package main

import (
	"bytes"
	"crypto/rand"
    "flag"
	"fmt"
	"math/big"
	"os"
	"path/filepath"
	"time"
)

func main() {
    flag.Usage = func() {
        fmt.Fprintf(os.Stdout, "pickrand.go: picks a random file.\n")
        fmt.Fprintf(os.Stdout, "\n")
        flag.PrintDefaults()
    }
    prevDays := flag.Int("n", 0, "The number of days back to look.")
    flag.Parse()
    var maxDuration time.Duration
    if (*prevDays != 0) {
        maxDuration = time.Duration(int64(*prevDays) * (24 * 60 * 60 * 1e9))
    }
    
	root := flag.Arg(0)
    if (root == "") {
        root = "."
    }
	files := make([]string, 0, 32)
	err := filepath.Walk(root, func(p string, f os.FileInfo, err error) error {
		if err != nil {
			return err
		}
        if maxDuration != 0 {
            var duration = time.Now().Sub(f.ModTime())
            if duration > maxDuration {
                return nil
            }
        }
		if !f.IsDir() {
			files = append(files, p)
		}
		return nil
	})
	if err != nil {
		fmt.Fprintf(os.Stderr, "** Error: %s\n", err.Error())
		os.Exit(1)
	}
    if len(files) == 0 {
		fmt.Fprintf(os.Stderr, "No matching files found.\n")
		os.Exit(1)
    }
	var b [16]byte
	_, err = rand.Read(b[:])
	if err != nil {
		fmt.Fprintf(os.Stderr, "Failed to access cryptographic randomness.  " +
			"Error: %s\n\n", err.Error())
		os.Exit(1)
	}
	i, err := rand.Int(bytes.NewReader(b[:]), big.NewInt(int64(len(files))))
	if err != nil {
		fmt.Fprintf(os.Stderr, "Failed to get a random int.  Error: %s\n", err.Error())
		os.Exit(1)
	}
	j := int(i.Uint64())

	fmt.Printf("%s\n", files[j])
}