/*
The MIT License (MIT)

Copyright © 2020-2025 pacman64

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.
*/

/*
Single-file source-code for primes.

To compile a smaller-sized command-line app, you can use the `go` command as
follows:

go build -ldflags "-s -w" -trimpath primes.go
*/

package main

import (
	"bufio"
	"math"
	"os"
	"strconv"
)

// Note: the code is avoiding using the fmt package to save hundreds of
// kilobytes on the resulting executable, which is a noticeable difference.

const info = `
primes [options...] [count...]


Show the first few prime numbers, starting from the lowest and showing one
per line. When not given how many primes to find, the default is 1 million.

All (optional) leading options start with either single or double-dash:

    -h          show this help message
    -help       show this help message
`

func main() {
	howMany := 1_000_000
	if len(os.Args) > 1 {
		switch os.Args[1] {
		case `-h`, `--h`, `-help`, `--help`:
			os.Stderr.WriteString(info[1:])
			return
		}

		n, err := strconv.Atoi(os.Args[1])
		if err != nil {
			os.Stderr.WriteString("\x1b[31m")
			os.Stderr.WriteString(err.Error())
			os.Stderr.WriteString("\x1b[0m\n")
			os.Exit(1)
		}

		if n < 0 {
			n = 0
		}
		howMany = n
	}

	primes(howMany)
}

func primes(left int) {
	bw := bufio.NewWriter(os.Stdout)
	defer bw.Flush()

	// 24 bytes are always enough for any 64-bit integer
	var buf [24]byte

	// 2 is the only even prime number
	if left > 0 {
		bw.WriteString("2\n")
		left--
	}

	for n := uint64(3); left > 0; n += 2 {
		if oddPrime(n) {
			bw.Write(strconv.AppendUint(buf[:0], n, 10))
			if err := bw.WriteByte('\n'); err != nil {
				// assume errors come from closed stdout pipes
				return
			}
			left--
		}
	}
}

// oddPrime assumes the number given to it is odd
func oddPrime(n uint64) bool {
	max := uint64(math.Sqrt(float64(n)))
	for div := uint64(3); div <= max; div += 2 {
		if n%div == 0 {
			return false
		}
	}
	return true
}