feat(solutions): add caesar-cipher/rust/function

challenges/caesar-cipher/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/caesar-cipher/solutions/rust/function/Cargo.toml

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

challenges/caesar-cipher/solutions/rust/function/README.md

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+# caesar-cipher/rust/function
+
+Created by [@Divlo](https://github.com/Divlo) on 4 January 2023.

challenges/caesar-cipher/solutions/rust/function/src/main.rs

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+use std::io;
+
+pub struct ShiftedLetter {
+    pub origin: char,
+    pub shifted: char,
+}
+
+pub fn shift_alphabet(shift: i32) -> Vec<ShiftedLetter> {
+    let mut shift = shift;
+    let mut result: Vec<ShiftedLetter> = Vec::new();
+    let mut alphabet: Vec<char> = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".chars().collect();
+    let is_negative_shift = shift < 0;
+    if is_negative_shift {
+        alphabet.reverse();
+        shift = shift.abs();
+    }
+    for (index, letter) in alphabet.iter().enumerate() {
+        let mut shifted_index = (index as i32 + shift) as i32;
+        if shifted_index > (alphabet.len() - 1) as i32 {
+            shifted_index = (alphabet.len() as i32 - shifted_index).abs();
+        }
+        let shifted_letter = alphabet[shifted_index as usize];
+        result.push(ShiftedLetter {
+            origin: *letter,
+            shifted: shifted_letter,
+        });
+    }
+    return result;
+}
+
+pub fn caesar_cipher(string: &str, shift: i32) -> String {
+    if shift == 0 {
+        return string.to_string();
+    }
+    let shifted_alphabet = shift_alphabet(shift);
+    let mut result = String::new();
+    for letter in string.chars() {
+        if letter.is_whitespace() {
+            result.push(letter);
+        } else {
+            let mut found_letter = &shifted_alphabet[0];
+            for shifted_letter in shifted_alphabet.iter() {
+                if shifted_letter.origin == letter {
+                    found_letter = shifted_letter;
+                    break;
+                }
+            }
+            result.push(found_letter.shifted);
+        }
+    }
+    return result;
+}
+
+fn main() {
+    let mut string = String::new();
+    io::stdin()
+        .read_line(&mut string)
+        .expect("Failed to read `stdin` line.");
+    let string = string.trim();
+
+    let mut shift = String::new();
+    io::stdin()
+        .read_line(&mut shift)
+        .expect("Failed to read `stdin` line.");
+    let shift: i32 = shift
+        .trim()
+        .parse()
+        .expect("Failed to convert `width` as an `i32`.");
+    println!("{}", caesar_cipher(&string, shift));
+}