C言語で書いてみました

■ Base64デコード（平成16年午後1問5）

#include <stdio.h>

/* 平成16年午後1問5 */

/* Base64 デコード */
const char base64[] =
    "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

char *_strchr(const char *s, char c)
{
    while (*s != '\0') {
        if (*s == c) return (char *)s;
        s++;
    }
    return NULL;
}

char _decode(char c1, char c2, int no)
{
    char b1, b2, r;
    b1 = _strchr(base64, c1) - base64;
    b2 = _strchr(base64, c2) - base64;
    switch (no) {
        case 0:
            r = (b1 << 2) | (b2 >> 4);
            break;
        case 1:
            r = (b1 << 4) | (b2 >> 2);
            break;
        case 2:
            r = (b1 << 6) | b2;
            break;
    }
    return r;
}

int main(int argc, char *argv[])
{
    int ch, i, n;
    char b[3], c[4];
    FILE *fi = NULL, *fo = NULL;
    
    if (argc < 3) return -1;
    fi = fopen(argv[1], "rt");
    fo = fopen(argv[2], "wb");

    i = 0;
    while (feof(fi) == 0) {
        ch = fgetc(fi);
        if (ch != '\n') {
            c[i] = ch;
            i++;
            if (i == 4) {
                b[0] = _decode(c[0], c[1], 0);
                b[1] = _decode(c[1], c[2], 1);
                b[2] = _decode(c[2], c[3], 2);
                n = 3;
                if (c[3] == '=') n = 2;
                if (c[2] == '=') n = 1;
                fwrite(b, 1, n, fo);
                i = 0;
            }
        }
    }
    if (i > 0)
        printf("入力データの形式が正しくない");
    
    fclose(fi);
    fclose(fo);

    return 0;
}