1 files changed, 100 insertions, 0 deletions
diff --git a/src/model/collapse.rs b/src/model/collapse.rs
new file mode 100644
index 00000000..5e7a25f8
--- /dev/null
+++ b/src/model/collapse.rs
@@ -0,0 +1,100 @@
+use super::{StyleChain, StyleVec, StyleVecBuilder};
+
+/// A wrapper around a [`StyleVecBuilder`] that allows to collapse items.
+pub struct CollapsingBuilder<'a, T> {
+    builder: StyleVecBuilder<'a, T>,
+    staged: Vec<(T, StyleChain<'a>, Option<u8>)>,
+    last: Last,
+}
+
+/// What the last non-ignorant item was.
+#[derive(Debug, Copy, Clone, Eq, PartialEq)]
+enum Last {
+    Weak,
+    Destructive,
+    Supportive,
+}
+
+impl<'a, T> CollapsingBuilder<'a, T> {
+    /// Create a new style-vec builder.
+    pub fn new() -> Self {
+        Self {
+            builder: StyleVecBuilder::new(),
+            staged: vec![],
+            last: Last::Destructive,
+        }
+    }
+
+    /// Can only exist when there is at least one supportive item to its left
+    /// and to its right, with no destructive items or weak items in between to
+    /// its left and no destructive items in between to its right. There may be
+    /// ignorant items in between in both directions.
+    pub fn weak(&mut self, item: T, strength: u8, styles: StyleChain<'a>) {
+        if self.last != Last::Destructive {
+            if self.last == Last::Weak {
+                if let Some(i) = self
+                    .staged
+                    .iter()
+                    .position(|(.., prev)| prev.map_or(false, |p| p < strength))
+                {
+                    self.staged.remove(i);
+                } else {
+                    return;
+                }
+            }
+
+            self.staged.push((item, styles, Some(strength)));
+            self.last = Last::Weak;
+        }
+    }
+
+    /// Forces nearby weak items to collapse.
+    pub fn destructive(&mut self, item: T, styles: StyleChain<'a>) {
+        self.flush(false);
+        self.push(item, styles);
+        self.last = Last::Destructive;
+    }
+
+    /// Allows nearby weak items to exist.
+    pub fn supportive(&mut self, item: T, styles: StyleChain<'a>) {
+        self.flush(true);
+        self.push(item, styles);
+        self.last = Last::Supportive;
+    }
+
+    /// Has no influence on other items.
+    pub fn ignorant(&mut self, item: T, styles: StyleChain<'a>) {
+        self.staged.push((item, styles, None));
+    }
+
+    /// Iterate over the contained items.
+    pub fn items(&self) -> impl DoubleEndedIterator<Item = &T> {
+        self.builder.items().chain(self.staged.iter().map(|(item, ..)| item))
+    }
+
+    /// Return the finish style vec and the common prefix chain.
+    pub fn finish(mut self) -> (StyleVec<T>, StyleChain<'a>) {
+        self.flush(false);
+        self.builder.finish()
+    }
+
+    /// Push the staged items, filtering out weak items if `supportive` is false.
+    fn flush(&mut self, supportive: bool) {
+        for (item, styles, strength) in self.staged.drain(..) {
+            if supportive || strength.is_none() {
+                self.builder.push(item, styles);
+            }
+        }
+    }
+
+    /// Push a new item into the style vector.
+    fn push(&mut self, item: T, styles: StyleChain<'a>) {
+        self.builder.push(item, styles);
+    }
+}
+
+impl<'a, T> Default for CollapsingBuilder<'a, T> {
+    fn default() -> Self {
+        Self::new()
+    }
+}