perf(map): optimize geoPathsFor method

packages/@ourworldindata/grapher/src/mapCharts/GeoPathRoundingContext.ts

@@ -0,0 +1,38 @@
+import { GeoContext } from "d3-geo"
+
+// Can be used as a d3 projection context to convert a geojson feature to a SVG path
+// In contrast to what d3-geo does by default, it will round all coordinates to one decimal place
+// Adapted from https://github.com/d3/d3-geo/blob/8d3f3a98c034b087e2c808f752d2381d51c30015/src/path/string.js
+
+// FUTURE: Once we update to d3-geo@^3.10, d3-geo can do this by default: https://github.com/d3/d3-geo/pull/272
+
+// Rounds to one decimal place
+const round = (x: number): number => Math.round(x * 10) / 10
+
+export class GeoPathRoundingContext implements GeoContext {
+    _string: string = ""
+
+    beginPath(): void {
+        this._string = ""
+    }
+
+    moveTo(x: number, y: number): void {
+        this._string += `M${round(x)},${round(y)}`
+    }
+
+    lineTo(x: number, y: number): void {
+        this._string += `L${round(x)},${round(y)}`
+    }
+
+    arc(_x: number, _y: number, _radius: number): void {
+        throw new Error("Method not implemented.")
+    }
+
+    closePath(): void {
+        this._string += "Z"
+    }
+
+    result(): string {
+        return this._string
+    }
+}

packages/@ourworldindata/grapher/src/mapCharts/MapChart.tsx

@@ -4,7 +4,6 @@ import {
     DEFAULT_BOUNDS,
     flatten,
     getRelativeMouse,
-    identity,
     sortBy,
     guid,
     minBy,
@@ -24,6 +23,7 @@ import {
     HorizontalNumericColorLegend,
 } from "../horizontalColorLegend/HorizontalColorLegends"
 import { MapProjectionName, MapProjectionGeos } from "./MapProjections"
+import { GeoPathRoundingContext } from "./GeoPathRoundingContext"
 import { select } from "d3-selection"
 import { easeCubic } from "d3-ease"
 import { MapTooltip } from "./MapTooltip"
@@ -96,25 +96,18 @@ const geoPathCache = new Map<MapProjectionName, string[]>()
 const geoPathsFor = (projectionName: MapProjectionName): string[] => {
     if (geoPathCache.has(projectionName))
         return geoPathCache.get(projectionName)!
-    const projectionGeo = MapProjectionGeos[projectionName]
-    const strs = GeoFeatures.map((feature) => {
-        const s = projectionGeo(feature) as string
-        const paths = s.split(/Z/).filter(identity)
-
-        const newPaths = paths.map((path) => {
-            const points = path.split(/[MLZ]/).filter((f: any) => f)
-            const rounded = points.map((point) =>
-                point
-                    .split(/,/)
-                    .map((v) => parseFloat(v).toFixed(1))
-                    .join(",")
-            )
-            return "M" + rounded.join("L")
-        })
 
-        return newPaths.join("Z") + "Z"
+    // Use this context to round the path coordinates to a set number of decimal places
+    const ctx = new GeoPathRoundingContext()
+    const projectionGeo = MapProjectionGeos[projectionName].context(ctx)
+    const strs = GeoFeatures.map((feature) => {
+        ctx.beginPath() // restart the path
+        projectionGeo(feature)
+        return ctx.result()
     })
 
+    projectionGeo.context(null) // reset the context for future calls
+
     geoPathCache.set(projectionName, strs)
     return geoPathCache.get(projectionName)!
 }