Master the Rail Fence Cipher in Rust: A Step‑by‑Step Mini Challenge

Learning Rust Through Mini Challenges

In this mini Rust challenge we solve a problem using Rust. The goal is to become familiar with reading and writing Rust code by tackling a series of small tasks. Join the daily challenges and follow along to solve each one.

Challenge Description

Implement the Rail Fence Cipher (a transposition cipher) for both encoding and decoding. The cipher writes the message in a zig‑zag pattern across a number of rails and then reads the rows sequentially.

For example, using three rails and the message "WE ARE DISCOVERED FLEE AT ONCE", the encryption writes:

W . . . E . . . C . . . R . . . L . . . T . . . E
. E . R . D . S . O . E . E . F . E . A . O . C .
. . A . . . I . . . V . . . D . . . E . . . N . .

Reading the rows yields the ciphertext: WECRLTEERDSOEEFEAOCAIVDEN To decrypt, you recreate the zig‑zag shape and fill the ciphertext row by row:

? . . . ? . . . ? . . . ? . . . ? . . . ? . . . ? . . . ?
. ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? .
. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . . . ? .

The first row receives the first seven characters "WECRLTE":

W . . . E . . . C . . . R . . . L . . . T . . . E
. ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? .
. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . .

The second row is filled with "ERDSOEEFEAOC":

W . . . E . . . C . . . R . . . L . . . T . . . E
. E . R . D . S . O . E . E . F . E . A . O . C .
. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . .

The final row receives "AIVDEN":

W . . . E . . . C . . . R . . . L . . . T . . . E
. E . R . D . S . O . E . E . F . E . A . O . C .
. . A . . . I . . . V . . . D . . . E . . . N . .

Reading the characters along the zig‑zag path reconstructs the original message.

Solution

Below is a preliminary Rust solution. There may be more idiomatic ways, and suggestions for improvement are welcome.

pub struct RailFence {
    rails: u32,
}

impl RailFence {
    pub fn new(rails: u32) -> RailFence {
        RailFence { rails }
    }

    pub fn en