feat: add distance_at_time function

src/fbp/distance_at_time.rs

@@ -0,0 +1,53 @@
+use std::f64::consts::E;
+
+use super::FbpFuelType;
+
+/// Calculate head spread distance of a fire at `time`
+///
+/// * `ros` - Rate of spread (m/min)
+/// * `time` - Elapsed time (min)
+/// * `cfb` - Crow fraction burned
+pub fn distance_at_time(fuel_type: FbpFuelType, ros: f64, time: f64, cfb: f64) -> f64 {
+    let alpha = match fuel_type {
+        FbpFuelType::C1
+        | FbpFuelType::O1a
+        | FbpFuelType::O1b
+        | FbpFuelType::S1
+        | FbpFuelType::S2
+        | FbpFuelType::S3
+        | FbpFuelType::D1 => 0.115,
+        _ => 0.115 - 18.8 * cfb.powf(2.5) * E.powf(-8. * cfb),
+    };
+
+    ros * (time + E.powf(-alpha * time) / alpha - 1. / alpha)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::test_util::precision_f64;
+
+    #[derive(Debug, serde::Deserialize)]
+    struct TestRow {
+        fuel_type: FbpFuelType,
+        ros: f64,
+        time: f64,
+        cfb: f64,
+        distance: f64,
+    }
+
+    #[test]
+    fn test_distance_at_time() -> Result<(), Box<dyn std::error::Error>> {
+        let fixture = std::fs::File::open("./tests/fixtures/distance_at_time.csv")?;
+        let mut rdr = csv::Reader::from_reader(fixture);
+
+        for result in rdr.deserialize() {
+            let record: TestRow = result?;
+            let distance = distance_at_time(record.fuel_type, record.ros, record.time, record.cfb);
+
+            assert_eq!(precision_f64(distance, 4), record.distance);
+        }
+
+        Ok(())
+    }
+}

src/fbp/mod.rs

@@ -5,6 +5,7 @@ mod crown_base_height;
 mod crown_fraction_burned;
 mod crown_fuel_consumption;
 mod crown_fuel_load;
+mod distance_at_time;
 mod fire_intensity;
 mod foliar_moisture_content;
 mod length_to_breadth;
@@ -19,6 +20,7 @@ pub use crown_base_height::*;
 pub use crown_fraction_burned::*;
 pub use crown_fuel_consumption::*;
 pub use crown_fuel_load::*;
+pub use distance_at_time::*;
 pub use fire_intensity::*;
 pub use foliar_moisture_content::*;
 pub use length_to_breadth::*;