## 519. Random Flip Matrix

You are given the number of rows `n_rows` and number of columns `n_cols` of a 2D binary matrix where all values are initially 0. Write a function `flip` which chooses a 0 value uniformly at random, changes it to 1, and then returns the position `[row.id, col.id]` of that value. Also, write a function `reset` which sets all values back to 0. Try to minimize the number of calls to system's Math.random() and optimize the time and space complexity.

Note:

1. `1 <= n_rows, n_cols <= 10000`
2. `0 <= row.id < n_rows` and `0 <= col.id < n_cols`
3. `flip` will not be called when the matrix has no 0 values left.
4. the total number of calls to `flip` and `reset` will not exceed 1000.

Example 1:

```Input:
["Solution","flip","flip","flip","flip"]
[[2,3],[],[],[],[]]
Output: [null,[0,1],[1,2],[1,0],[1,1]]
```

Example 2:

```Input:
["Solution","flip","flip","reset","flip"]
[[1,2],[],[],[],[]]
Output: [null,[0,0],[0,1],null,[0,0]]```

Explanation of Input Syntax:

The input is two lists: the subroutines called and their arguments. `Solution`'s constructor has two arguments, `n_rows` and `n_cols``flip` and `reset` have no arguments. Arguments are always wrapped with a list, even if there aren't any.

## Rust Solution

``````use rand::prelude::*;
use std::collections::HashMap;

struct Solution {
size: usize,
indexes: HashMap<usize, usize>,
rng: ThreadRng,
rows: usize,
cols: usize,
}

impl Solution {
fn new(n_rows: i32, n_cols: i32) -> Self {
let rows = n_rows as usize;
let cols = n_cols as usize;
let size = rows * cols;
let indexes = HashMap::new();
let rng = thread_rng();
Solution {
size,
indexes,
rng,
rows,
cols,
}
}

fn flip(&mut self) -> Vec<i32> {
let r = self.rng.gen_range(0, self.size);
let x = *self.indexes.entry(r).or_insert(r);
self.size -= 1;
let y = *self.indexes.entry(self.size).or_insert(self.size);
*self.indexes.entry(r).or_default() = y;
let col = x % self.cols;
let row = x / self.cols;
vec![row as i32, col as i32]
}

fn reset(&mut self) {
self.size = self.rows * self.cols;
self.indexes = HashMap::new();
}
}
``````

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