chore: Document and refactor ForkContext (#17566)

* chore: Document and refactor ForkContext"

* Update lib/linter/code-path-analysis/fork-context.js

Co-authored-by: Francesco Trotta <github@fasttime.org>

* Update lib/linter/code-path-analysis/fork-context.js

Co-authored-by: Francesco Trotta <github@fasttime.org>

* Update lib/linter/code-path-analysis/fork-context.js

Co-authored-by: Francesco Trotta <github@fasttime.org>

* Clean up comments

---------

Co-authored-by: Francesco Trotta <github@fasttime.org>

lib/linter/code-path-analysis/fork-context.js

@@ -21,70 +21,131 @@ const assert = require("assert"),
 //------------------------------------------------------------------------------
 
 /**
- * Gets whether or not a given segment is reachable.
- * @param {CodePathSegment} segment A segment to get.
+ * Determines whether or not a given segment is reachable.
+ * @param {CodePathSegment} segment The segment to check.
  * @returns {boolean} `true` if the segment is reachable.
  */
 function isReachable(segment) {
     return segment.reachable;
 }
 
 /**
- * Creates new segments from the specific range of `context.segmentsList`.
+ * Creates a new segment for each fork in the given context and appends it
+ * to the end of the specified range of segments. Ultimately, this ends up calling
+ * `new CodePathSegment()` for each of the forks using the `create` argument
+ * as a wrapper around special behavior.
+ *
+ * The `startIndex` and `endIndex` arguments specify a range of segments in
+ * `context` that should become `allPrevSegments` for the newly created
+ * `CodePathSegment` objects.
  *
  * When `context.segmentsList` is `[[a, b], [c, d], [e, f]]`, `begin` is `0`, and
- * `end` is `-1`, this creates `[g, h]`. This `g` is from `a`, `c`, and `e`.
- * This `h` is from `b`, `d`, and `f`.
- * @param {ForkContext} context An instance.
- * @param {number} begin The first index of the previous segments.
- * @param {number} end The last index of the previous segments.
- * @param {Function} create A factory function of new segments.
- * @returns {CodePathSegment[]} New segments.
+ * `end` is `-1`, this creates two new segments, `[g, h]`. This `g` is appended to
+ * the end of the path from `a`, `c`, and `e`. This `h` is appended to the end of
+ * `b`, `d`, and `f`.
+ * @param {ForkContext} context An instance from which the previous segments
+ *      will be obtained.
+ * @param {number} startIndex The index of the first segment in the context
+ *      that should be specified as previous segments for the newly created segments.
+ * @param {number} endIndex The index of the last segment in the context
+ *      that should be specified as previous segments for the newly created segments.
+ * @param {Function} create A function that creates new `CodePathSegment`
+ *      instances in a particular way. See the `CodePathSegment.new*` methods.
+ * @returns {Array<CodePathSegment>} An array of the newly-created segments.
  */
-function makeSegments(context, begin, end, create) {
+function createSegments(context, startIndex, endIndex, create) {
+
+    /** @type {Array<Array<CodePathSegment>>} */
     const list = context.segmentsList;
 
-    const normalizedBegin = begin >= 0 ? begin : list.length + begin;
-    const normalizedEnd = end >= 0 ? end : list.length + end;
+    /*
+     * Both `startIndex` and `endIndex` work the same way: if the number is zero
+     * or more, then the number is used as-is. If the number is negative,
+     * then that number is added to the length of the segments list to
+     * determine the index to use. That means -1 for either argument
+     * is the last element, -2 is the second to last, and so on.
+     *
+     * So if `startIndex` is 0, `endIndex` is -1, and `list.length` is 3, the
+     * effective `startIndex` is 0 and the effective `endIndex` is 2, so this function
+     * will include items at indices 0, 1, and 2.
+     *
+     * Therefore, if `startIndex` is -1 and `endIndex` is -1, that means we'll only
+     * be using the last segment in `list`.
+     */
+    const normalizedBegin = startIndex >= 0 ? startIndex : list.length + startIndex;
+    const normalizedEnd = endIndex >= 0 ? endIndex : list.length + endIndex;
 
+    /** @type {Array<CodePathSegment>} */
     const segments = [];
 
     for (let i = 0; i < context.count; ++i) {
+
+        // this is passed into `new CodePathSegment` to add to code path.
         const allPrevSegments = [];
 
         for (let j = normalizedBegin; j <= normalizedEnd; ++j) {
             allPrevSegments.push(list[j][i]);
         }
 
+        // note: `create` is just a wrapper that augments `new CodePathSegment`.
         segments.push(create(context.idGenerator.next(), allPrevSegments));
     }
 
     return segments;
 }
 
 /**
- * `segments` becomes doubly in a `finally` block. Then if a code path exits by a
- * control statement (such as `break`, `continue`) from the `finally` block, the
- * destination's segments may be half of the source segments. In that case, this
- * merges segments.
- * @param {ForkContext} context An instance.
- * @param {CodePathSegment[]} segments Segments to merge.
- * @returns {CodePathSegment[]} The merged segments.
+ * Inside of a `finally` block we end up with two parallel paths. If the code path
+ * exits by a control statement (such as `break` or `continue`) from the `finally`
+ * block, then we need to merge the remaining parallel paths back into one.
+ * @param {ForkContext} context The fork context to work on.
+ * @param {Array<CodePathSegment>} segments Segments to merge.
+ * @returns {Array<CodePathSegment>} The merged segments.
  */
 function mergeExtraSegments(context, segments) {
     let currentSegments = segments;
 
+    /*
+     * We need to ensure that the array returned from this function contains no more
+     * than the number of segments that the context allows. `context.count` indicates
+     * how many items should be in the returned array to ensure that the new segment
+     * entries will line up with the already existing segment entries.
+     */
     while (currentSegments.length > context.count) {
         const merged = [];
 
-        for (let i = 0, length = currentSegments.length / 2 | 0; i < length; ++i) {
+        /*
+         * Because `context.count` is a factor of 2 inside of a `finally` block,
+         * we can divide the segment count by 2 to merge the paths together.
+         * This loops through each segment in the list and creates a new `CodePathSegment`
+         * that has the segment and the segment two slots away as previous segments.
+         *
+         * If `currentSegments` is [a,b,c,d], this will create new segments e and f, such
+         * that:
+         *
+         * When `i` is 0:
+         * a->e
+         * c->e
+         *
+         * When `i` is 1:
+         * b->f
+         * d->f
+         */
+        for (let i = 0, length = Math.floor(currentSegments.length / 2); i < length; ++i) {
             merged.push(CodePathSegment.newNext(
                 context.idGenerator.next(),
                 [currentSegments[i], currentSegments[i + length]]
             ));
         }
+
+        /*
+         * Go through the loop condition one more time to see if we have the
+         * number of segments for the context. If not, we'll keep merging paths
+         * of the merged segments until we get there.
+         */
         currentSegments = merged;
     }
+
     return currentSegments;
 }
 
@@ -93,25 +154,55 @@ function mergeExtraSegments(context, segments) {
 //------------------------------------------------------------------------------
 
 /**
- * A class to manage forking.
+ * Manages the forking of code paths.
  */
 class ForkContext {
 
     /**
+     * Creates a new instance.
      * @param {IdGenerator} idGenerator An identifier generator for segments.
-     * @param {ForkContext|null} upper An upper fork context.
-     * @param {number} count A number of parallel segments.
+     * @param {ForkContext|null} upper The preceding fork context.
+     * @param {number} count The number of parallel segments in each element
+     *      of `segmentsList`.
      */
     constructor(idGenerator, upper, count) {
+
+        /**
+         * The ID generator that will generate segment IDs for any new
+         * segments that are created.
+         * @type {IdGenerator}
+         */
         this.idGenerator = idGenerator;
+
+        /**
+         * The preceding fork context.
+         * @type {ForkContext|null}
+         */
         this.upper = upper;
+
+        /**
+         * The number of elements in each element of `segmentsList`. In most
+         * cases, this is 1 but can be 2 when there is a `finally` present,
+         * which forks the code path outside of normal flow. In the case of nested
+         * `finally` blocks, this can be a multiple of 2.
+         * @type {number}
+         */
         this.count = count;
+
+        /**
+         * The segments within this context. Each element in this array has
+         * `count` elements that represent one step in each fork. For example,
+         * when `segmentsList` is `[[a, b], [c, d], [e, f]]`, there is one path
+         * a->c->e and one path b->d->f, and `count` is 2 because each element
+         * is an array with two elements.
+         * @type {Array<Array<CodePathSegment>>}
+         */
         this.segmentsList = [];
     }
 
     /**
-     * The head segments.
-     * @type {CodePathSegment[]}
+     * The segments that begin this fork context.
+     * @type {Array<CodePathSegment>}
      */
     get head() {
         const list = this.segmentsList;
@@ -120,15 +211,15 @@ class ForkContext {
     }
 
     /**
-     * A flag which shows empty.
+     * Indicates if the context contains no segments.
      * @type {boolean}
      */
     get empty() {
         return this.segmentsList.length === 0;
     }
 
     /**
-     * A flag which shows reachable.
+     * Indicates if there are any segments that are reachable.
      * @type {boolean}
      */
     get reachable() {
@@ -138,75 +229,82 @@ class ForkContext {
     }
 
     /**
-     * Creates new segments from this context.
-     * @param {number} begin The first index of previous segments.
-     * @param {number} end The last index of previous segments.
-     * @returns {CodePathSegment[]} New segments.
+     * Creates new segments in this context and appends them to the end of the
+     * already existing `CodePathSegment`s specified by `startIndex` and
+     * `endIndex`.
+     * @param {number} startIndex The index of the first segment in the context
+     *      that should be specified as previous segments for the newly created segments.
+     * @param {number} endIndex The index of the last segment in the context
+     *      that should be specified as previous segments for the newly created segments.
+     * @returns {Array<CodePathSegment>} An array of the newly created segments.
      */
-    makeNext(begin, end) {
-        return makeSegments(this, begin, end, CodePathSegment.newNext);
+    makeNext(startIndex, endIndex) {
+        return createSegments(this, startIndex, endIndex, CodePathSegment.newNext);
     }
 
     /**
-     * Creates new segments from this context.
-     * The new segments is always unreachable.
-     * @param {number} begin The first index of previous segments.
-     * @param {number} end The last index of previous segments.
-     * @returns {CodePathSegment[]} New segments.
+     * Creates new unreachable segments in this context and appends them to the end of the
+     * already existing `CodePathSegment`s specified by `startIndex` and
+     * `endIndex`.
+     * @param {number} startIndex The index of the first segment in the context
+     *      that should be specified as previous segments for the newly created segments.
+     * @param {number} endIndex The index of the last segment in the context
+     *      that should be specified as previous segments for the newly created segments.
+     * @returns {Array<CodePathSegment>} An array of the newly created segments.
      */
-    makeUnreachable(begin, end) {
-        return makeSegments(this, begin, end, CodePathSegment.newUnreachable);
+    makeUnreachable(startIndex, endIndex) {
+        return createSegments(this, startIndex, endIndex, CodePathSegment.newUnreachable);
     }
 
     /**
-     * Creates new segments from this context.
-     * The new segments don't have connections for previous segments.
-     * But these inherit the reachable flag from this context.
-     * @param {number} begin The first index of previous segments.
-     * @param {number} end The last index of previous segments.
-     * @returns {CodePathSegment[]} New segments.
+     * Creates new segments in this context and does not append them to the end
+     *  of the already existing `CodePathSegment`s specified by `startIndex` and
+     * `endIndex`. The `startIndex` and `endIndex` are only used to determine if
+     * the new segments should be reachable. If any of the segments in this range
+     * are reachable then the new segments are also reachable; otherwise, the new
+     * segments are unreachable.
+     * @param {number} startIndex The index of the first segment in the context
+     *      that should be considered for reachability.
+     * @param {number} endIndex The index of the last segment in the context
+     *      that should be considered for reachability.
+     * @returns {Array<CodePathSegment>} An array of the newly created segments.
      */
-    makeDisconnected(begin, end) {
-        return makeSegments(this, begin, end, CodePathSegment.newDisconnected);
+    makeDisconnected(startIndex, endIndex) {
+        return createSegments(this, startIndex, endIndex, CodePathSegment.newDisconnected);
     }
 
     /**
-     * Adds segments into this context.
-     * The added segments become the head.
-     * @param {CodePathSegment[]} segments Segments to add.
+     * Adds segments to the head of this context.
+     * @param {Array<CodePathSegment>} segments The segments to add.
      * @returns {void}
      */
     add(segments) {
         assert(segments.length >= this.count, `${segments.length} >= ${this.count}`);
-
         this.segmentsList.push(mergeExtraSegments(this, segments));
     }
 
     /**
-     * Replaces the head segments with given segments.
+     * Replaces the head segments with the given segments.
      * The current head segments are removed.
-     * @param {CodePathSegment[]} segments Segments to add.
+     * @param {Array<CodePathSegment>} replacementHeadSegments The new head segments.
      * @returns {void}
      */
-    replaceHead(segments) {
-        assert(segments.length >= this.count, `${segments.length} >= ${this.count}`);
-
-        this.segmentsList.splice(-1, 1, mergeExtraSegments(this, segments));
+    replaceHead(replacementHeadSegments) {
+        assert(
+            replacementHeadSegments.length >= this.count,
+            `${replacementHeadSegments.length} >= ${this.count}`
+        );
+        this.segmentsList.splice(-1, 1, mergeExtraSegments(this, replacementHeadSegments));
     }
 
     /**
      * Adds all segments of a given fork context into this context.
-     * @param {ForkContext} context A fork context to add.
+     * @param {ForkContext} otherForkContext The fork context to add from.
      * @returns {void}
      */
-    addAll(context) {
-        assert(context.count === this.count);
-
-        const source = context.segmentsList;
-
-        for (let i = 0; i < source.length; ++i) {
-            this.segmentsList.push(source[i]);
-        }
+    addAll(otherForkContext) {
+        assert(otherForkContext.count === this.count);
+        this.segmentsList.push(...otherForkContext.segmentsList);
     }
 
     /**
@@ -218,7 +316,8 @@ class ForkContext {
     }
 
     /**
-     * Creates the root fork context.
+     * Creates a new root context, meaning that there are no parent
+     * fork contexts.
      * @param {IdGenerator} idGenerator An identifier generator for segments.
      * @returns {ForkContext} New fork context.
      */
@@ -233,14 +332,16 @@ class ForkContext {
     /**
      * Creates an empty fork context preceded by a given context.
      * @param {ForkContext} parentContext The parent fork context.
-     * @param {boolean} forkLeavingPath A flag which shows inside of `finally` block.
+     * @param {boolean} shouldForkLeavingPath Indicates that we are inside of
+     *      a `finally` block and should therefore fork the path that leaves
+     *      `finally`.
      * @returns {ForkContext} New fork context.
      */
-    static newEmpty(parentContext, forkLeavingPath) {
+    static newEmpty(parentContext, shouldForkLeavingPath) {
         return new ForkContext(
             parentContext.idGenerator,
             parentContext,
-            (forkLeavingPath ? 2 : 1) * parentContext.count
+            (shouldForkLeavingPath ? 2 : 1) * parentContext.count
         );
     }
 }