From 5ee3677fb77165ec1d9ea849bc3535d164fd7177 Mon Sep 17 00:00:00 2001
From: MathisWellmann <wellmannmathis@gmail.com>
Date: Tue, 9 Jun 2026 00:27:23 +0200
Subject: [PATCH] add `First`

---
 Cargo.lock           |   2 +-
 Cargo.toml           |   2 +-
 img/trend_flex.png   | Bin 94131 -> 94131 bytes
 src/rolling/first.rs |  47 +++++++++++++++++++++++++++++++++++++++++++
 src/rolling/mod.rs   |   2 ++
 5 files changed, 51 insertions(+), 2 deletions(-)
 create mode 100644 src/rolling/first.rs

diff --git a/Cargo.lock b/Cargo.lock
index 4bff427..5430dc1 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1237,7 +1237,7 @@ checksum = "0c790de23124f9ab44544d7ac05d60440adc586479ce501c1d6d7da3cd8c9cf5"
 
 [[package]]
 name = "sliding_features"
-version = "9.1.0"
+version = "9.2.0"
 dependencies = [
  "ballpark",
  "criterion",
diff --git a/Cargo.toml b/Cargo.toml
index 475b5e1..ec0456f 100644
--- a/Cargo.toml
+++ b/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"
diff --git a/img/trend_flex.png b/img/trend_flex.png
index e950d8d6e9d793e2840b154deb1d467202185f85..b02620d5dd3348b2a7eb8d061989b8e875eb5541 100644
GIT binary patch
delta 34
scmV+-0Nwwy-vzVZ1%R{xe_RmjuHyxwa@7O9*o*P_$McxCk6ZyH6FtZg@c;k-

delta 34
ocmdmdpLO$n)`l&N^)UkTA|xL$0D-5gpUXO@gr@ENF^nq00P8jk3IG5A

diff --git a/src/rolling/first.rs b/src/rolling/first.rs
new file mode 100644
index 0000000..3a07679
--- /dev/null
+++ b/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));
+    }
+}
diff --git a/src/rolling/mod.rs b/src/rolling/mod.rs
index 022d5aa..caab30d 100644
--- a/src/rolling/mod.rs
+++ b/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;