Data commit

Task/Base64-decode-data/C/base64-decode-data.c

@@ -0,0 +1,79 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+typedef unsigned char ubyte;
+const ubyte BASE64[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
+
+int findIndex(const ubyte val) {
+    if ('A' <= val && val <= 'Z') {
+        return val - 'A';
+    }
+    if ('a' <= val && val <= 'z') {
+        return val - 'a' + 26;
+    }
+    if ('0' <= val && val <= '9') {
+        return val - '0' + 52;
+    }
+    if (val == '+') {
+        return 62;
+    }
+    if (val == '/') {
+        return 63;
+    }
+    return -1;
+}
+
+int decode(const ubyte source[], ubyte sink[]) {
+    const size_t length = strlen(source);
+    const ubyte *it = source;
+    const ubyte *end = source + length;
+    int acc;
+
+    if (length % 4 != 0) {
+        return 1;
+    }
+
+    while (it != end) {
+        const ubyte b1 = *it++;
+        const ubyte b2 = *it++;
+        const ubyte b3 = *it++;         // might be the first padding byte
+        const ubyte b4 = *it++;         // might be the first or second padding byte
+
+        const int i1 = findIndex(b1);
+        const int i2 = findIndex(b2);
+
+        acc = i1 << 2;                  // six bits came from the first byte
+        acc |= i2 >> 4;                 // two bits came from the first byte
+        *sink++ = acc;                  // output the first byte
+
+        if (b3 != '=') {
+            const int i3 = findIndex(b3);
+
+            acc = (i2 & 0xF) << 4;      // four bits came from the second byte
+            acc += i3 >> 2;             // four bits came from the second byte
+            *sink++ = acc;              // output the second byte
+
+            if (b4 != '=') {
+                const int i4 = findIndex(b4);
+
+                acc = (i3 & 0x3) << 6;  // two bits came from the third byte
+                acc |= i4;              // six bits came from the third byte
+                *sink++ = acc;          // output the third byte
+            }
+        }
+    }
+
+    *sink = '\0';   // add the sigil for end of string
+    return 0;
+}
+
+int main() {
+    ubyte data[] = "VG8gZXJyIGlzIGh1bWFuLCBidXQgdG8gcmVhbGx5IGZvdWwgdGhpbmdzIHVwIHlvdSBuZWVkIGEgY29tcHV0ZXIuCiAgICAtLVBhdWwgUi5FaHJsaWNo";
+    ubyte decoded[1024];
+
+    printf("%s\n\n", data);
+    decode(data, decoded);
+    printf("%s\n\n", decoded);
+
+    return 0;
+}