feat(day01): day 1 solutions and initial commit

build.rs

@@ -0,0 +1,41 @@
+use std::{
+    fs::{self, File},
+    io::Write,
+    path::Path,
+};
+
+fn main() -> std::io::Result<()> {
+    let src_dir = Path::new("src");
+    let out_dir = src_dir;
+    let mut days = Vec::new();
+
+    // Scan for dayXX.rs files
+    for entry in fs::read_dir(src_dir)? {
+        let entry = entry?;
+        let path = entry.path();
+        if path.is_file() {
+            if let Some(name) = path.file_stem().and_then(|s| s.to_str()) {
+                if name.starts_with("day")
+                    && path.extension().and_then(|s| s.to_str()) == Some("rs")
+                {
+                    days.push(name.to_string());
+                }
+            }
+        }
+    }
+
+    // Sort days to maintain order
+    days.sort();
+
+    // Write the generated module declarations directly into src directory
+    let dest_path = Path::new(&out_dir).join("days_mod.inc");
+    let mut file = File::create(dest_path)?;
+
+    writeln!(file, "// generated by build.rs")?;
+    writeln!(file, "// do not include in source control!")?;
+    for day in days {
+        writeln!(file, "pub mod {};", day)?;
+    }
+
+    Ok(())
+}