ovldec/source/main.c

#include <stdint.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

// This is a LZ77-esque compression algorithm.
// However, unlike LZ77, it decompresses itself in-place.
// This is done by storing and reading the compressed data _backwards_,
//   and writing the decompressed data far beyond the end of the compressed blob.
// Of course, that's not a *requirement* of any sort.
void nitro_decompress(void *header)
{
    if (!header) return;

    unsigned dest_offset = *((uint32_t *)header - 1);
    unsigned data_offset = *((uint32_t *)header - 2) >> 24;
    unsigned end_offset = *((uint32_t *)header - 2) & 0xFFFFFF;

    // Compressed size: end_offset
    // Header + padding size: data_offset
    // Decompressed size: dest_offset + end_offset + 1

    uint8_t *dest = (uint8_t *)header + dest_offset;
    uint8_t *data = (uint8_t *)header - data_offset;
    uint8_t *end = (uint8_t *)header - end_offset;

    while (data > end) {
        uint8_t flags = *--data;

        for (int i = 8; i > 0; i--) {
            if (flags & 0x80) {
                unsigned length = ((*(data - 1) & 0xF0) >> 4) + 3;
                unsigned offset = (((*(data - 1) & 0x0F) << 8) | *(data - 2)) + 2;
                data -= 2;

                for (unsigned x = length; x > 0; x--) {
                    char d = *(dest + offset);
                    *--dest = d;
                }
            } else {
                *--dest = *--data;
            }
            flags <<= 1;
            if (data <= end) break;
        }
    }

    return;
}

void usage(const char *prgname)
{
    fprintf(stderr, "Usage: %s [-o offset] [--] <in> <out>\n", prgname);
    exit(1);
}

int main(int argc, char *argv[])
{
    FILE *f;
    int argi = 1;

    int offset = 0;

    for (;;) {
        if (argc <= argi) usage(argv[0]);
        if (argv[argi][0] != '-') break;

        if (strcmp(argv[argi], "--") == 0) {
            argi++;
            break;
        } else if (strcmp(argv[argi], "-o") == 0) {
            if (argc <= argi + 1) {
                fprintf(stderr, "-o: Missing offset\n");
                usage(argv[0]);
            }
            offset = strtol(argv[argi + 1], NULL, 0);
            argi += 2;
        } else {
            fprintf(stderr, "Unrecognized option: '%s'\n", argv[argi]);
            usage(argv[0]);
        }
    }

    if (argc < argi + 2) {
        fprintf(stderr, "Missing parameters\n");
        usage(argv[0]);
    }

    f = fopen(argv[argi + 0], "r");
    if (!f) {
        perror("fopen");
        return 1;
    }

    // Find the "header" in the file
    if (!offset) {
        fseek(f, 0, SEEK_END);
    } else if (offset < 0) {
        fseek(f, offset, SEEK_END);
    } else if (offset > 0) {
        fseek(f, offset, SEEK_SET);
    }

    // Figure out the length and decompressed length of the file
    uint32_t f_len;
    uint32_t f_declen;
    fseek(f, -(sizeof(f_declen) + sizeof(f_len)), SEEK_CUR);
    if (fread(&f_len, sizeof(f_len), 1, f) != 1) {
        perror("fread");
        return 1;
    }
    if (fread(&f_declen, sizeof(f_declen), 1, f) != 1) {
        perror("fread");
        return 1;
    }
    f_len = f_len & 0xFFFFFF;
    if (f_len % 4 != 0) f_len += 4 - (f_len % 4);
    f_declen += f_len;
    if (fseek(f, -(size_t)f_len, SEEK_CUR)) {
        perror("fseek");
        return 1;
    }

    // Read the file
    void *file = malloc(f_declen);
    if (!file) {
        perror("malloc");
        return 1;
    }
    if (fread(file, 1, f_len, f) != f_len) {
        perror("fread");
        return 1;
    }
    fclose(f);

    nitro_decompress((char *)file + f_len);

    // Save the output
    f = fopen(argv[argi + 1], "w");
    if (!f) {
        perror("fopen");
        return 1;
    }
    fwrite(file, f_declen, 1, f);
    fclose(f);
    free(file);

    return 0;
}