From 7096c6c2608b2f5af07f0f06fdbfbc365224091d Mon Sep 17 00:00:00 2001
From: David Buezas <david.buezas@gmail.com>
Date: Sat, 7 Jan 2023 16:25:37 +0100
Subject: [PATCH] implement delta filter

---
 readme.md              |  1 +
 src/filters/filters.ts | 22 ++++++++++++++++++++--
 2 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/readme.md b/readme.md
index 016ed59..ceee5b1 100644
--- a/readme.md
+++ b/readme.md
@@ -417,6 +417,7 @@ entities:
       - 0.0 -> 0.0
       - 40.0 -> 45.0
       - 100.0 -> 102.5
+    - delata # computes the delta between each two consecutive numeric y values.
     - derivate: h # computes rate of change per unit of time: h # ms (milisecond), s (second), m (minute), h (hour), d (day), w (week), M (month), y (year)
     - integrate: h # computes area under the curve per unit of time using Right hand riemann integration. Same units as the derivative
     - map_y_numbers: Math.sqrt(y + 10*100) # map the y coordinate of each datapoint.
diff --git a/src/filters/filters.ts b/src/filters/filters.ts
index c506b00..448650e 100644
--- a/src/filters/filters.ts
+++ b/src/filters/filters.ts
@@ -75,6 +75,24 @@ const filters = {
         meta: { ...meta, regression },
       };
     },
+  delta:
+    () =>
+    ({ ys, meta }) => {
+      const last = {
+        y: NaN,
+      };
+      return {
+        meta: {
+          ...meta,
+          unit_of_measurement: `Δ${meta.unit_of_measurement}`,
+        },
+        ys: mapNumbers(ys, (y) => {
+          const yDelta = y - last.y;
+          last.y = y;
+          return yDelta;
+        }),
+      };
+    },
   derivate:
     (unit: keyof typeof timeUnits = "h") =>
     ({ xs, ys, meta }) => {
@@ -91,10 +109,10 @@ const filters = {
         ys: mapNumbers(ys, (y, i) => {
           const x = +xs[i];
           const dateDelta = (x - last.x) / timeUnits[unit];
-          const yDelta = (y - last.y) / dateDelta;
+          const yDeriv = (y - last.y) / dateDelta;
           last.y = y;
           last.x = x;
-          return yDelta;
+          return yDeriv;
         }),
       };
     },