From d5f617e25c7ca3f2b76c1bf3bb5e576694251765 Mon Sep 17 00:00:00 2001
From: Paul Bienkowski <pb@opatut.de>
Date: Wed, 2 Dec 2020 21:10:41 +0100
Subject: [PATCH] feat: ignore bad distances when measuring track length

---
 src/_helpers/generators.js | 65 ++++++++++++++++++++++++++++++++++++++
 src/models/TrackData.js    | 46 ++++++++-------------------
 2 files changed, 78 insertions(+), 33 deletions(-)
 create mode 100644 src/_helpers/generators.js

diff --git a/src/_helpers/generators.js b/src/_helpers/generators.js
new file mode 100644
index 0000000..df5b241
--- /dev/null
+++ b/src/_helpers/generators.js
@@ -0,0 +1,65 @@
+function* pairwise(iter) {
+  let last;
+  let firstLoop = true;
+  for (const it of iter) {
+    if (firstLoop) {
+      firstLoop = false;
+    } else {
+      yield [last, it];
+    }
+    last = it;
+  }
+}
+
+function* enumerate(iter) {
+  let i = 0;
+  for (const it of iter) {
+    yield [i, it];
+    i++;
+  }
+}
+
+const map = (fn) =>
+  function* (iter) {
+    for (const [i, it] of enumerate(iter)) {
+      yield fn(it, i);
+    }
+  };
+
+const filter = (fn) =>
+  function* (iter) {
+    for (const it of iter) {
+      if (fn(it)) {
+        yield it;
+      }
+    }
+  };
+
+const reduce = (fn, init) => (iter) => {
+  let acc = init;
+  for (const it of iter) {
+    acc = fn(acc, it);
+  }
+  return acc;
+};
+
+const scan = (fn) =>
+  function* (iter, init) {
+    let acc = init;
+    for (const it of iter) {
+      acc = fn(acc, it);
+      yield acc;
+    }
+  };
+
+const flow = (...reducers) => (input) => reducers.reduce((c, fn) => fn(c), input);
+
+module.exports = {
+  filter,
+  map,
+  enumerate,
+  pairwise,
+  flow,
+  reduce,
+  scan,
+};
diff --git a/src/models/TrackData.js b/src/models/TrackData.js
index 2c7817e..9c92a90 100644
--- a/src/models/TrackData.js
+++ b/src/models/TrackData.js
@@ -2,6 +2,8 @@ const mongoose = require('mongoose');
 const uniqueValidator = require('mongoose-unique-validator');
 const turf = require('turf');
 
+const { flow, filter, map, pairwise, reduce } = require('../_helpers/generators');
+
 const schema = new mongoose.Schema(
   {
     slug: { type: String, lowercase: true, unique: true },
@@ -76,12 +78,17 @@ class TrackData extends mongoose.Model {
   }
 
   measureTrackLength() {
-    let totalLength = 0;
-    for (const [a, b] of pairwise(map(this.points, (p) => turf.point([p.longitude, p.latitude])))) {
-      const legLengthMeters = turf.distance(a, b) * 1000;
-      totalLength += legLengthMeters;
-    }
-    return totalLength;
+    return flow(
+      filter((p) => p.latitude != null && p.longitude != null),
+      map((p) => turf.point([p.longitude, p.latitude])),
+      pairwise,
+      map(([a, b]) => turf.distance(a, b) * 1000),
+
+      // Ignore distances between two points that are bigger than 100m, this
+      // must be a gap in the data or a mistake.
+      filter((d) => d <= 100),
+      reduce((c, d) => c + d, 0),
+    )(this.points);
   }
 
   get duration() {
@@ -93,33 +100,6 @@ class TrackData extends mongoose.Model {
   }
 }
 
-function* pairwise(iter) {
-  let last;
-  let firstLoop = true;
-  for (const it of iter) {
-    if (firstLoop) {
-      firstLoop = false;
-    } else {
-      yield [last, it];
-    }
-    last = it;
-  }
-}
-
-function* enumerate(iter) {
-  let i = 0;
-  for (const it of iter) {
-    yield [i, it];
-    i++;
-  }
-}
-
-function* map(iter, fn) {
-  for (const [i, it] of enumerate(iter)) {
-    yield fn(it, i);
-  }
-}
-
 mongoose.model(TrackData, schema);
 
 module.exports = TrackData;