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Checkstyle is a development tool to help programmers write Java code that adheres to a coding standard
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////////////////////////////////////////////////////////////////////////////////
// checkstyle: Checks Java source code for adherence to a set of rules.
// Copyright (C) 2001-2019 the original author or authors.
//
// This library is free software; you can redistribute it and/or
// modify it under the terms of the GNU Lesser General Public
// License as published by the Free Software Foundation; either
// version 2.1 of the License, or (at your option) any later version.
//
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
// Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public
// License along with this library; if not, write to the Free Software
// Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
////////////////////////////////////////////////////////////////////////////////

package com.puppycrawl.tools.checkstyle.checks.metrics;

import java.math.BigInteger;
import java.util.ArrayDeque;
import java.util.Deque;

import com.puppycrawl.tools.checkstyle.FileStatefulCheck;
import com.puppycrawl.tools.checkstyle.api.AbstractCheck;
import com.puppycrawl.tools.checkstyle.api.DetailAST;
import com.puppycrawl.tools.checkstyle.api.TokenTypes;

/**
 * 
 * Checks the NPATH complexity against a specified limit.
 * 
 * 
 * The NPATH metric computes the number of possible execution paths through a
 * function(method). It takes into account the nesting of conditional statements
 * and multi-part boolean expressions (A && B, C || D, E ? F :G and
 * their combinations).
 * 
 * 
 * The NPATH metric was designed base on Cyclomatic complexity to avoid problem
 * of Cyclomatic complexity metric like nesting level within a function(method).
 * 
 * 
 * Metric was described at 
 * "NPATH: a measure of execution pathcomplexity and its applications".
 * If you need detailed description of algorithm, please read that article,
 * it is well written and have number of examples and details.
 * 
 * 
 * Here is some quotes:
 * 
 * 
 * An NPATH threshold value of 200 has been established for a function.
 * The value 200 is based on studies done at AT&T Bell Laboratories [1988 year].
 * 
 * 
 * Some of the most effective methods of reducing the NPATH value include:
 * 
 * 
 * distributing functionality;
 * 
 * 
 * implementing multiple if statements as a switch statement;
 * 
 * 
 * creating a separate function for logical expressions with a high count of
 * variables and (&&) and or (||) operators.
 * 
 * 
 * 
 * 
 * Although strategies to reduce the NPATH complexity of functions are important,
 * care must be taken not to distort the logical clarity of the software by
 * applying a strategy to reduce the complexity of functions. That is, there is
 * a point of diminishing return beyond which a further attempt at reduction of
 * complexity distorts the logical clarity of the system structure.
 * 
 * 
 * 
 * 
 * 
 * 
 * 
 * 
 * 
 * 
 * 
 * 
 * 
 * 
 * 
 * 
 * 
 * 
 * 
 * 
 * 
 * ExamplesStructure Complexity expression
if ([expr]) { [if-range] } NP(if-range) + 1 + NP(expr)
if ([expr]) { [if-range] } else { [else-range] } NP(if-range)+ NP(else-range) + NP(expr)
while ([expr]) { [while-range] } NP(while-range) + NP(expr) + 1
do { [do-range] } while ([expr]) NP(do-range) + NP(expr) + 1
for([expr1]; [expr2]; [expr3]) { [for-range] } NP(for-range) + NP(expr1)+ NP(expr2) + NP(expr3) + 1
switch ([expr]) { case : [case-range] default: [default-range] } S(i=1:i=n)NP(case-range[i]) + NP(default-range) + NP(expr)
[expr1] ? [expr2] : [expr3] NP(expr1) + NP(expr2) + NP(expr3) + 2
goto label 1
break 1
Expressions Number of && and || operators in expression. No operators - 0
continue 1
return 1
Statement (even sequential statements) 1
Empty block {} 1
Function call 1
Function(Method) declaration or Block P(i=1:i=N)NP(Statement[i])
 * 
 * Rationale: Nejmeh says that his group had an informal NPATH limit of
 * 200 on individual routines; functions(methods) that exceeded this value were
 * candidates for further decomposition - or at least a closer look.
 * Please do not be fanatic with limit 200 - choose number that suites
 * your project style. Limit 200 is empirical number base on some sources of at
 * AT&T Bell Laboratories of 1988 year.
 * 
 * 
 * 
 * Property {@code max} - Specify the maximum threshold allowed.
 * Default value is {@code 200}.
 * 
 * 
 * 
 * To configure the check:
 * 
 *  * <module name="NPathComplexity"/>
 * 
 * 
 * To configure the check with a threshold of 1000:
 * 
 *  * <module name="NPathComplexity">
 *   <property name="max" value="1000"/>
 * </module>
 * 
 *
 * @since 3.4
 */
// -@cs[AbbreviationAsWordInName] Can't change check name
@FileStatefulCheck
public final class NPathComplexityCheck extends AbstractCheck {

    /**
     * A key is pointing to the warning message text in "messages.properties"
     * file.
     */
    public static final String MSG_KEY = "npathComplexity";

    /** Default allowed complexity. */
    private static final int DEFAULT_MAX = 200;

    /** The initial current value. */
    private static final BigInteger INITIAL_VALUE = BigInteger.ZERO;

    /**
     * Stack of NP values for ranges.
     */
    private final Deque rangeValues = new ArrayDeque<>();

    /** Stack of NP values for expressions. */
    private final Deque expressionValues = new ArrayDeque<>();

    /** Stack of belongs to range values for question operator. */
    private final Deque afterValues = new ArrayDeque<>();

    /**
     * Range of the last processed expression. Used for checking that ternary operation
     * which is a part of expression won't be processed for second time.
     */
    private final TokenEnd processingTokenEnd = new TokenEnd();

    /** NP value for current range. */
    private BigInteger currentRangeValue = INITIAL_VALUE;

    /** Specify the maximum threshold allowed. */
    private int max = DEFAULT_MAX;

    /** True, when branch is visited, but not leaved. */
    private boolean branchVisited;

    /**
     * Setter to specify the maximum threshold allowed.
     *
     * @param max the maximum threshold
     */
    public void setMax(int max) {
        this.max = max;
    }

    @Override
    public int[] getDefaultTokens() {
        return getRequiredTokens();
    }

    @Override
    public int[] getAcceptableTokens() {
        return getRequiredTokens();
    }

    @Override
    public int[] getRequiredTokens() {
        return new int[] {
            TokenTypes.CTOR_DEF,
            TokenTypes.METHOD_DEF,
            TokenTypes.STATIC_INIT,
            TokenTypes.INSTANCE_INIT,
            TokenTypes.LITERAL_WHILE,
            TokenTypes.LITERAL_DO,
            TokenTypes.LITERAL_FOR,
            TokenTypes.LITERAL_IF,
            TokenTypes.LITERAL_ELSE,
            TokenTypes.LITERAL_SWITCH,
            TokenTypes.CASE_GROUP,
            TokenTypes.LITERAL_TRY,
            TokenTypes.LITERAL_CATCH,
            TokenTypes.QUESTION,
            TokenTypes.LITERAL_RETURN,
            TokenTypes.LITERAL_DEFAULT,
        };
    }

    @Override
    public void beginTree(DetailAST rootAST) {
        rangeValues.clear();
        expressionValues.clear();
        afterValues.clear();
        processingTokenEnd.reset();
        currentRangeValue = INITIAL_VALUE;
        branchVisited = false;
    }

    @Override
    public void visitToken(DetailAST ast) {
        switch (ast.getType()) {
            case TokenTypes.LITERAL_IF:
            case TokenTypes.LITERAL_SWITCH:
            case TokenTypes.LITERAL_WHILE:
            case TokenTypes.LITERAL_DO:
            case TokenTypes.LITERAL_FOR:
                visitConditional(ast, 1);
                break;
            case TokenTypes.QUESTION:
                visitUnitaryOperator(ast, 2);
                break;
            case TokenTypes.LITERAL_RETURN:
                visitUnitaryOperator(ast, 0);
                break;
            case TokenTypes.CASE_GROUP:
                final int caseNumber = countCaseTokens(ast);
                branchVisited = true;
                pushValue(caseNumber);
                break;
            case TokenTypes.LITERAL_ELSE:
                branchVisited = true;
                if (currentRangeValue.equals(BigInteger.ZERO)) {
                    currentRangeValue = BigInteger.ONE;
                }
                pushValue(0);
                break;
            case TokenTypes.LITERAL_TRY:
            case TokenTypes.LITERAL_CATCH:
            case TokenTypes.LITERAL_DEFAULT:
                pushValue(1);
                break;
            case TokenTypes.CTOR_DEF:
            case TokenTypes.METHOD_DEF:
            case TokenTypes.INSTANCE_INIT:
            case TokenTypes.STATIC_INIT:
                pushValue(0);
                break;
            default:
                break;
        }
    }

    @Override
    public void leaveToken(DetailAST ast) {
        switch (ast.getType()) {
            case TokenTypes.LITERAL_WHILE:
            case TokenTypes.LITERAL_DO:
            case TokenTypes.LITERAL_FOR:
            case TokenTypes.LITERAL_IF:
            case TokenTypes.LITERAL_SWITCH:
                leaveConditional();
                break;
            case TokenTypes.LITERAL_TRY:
                leaveMultiplyingConditional();
                break;
            case TokenTypes.LITERAL_RETURN:
            case TokenTypes.QUESTION:
                leaveUnitaryOperator();
                break;
            case TokenTypes.LITERAL_CATCH:
                leaveAddingConditional();
                break;
            case TokenTypes.LITERAL_DEFAULT:
                leaveBranch();
                break;
            case TokenTypes.LITERAL_ELSE:
            case TokenTypes.CASE_GROUP:
                leaveBranch();
                branchVisited = false;
                break;
            case TokenTypes.CTOR_DEF:
            case TokenTypes.METHOD_DEF:
            case TokenTypes.INSTANCE_INIT:
            case TokenTypes.STATIC_INIT:
                leaveMethodDef(ast);
                break;
            default:
                break;
        }
    }

    /**
     * Visits if, while, do-while, for and switch tokens - all of them have expression in
     * parentheses which is used for calculation.
     * @param ast visited token.
     * @param basicBranchingFactor default number of branches added.
     */
    private void visitConditional(DetailAST ast, int basicBranchingFactor) {
        int expressionValue = basicBranchingFactor;
        DetailAST bracketed;
        for (bracketed = ast.findFirstToken(TokenTypes.LPAREN).getNextSibling();
                bracketed.getType() != TokenTypes.RPAREN;
                bracketed = bracketed.getNextSibling()) {
            expressionValue += countConditionalOperators(bracketed);
        }
        processingTokenEnd.setToken(bracketed);
        pushValue(expressionValue);
    }

    /**
     * Visits ternary operator (?:) and return tokens. They differ from those processed by
     * visitConditional method in that their expression isn't bracketed.
     * @param ast visited token.
     * @param basicBranchingFactor number of branches inherently added by this token.
     */
    private void visitUnitaryOperator(DetailAST ast, int basicBranchingFactor) {
        final boolean isAfter = processingTokenEnd.isAfter(ast);
        afterValues.push(isAfter);
        if (!isAfter) {
            processingTokenEnd.setToken(getLastToken(ast));
            final int expressionValue = basicBranchingFactor + countConditionalOperators(ast);
            pushValue(expressionValue);
        }
    }

    /**
     * Leaves ternary operator (?:) and return tokens.
     */
    private void leaveUnitaryOperator() {
        if (!afterValues.pop()) {
            final Values valuePair = popValue();
            BigInteger basicRangeValue = valuePair.getRangeValue();
            BigInteger expressionValue = valuePair.getExpressionValue();
            if (expressionValue.equals(BigInteger.ZERO)) {
                expressionValue = BigInteger.ONE;
            }
            if (basicRangeValue.equals(BigInteger.ZERO)) {
                basicRangeValue = BigInteger.ONE;
            }
            currentRangeValue = currentRangeValue.add(expressionValue).multiply(basicRangeValue);
        }
    }

    /** Leaves while, do, for, if, ternary (?::), return or switch. */
    private void leaveConditional() {
        final Values valuePair = popValue();
        final BigInteger expressionValue = valuePair.getExpressionValue();
        BigInteger basicRangeValue = valuePair.getRangeValue();
        if (currentRangeValue.equals(BigInteger.ZERO)) {
            currentRangeValue = BigInteger.ONE;
        }
        if (basicRangeValue.equals(BigInteger.ZERO)) {
            basicRangeValue = BigInteger.ONE;
        }
        currentRangeValue = currentRangeValue.add(expressionValue).multiply(basicRangeValue);
    }

    /** Leaves else, default or case group tokens. */
    private void leaveBranch() {
        final Values valuePair = popValue();
        final BigInteger basicRangeValue = valuePair.getRangeValue();
        final BigInteger expressionValue = valuePair.getExpressionValue();
        if (branchVisited && currentRangeValue.equals(BigInteger.ZERO)) {
            currentRangeValue = BigInteger.ONE;
        }
        currentRangeValue = currentRangeValue.subtract(BigInteger.ONE)
                .add(basicRangeValue)
                .add(expressionValue);
    }

    /**
     * Process the end of a method definition.
     * @param ast the token type representing the method definition
     */
    private void leaveMethodDef(DetailAST ast) {
        final BigInteger bigIntegerMax = BigInteger.valueOf(max);
        if (currentRangeValue.compareTo(bigIntegerMax) > 0) {
            log(ast, MSG_KEY, currentRangeValue, bigIntegerMax);
        }
        popValue();
        currentRangeValue = INITIAL_VALUE;
    }

    /** Leaves catch. */
    private void leaveAddingConditional() {
        currentRangeValue = currentRangeValue.add(popValue().getRangeValue().add(BigInteger.ONE));
    }

    /**
     * Pushes the current range value on the range value stack. Pushes this token expression value
     * on the expression value stack.
     * @param expressionValue value of expression calculated for current token.
     */
    private void pushValue(Integer expressionValue) {
        rangeValues.push(currentRangeValue);
        expressionValues.push(expressionValue);
        currentRangeValue = INITIAL_VALUE;
    }

    /**
     * Pops values from both stack of expression values and stack of range values.
     * @return pair of head values from both of the stacks.
     */
    private Values popValue() {
        final int expressionValue = expressionValues.pop();
        return new Values(rangeValues.pop(), BigInteger.valueOf(expressionValue));
    }

    /** Leaves try. */
    private void leaveMultiplyingConditional() {
        currentRangeValue = currentRangeValue.add(BigInteger.ONE)
                .multiply(popValue().getRangeValue().add(BigInteger.ONE));
    }

    /**
     * Calculates number of conditional operators, including inline ternary operator, for a token.
     * @param ast inspected token.
     * @return number of conditional operators.
     * @see 
     * Java Language Specification, §15.23
     * @see 
     * Java Language Specification, §15.24
     * @see 
     * Java Language Specification, §15.25
     */
    private static int countConditionalOperators(DetailAST ast) {
        int number = 0;
        for (DetailAST child = ast.getFirstChild(); child != null;
                child = child.getNextSibling()) {
            final int type = child.getType();
            if (type == TokenTypes.LOR || type == TokenTypes.LAND) {
                number++;
            }
            else if (type == TokenTypes.QUESTION) {
                number += 2;
            }
            number += countConditionalOperators(child);
        }
        return number;
    }

    /**
     * Finds a leaf, which is the most distant from the root.
     * @param ast the root of tree.
     * @return the leaf.
     */
    private static DetailAST getLastToken(DetailAST ast) {
        final DetailAST lastChild = ast.getLastChild();
        final DetailAST result;
        if (lastChild.getFirstChild() == null) {
            result = lastChild;
        }
        else {
            result = getLastToken(lastChild);
        }
        return result;
    }

    /**
     * Counts number of case tokens subject to a case group token.
     * @param ast case group token.
     * @return number of case tokens.
     */
    private static int countCaseTokens(DetailAST ast) {
        int counter = 0;
        for (DetailAST iterator = ast.getFirstChild(); iterator != null;
                iterator = iterator.getNextSibling()) {
            if (iterator.getType() == TokenTypes.LITERAL_CASE) {
                counter++;
            }
        }
        return counter;
    }

    /**
     * Coordinates of token end. Used to prevent inline ternary
     * operator from being processed twice.
     */
    private static class TokenEnd {

        /** End line of token. */
        private int endLineNo;

        /** End column of token. */
        private int endColumnNo;

        /**
         * Sets end coordinates from given token.
         * @param endToken token.
         */
        public void setToken(DetailAST endToken) {
            if (!isAfter(endToken)) {
                endLineNo = endToken.getLineNo();
                endColumnNo = endToken.getColumnNo();
            }
        }

        /** Sets end token coordinates to the start of the file. */
        public void reset() {
            endLineNo = 0;
            endColumnNo = 0;
        }

        /**
         * Checks if saved coordinates located after given token.
         * @param ast given token.
         * @return true, if saved coordinates located after given token.
         */
        public boolean isAfter(DetailAST ast) {
            final int lineNo = ast.getLineNo();
            final int columnNo = ast.getColumnNo();
            boolean isAfter = true;
            if (lineNo > endLineNo
                    || lineNo == endLineNo
                    && columnNo > endColumnNo) {
                isAfter = false;
            }
            return isAfter;
        }

    }

    /**
     * Class that store range value and expression value.
     */
    private static class Values {

        /** NP value for range. */
        private final BigInteger rangeValue;

        /** NP value for expression. */
        private final BigInteger expressionValue;

        /**
         * Constructor that assigns all of class fields.
         * @param valueOfRange NP value for range
         * @param valueOfExpression NP value for expression
         */
        /* package */ Values(BigInteger valueOfRange, BigInteger valueOfExpression) {
            rangeValue = valueOfRange;
            expressionValue = valueOfExpression;
        }

        /**
         * Returns NP value for range.
         * @return NP value for range
         */
        public BigInteger getRangeValue() {
            return rangeValue;
        }

        /**
         * Returns NP value for expression.
         * @return NP value for expression
         */
        public BigInteger getExpressionValue() {
            return expressionValue;
        }

    }

}
Structure	Complexity expression
if ([expr]) { [if-range] }	NP(if-range) + 1 + NP(expr)
if ([expr]) { [if-range] } else { [else-range] }	NP(if-range)+ NP(else-range) + NP(expr)
while ([expr]) { [while-range] }	NP(while-range) + NP(expr) + 1
do { [do-range] } while ([expr])	NP(do-range) + NP(expr) + 1
for([expr1]; [expr2]; [expr3]) { [for-range] }	NP(for-range) + NP(expr1)+ NP(expr2) + NP(expr3) + 1
switch ([expr]) { case : [case-range] default: [default-range] }	S(i=1:i=n)NP(case-range[i]) + NP(default-range) + NP(expr)
[expr1] ? [expr2] : [expr3]	NP(expr1) + NP(expr2) + NP(expr3) + 2
goto label	1
break	1
Expressions	Number of && and \|\| operators in expression. No operators - 0
continue	1
return	1
Statement (even sequential statements)	1
Empty block {}	1
Function call	1
Function(Method) declaration or Block	P(i=1:i=N)NP(Statement[i])