Made of little things.

import { cell, top } from './world';
import type { World } from './world';
type Point = { x: number; y: number };
type Project = (x: number, y: number, z: number) => Point;

function inside(point: Point, polygon: Point[]) {
  let sign = 0;
  for (let i = 0; i < polygon.length; i++) {
    const a = polygon[i], b = polygon[(i + 1) % polygon.length];
    const cross = (b.x-a.x)*(point.y-a.y)-(b.y-a.y)*(point.x-a.x);
    if (Math.abs(cross) < 0.001) continue;
    const next = Math.sign(cross);
    if (sign && next !== sign) return false;
    sign = next;
  }
  return true;
}

// Reverse the renderer's painter order: a visible wall must win over a hidden
// top face behind it. Selection still edits the top of this supported column.
export function pickColumn(world: World, project: Project, sx: number, sy: number) {
  const point = {x:sx,y:sy};
  for (let sum = 34; sum >= 0; sum--) {
    for (let x = 17; x >= 0; x--) {
      const z = sum-x;
      if (z < 0 || z > 17) continue;
      for (let y = top(world,x,z); y >= 0; y--) {
        if (!world.blocks[cell(x,y,z)]) continue;
        const a=project(x,y+1,z), b=project(x+1,y+1,z),
          d=project(x,y+1,z+1), e=project(x+1,y+1,z+1), bottom=project(x+1,y,z+1);
        if (inside(point,[a,b,e,d]) ||
            inside(point,[b,e,bottom,project(x+1,y,z)]) ||
            inside(point,[d,e,bottom,project(x,y,z+1)])) return {x,z};
      }
    }
  }
}