80 lines
2.2 KiB
C
80 lines
2.2 KiB
C
#include <stdio.h>
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#include <stdlib.h>
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typedef unsigned char ubyte;
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const ubyte BASE64[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
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int findIndex(const ubyte val) {
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if ('A' <= val && val <= 'Z') {
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return val - 'A';
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}
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if ('a' <= val && val <= 'z') {
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return val - 'a' + 26;
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}
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if ('0' <= val && val <= '9') {
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return val - '0' + 52;
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}
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if (val == '+') {
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return 62;
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}
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if (val == '/') {
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return 63;
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}
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return -1;
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}
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int decode(const ubyte source[], ubyte sink[]) {
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const size_t length = strlen(source);
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const ubyte *it = source;
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const ubyte *end = source + length;
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int acc;
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if (length % 4 != 0) {
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return 1;
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}
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while (it != end) {
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const ubyte b1 = *it++;
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const ubyte b2 = *it++;
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const ubyte b3 = *it++; // might be the first padding byte
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const ubyte b4 = *it++; // might be the first or second padding byte
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const int i1 = findIndex(b1);
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const int i2 = findIndex(b2);
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acc = i1 << 2; // six bits came from the first byte
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acc |= i2 >> 4; // two bits came from the first byte
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*sink++ = acc; // output the first byte
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if (b3 != '=') {
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const int i3 = findIndex(b3);
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acc = (i2 & 0xF) << 4; // four bits came from the second byte
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acc += i3 >> 2; // four bits came from the second byte
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*sink++ = acc; // output the second byte
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if (b4 != '=') {
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const int i4 = findIndex(b4);
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acc = (i3 & 0x3) << 6; // two bits came from the third byte
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acc |= i4; // six bits came from the third byte
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*sink++ = acc; // output the third byte
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}
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}
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}
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*sink = '\0'; // add the sigil for end of string
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return 0;
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}
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int main() {
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ubyte data[] = "VG8gZXJyIGlzIGh1bWFuLCBidXQgdG8gcmVhbGx5IGZvdWwgdGhpbmdzIHVwIHlvdSBuZWVkIGEgY29tcHV0ZXIuCiAgICAtLVBhdWwgUi5FaHJsaWNo";
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ubyte decoded[1024];
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printf("%s\n\n", data);
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decode(data, decoded);
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printf("%s\n\n", decoded);
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return 0;
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}
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