package main

import (
	"bufio"
	"fmt"
	"log"
	"math"
	"math/rand"
	"os"
	"time"
)

const (
	randomMin  = 1                  // We specify a range which random numbers will be generated, we must start at the first possible left-most digit
	randomMax  = 999999999999999999 // int64 max value is 9223372036854775807.  We use one digit less than that with all 9's in order to not give bias to any digits.
	numSamples = 100000000          // A nice rounded human number
)

func main() {

	// In results, we store a count of each left most leading digits as numbers are randomly genereated
	results := [9]int{} // There are 9 possible leading digits and 0 does not count, offset by 1 for index to actual value. Examples: To access count for 1 use [0].  To access 5 use [4].  To access 9 use [8].
	currentSample := 0  // A counter that increments each time a random number sample has been generated.  Used for status messages

	// Start a little goroutine to output status and attach a ticker to execute it each second
	statusTicker := time.NewTicker(time.Second)
	go func() {
		for {
			<-statusTicker.C // Wait for heartbeat from ticker channel
			percentCompleted := (currentSample * 100) / numSamples
			log.Printf("%d%% completed generating and analyzing samples", percentCompleted)
		}
	}()

	log.Printf("generating numbers...")

	rand.Seed(time.Now().UnixNano())
	for currentSample = 0; currentSample < numSamples; currentSample++ {
		results[firstDigit(rand.Intn(randomMax-randomMin+1)+randomMin)-1]++ // Generate a random number between randomMin and randomMax, get the first digit then increment the counter in results array, remember, it's offset by 1
	}

	// Done generating and counting digits, stop the status ticker
	statusTicker.Stop()
	log.Printf("done.")

	// Output results
	for digitMinusOne, digitCount := range results {
		fmt.Printf("%d: %d (%f%%)\n", digitMinusOne+1, digitCount, float64(digitCount*100)/float64(numSamples))
	}

	// Wait indefinitely until Enter Key is pressed, avoid terminating terminal before viewing results if ran from a shell
	fmt.Print("Press 'Enter' to continue...")
	bufio.NewReader(os.Stdin).ReadBytes('\n')
}

// firstDigit returns the first/leftmost digit of the base10 representation of an integer
func firstDigit(x int) int {
	return int(math.Abs(float64(x)) / math.Pow(10, float64(numDigits(x)-1)))
}

// numDigits returns the number of digits
func numDigits(x int) int {
	if x == 0 {
		return 1
	}
	return int(math.Floor(math.Log10(math.Abs(float64(x))))) + 1
}