Cache TaM information on disk

.gitignore

@@ -1,3 +1,4 @@
 node_modules
 dist
 .cache
+cache

src/tam/sources/tam.js

@@ -1,5 +1,7 @@
 const csv = require("csv-parse");
 const axios = require("axios");
+const path = require("path");
+const fs = require("fs").promises;
 const { snakeToCamelCase, unzipFile } = require("../../util");
 
 /**
@@ -17,15 +19,65 @@ const { snakeToCamelCase, unzipFile } = require("../../util");
  * @property {string} directionId Route identifier inside the line.
  * @property {string} departureTime Theoretical time at which the
  * vehicle will depart the stop (HH:MM:SS format).
- * @property {string} isTheorical (sic) Whether the arrival time is only
- * a theoretical information.
+ * @property {string} isTheorical (sic) True if this is only the planned
+ * passing time, false if this is real-time information.
  * @property {string} delaySec Number of seconds before the vehicle arrives
- * at the station.
+ * at the station (only if isTheorical is false).
  * @property {string} destArCode Unique network identifier for the final
- * stop of this trip.
+ * stop of this trip (only if isTheorical is false).
  */
 
+/**
+ * Wrap a passing-fetching function to use a filesystem-based cache.
+ *
+ * @param {function} func Fetching function to wrap.
+ * @param {string} cachePath Path to the file to use as a cache (will be
+ * overwritten, may be non-existing).
+ * @return {function} Wrapped function.
+ */
+const makeCached = (func, cachePath) => {
+    return async function *() {
+        try {
+            const cacheRaw = await fs.readFile(cachePath, {encoding: "utf8"});
+            const cache = JSON.parse(cacheRaw);
+
+            if (Date.now() < cache.timing.nextUpdate) {
+                yield cache.timing;
+
+                for (const passing of cache.passings) {
+                    yield passing;
+                }
+
+                return;
+            }
+        } catch (err) {
+            // Ignore missing cache file
+            if (err.code !== 'ENOENT') {
+                throw err;
+            }
+        }
+
+        const passings = func();
+        const newCache = {
+            timing: (await passings.next()).value,
+            passings: [],
+        };
+
+        yield newCache.timing;
+
+        for await (const passing of passings) {
+            newCache.passings.push(passing);
+            yield passing;
+        }
+
+        fs.writeFile(cachePath, JSON.stringify(newCache));
+    };
+};
+
+const cacheDir = path.join(__dirname, "..", "..", "..", "cache");
+
 const realtimeEndpoint = "http://data.montpellier3m.fr/node/10732/download";
+const realtimeCachePath = path.join(cacheDir, "realtime.json");
 
 /**
  * Fetch real time passings of vehicles across the network.
@@ -54,9 +106,10 @@ const fetchRealtime = async function *() {
     }
 };
 
-exports.fetchRealtime = fetchRealtime;
+exports.fetchRealtime = makeCached(fetchRealtime, realtimeCachePath);
 
 const theoreticalEndpoint = "http://data.montpellier3m.fr/node/10731/download";
+const theoreticalCachePath = path.join(cacheDir, "theoretical.json");
 
 /**
  * Fetch theoretical passings for the current day across the network.
@@ -101,4 +154,4 @@ const fetchTheoretical = async function *() {
     }
 };
 
-exports.fetchTheoretical = fetchTheoretical;
+exports.fetchTheoretical = makeCached(fetchTheoretical, theoreticalCachePath);