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Filename : fork-context.js
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/**
 * @fileoverview A class to operate forking.
 *
 * This is state of forking.
 * This has a fork list and manages it.
 *
 * @author Toru Nagashima
 */

"use strict";

//------------------------------------------------------------------------------
// Requirements
//------------------------------------------------------------------------------

const assert = require("assert"),
    CodePathSegment = require("./code-path-segment");

//------------------------------------------------------------------------------
// Helpers
//------------------------------------------------------------------------------

/**
 * 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 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 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 createSegments(context, startIndex, endIndex, create) {

    /** @type {Array<Array<CodePathSegment>>} */
    const list = context.segmentsList;

    /*
     * 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;
}

/**
 * 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 = [];

        /*
         * 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;
}

//------------------------------------------------------------------------------
// Public Interface
//------------------------------------------------------------------------------

/**
 * Manages the forking of code paths.
 */
class ForkContext {

    /**
     * Creates a new instance.
     * @param {IdGenerator} idGenerator An identifier generator for 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 segments that begin this fork context.
     * @type {Array<CodePathSegment>}
     */
    get head() {
        const list = this.segmentsList;

        return list.length === 0 ? [] : list[list.length - 1];
    }

    /**
     * Indicates if the context contains no segments.
     * @type {boolean}
     */
    get empty() {
        return this.segmentsList.length === 0;
    }

    /**
     * Indicates if there are any segments that are reachable.
     * @type {boolean}
     */
    get reachable() {
        const segments = this.head;

        return segments.length > 0 && segments.some(isReachable);
    }

    /**
     * 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(startIndex, endIndex) {
        return createSegments(this, startIndex, endIndex, CodePathSegment.newNext);
    }

    /**
     * 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(startIndex, endIndex) {
        return createSegments(this, startIndex, endIndex, CodePathSegment.newUnreachable);
    }

    /**
     * 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(startIndex, endIndex) {
        return createSegments(this, startIndex, endIndex, CodePathSegment.newDisconnected);
    }

    /**
     * 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 the given segments.
     * The current head segments are removed.
     * @param {Array<CodePathSegment>} replacementHeadSegments The new head segments.
     * @returns {void}
     */
    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} otherForkContext The fork context to add from.
     * @returns {void}
     */
    addAll(otherForkContext) {
        assert(otherForkContext.count === this.count);
        this.segmentsList.push(...otherForkContext.segmentsList);
    }

    /**
     * Clears all segments in this context.
     * @returns {void}
     */
    clear() {
        this.segmentsList = [];
    }

    /**
     * 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.
     */
    static newRoot(idGenerator) {
        const context = new ForkContext(idGenerator, null, 1);

        context.add([CodePathSegment.newRoot(idGenerator.next())]);

        return context;
    }

    /**
     * Creates an empty fork context preceded by a given context.
     * @param {ForkContext} parentContext The parent fork context.
     * @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, shouldForkLeavingPath) {
        return new ForkContext(
            parentContext.idGenerator,
            parentContext,
            (shouldForkLeavingPath ? 2 : 1) * parentContext.count
        );
    }
}

module.exports = ForkContext;