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  1. import java.io.*;
  2.  
  3. public class Base64Decode {
  4.     public static void main(String[] args) {
  5.         // 変換テーブル
  6.         char[] table = {	
  7.             'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H',
  8.             'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P',
  9.             'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 
  10.             'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f',
  11.             'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 
  12.             'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 
  13.             'w', 'x', 'y', 'z', '0', '1', '2', '3', 
  14.             '4', '5', '6', '7', '8', '9', '+', '/'};
  15.  
  16.         InputStream in = null;    // 入力データ
  17.         OutputStream out = null;  // 出力先
  18.  
  19.         try {
  20.             in = new FileInputStream(args[0]);
  21.             out = new FileOutputStream(args[1]);
  22.             char[] cs;
  23.             while((cs=read4(in))!=null) {
  24.                 write3(out, convert6to8(decode(cs, table)));
  25.             }
  26.         } catch (Exception e) {
  27.             e.printStackTrace();  // 例外の情報を表示する
  28.         } finally {    // in, out を閉じる
  29.             try {
  30.                 in.close();
  31.                 out.close();
  32.             }
  33.             catch (Exception e) {
  34.             }
  35.         }
  36.     }
  37.  
  38.     public static int[] decode(char[] cs, char[] table) {
  39.         int[] ret = new int[cs.length];
  40.         for(int i=0; i<cs.length; i++) {
  41.             for(int j=0; j<table.length; j++) {
  42.                 if(cs[i] == table[j]) {
  43.                     ret[i] = j;
  44.                     break;
  45.                 }
  46.             }
  47.         }
  48.         return ret;
  49.     }
  50.  
  51.     public static int[] convert6to8(int[] buf) {
  52.         StringBuilder fb = new StringBuilder();
  53.         for(int i=0; i<buf.length; i++) {
  54.             fb.append(toBinary(buf[i], 6));
  55.         }
  56.         int[] ret = new int[fb.length()/6-1];
  57.         for(int i=0; i<ret.length; i++) {
  58.             String f = fb.substring(i*8, i*8+8);
  59.             ret[i] = fromBinary(f);
  60.         }
  61.         return ret;
  62.     }
  63.  
  64.     /**
  65.      * バイト列 bt の数を順に出力する.
  66.      * @param bt 数の配列。長さは 3以下. 各数は8ビットの整数
  67.      */
  68.     public static void write3(OutputStream out, int[] bt) throws IOException {
  69.         for (int i=0; i<3; i++) {
  70.             if (i<bt.length) {
  71.                 out.write(bt[i]);
  72.             }
  73.         }
  74.     }
  75.  
  76.     /**
  77.      * in から文字を最大4つ読み出す.
  78.      * @param in 入力ストリーム
  79.      * @return 文字の配列。配列長は最大4. 入力終了したときには null を返す.
  80.      */
  81.     public static char[] read4(InputStream in) throws IOException {
  82.         char[] bs;
  83.         int n0=in.read();
  84.         int n1=in.read();
  85.         int n2=in.read();
  86.         int n3=in.read();
  87.  
  88.         if (n0 < 0) {  // 読み込み終了
  89.             bs = null;
  90.         } else if (n2 < 0 || (char) n2=='=') {
  91.             bs = new char[2];
  92.             bs[0] = (char) n0;
  93.             bs[1] = (char) n1;
  94.         } else if (n3 < 0 || (char) n3=='=') {
  95.             bs = new char[3];
  96.             bs[0] = (char) n0;
  97.             bs[1] = (char) n1;
  98.             bs[2] = (char) n2;
  99.         } else {
  100.             bs = new char[4];
  101.             bs[0] = (char) n0;
  102.             bs[1] = (char) n1;
  103.             bs[2] = (char) n2;
  104.             bs[3] = (char) n3;
  105.         }
  106.         return bs;
  107.     }
  108.  
  109.     /**
  110.      * 数を読み取って、nビットの2進数を表す文字列に変換する
  111.      * @param bt 1バイトの数
  112.      * @param n 2進数のビット数
  113.      * @return 2進数を表す文字列
  114.      */
  115.     public static String toBinary(int bt, int n) {
  116.         String s = Integer.toBinaryString(bt);
  117.         for (int i=s.length(); i<n; i++) {
  118.             s = "0" + s;
  119.         }
  120.         return s;
  121.     }
  122.  
  123.     /**
  124.      * 2進数を表す文字列を数に変換する
  125.      * @param b 2進数を表す文字列
  126.      * @return b が表す数
  127.      */
  128.     public static int fromBinary(String b) {
  129.         return Integer.parseInt(b, 2);
  130.     }
  131. }
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https://ideone.com/18oM0
language:
Java (HotSpot 12)
created:
12 years ago
visibility:
secret

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