From 372db851665c9fa8e56f00e9181b0e42b288d6a8 Mon Sep 17 00:00:00 2001
From: Karsten Schmidt <k@thi.ng>
Date: Mon, 9 Jan 2023 14:05:47 +0100
Subject: [PATCH] feat(geom-clip-line): add clipPolylineWith()

---
 packages/geom-clip-line/package.json     |  3 +
 packages/geom-clip-line/src/clip-with.ts | 75 ++++++++++++++++++++++++
 packages/geom-clip-line/src/index.ts     |  1 +
 3 files changed, 79 insertions(+)
 create mode 100644 packages/geom-clip-line/src/clip-with.ts

diff --git a/packages/geom-clip-line/package.json b/packages/geom-clip-line/package.json
index f00fb51f5a..31d788b8f4 100644
--- a/packages/geom-clip-line/package.json
+++ b/packages/geom-clip-line/package.json
@@ -73,6 +73,9 @@
 		"./clip-poly": {
 			"default": "./clip-poly.js"
 		},
+		"./clip-with": {
+			"default": "./clip-with.js"
+		},
 		"./liang-barsky": {
 			"default": "./liang-barsky.js"
 		}
diff --git a/packages/geom-clip-line/src/clip-with.ts b/packages/geom-clip-line/src/clip-with.ts
new file mode 100644
index 0000000000..dd60b8262e
--- /dev/null
+++ b/packages/geom-clip-line/src/clip-with.ts
@@ -0,0 +1,75 @@
+import type { Predicate2 } from "@thi.ng/api";
+import type { ReadonlyVec } from "@thi.ng/vectors";
+
+/**
+ * Segments given polyline based on given predicate and `step` size. Returns
+ * array of segmented vertex chunks.
+ *
+ * @remarks
+ * Vertices are processed pairwise using given `step` size (default: 1). As long
+ * as the predicate is true, vertices are collected into the current chunk. If
+ * the predicate returns false, the current chunk is terminated and depending on
+ * `keepLast`, the current vertex (i.e. for which the predicate failed) is still
+ * added or not. If `keepLast` is false and the current chunk only included one
+ * other vertex, that entire chunk will be discarded.
+ *
+ * @example
+ * ```ts
+ * const pts = [[0, 0], [1, 0], [1, 1], [2, 1], [3, 1], [4, 0], [5, 0]];
+ *
+ * // isolate horizontal chunks
+ * clipPolylineWith((a, b) => a[1] == b[1], pts)
+ * // [
+ * //   [[0, 0], [1, 0]],
+ * //   [[1, 1], [2, 1], [3, 1]],
+ * //   [[4, 0], [5, 0]]
+ * // ]
+ *
+ * // isolate sloped chunks
+ * clipPolylineWith((a, b) => a[1] != b[1], pts)
+ * // [
+ * //   [[1, 0], [1, 1]],
+ * //   [[3, 1], [4, 0]]
+ * // ]
+ * ```
+ *
+ * @param pred
+ * @param pts
+ * @param step
+ * @param keepLast
+ */
+export const clipPolylineWith = (
+	pred: Predicate2<ReadonlyVec>,
+	pts: ReadonlyVec[],
+	step = 1,
+	keepLast = true
+) => {
+	const segs: ReadonlyVec[][] = [];
+	let last = -1;
+
+	const $terminate = (i: number) => {
+		if (last !== -1) {
+			if (keepLast) {
+				segs[segs.length - 1].push(pts[i]);
+			} else if (segs[segs.length - 1].length < 2) {
+				segs.pop();
+			}
+		}
+		last = -1;
+	};
+
+	for (let i = 0, n = pts.length - step; i < n; i++) {
+		if (pred(pts[i], pts[i + step])) {
+			if (last === -1) {
+				segs.push([pts[i]]);
+				last = i;
+			} else {
+				segs[segs.length - 1].push(pts[i]);
+			}
+		} else {
+			$terminate(i);
+		}
+	}
+	$terminate(pts.length - step);
+	return segs;
+};
diff --git a/packages/geom-clip-line/src/index.ts b/packages/geom-clip-line/src/index.ts
index 11d2fbcf12..76e2fc672c 100644
--- a/packages/geom-clip-line/src/index.ts
+++ b/packages/geom-clip-line/src/index.ts
@@ -1,2 +1,3 @@
 export * from "./clip-poly.js";
+export * from "./clip-with.js";
 export * from "./liang-barsky.js";