add some comments

script.js

@@ -1572,6 +1572,9 @@ class Game {
   }
 }
 
+/**
+ * Base class for orb placer.
+ */
 class BaseOrbPlacer {
   constructor(game, template) {
     this.template = template;
@@ -1581,6 +1584,12 @@ class BaseOrbPlacer {
     this.board = game.board;
   }
 
+  /**
+   * Place an orb. The position must be inside template and free.
+   *
+   * @param n - position
+   * @param symbol - symbol to place
+   */
   placeOrb(n, symbol) {
     if (!this.templateMap[n]) {
       throw Error(`Position ${n} not allowed by template`);
@@ -1592,6 +1601,11 @@ class BaseOrbPlacer {
     this.board.grid[n] = symbol;
   }
 
+  /**
+   * Remove an orb, if any.
+   *
+   * @param n - position
+   */
   removeOrb(n) {
     let old = this.board.grid[n];
     this.board.grid[n] = null;
@@ -1599,10 +1613,12 @@ class BaseOrbPlacer {
     return old;
   }
 
+  /** Check if a cell is available for selection - 3 free slots */
   isAvailable(n) {
     return this.board.isAvailable(n);
   }
 
+  /** Get cell info by number */
   getCellInfo(n) {
     return this.board.getCellInfo(n);
   }
@@ -1627,6 +1643,13 @@ class BaseOrbPlacer {
     this.game.error(...args);
   }
 
+  /**
+   * Build a list of orbs in the right order.
+   * They are always grouped in matching pairs, and metals are always in
+   * the correct reaction order.
+   *
+   * @return {string[]}
+   */
   buildPlacementList() {
     let toPlace = [
       ['air', 'air'],
@@ -1713,7 +1736,9 @@ class BaseOrbPlacer {
     }, []);
   }
 
-
+  /**
+   * Run the placement logic.
+   */
   place() {
     this.board.removeAllOrbs();
     this.solution = [];
@@ -1747,7 +1772,8 @@ class BaseOrbPlacer {
     this.placeOrb(60, 'gold');
     this.solution.push(['gold', 60]);
 
-    let rv = this.doPlace();
+    let toPlace = this.buildPlacementList();
+    let rv = this.doPlace(toPlace);
 
     let solution = this.solution;
 
@@ -1765,12 +1791,42 @@ class BaseOrbPlacer {
     return rv;
   }
 
-  doPlace() {
+  /**
+   * Perform the orbs placement.
+   *
+   * Orb symbols to place are given as an argument.
+   *
+   * The layout template is in `this.template`, also `this.templateMap` as an array
+   * with indices 0-121 containing true/false to indicate template membership.
+   *
+   * `this.outsideTemplate` is a list of cell indices that are not in the template.
+   *
+   * this.solution is an array to populate with orb placements in the format [n, symbol].
+   *
+   * The board is cleared now. When the function ends, the board should contain
+   * the new layout (as symbol name strings)
+   *
+   * After placing each orb, make sure to call `this.solution.push([symbol, index])`;
+   * in case of backtracking, pop it again.
+   *
+   * Return object to return to parent; 'solution' will be added automatically.
+   */
+  doPlace(toPlace) {
     throw new Error("Not implemented");
   }
 }
 
+/**
+ * Orb placement algorithm that starts in the center and places orbs in rings, with some
+ * small jitter allowed.
+ */
 class RadialOrbPlacer extends BaseOrbPlacer {
+  /**
+   * Find a candidate cell
+   *
+   * @param {number[]|null} except - indices to exclude
+   * @return {number}
+   */
   findBestCandidate(except = null) {
     let candidates = [];
     for (let n of this.template) {
@@ -1810,6 +1866,12 @@ class RadialOrbPlacer extends BaseOrbPlacer {
     return false;
   }
 
+  /**
+   * Find index for next placement
+   *
+   * @param {number[]|null} except - indices to exclude
+   * @return {number} - index
+   */
   findAvailableIndex(except = null) {
     const n = this.findBestCandidate(except);
     if (n !== false) return n;
@@ -1846,9 +1908,8 @@ class RadialOrbPlacer extends BaseOrbPlacer {
     throw Error("Failed to find available board tile.");
   }
 
-  doPlace() {
+  doPlace(toPlace) {
     this.tilesAdded = 0;
-    const toPlace = this.buildPlacementList();
 
     while (toPlace.length > 0) {
       this.trace('placing a pair.');