Quickly add some new walking uptake functions

docs/config_uptake.md

@@ -7,3 +7,5 @@ The possible values for `"uptake"`:
 - `"identity"` -- every route counts as 1, equivalent to just counting every trip
 - `{ "CutoffMaxDistanceMeters": 16000 }` -- trips over 16km are skipped entirely, otherwise they count as 1
 - `"GovTargetPCT"` and `"GoDutchPCT"` are uptake models from the PCT, using distance and gradient (**currently hardcoded to 0**)
+
+TODO: New walking uptake functions not described yet

od2net/src/config.rs

@@ -150,6 +150,11 @@ pub enum Uptake {
     GovTargetPCT,
     /// Defined by https://github.com/ITSLeeds/pct/blob/HEAD/R/uptake.R
     GoDutchPCT,
+    // TODO describe
+    WalkToSchool {
+        upper_limit: f64,
+        exponent: f64,
+    },
 }
 
 #[derive(Serialize, Deserialize)]

od2net/src/plugins/uptake.rs

@@ -16,6 +16,10 @@ pub fn calculate_uptake(uptake: &Uptake, total_distance_meters: f64) -> f64 {
         }
         Uptake::GovTargetPCT => pct_gov_target(total_distance_meters, gradient),
         Uptake::GoDutchPCT => pct_go_dutch(total_distance_meters, gradient),
+        Uptake::WalkToSchool {
+            upper_limit,
+            exponent,
+        } => walk_to_school(total_distance_meters, *upper_limit, *exponent),
     }
 }
 
@@ -78,6 +82,12 @@ fn inverse_logit(p: f64) -> f64 {
     result
 }
 
+fn walk_to_school(distance_meters: f64, upper_limit: f64, exponent: f64) -> f64 {
+    let distance_km = distance_meters / 1000.0;
+    let p = (-distance_km * exponent).exp();
+    p.min(upper_limit)
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;