From 1293ea92bdbf3eb2086ec117e355714b6ee54ae9 Mon Sep 17 00:00:00 2001
From: A Farzat <a@farzat.xyz>
Date: Mon, 22 Jun 2026 19:56:40 +0300
Subject: Make Logger's output destination customizable

This aids in testing and might allow logging to a file at a later point.
---
 src/logger.rs | 119 +++++++++++++++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 109 insertions(+), 10 deletions(-)

(limited to 'src/logger.rs')

diff --git a/src/logger.rs b/src/logger.rs
index 0352d61..0a0bdc2 100644
--- a/src/logger.rs
+++ b/src/logger.rs
@@ -1,5 +1,10 @@
 //! Minimal stderr logging utilities.
 
+use std::{
+    cell::RefCell,
+    io::{self, Write},
+};
+
 /// Logging verbosity level.
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 pub enum Verbosity {
@@ -12,36 +17,130 @@ pub enum Verbosity {
 }
 
 /// Lightweight logger used by renderers and generators.
-#[derive(Copy, Clone, Debug, PartialEq, Eq)]
-pub struct Logger {
+#[derive(Debug)]
+pub struct Logger<W: Write> {
     verbosity: Verbosity,
+    writer: RefCell<W>,
 }
 
-impl Logger {
+impl Logger<io::Stderr> {
     /// Creates a logger with the given verbosity.
     pub fn new(verbosity: Verbosity) -> Self {
-        Self { verbosity }
+        Self {
+            verbosity,
+            writer: RefCell::new(io::stderr()),
+        }
+    }
+}
+
+impl<W: Write> Logger<W> {
+    pub fn with_writer(verbosity: Verbosity, writer: W) -> Self {
+        Self {
+            verbosity,
+            writer: RefCell::new(writer),
+        }
     }
 
     /// Emits a warning unless verbosity is [`Verbosity::Quiet`].
     pub fn warn(&self, message: impl AsRef<str>) {
         if self.verbosity != Verbosity::Quiet {
-            eprintln!("Warning: {}", message.as_ref());
+            let _ = writeln!(self.writer.borrow_mut(), "Warning: {}", message.as_ref());
         }
     }
 
     /// Emits an informational message only when verbosity is [`Verbosity::Verbose`].
     pub fn info(&self, message: impl AsRef<str>) {
         if self.verbosity == Verbosity::Verbose {
-            eprintln!("Info: {}", message.as_ref());
+            let _ = writeln!(self.writer.borrow_mut(), "Info: {}", message.as_ref());
         }
     }
 }
 
-impl Default for Logger {
-    fn default() -> Self {
-        Self {
-            verbosity: Verbosity::Quiet,
+#[cfg(test)]
+mod tests {
+    use std::io::{self, Write};
+
+    use super::{Logger, Verbosity};
+
+    fn logged_output(logger: &Logger<Vec<u8>>) -> String {
+        String::from_utf8(logger.writer.borrow().clone()).unwrap()
+    }
+
+    #[test]
+    fn quiet_suppresses_warnings() {
+        let logger = Logger::with_writer(Verbosity::Quiet, Vec::new());
+        logger.warn("skipped file");
+        assert_eq!(logged_output(&logger), "");
+    }
+
+    #[test]
+    fn quiet_suppresses_info_messages() {
+        let logger = Logger::with_writer(Verbosity::Quiet, Vec::new());
+        logger.info("rendering file");
+        assert_eq!(logged_output(&logger), "");
+    }
+
+    #[test]
+    fn normal_writes_warnings() {
+        let logger = Logger::with_writer(Verbosity::Normal, Vec::new());
+        logger.warn("skipped file");
+        assert_eq!(logged_output(&logger), "Warning: skipped file\n");
+    }
+
+    #[test]
+    fn normal_suppresses_info_messages() {
+        let logger = Logger::with_writer(Verbosity::Normal, Vec::new());
+        logger.info("rendering file");
+        assert_eq!(logged_output(&logger), "");
+    }
+
+    #[test]
+    fn verbose_writes_warnings() {
+        let logger = Logger::with_writer(Verbosity::Verbose, Vec::new());
+        logger.warn("skipped file");
+        assert_eq!(logged_output(&logger), "Warning: skipped file\n");
+    }
+
+    #[test]
+    fn verbose_writes_info_messages() {
+        let logger = Logger::with_writer(Verbosity::Verbose, Vec::new());
+        logger.info("rendering file");
+        assert_eq!(logged_output(&logger), "Info: rendering file\n");
+    }
+
+    #[test]
+    fn verbose_preserves_log_order() {
+        let logger = Logger::with_writer(Verbosity::Verbose, Vec::new());
+
+        logger.info("rendering a.rs");
+        logger.warn("skipping duplicate file: a.rs");
+        logger.info("rendering b.rs");
+
+        assert_eq!(
+            logged_output(&logger),
+            "Info: rendering a.rs\n\
+             Warning: skipping duplicate file: a.rs\n\
+             Info: rendering b.rs\n"
+        );
+    }
+
+    struct FailingWriter;
+
+    impl Write for FailingWriter {
+        fn write(&mut self, _buf: &[u8]) -> io::Result<usize> {
+            Err(io::Error::other("intentional write failure"))
         }
+
+        fn flush(&mut self) -> io::Result<()> {
+            Ok(())
+        }
+    }
+
+    #[test]
+    fn logging_ignores_writer_errors() {
+        let logger = Logger::with_writer(Verbosity::Verbose, FailingWriter);
+
+        logger.warn("this should not panic");
+        logger.info("this should not panic");
     }
 }
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