feat(solutions): add first-non-repeating-character/rust/function

challenges/first-non-repeating-character/solutions/rust/function/Cargo.lock

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+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "rust_application"
+version = "1.0.0"

challenges/first-non-repeating-character/solutions/rust/function/Cargo.toml

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+[package]
+name = "rust_application"
+version = "1.0.0"
+edition = "2021"
+
+[dependencies]

challenges/first-non-repeating-character/solutions/rust/function/README.md

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+# first-non-repeating-character/rust/function
+
+Created by [@Divlo](https://github.com/Divlo) on 5 January 2023.

challenges/first-non-repeating-character/solutions/rust/function/src/main.rs

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+use std::{collections::HashMap, io};
+
+#[derive(Debug, Copy, Clone)]
+pub struct CharacterOccurence {
+    pub total: usize,
+    pub first_index: usize,
+    pub value: char,
+}
+
+pub fn first_non_repeating_character(string: &str) -> String {
+    let mut characters_occurences: HashMap<char, CharacterOccurence> = HashMap::new();
+    for (index, character) in string.chars().enumerate() {
+        match characters_occurences.get(&character) {
+            Some(value) => {
+                characters_occurences.insert(
+                    character,
+                    CharacterOccurence {
+                        total: value.total + 1,
+                        ..value.clone()
+                    },
+                );
+            }
+            None => {
+                characters_occurences.insert(
+                    character,
+                    CharacterOccurence {
+                        total: 1,
+                        first_index: index,
+                        value: character,
+                    },
+                );
+            }
+        }
+    }
+    let mut result: Option<CharacterOccurence> = Option::None;
+    for (_, character_occurence) in &characters_occurences {
+        if character_occurence.total == 1 {
+            match result {
+                Some(ref mut result) => {
+                    if character_occurence.first_index < result.first_index {
+                        *result = *character_occurence;
+                    }
+                }
+                None => {
+                    result = Some(*character_occurence);
+                }
+            }
+        }
+    }
+    match result {
+        Some(result) => result.value.to_string(),
+        None => "".to_string(),
+    }
+}
+
+fn main() {
+    let mut string = String::new();
+    io::stdin()
+        .read_line(&mut string)
+        .expect("Failed to read `stdin` line.");
+    let string = string.trim();
+    println!("{}", first_non_repeating_character(&string));
+}