Base64 encode and decode, line by line.

.gitignore

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+bin/

README.md

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+# Base64 line-by-line
+
+These applications encode and decode strins in Base64 by reading their inputs **line by line**.
+
+Advantages:
+- Ultra fast, not even a single memory allocation is performed
+- Readable source code, it's just a function call and a loop over input

makefile

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+all: build
+
+build:
+	[ -d ./bin ] || mkdir bin
+	cc -o bin/b64-lbl-encode src/b64-lbl-encode.c
+	cc -o bin/b64-lbl-decode src/b64-lbl-decode.c

src/b64-lbl-decode.c

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+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#define BUFFER_LENGTH 50000
+
+static const char *base64_chars = 
+    "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+    "abcdefghijklmnopqrstuvwxyz"
+    "0123456789+/";
+
+int base64_decode(const char *input, unsigned char *output, size_t *output_length) {
+    int input_length = strlen(input);
+    if (input_length % 4 != 0) return -1; // Base64 input must be a multiple of 4.
+    
+    // Calculate output length
+    *output_length = input_length / 4 * 3;
+    if (input[input_length - 1] == '=') (*output_length)--;
+    if (input[input_length - 2] == '=') (*output_length)--;
+
+    for (int i = 0, j = 0; i < input_length; ) {
+        unsigned int a = input[i] == '=' ? 0 : strchr(base64_chars, input[i]) - base64_chars; i++;
+        unsigned int b = input[i] == '=' ? 0 : strchr(base64_chars, input[i]) - base64_chars; i++;
+        unsigned int c = input[i] == '=' ? 0 : strchr(base64_chars, input[i]) - base64_chars; i++;
+        unsigned int d = input[i] == '=' ? 0 : strchr(base64_chars, input[i]) - base64_chars; i++;
+        
+        output[j++] = (a << 2) | (b >> 4);
+        if (j < *output_length) output[j++] = (b << 4) | (c >> 2);
+        if (j < *output_length) output[j++] = (c << 6) | d;
+    }
+    
+    return 0;
+}
+
+int main(void) {
+    char input[BUFFER_LENGTH];
+    unsigned char output[BUFFER_LENGTH]; // Maximum size for decoded output from a Base64 string
+    size_t output_length;
+
+    while (fgets(input, sizeof(input), stdin)) {
+        // Remove the newline character if present
+        input[strcspn(input, "\n")] = 0;
+
+        if (base64_decode(input, output, &output_length) == 0) {
+            fwrite(output, 1, output_length, stdout);
+            putchar('\n'); // Just to separate outputs if needed
+        } else {
+            fprintf(stderr, "Error decoding: %s\n", input);
+        }
+    }
+
+    return 0;
+}

src/b64-lbl-encode.c

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+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#define BUFFER_LENGTH 50000
+
+static const char *base64_chars = 
+    "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+    "abcdefghijklmnopqrstuvwxyz"
+    "0123456789+/";
+
+void base64_encode(const unsigned char *input, size_t input_length, char *output, size_t *output_length) {
+    int i, j;
+    *output_length = 4 * ((input_length + 2) / 3); // Calculate output size
+
+    for (i = 0, j = 0; i < input_length;) {
+        unsigned int a = i < input_length ? input[i++] : 0;
+        unsigned int b = i < input_length ? input[i++] : 0;
+        unsigned int c = i < input_length ? input[i++] : 0;
+
+        output[j++] = base64_chars[a >> 2];
+        output[j++] = base64_chars[((a & 3) << 4) | (b >> 4)];
+        output[j++] = (i > input_length + 1) ? '=' : base64_chars[((b & 15) << 2) | (c >> 6)];
+        output[j++] = (i > input_length) ? '=' : base64_chars[c & 63];
+    }
+    output[j] = '\0'; // Null-terminate the output string
+}
+
+int main() {
+    char input[BUFFER_LENGTH];
+    char output[BUFFER_LENGTH*4/3]; // Maximum size for encoded output (4/3 per byte)
+    size_t output_length;
+
+    while (fgets(input, sizeof(input), stdin)) {
+        // Remove the newline character if present
+        input[strcspn(input, "\n")] = 0;
+
+        base64_encode((unsigned char *)input, strlen(input), output, &output_length);
+        printf("%s\n", output);
+    }
+
+    return 0;
+}