add First

Cargo.toml

@@ -2,7 +2,7 @@ cargo-features = ["edition2024"]
 
 [package]
 name = "sliding_features"
-version = "9.1.0"
+version = "9.2.0"
 authors = ["MathisWellmann <wellmannmathis@gmail.com>"]
 edition = "2024"
 license-file = "LICENSE"

src/rolling/first.rs

@@ -0,0 +1,47 @@
+//! Retains the first value it sees and never updates again.
+
+use crate::View;
+
+/// Retains the first value it ever sees and never updates it again.
+#[derive(Default, Clone, Debug)]
+pub struct First<T> {
+    out: Option<T>,
+}
+
+impl<T> First<T> {
+    /// Create a new First View
+    #[inline(always)]
+    pub fn new() -> First<T> {
+        First { out: None }
+    }
+}
+
+impl<T: num::Float> View<T> for First<T> {
+    fn update(&mut self, val: T) {
+        debug_assert!(val.is_finite(), "value must be finite");
+        if self.out.is_none() {
+            self.out = Some(val);
+        }
+    }
+
+    fn last(&self) -> Option<T> {
+        self.out
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn first() {
+        let mut f = First::new();
+        assert_eq!(f.last(), None);
+        f.update(42.0);
+        assert_eq!(f.last(), Some(42.0));
+        f.update(7.0);
+        assert_eq!(f.last(), Some(42.0));
+        f.update(99.0);
+        assert_eq!(f.last(), Some(42.0));
+    }
+}

src/rolling/mod.rs

@@ -1,9 +1,11 @@
 //! This module contains `View` implementations that are updated on a rolling basis, but don't maintain a sliding window with history.
 
 mod drawdown;
+mod first;
 mod ln_return;
 mod welford_rolling;
 
 pub use drawdown::Drawdown;
+pub use first::First;
 pub use ln_return::LnReturn;
 pub use welford_rolling::WelfordRolling;