Add loop.changed() (#91)

minijinja/src/lib.rs

@@ -110,7 +110,8 @@
 //! - `sync`: this feature makes MiniJinja's type `Send` and `Sync`.  If this feature
 //!   is disabled sending types across threads is often not possible.  Thread bounds
 //!   of things like callbacks however are not changing which means code that uses
-//!   MiniJinja still needs to be threadsafe.
+//!   MiniJinja still needs to be threadsafe.  This also disables some features that
+//!   require synchronization such as the `loop.changed` feature.
 //! - `debug`: if this feature is removed some debug functionality of the engine is
 //!   removed as well.  This mainly affects the quality of error reporting.
 //! - `key_interning`: if this feature is removed the automatic string interning in

minijinja/src/syntax.rs

@@ -195,16 +195,31 @@
 //! - `loop.cycle`: A helper function to cycle between a list of sequences. See the explanation below.
 //! - `loop.depth`: Indicates how deep in a recursive loop the rendering currently is. Starts at level 1
 //! - `loop.depth0`: Indicates how deep in a recursive loop the rendering currently is. Starts at level 0
+//! - `loop.changed(...args)`: Returns true if the passed values have changed since the last time it was called with the same arguments.
+//! - `loop.cycle(...args)`: Returns a value from the passed sequence in a cycle.
 //!
 //! Within a for-loop, it’s possible to cycle among a list of strings/variables each time through
-//! the loop by using the special loop.cycle helper:
+//! the loop by using the special `loop.cycle` helper:
 //!
 //! ```jinja
 //! {% for row in rows %}
 //!   <li class="{{ loop.cycle('odd', 'even') }}">{{ row }}</li>
 //! {% endfor %}
 //! ```
 //!
+//! If the `sync` feature is not disabled, the `loop.changed` helper is also available
+//! which can be used to detect when a value changes between the last iteration and the
+//! current one.  The method takes one or more arguments that are all compared.
+//!
+//! ```jinja
+//! {% for entry in entries %}
+//!   {% if loop.changed(entry.category) %}
+//!     <h2>{{ entry.category }}</h2>
+//!   {% endif %}
+//!   <p>{{ entry.message }}</p>
+//! {% endfor %}
+//! ```
+//!
 //! Unlike in Rust or Python, it’s not possible to break or continue in a loop. You can,
 //! however, filter the sequence during iteration, which allows you to skip items.  The
 //! following example skips all the users which are hidden:

minijinja/src/vm.rs

@@ -1,6 +1,8 @@
 use std::collections::{BTreeMap, HashSet};
 use std::fmt::{self, Write};
 use std::sync::atomic::{AtomicUsize, Ordering};
+#[cfg(feature = "sync")]
+use std::sync::Mutex;
 
 use crate::environment::Environment;
 use crate::error::{Error, ErrorKind};
@@ -16,6 +18,8 @@ pub struct LoopState {
     len: usize,
     idx: AtomicUsize,
     depth: usize,
+    #[cfg(feature = "sync")]
+    last_changed_value: Mutex<Option<Vec<Value>>>,
 }
 
 impl fmt::Debug for LoopState {
@@ -68,6 +72,19 @@ impl Object for LoopState {
     }
 
     fn call_method(&self, _state: &State, name: &str, args: Vec<Value>) -> Result<Value, Error> {
+        #[cfg(feature = "sync")]
+        {
+            if name == "changed" {
+                let mut last_changed_value = self.last_changed_value.lock().unwrap();
+                let changed = last_changed_value.as_ref() != Some(&args);
+                if changed {
+                    *last_changed_value = Some(args);
+                    return Ok(Value::from(true));
+                }
+                return Ok(Value::from(false));
+            }
+        }
+
         if name == "cycle" {
             let idx = self.idx.load(Ordering::Relaxed);
             match args.get(idx % args.len()) {
@@ -767,6 +784,8 @@ impl<'env> Vm<'env> {
                                 idx: AtomicUsize::new(!0usize),
                                 len,
                                 depth,
+                                #[cfg(feature = "sync")]
+                                last_changed_value: Mutex::default(),
                             }),
                         }),
                         ..Frame::default()

minijinja/tests/test_templates.rs

@@ -118,3 +118,17 @@ fn test_auto_escaping() {
     let rv = tmpl.render(context!(var => "foo\"bar'baz")).unwrap();
     insta::assert_snapshot!(rv, @r###"foo"bar'baz"###);
 }
+
+#[cfg(feature = "sync")]
+#[test]
+fn test_loop_changed() {
+    let rv = minijinja::render!(
+        r#"
+        {%- for i in items -%}
+          {% if loop.changed(i) %}{{ i }}{% endif %}
+        {%- endfor -%}
+        "#,
+        items => vec![1, 1, 1, 2, 3, 4, 4, 5],
+    );
+    assert_eq!(rv, "12345");
+}