Minimum Window Substring LeetCode solution in Rust

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impl Solution {
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  pub fn min_window(s: String, t: String) -> String {
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    let mut state = [0 as isize; (b'z' - b'A' + 1) as usize];
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    let mut target = [0 as isize; (b'z' - b'A' + 1) as usize];
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    for b in t.bytes() {
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      target[(b - b'A') as usize] += 1;
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    }
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    let target = target;
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    let string = s.as_bytes();
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    let mut best_from = 0;
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    let mut best_to = 0;
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    let mut from = 0;
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    let mut to = 0;
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    let chars_expected = target.iter().filter(|&&ch| ch > 0).count();
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    let mut chars_found = 0;
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    while to < string.len() {
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      let ch = (string[to] - b'A') as usize;
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      if target[ch] > 0 {
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        if state[ch] + 1 == target[ch] {
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          chars_found += 1;
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        }
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        state[ch] += 1;
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      }
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      if chars_found == chars_expected {
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        let mut fch = (string[from] - b'A') as usize;
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        while target[fch] == 0 || state[fch] > target[fch] {
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          if state[fch] > 0 {
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            state[fch] -= 1;
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          }
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          from += 1;
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          fch = (string[from] - b'A') as usize;
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        }
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        if to - from < best_to - best_from || best_to == 0 {
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          best_from = from;
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          best_to = to + 1;
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        }
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      }
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      to += 1;
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    }
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    s[best_from..best_to].to_string()
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  }
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}