Add trim filter

sailfish/src/runtime/filter.rs

@@ -1,6 +1,9 @@
+//! Build-in filters
+
 // TODO: performance improvement
 
 use std::fmt;
+use std::ptr;
 
 use super::{Buffer, Render, RenderError};
 
@@ -86,6 +89,60 @@ pub fn lower<T: Render>(expr: &T) -> Lower<T> {
     Lower(expr)
 }
 
+pub struct Trim<'a, T>(&'a T);
+
+impl<'a, T: Render> Render for Trim<'a, T> {
+    #[inline]
+    fn render(&self, b: &mut Buffer) -> Result<(), RenderError> {
+        let old_len = b.len();
+        self.0.render(b)?;
+        trim_impl(b, old_len);
+        Ok(())
+    }
+
+    #[inline]
+    fn render_escaped(&self, b: &mut Buffer) -> Result<(), RenderError> {
+        let old_len = b.len();
+        self.0.render_escaped(b)?;
+        trim_impl(b, old_len);
+        Ok(())
+    }
+}
+
+fn trim_impl(b: &mut Buffer, old_len: usize) {
+    debug_assert!(b.len() >= old_len);
+    let new_contents = &b.as_str()[old_len..];
+    let trimmed = new_contents.trim();
+    let trimmed_len = trimmed.len();
+
+    if new_contents.len() != trimmed_len {
+        // performs inplace trimming
+
+        if new_contents.as_ptr() != trimmed.as_ptr() {
+            debug_assert!(new_contents.as_ptr() < trimmed.as_ptr());
+            let offset = trimmed.as_ptr() as usize - new_contents.as_ptr() as usize;
+            unsafe {
+                ptr::copy(
+                    b.as_mut_ptr().add(old_len + offset),
+                    b.as_mut_ptr().add(old_len),
+                    trimmed_len,
+                );
+            }
+        }
+
+        debug_assert!(b.capacity() >= old_len + trimmed_len);
+        unsafe {
+            b._set_len(old_len + trimmed_len);
+        }
+    }
+}
+
+/// convert the rendered contents to lowercase
+#[inline]
+pub fn trim<T: Render>(expr: &T) -> Trim<T> {
+    Trim(expr)
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -104,4 +161,23 @@ mod tests {
         lower(&"<h1>TITLE</h1>").render_escaped(&mut buf).unwrap();
         assert_eq!(buf.as_str(), "&lt;h1&gt;title&lt;/h1&gt;");
     }
+
+    #[test]
+    fn trim_test() {
+        let mut buf = Buffer::new();
+        trim(&" hello  ").render(&mut buf).unwrap();
+        trim(&"hello ").render(&mut buf).unwrap();
+        trim(&"   hello").render(&mut buf).unwrap();
+        assert_eq!(buf.as_str(), "hellohellohello");
+
+        let mut buf = Buffer::new();
+        trim(&"hello ").render(&mut buf).unwrap();
+        trim(&" hello").render(&mut buf).unwrap();
+        trim(&"hello").render(&mut buf).unwrap();
+        assert_eq!(buf.as_str(), "hellohellohello");
+
+        let mut buf = Buffer::new();
+        trim(&" hello").render(&mut buf).unwrap();
+        assert_eq!(buf.as_str(), "hello");
+    }
 }