package main

import (
        "crypto/tls"
        "flag"
        "fmt"
        "net"
        "os"
        "regexp"
        "strings"
        "sync"
        "time"

        "github.com/dustin/go-humanize"
)

func main() {

        warning_period := flag.Int("warn", 7, "warning period in days")
        timeout := flag.Duration("timeout", 2*time.Second, "timeout for connection")
        concurrency := flag.Int("c", 128, "number of concurrent checks")
        flag.Usage = func() {
                fmt.Fprintf(os.Stderr, "Usage: %s [options] <host:port>...\n", os.Args[0])
                flag.PrintDefaults()
        }
        flag.Parse()

        // endpoints to check
        endpoints := flag.Args()

        dialer := &net.Dialer{
                Timeout: *timeout,
        }

        warn_if_expired_at := time.Now().AddDate(0, 0, *warning_period)

        checker := NewSimpleHostChecker(dialer, warn_if_expired_at)

        os.Exit(RunChecks(checker, endpoints, *concurrency))
}

type HostChecker interface {
        CheckHost(host string) (bool, error)
}

func RunChecks(
        checker HostChecker,
        endpoints []string,
        concurrency int,
) (exitcode int) {

        // semaphore to limit concurrency to a reasonable number
        semaphore := make(chan struct{}, concurrency)

        wg := new(sync.WaitGroup)
        wg.Add(len(endpoints))

        for _, i := range endpoints {

                // sleep 1ms to avoid hitting DNS resolver limits
                time.Sleep(time.Millisecond)

                // acquire semaphore
                semaphore <- struct{}{}

                go func(i string) {

                        // mark as done when we're finished
                        defer wg.Done()

                        // release semaphore
                        defer func() { <-semaphore }()

                        expires_soon, err := checker.CheckHost(i)
                        if err != nil {
                                fmt.Printf("can't check %s: %s\n", i, err)
                                exitcode = 1
                        } else if expires_soon {
                                exitcode = 1
                        }

                }(i)
        }

        wg.Wait()

        return exitcode

}

var addrOverride = regexp.MustCompile(`^([^:]+):(((\[[0-9a-f:]+\])|([^:]+)):\d+)$`)

type SimpleHostChecker struct {
        dialer             *net.Dialer
        warn_if_expired_at time.Time
}

func NewSimpleHostChecker(
        dialer *net.Dialer,
        warn_if_expired_at time.Time,
) *SimpleHostChecker {
        return &SimpleHostChecker{
                dialer:             dialer,
                warn_if_expired_at: warn_if_expired_at,
        }
}

func expiresInOrExpired(t time.Time) string {
        if time.Now().After(t) {
                return "expired"
        } else {
                return "expires in"
        }
}

func (c *SimpleHostChecker) CheckHost(
        host string,
) (expires_soon bool, err error) {

        config := tls.Config{
                // we still want to get connection even if the cert is expired, or if
                // the hostname doesn't match
                InsecureSkipVerify: true,
        }

        // custom address parsing to allow default port and address override
        if !strings.Contains(host, ":") {
                host = host + ":443"
        } else if match := addrOverride.FindStringSubmatch(host); match != nil {
                config.ServerName = match[1]
                host = match[2]
        }

        // make a connection to get the certificate
        conn, err := tls.DialWithDialer(c.dialer, "tcp", host, &config)
        if err != nil {
                return
        }
        conn.Close()

        // check all certificates in the chain for expiration
        for _, cert := range conn.ConnectionState().PeerCertificates {
                if c.warn_if_expired_at.After(cert.NotAfter) {
                        expires_soon = true
                        fmt.Printf("Certificate for %s (%s) %s %s.\n",
                                host, cert.Subject.CommonName,
                                expiresInOrExpired(cert.NotAfter),
                                humanize.Time(cert.NotAfter))
                }
        }

        // TODO: validate hostname and chain of trust

        return
}