0000 0001
	directionRight = 0
	directionLeft  = 1
	defaultWidth   = 72
	indicatorRune  = '*'
	spaceRune      = ' '
)

// progressBar is a simple progress bar that uses bits and bitwise operators to track and move
// an indicator back and forth across a "progress bar".
type progressBar struct {
	sync.RWMutex
	barState uint16
	bar      []byte
	op       int
	stat     string
	width    int
	drawBuf  []byte
}

// NewProgressBar creates a new progress bar
func NewProgressBar() *progressBar {
	p := &progressBar{
		barState: indicatorLeft,
		width:    defaultWidth,
		bar:      make([]byte, 16),
		drawBuf:  make([]byte, defaultWidth),
	}
	p.drawBuf[0] = byte('[')
	p.drawBuf[18] = byte(']')
	p.drawBuf[19] = byte(' ')
	return p
}

// Update updates the progress bar representation
func (p *progressBar) Update(stat string) {
	// write lock
	p.Lock()
	defer p.Unlock()

	// move the indicator in the appropriate direction
	switch p.op {
	case directionRight:
		p.barState >>= 1
	case directionLeft:
		p.barState <<= 1
	}

	switch p.barState {
	case indicatorLeft:
		// indicator at far left, change direction to right
		p.op = directionRight
	case indicatorRight:
		// indicator at far right, change direction to left
		p.op = directionLeft
	}

	// convert bits to runes
	c := uint16(p.barState)
	for i := 15; i >= 0; i-- {
		if c&1 == 1 {
			p.bar[i] = indicatorRune
		} else {
			p.bar[i] = spaceRune
		}
		c >>= 1
	}

	// update the stat message
	p.stat = stat
}

// String returns a string representation of the progress bar
func (p *progressBar) String() string {
	// read lock
	p.RLock()
	defer p.RUnlock()

	// copy the progress bar into the buffer
	copy(p.drawBuf[1:17], p.bar)

	// copy the stat message into the buffer
	copy(p.drawBuf[20:20+len(p.stat)], []byte(p.stat))

	// add the padding from right to left
	for i := (20 + len(p.stat)); i < p.width; i++ {
		p.drawBuf[i] = byte(' ')
	}

	// spit out the results
	return string(p.drawBuf)
}

// main is just an example use
func main() {

	// create a new progress bar
	progress := NewProgressBar()

	// update a couple times to see it in action
	for i := 100; i >= 0; i-- {
		var stat string
		if i == 0 {
			stat = fmt.Sprint("WE'RE DONE!")
		} else {
			stat = fmt.Sprintf("%d more to go", i)
		}

		progress.Update(stat)

		// make sure to reset the line first with \r
		fmt.Printf("\r%s", progress.String())

		// artificial pause so we can actually see it
		time.Sleep(time.Millisecond * 50)
	}
	fmt.Println("\ndone")
}