Aconcagua text decompressor

//(WIP) Aconcagua (PSX) text decompressor (and compressor later)
//First argument is input file and second argument is output file

#include <stdio.h>

#define BUFFER_SIZE 4096

void push(char*, char);

int main(int argc, char **argv) {
    FILE *input, *output;
    short int i;
    unsigned short int mask = 0;
    unsigned short int offset;
    char buf[BUFFER_SIZE];
    unsigned char newByte, byte1, byte2, length;
    for(i = 0; i < BUFFER_SIZE; i++) {
        buf[i] = 0x0;
    }

    if ((input  = fopen(argv[1], "rb")) == NULL) {
        printf("? %s\n", argv[1]);  return 1;
    }

    if ((output = fopen(argv[2], "wb")) == NULL) {
        printf("? %s\n", argv[2]);  return 1;
    }

    while((byte1 = fgetc(input)) != EOF) {
        if(!(mask & 0x0100)) {
            mask = byte1 | 0xFF00;
        } else if (mask & 0x01) {
            if ((newByte = fgetc(input)) != EOF) byte2 = newByte;
            offset = ((byte2 & 0x0F) << 8) | byte1;
            length = (byte2 >> 4) + 3; //this 3 is the minimum copy length

            for(i = 0; i < length; i++) {
                byte1 = buf[offset];
                fputc(byte1, output);
                push(buf, byte1);
            }

            mask >>= 1;
        } else {
            fputc(byte1, output);
            push(buf, byte1);
            mask >>= 1;
        }
    }

    return 0;
}

void push(char *array, char c) {
    int i;

    //begin loop in 1 because you can't move [0] to [-1]
    for(i = 1; i < BUFFER_SIZE; i++) {
        array[i - 1] = array[i];
    }

    array[BUFFER_SIZE - 1] = c;

}