code:
/**
 * @fileoverview A class of the code path analyzer.
 * @author Toru Nagashima
 */

"use strict";

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

const assert = require("assert"),
    { breakableTypePattern } = require("../../shared/ast-utils"),
    CodePath = require("./code-path"),
    CodePathSegment = require("./code-path-segment"),
    IdGenerator = require("./id-generator"),
    debug = require("./debug-helpers");

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

/**
 * Checks whether or not a given node is a `case` node (not `default` node).
 * @param {ASTNode} node A `SwitchCase` node to check.
 * @returns {boolean} `true` if the node is a `case` node (not `default` node).
 */
function isCaseNode(node) {
    return Boolean(node.test);
}

/**
 * Checks if a given node appears as the value of a PropertyDefinition node.
 * @param {ASTNode} node THe node to check.
 * @returns {boolean} `true` if the node is a PropertyDefinition value,
 *      false if not.
 */
function isPropertyDefinitionValue(node) {
    const parent = node.parent;

    return parent && parent.type === "PropertyDefinition" && parent.value === node;
}

/**
 * Checks whether the given logical operator is taken into account for the code
 * path analysis.
 * @param {string} operator The operator found in the LogicalExpression node
 * @returns {boolean} `true` if the operator is "&&" or "||" or "??"
 */
function isHandledLogicalOperator(operator) {
    return operator === "&&" || operator === "||" || operator === "??";
}

/**
 * Checks whether the given assignment operator is a logical assignment operator.
 * Logical assignments are taken into account for the code path analysis
 * because of their short-circuiting semantics.
 * @param {string} operator The operator found in the AssignmentExpression node
 * @returns {boolean} `true` if the operator is "&&=" or "||=" or "??="
 */
function isLogicalAssignmentOperator(operator) {
    return operator === "&&=" || operator === "||=" || operator === "??=";
}

/**
 * Gets the label if the parent node of a given node is a LabeledStatement.
 * @param {ASTNode} node A node to get.
 * @returns {string|null} The label or `null`.
 */
function getLabel(node) {
    if (node.parent.type === "LabeledStatement") {
        return node.parent.label.name;
    }
    return null;
}

/**
 * Checks whether or not a given logical expression node goes different path
 * between the `true` case and the `false` case.
 * @param {ASTNode} node A node to check.
 * @returns {boolean} `true` if the node is a test of a choice statement.
 */
function isForkingByTrueOrFalse(node) {
    const parent = node.parent;

    switch (parent.type) {
        case "ConditionalExpression":
        case "IfStatement":
        case "WhileStatement":
        case "DoWhileStatement":
        case "ForStatement":
            return parent.test === node;

        case "LogicalExpression":
            return isHandledLogicalOperator(parent.operator);

        case "AssignmentExpression":
            return isLogicalAssignmentOperator(parent.operator);

        default:
            return false;
    }
}

/**
 * Gets the boolean value of a given literal node.
 *
 * This is used to detect infinity loops (e.g. `while (true) {}`).
 * Statements preceded by an infinity loop are unreachable if the loop didn't
 * have any `break` statement.
 * @param {ASTNode} node A node to get.
 * @returns {boolean|undefined} a boolean value if the node is a Literal node,
 *   otherwise `undefined`.
 */
function getBooleanValueIfSimpleConstant(node) {
    if (node.type === "Literal") {
        return Boolean(node.value);
    }
    return void 0;
}

/**
 * Checks that a given identifier node is a reference or not.
 *
 * This is used to detect the first throwable node in a `try` block.
 * @param {ASTNode} node An Identifier node to check.
 * @returns {boolean} `true` if the node is a reference.
 */
function isIdentifierReference(node) {
    const parent = node.parent;

    switch (parent.type) {
        case "LabeledStatement":
        case "BreakStatement":
        case "ContinueStatement":
        case "ArrayPattern":
        case "RestElement":
        case "ImportSpecifier":
        case "ImportDefaultSpecifier":
        case "ImportNamespaceSpecifier":
        case "CatchClause":
            return false;

        case "FunctionDeclaration":
        case "FunctionExpression":
        case "ArrowFunctionExpression":
        case "ClassDeclaration":
        case "ClassExpression":
        case "VariableDeclarator":
            return parent.id !== node;

        case "Property":
        case "PropertyDefinition":
        case "MethodDefinition":
            return (
                parent.key !== node ||
                parent.computed ||
                parent.shorthand
            );

        case "AssignmentPattern":
            return parent.key !== node;

        default:
            return true;
    }
}

/**
 * Updates the current segment with the head segment.
 * This is similar to local branches and tracking branches of git.
 *
 * To separate the current and the head is in order to not make useless segments.
 *
 * In this process, both "onCodePathSegmentStart" and "onCodePathSegmentEnd"
 * events are fired.
 * @param {CodePathAnalyzer} analyzer The instance.
 * @param {ASTNode} node The current AST node.
 * @returns {void}
 */
function forwardCurrentToHead(analyzer, node) {
    const codePath = analyzer.codePath;
    const state = CodePath.getState(codePath);
    const currentSegments = state.currentSegments;
    const headSegments = state.headSegments;
    const end = Math.max(currentSegments.length, headSegments.length);
    let i, currentSegment, headSegment;

    // Fires leaving events.
    for (i = 0; i < end; ++i) {
        currentSegment = currentSegments[i];
        headSegment = headSegments[i];

        if (currentSegment !== headSegment && currentSegment) {

            const eventName = currentSegment.reachable
                ? "onCodePathSegmentEnd"
                : "onUnreachableCodePathSegmentEnd";

            debug.dump(`${eventName} ${currentSegment.id}`);

            analyzer.emitter.emit(
                eventName,
                currentSegment,
                node
            );
        }
    }

    // Update state.
    state.currentSegments = headSegments;

    // Fires entering events.
    for (i = 0; i < end; ++i) {
        currentSegment = currentSegments[i];
        headSegment = headSegments[i];

        if (currentSegment !== headSegment && headSegment) {

            const eventName = headSegment.reachable
                ? "onCodePathSegmentStart"
                : "onUnreachableCodePathSegmentStart";

            debug.dump(`${eventName} ${headSegment.id}`);

            CodePathSegment.markUsed(headSegment);
            analyzer.emitter.emit(
                eventName,
                headSegment,
                node
            );
        }
    }

}

/**
 * Updates the current segment with empty.
 * This is called at the last of functions or the program.
 * @param {CodePathAnalyzer} analyzer The instance.
 * @param {ASTNode} node The current AST node.
 * @returns {void}
 */
function leaveFromCurrentSegment(analyzer, node) {
    const state = CodePath.getState(analyzer.codePath);
    const currentSegments = state.currentSegments;

    for (let i = 0; i < currentSegments.length; ++i) {
        const currentSegment = currentSegments[i];
        const eventName = currentSegment.reachable
            ? "onCodePathSegmentEnd"
            : "onUnreachableCodePathSegmentEnd";

        debug.dump(`${eventName} ${currentSegment.id}`);

        analyzer.emitter.emit(
            eventName,
            currentSegment,
            node
        );
    }

    state.currentSegments = [];
}

/**
 * Updates the code path due to the position of a given node in the parent node
 * thereof.
 *
 * For example, if the node is `parent.consequent`, this creates a fork from the
 * current path.
 * @param {CodePathAnalyzer} analyzer The instance.
 * @param {ASTNode} node The current AST node.
 * @returns {void}
 */
function preprocess(analyzer, node) {
    const codePath = analyzer.codePath;
    const state = CodePath.getState(codePath);
    const parent = node.parent;

    switch (parent.type) {

        // The `arguments.length == 0` case is in `postprocess` function.
        case "CallExpression":
            if (parent.optional === true && parent.arguments.length >= 1 && parent.arguments[0] === node) {
                state.makeOptionalRight();
            }
            break;
        case "MemberExpression":
            if (parent.optional === true && parent.property === node) {
                state.makeOptionalRight();
            }
            break;

        case "LogicalExpression":
            if (
                parent.right === node &&
                isHandledLogicalOperator(parent.operator)
            ) {
                state.makeLogicalRight();
            }
            break;

        case "AssignmentExpression":
            if (
                parent.right === node &&
                isLogicalAssignmentOperator(parent.operator)
            ) {
                state.makeLogicalRight();
            }
            break;

        case "ConditionalExpression":
        case "IfStatement":

            /*
             * Fork if this node is at `consequent`/`alternate`.
             * `popForkContext()` exists at `IfStatement:exit` and
             * `ConditionalExpression:exit`.
             */
            if (parent.consequent === node) {
                state.makeIfConsequent();
            } else if (parent.alternate === node) {
                state.makeIfAlternate();
            }
            break;

        case "SwitchCase":
            if (parent.consequent[0] === node) {
                state.makeSwitchCaseBody(false, !parent.test);
            }
            break;

        case "TryStatement":
            if (parent.handler === node) {
                state.makeCatchBlock();
            } else if (parent.finalizer === node) {
                state.makeFinallyBlock();
            }
            break;

        case "WhileStatement":
            if (parent.test === node) {
                state.makeWhileTest(getBooleanValueIfSimpleConstant(node));
            } else {
                assert(parent.body === node);
                state.makeWhileBody();
            }
            break;

        case "DoWhileStatement":
            if (parent.body === node) {
                state.makeDoWhileBody();
            } else {
                assert(parent.test === node);
                state.makeDoWhileTest(getBooleanValueIfSimpleConstant(node));
            }
            break;

        case "ForStatement":
            if (parent.test === node) {
                state.makeForTest(getBooleanValueIfSimpleConstant(node));
            } else if (parent.update === node) {
                state.makeForUpdate();
            } else if (parent.body === node) {
                state.makeForBody();
            }
            break;

        case "ForInStatement":
        case "ForOfStatement":
            if (parent.left === node) {
                state.makeForInOfLeft();
            } else if (parent.right === node) {
                state.makeForInOfRight();
            } else {
                assert(parent.body === node);
                state.makeForInOfBody();
            }
            break;

        case "AssignmentPattern":

            /*
             * Fork if this node is at `right`.
             * `left` is executed always, so it uses the current path.
             * `popForkContext()` exists at `AssignmentPattern:exit`.
             */
            if (parent.right === node) {
                state.pushForkContext();
                state.forkBypassPath();
                state.forkPath();
            }
            break;

        default:
            break;
    }
}

/**
 * Updates the code path due to the type of a given node in entering.
 * @param {CodePathAnalyzer} analyzer The instance.
 * @param {ASTNode} node The current AST node.
 * @returns {void}
 */
function processCodePathToEnter(analyzer, node) {
    let codePath = analyzer.codePath;
    let state = codePath && CodePath.getState(codePath);
    const parent = node.parent;

    /**
     * Creates a new code path and trigger the onCodePathStart event
     * based on the currently selected node.
     * @param {string} origin The reason the code path was started.
     * @returns {void}
     */
    function startCodePath(origin) {
        if (codePath) {

            // Emits onCodePathSegmentStart events if updated.
            forwardCurrentToHead(analyzer, node);
            debug.dumpState(node, state, false);
        }

        // Create the code path of this scope.
        codePath = analyzer.codePath = new CodePath({
            id: analyzer.idGenerator.next(),
            origin,
            upper: codePath,
            onLooped: analyzer.onLooped
        });
        state = CodePath.getState(codePath);

        // Emits onCodePathStart events.
        debug.dump(`onCodePathStart ${codePath.id}`);
        analyzer.emitter.emit("onCodePathStart", codePath, node);
    }

    /*
     * Special case: The right side of class field initializer is considered
     * to be its own function, so we need to start a new code path in this
     * case.
     */
    if (isPropertyDefinitionValue(node)) {
        startCodePath("class-field-initializer");

        /*
         * Intentional fall through because `node` needs to also be
         * processed by the code below. For example, if we have:
         *
         * class Foo {
         *     a = () => {}
         * }
         *
         * In this case, we also need start a second code path.
         */

    }

    switch (node.type) {
        case "Program":
            startCodePath("program");
            break;

        case "FunctionDeclaration":
        case "FunctionExpression":
        case "ArrowFunctionExpression":
            startCodePath("function");
            break;

        case "StaticBlock":
            startCodePath("class-static-block");
            break;

        case "ChainExpression":
            state.pushChainContext();
            break;
        case "CallExpression":
            if (node.optional === true) {
                state.makeOptionalNode();
            }
            break;
        case "MemberExpression":
            if (node.optional === true) {
                state.makeOptionalNode();
            }
            break;

        case "LogicalExpression":
            if (isHandledLogicalOperator(node.operator)) {
                state.pushChoiceContext(
                    node.operator,
                    isForkingByTrueOrFalse(node)
                );
            }
            break;

        case "AssignmentExpression":
            if (isLogicalAssignmentOperator(node.operator)) {
                state.pushChoiceContext(
                    node.operator.slice(0, -1), // removes `=` from the end
                    isForkingByTrueOrFalse(node)
                );
            }
            break;

        case "ConditionalExpression":
        case "IfStatement":
            state.pushChoiceContext("test", false);
            break;

        case "SwitchStatement":
            state.pushSwitchContext(
                node.cases.some(isCaseNode),
                getLabel(node)
            );
            break;

        case "TryStatement":
            state.pushTryContext(Boolean(node.finalizer));
            break;

        case "SwitchCase":

            /*
             * Fork if this node is after the 2st node in `cases`.
             * It's similar to `else` blocks.
             * The next `test` node is processed in this path.
             */
            if (parent.discriminant !== node && parent.cases[0] !== node) {
                state.forkPath();
            }
            break;

        case "WhileStatement":
        case "DoWhileStatement":
        case "ForStatement":
        case "ForInStatement":
        case "ForOfStatement":
            state.pushLoopContext(node.type, getLabel(node));
            break;

        case "LabeledStatement":
            if (!breakableTypePattern.test(node.body.type)) {
                state.pushBreakContext(false, node.label.name);
            }
            break;

        default:
            break;
    }

    // Emits onCodePathSegmentStart events if updated.
    forwardCurrentToHead(analyzer, node);
    debug.dumpState(node, state, false);
}

/**
 * Updates the code path due to the type of a given node in leaving.
 * @param {CodePathAnalyzer} analyzer The instance.
 * @param {ASTNode} node The current AST node.
 * @returns {void}
 */
function processCodePathToExit(analyzer, node) {

    const codePath = analyzer.codePath;
    const state = CodePath.getState(codePath);
    let dontForward = false;

    switch (node.type) {
        case "ChainExpression":
            state.popChainContext();
            break;

        case "IfStatement":
        case "ConditionalExpression":
            state.popChoiceContext();
            break;

        case "LogicalExpression":
            if (isHandledLogicalOperator(node.operator)) {
                state.popChoiceContext();
            }
            break;

        case "AssignmentExpression":
            if (isLogicalAssignmentOperator(node.operator)) {
                state.popChoiceContext();
            }
            break;

        case "SwitchStatement":
            state.popSwitchContext();
            break;

        case "SwitchCase":

            /*
             * This is the same as the process at the 1st `consequent` node in
             * `preprocess` function.
             * Must do if this `consequent` is empty.
             */
            if (node.consequent.length === 0) {
                state.makeSwitchCaseBody(true, !node.test);
            }
            if (state.forkContext.reachable) {
                dontForward = true;
            }
            break;

        case "TryStatement":
            state.popTryContext();
            break;

        case "BreakStatement":
            forwardCurrentToHead(analyzer, node);
            state.makeBreak(node.label && node.label.name);
            dontForward = true;
            break;

        case "ContinueStatement":
            forwardCurrentToHead(analyzer, node);
            state.makeContinue(node.label && node.label.name);
            dontForward = true;
            break;

        case "ReturnStatement":
            forwardCurrentToHead(analyzer, node);
            state.makeReturn();
            dontForward = true;
            break;

        case "ThrowStatement":
            forwardCurrentToHead(analyzer, node);
            state.makeThrow();
            dontForward = true;
            break;

        case "Identifier":
            if (isIdentifierReference(node)) {
                state.makeFirstThrowablePathInTryBlock();
                dontForward = true;
            }
            break;

        case "CallExpression":
        case "ImportExpression":
        case "MemberExpression":
        case "NewExpression":
        case "YieldExpression":
            state.makeFirstThrowablePathInTryBlock();
            break;

        case "WhileStatement":
        case "DoWhileStatement":
        case "ForStatement":
        case "ForInStatement":
        case "ForOfStatement":
            state.popLoopContext();
            break;

        case "AssignmentPattern":
            state.popForkContext();
            break;

        case "LabeledStatement":
            if (!breakableTypePattern.test(node.body.type)) {
                state.popBreakContext();
            }
            break;

        default:
            break;
    }

    // Emits onCodePathSegmentStart events if updated.
    if (!dontForward) {
        forwardCurrentToHead(analyzer, node);
    }
    debug.dumpState(node, state, true);
}

/**
 * Updates the code path to finalize the current code path.
 * @param {CodePathAnalyzer} analyzer The instance.
 * @param {ASTNode} node The current AST node.
 * @returns {void}
 */
function postprocess(analyzer, node) {

    /**
     * Ends the code path for the current node.
     * @returns {void}
     */
    function endCodePath() {
        let codePath = analyzer.codePath;

        // Mark the current path as the final node.
        CodePath.getState(codePath).makeFinal();

        // Emits onCodePathSegmentEnd event of the current segments.
        leaveFromCurrentSegment(analyzer, node);

        // Emits onCodePathEnd event of this code path.
        debug.dump(`onCodePathEnd ${codePath.id}`);
        analyzer.emitter.emit("onCodePathEnd", codePath, node);
        debug.dumpDot(codePath);

        codePath = analyzer.codePath = analyzer.codePath.upper;
        if (codePath) {
            debug.dumpState(node, CodePath.getState(codePath), true);
        }

    }

    switch (node.type) {
        case "Program":
        case "FunctionDeclaration":
        case "FunctionExpression":
        case "ArrowFunctionExpression":
        case "StaticBlock": {
            endCodePath();
            break;
        }

        // The `arguments.length >= 1` case is in `preprocess` function.
        case "CallExpression":
            if (node.optional === true && node.arguments.length === 0) {
                CodePath.getState(analyzer.codePath).makeOptionalRight();
            }
            break;

        default:
            break;
    }

    /*
     * Special case: The right side of class field initializer is considered
     * to be its own function, so we need to end a code path in this
     * case.
     *
     * We need to check after the other checks in order to close the
     * code paths in the correct order for code like this:
     *
     *
     * class Foo {
     *     a = () => {}
     * }
     *
     * In this case, The ArrowFunctionExpression code path is closed first
     * and then we need to close the code path for the PropertyDefinition
     * value.
     */
    if (isPropertyDefinitionValue(node)) {
        endCodePath();
    }
}

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

/**
 * The class to analyze code paths.
 * This class implements the EventGenerator interface.
 */
class CodePathAnalyzer {

    /**
     * @param {EventGenerator} eventGenerator An event generator to wrap.
     */
    constructor(eventGenerator) {
        this.original = eventGenerator;
        this.emitter = eventGenerator.emitter;
        this.codePath = null;
        this.idGenerator = new IdGenerator("s");
        this.currentNode = null;
        this.onLooped = this.onLooped.bind(this);
    }

    /**
     * Does the process to enter a given AST node.
     * This updates state of analysis and calls `enterNode` of the wrapped.
     * @param {ASTNode} node A node which is entering.
     * @returns {void}
     */
    enterNode(node) {
        this.currentNode = node;

        // Updates the code path due to node's position in its parent node.
        if (node.parent) {
            preprocess(this, node);
        }

        /*
         * Updates the code path.
         * And emits onCodePathStart/onCodePathSegmentStart events.
         */
        processCodePathToEnter(this, node);

        // Emits node events.
        this.original.enterNode(node);

        this.currentNode = null;
    }

    /**
     * Does the process to leave a given AST node.
     * This updates state of analysis and calls `leaveNode` of the wrapped.
     * @param {ASTNode} node A node which is leaving.
     * @returns {void}
     */
    leaveNode(node) {
        this.currentNode = node;

        /*
         * Updates the code path.
         * And emits onCodePathStart/onCodePathSegmentStart events.
         */
        processCodePathToExit(this, node);

        // Emits node events.
        this.original.leaveNode(node);

        // Emits the last onCodePathStart/onCodePathSegmentStart events.
        postprocess(this, node);

        this.currentNode = null;
    }

    /**
     * This is called on a code path looped.
     * Then this raises a looped event.
     * @param {CodePathSegment} fromSegment A segment of prev.
     * @param {CodePathSegment} toSegment A segment of next.
     * @returns {void}
     */
    onLooped(fromSegment, toSegment) {
        if (fromSegment.reachable && toSegment.reachable) {
            debug.dump(`onCodePathSegmentLoop ${fromSegment.id} -> ${toSegment.id}`);
            this.emitter.emit(
                "onCodePathSegmentLoop",
                fromSegment,
                toSegment,
                this.currentNode
            );
        }
    }
}

module.exports = CodePathAnalyzer;


code:
/**
 * @fileoverview The CodePathSegment class.
 * @author Toru Nagashima
 */

"use strict";

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

const debug = require("./debug-helpers");

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

/**
 * Checks whether or not a given segment is reachable.
 * @param {CodePathSegment} segment A segment to check.
 * @returns {boolean} `true` if the segment is reachable.
 */
function isReachable(segment) {
    return segment.reachable;
}

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

/**
 * A code path segment.
 *
 * Each segment is arranged in a series of linked lists (implemented by arrays)
 * that keep track of the previous and next segments in a code path. In this way,
 * you can navigate between all segments in any code path so long as you have a
 * reference to any segment in that code path.
 *
 * When first created, the segment is in a detached state, meaning that it knows the
 * segments that came before it but those segments don't know that this new segment
 * follows it. Only when `CodePathSegment#markUsed()` is called on a segment does it
 * officially become part of the code path by updating the previous segments to know
 * that this new segment follows.
 */
class CodePathSegment {

    /**
     * Creates a new instance.
     * @param {string} id An identifier.
     * @param {CodePathSegment[]} allPrevSegments An array of the previous segments.
     *   This array includes unreachable segments.
     * @param {boolean} reachable A flag which shows this is reachable.
     */
    constructor(id, allPrevSegments, reachable) {

        /**
         * The identifier of this code path.
         * Rules use it to store additional information of each rule.
         * @type {string}
         */
        this.id = id;

        /**
         * An array of the next reachable segments.
         * @type {CodePathSegment[]}
         */
        this.nextSegments = [];

        /**
         * An array of the previous reachable segments.
         * @type {CodePathSegment[]}
         */
        this.prevSegments = allPrevSegments.filter(isReachable);

        /**
         * An array of all next segments including reachable and unreachable.
         * @type {CodePathSegment[]}
         */
        this.allNextSegments = [];

        /**
         * An array of all previous segments including reachable and unreachable.
         * @type {CodePathSegment[]}
         */
        this.allPrevSegments = allPrevSegments;

        /**
         * A flag which shows this is reachable.
         * @type {boolean}
         */
        this.reachable = reachable;

        // Internal data.
        Object.defineProperty(this, "internal", {
            value: {

                // determines if the segment has been attached to the code path
                used: false,

                // array of previous segments coming from the end of a loop
                loopedPrevSegments: []
            }
        });

        /* c8 ignore start */
        if (debug.enabled) {
            this.internal.nodes = [];
        }/* c8 ignore stop */
    }

    /**
     * Checks a given previous segment is coming from the end of a loop.
     * @param {CodePathSegment} segment A previous segment to check.
     * @returns {boolean} `true` if the segment is coming from the end of a loop.
     */
    isLoopedPrevSegment(segment) {
        return this.internal.loopedPrevSegments.includes(segment);
    }

    /**
     * Creates the root segment.
     * @param {string} id An identifier.
     * @returns {CodePathSegment} The created segment.
     */
    static newRoot(id) {
        return new CodePathSegment(id, [], true);
    }

    /**
     * Creates a new segment and appends it after the given segments.
     * @param {string} id An identifier.
     * @param {CodePathSegment[]} allPrevSegments An array of the previous segments
     *      to append to.
     * @returns {CodePathSegment} The created segment.
     */
    static newNext(id, allPrevSegments) {
        return new CodePathSegment(
            id,
            CodePathSegment.flattenUnusedSegments(allPrevSegments),
            allPrevSegments.some(isReachable)
        );
    }

    /**
     * Creates an unreachable segment and appends it after the given segments.
     * @param {string} id An identifier.
     * @param {CodePathSegment[]} allPrevSegments An array of the previous segments.
     * @returns {CodePathSegment} The created segment.
     */
    static newUnreachable(id, allPrevSegments) {
        const segment = new CodePathSegment(id, CodePathSegment.flattenUnusedSegments(allPrevSegments), false);

        /*
         * In `if (a) return a; foo();` case, the unreachable segment preceded by
         * the return statement is not used but must not be removed.
         */
        CodePathSegment.markUsed(segment);

        return segment;
    }

    /**
     * Creates a segment that follows given segments.
     * This factory method does not connect with `allPrevSegments`.
     * But this inherits `reachable` flag.
     * @param {string} id An identifier.
     * @param {CodePathSegment[]} allPrevSegments An array of the previous segments.
     * @returns {CodePathSegment} The created segment.
     */
    static newDisconnected(id, allPrevSegments) {
        return new CodePathSegment(id, [], allPrevSegments.some(isReachable));
    }

    /**
     * Marks a given segment as used.
     *
     * And this function registers the segment into the previous segments as a next.
     * @param {CodePathSegment} segment A segment to mark.
     * @returns {void}
     */
    static markUsed(segment) {
        if (segment.internal.used) {
            return;
        }
        segment.internal.used = true;

        let i;

        if (segment.reachable) {

            /*
             * If the segment is reachable, then it's officially part of the
             * code path. This loops through all previous segments to update
             * their list of next segments. Because the segment is reachable,
             * it's added to both `nextSegments` and `allNextSegments`.
             */
            for (i = 0; i < segment.allPrevSegments.length; ++i) {
                const prevSegment = segment.allPrevSegments[i];

                prevSegment.allNextSegments.push(segment);
                prevSegment.nextSegments.push(segment);
            }
        } else {

            /*
             * If the segment is not reachable, then it's not officially part of the
             * code path. This loops through all previous segments to update
             * their list of next segments. Because the segment is not reachable,
             * it's added only to `allNextSegments`.
             */
            for (i = 0; i < segment.allPrevSegments.length; ++i) {
                segment.allPrevSegments[i].allNextSegments.push(segment);
            }
        }
    }

    /**
     * Marks a previous segment as looped.
     * @param {CodePathSegment} segment A segment.
     * @param {CodePathSegment} prevSegment A previous segment to mark.
     * @returns {void}
     */
    static markPrevSegmentAsLooped(segment, prevSegment) {
        segment.internal.loopedPrevSegments.push(prevSegment);
    }

    /**
     * Creates a new array based on an array of segments. If any segment in the
     * array is unused, then it is replaced by all of its previous segments.
     * All used segments are returned as-is without replacement.
     * @param {CodePathSegment[]} segments The array of segments to flatten.
     * @returns {CodePathSegment[]} The flattened array.
     */
    static flattenUnusedSegments(segments) {
        const done = new Set();

        for (let i = 0; i < segments.length; ++i) {
            const segment = segments[i];

            // Ignores duplicated.
            if (done.has(segment)) {
                continue;
            }

            // Use previous segments if unused.
            if (!segment.internal.used) {
                for (let j = 0; j < segment.allPrevSegments.length; ++j) {
                    const prevSegment = segment.allPrevSegments[j];

                    if (!done.has(prevSegment)) {
                        done.add(prevSegment);
                    }
                }
            } else {
                done.add(segment);
            }
        }

        return [...done];
    }
}

module.exports = CodePathSegment;

license: mit