thi-ng/umbrella

import { intersectLinePolylineAll } from "@thi.ng/geom-isec/line-poly";
import { pointInPolygon2 } from "@thi.ng/geom-isec/point";
import { intersectRayPolylineAll } from "@thi.ng/geom-isec/ray-poly";
import type { ReadonlyVec, Vec } from "@thi.ng/vectors";
import { direction2 } from "@thi.ng/vectors/direction";

/**
 * Computes all intersection points of the **infinite** line defined by `a`, `b`
 * with the given polygon. Returns an array of segments where the line is inside
 * the polygon.
 *
 * @remarks
 * "Infinite" here means the line extends indefinitely on either side and
 * intersections are found also outside the segment a..b. If only intra-segment
 * intersections are desired, use {@link clipLineSegmentPoly} instead.
 *
 * @param a
 * @param b
 * @param poly
 */
export const clipLinePoly = (
    a: ReadonlyVec,
    b: ReadonlyVec,
    poly: ReadonlyVec[]
) => {
    const isecs = intersectRayPolylineAll(
        a,
        direction2([], a, b),
        poly,
        true
    ).isec;
    return isecs ? collectSegments(<Vec[]>isecs) : undefined;
};

/**
 * Similar to {@link clipLinePoly}, but only considers intersections within the
 * given line segment.
 *
 * @param a
 * @param b
 * @param poly
 */
export const clipLineSegmentPoly = (
    a: ReadonlyVec,
    b: ReadonlyVec,
    poly: ReadonlyVec[]
) => {
    const isecs = intersectLinePolylineAll(a, b, poly, true).isec;
    const isAInside = pointInPolygon2(a, poly);
    const isBInside = pointInPolygon2(b, poly);
    if (!isecs) {
        return isAInside && isBInside ? [[a, b]] : undefined;
    }
    isAInside && (<Vec[]>isecs).unshift(a);
    isBInside && (<Vec[]>isecs).push(b);
    return collectSegments(<Vec[]>isecs);
};

/**
 * Similar to {@link clipLineSegmentPoly}, but for polylines. Returns array of
 * new vertex chunks where the given polyline is inside the given bounding poly.
 *
 * @param pts
 * @param poly
 */
export const clipPolylinePoly = (pts: ReadonlyVec[], poly: ReadonlyVec[]) => {
    let res: ReadonlyVec[][] = [];
    if (pts.length < 2) return res;
    let isAInside = pointInPolygon2(pts[0], poly);
    for (let i = 0, n = pts.length - 1; i < n; i++) {
        const a = pts[i];
        const b = pts[i + 1];
        const isBInside = pointInPolygon2(b, poly);
        const isecs = intersectLinePolylineAll(a, b, poly, true).isec;
        if (!isecs) {
            if (isAInside && isBInside) {
                if (res.length) res[res.length - 1].push(b);
                else res.push([a, b]);
            }
        } else {
            isAInside && (<Vec[]>isecs).unshift(a);
            isBInside && (<Vec[]>isecs).push(b);
            const [first, ...segments] = collectSegments(<Vec[]>isecs);
            if (isAInside) {
                if (res.length) res[res.length - 1].push(first[1]);
                else res.push(first);
            } else {
                res.push(first);
            }
            res.push(...segments);
        }
        isAInside = isBInside;
    }
    return res;
};

const collectSegments = (isecs: Vec[]) => {
    const segments: Vec[][] = [];
    for (let i = 0, n = isecs.length - 1; i < n; i += 2) {
        segments.push([isecs[i], isecs[i + 1]]);
    }
    return segments;
};