From a49dc188e003205e685c1872cc133acc05c598ec Mon Sep 17 00:00:00 2001
From: Gabriel Augendre <gabriel@augendre.info>
Date: Sat, 14 Aug 2021 07:54:13 +0200
Subject: [PATCH] Implement primes counter in go

---
 .gitignore |  2 ++
 README.md  |  6 ++++
 go.mod     |  3 ++
 main.go    | 91 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 102 insertions(+)
 create mode 100644 .gitignore
 create mode 100644 README.md
 create mode 100644 go.mod
 create mode 100644 main.go

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..1099ec2
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,2 @@
+.idea
+go-primes-counter
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..128d8b9
--- /dev/null
+++ b/README.md
@@ -0,0 +1,6 @@
+# Primes counter
+````shell
+go build
+./go-primes-counter -h
+./go-primes-counter -start 10 -max 1000 -print-count -1 -step-size 100
+````
\ No newline at end of file
diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..a48c0dd
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,3 @@
+module go-primes-counter
+
+go 1.16
diff --git a/main.go b/main.go
new file mode 100644
index 0000000..1a1c2cf
--- /dev/null
+++ b/main.go
@@ -0,0 +1,91 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"sync"
+)
+
+func main() {
+	start, max, step, printCount := parseArgs()
+	if max < start {
+		start, max = max, start
+	}
+	totalLength := max - start
+	if totalLength < step {
+		step = totalLength
+	}
+
+	// Start jobs
+	var wg sync.WaitGroup
+	results := make(chan int, 100)
+	for i := start; i < max; i += step {
+		wg.Add(1)
+		go worker(i, i+step, &wg, results)
+	}
+
+	// Collect results
+	go waitForWorkers(&wg, results)
+	var primes []int
+	for prime := range results {
+		primes = append(primes, prime)
+	}
+
+	// Print results
+	fmt.Println("There are", len(primes), "primes between", start, "and", max)
+	if printCount > len(primes) {
+		printCount = -1
+	}
+	if printCount < 0 {
+		fmt.Println("Here they are:")
+		fmt.Println(primes)
+	} else {
+		fmt.Println("Here's a free sample:")
+		fmt.Println(primes[:printCount])
+	}
+}
+
+func worker(start, end int, wg *sync.WaitGroup, results chan<- int) {
+	defer wg.Done()
+	for i := start; i < end; i++ {
+		if IsPrime(i) {
+			results <- i
+		}
+	}
+}
+
+func waitForWorkers(wg *sync.WaitGroup, results chan int) {
+	wg.Wait()
+	close(results)
+}
+
+// IsPrime determines if the given int is a prime number or not.
+// The method currently implemented by this method is not smart and not
+// optimized for large numbers.
+func IsPrime(n int) bool {
+	if n <= 3 {
+		return n > 1
+	}
+	if n%2 == 0 || n%3 == 0 {
+		return false
+	}
+	i := 5
+	for i*i <= n {
+		if n%i == 0 || n%(i+2) == 0 {
+			return false
+		}
+		i += 6
+	}
+	return true
+}
+
+func parseArgs() (int, int, int, int) {
+	start := flag.Int("start", 0, "The start number")
+	max := flag.Int("max", 10_000, "The end value")
+	step := flag.Int("step-size", 1_000, "The job size")
+	printCountDesc := `Number of primes to print. -1 to print all.
+Primes are not guaranteed to be sorted.`
+	printCount := flag.Int("print-count", 20, printCountDesc)
+	flag.Parse()
+	return *start, *max, *step, *printCount
+}