Corrupted memory issue when deleting allocated memory - c++

Question

I am trying to store a sparse vector using a bit mask. I allocate a char* to represent the bit mask. However, when I delete [] the mask, I get a memory corruption error. Upon investigation, I'm seeing that it's because I'm freeing memory that I'm not supposed to. This is confusing, since I don't see how this could be the case.
When I run this on my case, it prints out "ALLOCATED" and "DEALLOCATING" but nothing further.
void set_i_bit(char* mask, int i) {
int field_num = floor(i/8);
int bit_num = i %8;
mask[field_num] = (1 << bit_num) | mask[field_num];
}
int write_sparse_with_bitmask(vector<float> arr, ofstream* fout) {
int mx_sz = arr.size() - 1;
float tol = 0.5;
char* mask = 0;
for(int i = arr.size() -1; i>=0; i-=1) {
if (fabs(arr[i]) > tol) break;
mx_sz = i;
}
int sprse_cnt = 0;
for(int i = 0; i<=mx_sz; i+=1) {
if (fabs(arr[i]) < tol) sprse_cnt++;
}
int bitmask_sz = ceil(mx_sz/8);
if (sprse_cnt*sizeof(int16_t) + sizeof(int16_t) > bitmask_sz) {
cout<<"ALLOCATED"<<endl;
mask = new char[bitmask_sz];
for (int i =0; i<bitmask_sz; i++) mask[i] = 0;
for(int i = 0; i<=mx_sz; i+=1) {
if (fabs(arr[i]) > coef_tol) {
set_i_bit(mask, i);
}
}
}
else {
bitmask_sz = 0;
}
uint16_t sz = mx_sz + 1;
uint16_t bt_msk = bitmask_sz + 1;
char flag = 0;
if (bitmask_sz > 0) {
flag = flag | 1;
}
fout->write((char*)&sz, sizeof(uint16_t));
fout->write((char*)&flag, sizeof(char));
int w_size = sizeof(uint16_t) + sizeof(char);
if (flag & 1) {
fout->write((char*)&bt_msk, sizeof(uint16_t));
fout->write(mask, sizeof(char)*bt_msk);
cout<<"DEALLOCATING"<<endl;
delete [] mask;
cout<<"THIS DOESN'T PRINT"<<endl;
w_size += sizeof(uint16_t) + sizeof(char)*bt_msk;
}
for(int i = 0; i<=mx_sz; i+=1) {
if (fabs(arr[i]) > tol || !(flag & 1)) {
int16_t vl = arr[i];
fout->write((char*) &vl, sizeof(int16_t));
w_size += sizeof(int16_t);
}
}
return w_size;
}