Implement xor_compress in C

1 files changed, 115 insertions, 0 deletions

diff --git a/tools/xor_compress.c b/tools/xor_compress.c
new file mode 100644
index 0000000..03ea451
--- /dev/null
+++ b/tools/xor_compress.c
@@ -0,0 +1,115 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <errno.h>
+
+#define PROGRAM_NAME "xor_compress"
+
+extern int errno;
+
+size_t file_size(FILE *f, int *err) {
+    int check = fseek(f, 0, SEEK_END);
+    if (check == -1) goto failure;
+
+    long f_size = ftell(f);
+    if (f_size == -1) goto failure;
+
+    check = fseek(f, 0, SEEK_SET);
+    if (check == -1) goto failure;
+
+    *err = 0;
+    return (size_t)f_size;
+
+failure:
+    *err = errno;
+    return 0;
+}
+
+unsigned char *read_files(char *filenames[], int num_files, size_t *buf_size, int *err) {
+    *buf_size = 0;
+    unsigned char *buffer = NULL;
+
+    const char *filename = NULL;
+    for (int i = 0; i < num_files; i++) {
+        filename = filenames[i];
+
+        FILE *f = fopen(filename, "rb");
+        if (!f) goto failure;
+
+        size_t f_size = file_size(f, err);
+        if (*err > 0) goto failure;
+        if (f_size == 0) continue;
+
+        *buf_size += f_size;
+        buffer = realloc(buffer, *buf_size);
+        if (!buffer) goto failure;
+
+        unsigned char *buf_end = buffer + (*buf_size - f_size);
+        size_t read_size = fread(buf_end, 1, f_size, f);
+        if (read_size != f_size) {
+            // fread does not set errno
+            fprintf(stderr, PROGRAM_NAME ": %s: Read error\n", filename);
+            *err = 1;
+            return buffer;
+        }
+    }
+
+    *err = 0;
+    return buffer;
+
+failure:
+    fprintf(stderr, PROGRAM_NAME ": %s: %s\n", filename, strerror(errno));
+    *err = errno;
+    return buffer;
+}
+
+int compress_files(char *filenames[], int num_files) {
+    int err = 0;
+    size_t n = 0;
+    unsigned char *data = read_files(filenames, num_files, &n, &err);
+    if (err > 0) {
+        if (data) free(data);
+        return err;
+    }
+
+    unsigned char v = 0x00;
+    for (size_t i = 0; i < n;) {
+        unsigned char byte = data[i++];
+        unsigned char size = 0;
+        if (data[i] == v) {
+            // Alternating (>= 0x80)
+            // Run stops at 0x80 bytes or when the values stop alternating
+            for (; i < n && size < 0x80 && data[i] == (size % 2 ? byte : v); size++, i++);
+            putchar(size + 0x7f);
+            putchar(v ^ byte);
+            if (size % 2 == 0) v = byte;
+        } else {
+            // Sequential (< 0x80)
+            // Run stops at 0x80 bytes or when the value two ahead is equal to v
+            unsigned char buffer[256] = {0};
+            buffer[size++] = v ^ byte;
+            for (; i < n; i++) {
+                v = byte;
+                if (size > 0x7f || (i + 1 < n && data[i + 1] == v)) break;
+                byte = data[i];
+                buffer[size++] = v ^ byte;
+            }
+            putchar(size - 1);
+            fwrite(buffer, 1, size, stdout);
+        }
+    }
+
+    fflush(stdout);
+    free(data);
+    return 0;
+}
+
+int main(int argc, char *argv[]) {
+    if (argc < 2) {
+        fputs("Usage: " PROGRAM_NAME " file... > files.xor\n", stderr);
+        exit(1);
+    }
+    argv++;
+    argc--;
+    return compress_files(argv, argc);
+}