Generate random, yet compressible data.

gen-data.c

static void gen_pre_buf(uint8_t * pre_buf, uint8_t (*get_byte)(void *), void * opaque) {
    int i;

    memset(pre_buf, 0, 1024);

    for (i = 0; i < 256; i++) {
        pre_buf[i + 256] = i;
        pre_buf[i * 2 + 512] = i;
    }

    for (i = 0; i < 384; i++) {
        uint8_t l = get_byte(opaque);
        uint8_t b = get_byte(opaque);
        pre_buf[l * 4 + (i & 3)] = b;
    }
}

void gen_data(uint8_t * out, size_t out_size, uint8_t (*get_byte)(void *), void * opaque) {
    gen_pre_buf(out, get_byte, opaque);
    out += 1024;

    while (1) {
        uint8_t mask;
        int i;

        mask = get_byte(opaque);

        for (i = 0; i < 8; i++) {
            if (mask & 1) {
                uint8_t b1, b2;
                uint16_t n;
                uint16_t offset;
                uint8_t * ptr;

                b1 = get_byte(opaque);
                b2 = get_byte(opaque);
                n = (b1 << 8) | b2;
                offset = (n >> 6) + 1;
                n &= 0x3f;
                n += 3;
                ptr = out - offset;
                if (n > out_size)
                    n = out_size;
                out_size -= n;
                while (n--) {
                    *out++ = *ptr++;
                }
            } else {
                *out++ = get_byte(opaque);
                out_size--;
            }
            if (out_size == 0)
                return;
            mask >>= 1;
        }
    }
}