On the web, modify the current URL as we change maps and scenarios.

game/Cargo.toml

@@ -12,7 +12,7 @@ crate-type = ["cdylib", "lib"]
 
 [features]
 default = ["built", "map_gui/native", "widgetry/native-backend"]
-wasm = ["getrandom/js", "map_gui/wasm", "wasm-bindgen", "widgetry/wasm-backend"]
+wasm = ["getrandom/js", "map_gui/wasm", "wasm-bindgen", "web-sys", "widgetry/wasm-backend"]
 
 [dependencies]
 aabb-quadtree = "0.1.0"
@@ -45,6 +45,7 @@ serde_json = "1.0.61"
 svg_face = "0.1.3"
 sim = { path = "../sim" }
 wasm-bindgen = { version = "0.2.70", optional = true }
+web-sys = { version = "0.3.47", optional = true, features=["History", "Location", "Window"] }
 widgetry = { path = "../widgetry" }
 
 [build-dependencies]

game/src/common/mod.rs

@@ -1,5 +1,7 @@
 use std::collections::BTreeSet;
 
+use anyhow::Result;
+
 use geom::{Duration, Polygon};
 use map_gui::ID;
 use map_model::{IntersectionID, Map, RoadID};
@@ -407,3 +409,103 @@ pub fn checkbox_per_mode(
     }
     Widget::custom_row(filters)
 }
+
+/// This does nothing on native. On web, it modifies the current URL to change the first free
+/// parameter in the HTTP GET params to the specified value, adding it if needed.
+#[allow(unused_variables)]
+pub fn update_url(free_param: &str) -> Result<()> {
+    #[cfg(target_arch = "wasm32")]
+    {
+        let window = web_sys::window().ok_or(anyhow!("no window?"))?;
+        let url = window.location().href().map_err(|err| {
+            anyhow!(err
+                .as_string()
+                .unwrap_or("window.location.href failed".to_string()))
+        })?;
+        let new_url = change_url_free_query_param(url, free_param);
+
+        // Setting window.location.href may seem like the obvious thing to do, but that actually
+        // refreshes the page. This method just changes the URL and doesn't mess up history. See
+        // https://developer.mozilla.org/en-US/docs/Web/API/History_API/Working_with_the_History_API.
+        let history = window.history().map_err(|err| {
+            anyhow!(err
+                .as_string()
+                .unwrap_or("window.history failed".to_string()))
+        })?;
+        history
+            .replace_state_with_url(&wasm_bindgen::JsValue::NULL, "", Some(&new_url))
+            .map_err(|err| {
+                anyhow!(err
+                    .as_string()
+                    .unwrap_or("window.history.replace_state failed".to_string()))
+            })?;
+    }
+    Ok(())
+}
+
+#[allow(unused)]
+fn change_url_free_query_param(url: String, free_param: &str) -> String {
+    // The URL parsing crates I checked had lots of dependencies and didn't even expose such a nice
+    // API for doing this anyway.
+    let url_parts = url.split("?").collect::<Vec<_>>();
+    if url_parts.len() == 1 {
+        return format!("{}?{}", url, free_param);
+    }
+    let mut query_params = String::new();
+    let mut found_free = false;
+    let mut first = true;
+    for x in url_parts[1].split("&") {
+        if !first {
+            query_params.push('&');
+        }
+        first = false;
+
+        if x.starts_with("--") {
+            query_params.push_str(x);
+        } else if !found_free {
+            // Replace the first free parameter
+            query_params.push_str(free_param);
+            found_free = true;
+        } else {
+            query_params.push_str(x);
+        }
+    }
+    if !found_free {
+        if !first {
+            query_params.push('&');
+        }
+        query_params.push_str(free_param);
+    }
+
+    format!("{}?{}", url_parts[0], query_params)
+}
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn test_change_url() {
+        use super::change_url_free_query_param;
+
+        assert_eq!(
+            "http://0.0.0.0:8000/?--dev&seattle/maps/montlake.bin",
+            change_url_free_query_param(
+                "http://0.0.0.0:8000/?--dev".to_string(),
+                "seattle/maps/montlake.bin"
+            )
+        );
+        assert_eq!(
+            "http://0.0.0.0:8000/?--dev&seattle/maps/qa.bin",
+            change_url_free_query_param(
+                "http://0.0.0.0:8000/?--dev&seattle/maps/montlake.bin".to_string(),
+                "seattle/maps/qa.bin"
+            )
+        );
+        assert_eq!(
+            "http://0.0.0.0:8000?seattle/maps/montlake.bin",
+            change_url_free_query_param(
+                "http://0.0.0.0:8000".to_string(),
+                "seattle/maps/montlake.bin"
+            )
+        );
+    }
+}

game/src/sandbox/gameplay/freeform.rs

@@ -15,7 +15,7 @@ use widgetry::{
 };
 
 use crate::app::{App, Transition};
-use crate::common::CommonState;
+use crate::common::{update_url, CommonState};
 use crate::edit::EditMode;
 use crate::sandbox::gameplay::{GameplayMode, GameplayState};
 use crate::sandbox::{Actions, SandboxControls, SandboxMode};
@@ -26,7 +26,18 @@ pub struct Freeform {
 }
 
 impl Freeform {
-    pub fn new(ctx: &mut EventCtx) -> Box<dyn GameplayState> {
+    pub fn new(ctx: &mut EventCtx, app: &App) -> Box<dyn GameplayState> {
+        if let Err(err) = update_url(
+            app.primary
+                .map
+                .get_name()
+                .path()
+                .strip_prefix(&abstio::path(""))
+                .unwrap(),
+        ) {
+            warn!("Couldn't update URL: {}", err);
+        }
+
         Box::new(Freeform {
             top_center: Panel::empty(ctx),
         })

game/src/sandbox/gameplay/mod.rs

@@ -207,9 +207,9 @@ impl GameplayMode {
     /// after this, so each constructor doesn't need to.
     pub fn initialize(&self, ctx: &mut EventCtx, app: &mut App) -> Box<dyn GameplayState> {
         match self {
-            GameplayMode::Freeform(_) => freeform::Freeform::new(ctx),
+            GameplayMode::Freeform(_) => freeform::Freeform::new(ctx, app),
             GameplayMode::PlayScenario(_, ref scenario, ref modifiers) => {
-                play_scenario::PlayScenario::new(ctx, scenario, modifiers.clone())
+                play_scenario::PlayScenario::new(ctx, app, scenario, modifiers.clone())
             }
             GameplayMode::FixTrafficSignals => {
                 fix_traffic_signals::FixTrafficSignals::new(ctx, app)

game/src/sandbox/gameplay/play_scenario.rs

@@ -10,7 +10,7 @@ use widgetry::{
 };
 
 use crate::app::{App, Transition};
-use crate::common::checkbox_per_mode;
+use crate::common::{checkbox_per_mode, update_url};
 use crate::edit::EditMode;
 use crate::sandbox::gameplay::freeform::ChangeScenario;
 use crate::sandbox::gameplay::{GameplayMode, GameplayState};
@@ -25,9 +25,21 @@ pub struct PlayScenario {
 impl PlayScenario {
     pub fn new(
         ctx: &mut EventCtx,
+        app: &App,
         name: &String,
         modifiers: Vec<ScenarioModifier>,
     ) -> Box<dyn GameplayState> {
+        if let Err(err) = update_url(
+            // For dynamiclly generated scenarios like "random" and "home_to_work", this winds up
+            // making up a filename that doesn't actually exist. But if you pass that in, it winds
+            // up working, because we call abstio::parse_scenario_path() on the other side.
+            abstio::path_scenario(app.primary.map.get_name(), name)
+                .strip_prefix(&abstio::path(""))
+                .unwrap(),
+        ) {
+            warn!("Couldn't update URL: {}", err);
+        }
+
         Box::new(PlayScenario {
             top_center: Panel::empty(ctx),
             scenario_name: name.to_string(),