Use the file name to parse the day value

src/template.txt

@@ -1,4 +1,4 @@
-advent_of_code::solution!(%DAY_NUMBER%);
+advent_of_code::solution!();
 
 pub fn part_one(input: &str) -> Option<u32> {
     None

src/template/commands/scaffold.rs

@@ -40,11 +40,7 @@ pub fn handle(day: Day, overwrite: bool) {
         }
     };
 
-    match file.write_all(
-        MODULE_TEMPLATE
-            .replace("%DAY_NUMBER%", &day.into_inner().to_string())
-            .as_bytes(),
-    ) {
+    match file.write_all(MODULE_TEMPLATE.as_bytes()) {
         Ok(()) => {
             println!("Created module file \"{}\"", &module_path);
         }

src/template/day.rs

@@ -24,7 +24,7 @@ pub struct Day(u8);
 impl Day {
     /// Creates a [`Day`] from the provided value if it's in the valid range,
     /// returns [`None`] otherwise.
-    pub fn new(day: u8) -> Option<Self> {
+    pub const fn new(day: u8) -> Option<Self> {
         if day == 0 || day > 25 {
             return None;
         }
@@ -150,6 +150,58 @@ macro_rules! day {
     }};
 }
 
+/// Creates a [`Day`] using the current file name.
+///
+/// The file name must be a valid integer in the range 1-25.
+#[macro_export]
+macro_rules! day_from_file_name {
+    () => {
+        const {
+            let mut path = ::core::panic::Location::caller().file().as_bytes();
+
+            // Get the last part of the path (e.g. "11.rs")
+            // path = path.rsplit_once(|c| c == '\\' || c == '/').map(|(_, s)| s).unwrap_or(path)
+            let mut i = path.len();
+            while i > 0 {
+                let c = path[i - 1];
+                if c == b'\\' || c == b'/' {
+                    path = path.split_at(i).1;
+                    break;
+                }
+
+                i -= 1;
+            }
+
+            // Remove the extension
+            // path = path.split_once('.').map(|(s, _)| s).unwrap_or(path)
+            let mut i = 0;
+            while i < path.len() {
+                if path[i] == b'.' {
+                    path = path.split_at(i).0;
+                    break;
+                }
+
+                i += 1;
+            }
+
+            // Convert the path back into a &str
+            // Note: as we only split at ascii chars (/, \ or .) the path should be a valid &str
+            let path = match ::core::str::from_utf8(path) {
+                Ok(path) => path,
+                Err(_) => unreachable!(),
+            };
+
+            let day = match ::core::primitive::u8::from_str_radix(path, 10) {
+                Ok(day) => day,
+                Err(_) => panic!("the file name is expected to be a number"),
+            };
+
+            $crate::template::Day::new(day)
+                .expect("the file name should be a number between 1 and 25")
+        }
+    };
+}
+
 /* -------------------------------------------------------------------------- */
 
 #[cfg(feature = "test_lib")]

src/template/mod.rs

@@ -42,18 +42,31 @@ pub fn read_file_part(folder: &str, day: Day, part: u8) -> String {
 #[macro_export]
 macro_rules! solution {
     ($day:expr) => {
-        $crate::solution!(@impl $day, [part_one, 1] [part_two, 2]);
+        $crate::solution!(@impl $crate::day!($day), [part_one, 1] [part_two, 2]);
     };
     ($day:expr, 1) => {
-        $crate::solution!(@impl $day, [part_one, 1]);
+        $crate::solution!(@impl $crate::day!($day), [part_one, 1]);
     };
     ($day:expr, 2) => {
-        $crate::solution!(@impl $day, [part_two, 2]);
+        $crate::solution!(@impl $crate::day!($day), [part_two, 2]);
+    };
+
+    () => {
+        $crate::solution!(@impl $crate::day_from_file_name!(), [part_one, 1] [part_two, 2]);
+    };
+    (*) => {
+        $crate::solution!();
+    };
+    (*, 1) => {
+        $crate::solution!(@impl $crate::day_from_file_name!(), [part_one, 1]);
+    };
+    (*, 2) => {
+        $crate::solution!(@impl $crate::day_from_file_name!(), [part_two, 2]);
     };
 
     (@impl $day:expr, $( [$func:expr, $part:expr] )*) => {
         /// The current day.
-        const DAY: $crate::template::Day = $crate::day!($day);
+        const DAY: $crate::template::Day = $day;
 
         #[cfg(feature = "dhat-heap")]
         #[global_allocator]