feat: support environment variable containing session

fixes #8

README.md

@@ -89,7 +89,11 @@ obtain your session cookie, login to the
 [Advent of Code](https://adventofcode.com) website and inspect the `session`
 value of the cookie that gets stored in your browser. Put the session number (a
 long hex string) either in a file called `.adventofcode.session` in your home
-directory or `adventofcode.session` in your users config directory (`~/.config` on Linux, `~/Library/Application Support` on macOS or `~\AppData\Roaming` on Windows). This file should only contain your session number, in a single line.
+directory or `adventofcode.session` in your users config directory (`~/.config`
+on Linux, `~/Library/Application Support` on macOS or `~\AppData\Roaming` on
+Windows). This file should only contain your session number, in a single line.
+Alternatively you can export your session as the environment variable 
+`ADVENT_OF_CODE_SESSION`.
 
 ## Usage
 

src/main.rs

@@ -4,12 +4,14 @@ mod args;
 use aoc::*;
 use args::*;
 use clap::Parser;
+use std::env;
 use std::fs::read_to_string;
 use std::path::PathBuf;
 use std::process::exit;
 
 const SESSION_COOKIE_FILE: &str = "adventofcode.session";
 const HIDDEN_SESSION_COOKIE_FILE: &str = ".adventofcode.session";
+const SESSION_COOKIE_ENV: &str = "ADVENT_OF_CODE_SESSION";
 const DEFAULT_COL_WIDTH: usize = 80;
 
 fn main() -> Result<(), String> {
@@ -34,6 +36,8 @@ fn main() -> Result<(), String> {
 fn read_session_cookie(session_file: &Option<String>) -> String {
     let path = if let Some(file) = session_file {
         PathBuf::from(file)
+    } else if let Ok(cookie) = env::var(SESSION_COOKIE_ENV) {
+        cookie
     } else if let Some(file) = dirs::home_dir()
         .map(|dir| dir.join(HIDDEN_SESSION_COOKIE_FILE))
         .filter(|file| file.exists())