implement validations of pagination URLs

java/org/chromium/distiller/PageParamInfo.java

Index: java/org/chromium/distiller/PageParamInfo.java
diff --git a/java/org/chromium/distiller/PageParamInfo.java b/java/org/chromium/distiller/PageParamInfo.java
index 63d5c722e79096b176f41d64bb2a74a3bdff1f78..d7047df4277ba03aa6ab56fade82df9455924097 100644
--- a/java/org/chromium/distiller/PageParamInfo.java
+++ b/java/org/chromium/distiller/PageParamInfo.java
@@ -1,11 +1,13 @@
-// Copyright 2016 The Chromium Authors. All rights reserved.
+// Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package org.chromium.distiller;
 
 import java.util.ArrayList;
+import java.util.HashSet;
 import java.util.List;
+import java.util.Set;
 
 /**
  * This class stores information about the page parameter detected from potential pagination URLs
@@ -13,10 +15,13 @@ import java.util.List;
  * - type of page parameter detected
  * - list of pagination URLs with their page numbers
  * - coefficient and delta values of the linear formula formed by the pagination URLs:
- *     pageParamValue = coefficient * pageNum + delta
- * - detected single page URL.
+ *     pageParamValue = coefficient * pageNum + delta.
  */
 class PageParamInfo implements Cloneable {
+    private static final int MIN_LINKS_TO_JUSTIFY_LINEAR_MAP = 2;
+
+    static final int PAGE_NUM_ADJACENT_MASK = 1 << 0;
+    static final int PAGE_NUM_CONSECUTIVE_MASK = 1 << 1;
 
     /**
      * Stores potential pagination info:
@@ -50,7 +55,7 @@ class PageParamInfo implements Cloneable {
     /**
      * Stores the coefficient and delta values of the linear formula:
      *   pageParamValue = coefficient * pageNum + delta.
-     * See PageParamterDetector.getPageParamLinearFormula() for details.
+     * See getLinearFormula() for details.
      */
     static class LinearFormula {
         final int mCoefficient;
@@ -77,26 +82,103 @@ class PageParamInfo implements Cloneable {
     }
 
     Type mType = Type.UNSET;
-    List<PageInfo> mAllPageInfo = new ArrayList<PageInfo>();
+    List<PageInfo> mAllPageInfo;
     LinearFormula mFormula = null;
-    String mSinglePageUrl = "";
+
+    PageParamInfo() {
+        mAllPageInfo = new ArrayList<PageInfo>();
+    }
+
+    PageParamInfo(Type type, List<PageInfo> allPageInfo, LinearFormula formula) {
+        mType = type;
+        mAllPageInfo = allPageInfo;
+        mFormula = formula;
+    }
+
+    /**
+     * Evaluates if the given list of PageLinkInfo's is a list of paging URLs:
+     * - page numbers in list of PageLinkInfo's must be adjacent
+     * - page numbers in list of ascending numbers must either
+     *   - be consecutive and form a page number sequence, or
+     *   - must construct a linear map with a linear formula: page_parameter = a * page_number + b
+     * - if there's only 1 PageLinkInfo, the first ascending number must be page 1, first page URL
+     *   must match page pattern, and the only outlink must be 2nd or 3rd page.
+     *
+     * Returns a populated PageParamInfo if evaluated true.  Otherwise, returns null.
+     *
+     * @param allLinkInfo the list of PageLinkInfo's to evaluate
+     * @param pagePattern the URL pattern to use
+     * @param ascendingNumbers list of PageInfo's with ascending mPageNum's
+     * @param firstPageUrl the URL of the PageInfo with mPageNum=1
+     */
+    static PageParamInfo evaluate(PageParameterDetector.PagePattern pagePattern,
+            List<PageLinkInfo> allLinkInfo, List<PageInfo> ascendingNumbers, String firstPageUrl) {
+        if (allLinkInfo.size() >= MIN_LINKS_TO_JUSTIFY_LINEAR_MAP) {
+            int result = arePageNumsAdjacentAndConsecutive(allLinkInfo, ascendingNumbers);
+            if ((result & PAGE_NUM_ADJACENT_MASK) == 0) return null;
+
+            LinearFormula linearFormula = getLinearFormula(allLinkInfo);
+
+            // Type.PAGE_NUMBER: ascending numbers must be consecutive and form a page number
+            // sequence.
+            if ((result & PAGE_NUM_CONSECUTIVE_MASK) == 0) return null;
+            if (!isPageNumberSequence(ascendingNumbers)) return null;
+
+            List<PageInfo> allPageInfo = new ArrayList<PageInfo>();
+            for (PageLinkInfo link : allLinkInfo) {
+                allPageInfo.add(new PageInfo(link.mPageNum,
+                        ascendingNumbers.get(link.mPosInAscendingList).mUrl));
+            }
+            return new PageParamInfo(Type.PAGE_NUMBER, allPageInfo, linearFormula);
+        }
+
+        // Most of news article have no more than 3 pages and the first page probably doesn't have
+        // any page parameter.  If the first page url matches the the page pattern, we treat it as
+        // the first page of this pattern.
+        if (allLinkInfo.size() == 1 && !firstPageUrl.isEmpty()) {
+            final PageLinkInfo onlyLink = allLinkInfo.get(0);
+            boolean secondPageIsOutlink = onlyLink.mPageNum == 2 &&
+                    onlyLink.mPosInAscendingList == 1;
+            boolean thirdPageIsOutlink = onlyLink.mPageNum == 3 &&
+                    onlyLink.mPosInAscendingList == 2 &&
+                    // onlyLink's pos is 2 (evaluated right before), so ascendingNumbers has >= 3
+                    // elements; check if previous element is previous page.
+                    ascendingNumbers.get(1).mPageNum == 2;
+            // 1 PageLinkInfo means ascendingNumbers has >= 1 element.
+            if (ascendingNumbers.get(0).mPageNum == 1 &&
+                    (secondPageIsOutlink || thirdPageIsOutlink) &&
+                    pagePattern.isPagingUrl(firstPageUrl)) {
+                // Has valid PageParamInfo, create and populate it.
+                int coefficient;
+                int delta = onlyLink.mPageParamValue - onlyLink.mPageNum;
+                if (delta == 0 || delta == 1) {
+                    coefficient = 1;
+                } else {
+                    coefficient = onlyLink.mPageParamValue;
+                    delta = 0;
+                }
+                List<PageInfo> allPageInfo = new ArrayList<PageInfo>();
+                allPageInfo.add(new PageInfo(1, firstPageUrl));
+                allPageInfo.add(new PageInfo(onlyLink.mPageNum,
+                        ascendingNumbers.get(onlyLink.mPosInAscendingList).mUrl));
+                return new PageParamInfo(Type.PAGE_NUMBER, allPageInfo,
+                        new LinearFormula(coefficient, delta));
+            }
+        }
+
+        return null;
+    }
 
     /**
      * Returns a new PageParamInfo that is a clone of this object.
      */
     public Object clone() {
-        PageParamInfo info = new PageParamInfo();
-        info.mType = this.mType;
-        info.mAllPageInfo = this.mAllPageInfo;
-        info.mFormula = this.mFormula;
-        info.mSinglePageUrl = this.mSinglePageUrl;
-        return info;
+        return new PageParamInfo(this.mType, this.mAllPageInfo, this.mFormula);
     }
    
     /**
      * Evaluates which PageParamInfo is better based on the properties of PageParamInfo.
-     * We prefer the one if the list of PageParameterDetector.LinkInfo forms a linear formula, see
-     * PageParamterDetector.getPageParamLinearFormula().
+     * We prefer the one if the list of PageLinkInfo forms a linear formula, see getLinearFormula().
      *
      * Returns 1 if this is better, -1 if other is better and 0 if they are equal.
      *
@@ -121,9 +203,190 @@ class PageParamInfo implements Cloneable {
        for (PageInfo page : mAllPageInfo) {
            str += "\n  " + page.toString();
        }
-       str += "\nsinglePageUrl: " + mSinglePageUrl +
-               "\nformula: " + (mFormula == null ? "null" : mFormula.toString());
+       str += "\nformula: " + (mFormula == null ? "null" : mFormula.toString());
        return str;
     }
 
+    /**
+     * Detects if page numbers in list of PageLinkInfo's are adjacent, and if page numbers in list
+     * of PageInfo's are consecutive.
+     *
+     * For adjacency, the page numbers in list of PageLinkInfo's must either be adjacent, or
+     * separated by at most 1 plain text number which must represent the current page number in one
+     * of the PageInfo's.
+     * For consecutiveness, there must be at least one pair of consecutive number values in the list
+     * of PageLinkInfo's, or between a PageLinkInfo and a plain text number.
+     *
+     * Returns a int value that is a combination of bits:
+     * - bit for PAGE_PARAM_ADJACENT_MASK is set if allLinkInfo are adjacent
+     * - bit for PAGE_PARAM_CONSECUTIVE_MASK is set if ascendingNumbers are consecutive.
+     *
+     * @param allLinkInfo the list of PageLinkInfo's to evaluate
+     * @param ascendingNumbers list of PageInfo's with ascending mPageNum's
+     */
+    static int arePageNumsAdjacentAndConsecutive(List<PageLinkInfo> allLinkInfo,
+            List<PageInfo> ascendingNumbers) {
+        int result = 0;
+
+        // Check if elements in allLinkInfo are adjacent or there's only 1 gap i.e. the gap is
+        // current page number respresented in plain text.
+        int firstPos = -1;
+        int lastPos = -1;
+        int gapPos = -1;
+        Set<Integer> pageParamSet = new HashSet<Integer>();  // To check that page number is unique.
+        for (PageLinkInfo linkInfo : allLinkInfo) {
+            final int currPos = linkInfo.mPosInAscendingList;
+            if (lastPos == -1) {
+                firstPos = currPos;
+            } else if (currPos != lastPos + 1) {
+                // If position is not strictly ascending, or the gap size is > 1 (e.g. "[3] [4] 5 6
+                // [7]"), or there's more than 1 gap (e.g. "[3] 4 [5] 6 [7]"), allLinkInfo is not
+                // adjacent.
+                if (currPos <= lastPos || currPos != lastPos + 2 || gapPos != -1) return result;
+                gapPos = currPos - 1;
+            }
+            // Make sure page param value, i.e. page number represented in plain text, is unique.
+            if (!pageParamSet.add(linkInfo.mPageParamValue)) return result;
+            lastPos = currPos;
+        }  // for all LinkInfo's
+
+        result |= PAGE_NUM_ADJACENT_MASK;
+
+        // Now, determine if page numbers in ascendingNumbers are consecutive.
+
+        // First, handle the gap.
+        if (gapPos != -1) {
+           if (gapPos <= 0 || gapPos >= ascendingNumbers.size() - 1) return result;
+           // The "gap" should represent current page number in plain text.
+           // Check if its adjacent page numbers are consecutive.
+           // e.g. "[1] [5] 6 [7] [12]" is accepted; "[4] 8 [16]" is rejected.
+           // This can eliminate links affecting the number of items on a page.
+           final int currPageNum = ascendingNumbers.get(gapPos).mPageNum;
+           if (ascendingNumbers.get(gapPos - 1).mPageNum == currPageNum - 1 &&
+                   ascendingNumbers.get(gapPos + 1).mPageNum == currPageNum + 1) {
+               return result | PAGE_NUM_CONSECUTIVE_MASK;
+           }
+           return result;
+        }
+
+        // There is no gap.  Check if at least one of the following cases is satisfied:
+        // Case #1: "[1] [2] ..." or "1 [2] ... ".
+        if ((firstPos == 0 || firstPos == 1) && ascendingNumbers.get(0).mPageNum == 1 &&
+                ascendingNumbers.get(1).mPageNum == 2) {
+            return result | PAGE_NUM_CONSECUTIVE_MASK;
+        }
+        // Case #2: "[1] 2 [3] ..." where [1] doesn't belong to current pattern.
+        if (firstPos == 2 && ascendingNumbers.get(2).mPageNum == 3 &&
+                ascendingNumbers.get(1).mUrl.isEmpty() && !ascendingNumbers.get(0).mUrl.isEmpty()) {
+            return result | PAGE_NUM_CONSECUTIVE_MASK;
+        }
+        // Case #3: "... [n-1] [n]" or "... [n - 1] n".
+        final int numbersSize = ascendingNumbers.size();
+        if ((lastPos == numbersSize - 1 || lastPos == numbersSize - 2) &&
+                ascendingNumbers.get(numbersSize - 2).mPageNum + 1 ==
+                        ascendingNumbers.get(numbersSize - 1).mPageNum) {
+            return result | PAGE_NUM_CONSECUTIVE_MASK;
+        }
+        // Case #4: "... [i-1] [i] [i+1] ...".
+        for (int i = firstPos + 1; i < lastPos; i++) {
+            if (ascendingNumbers.get(i - 1).mPageNum + 2 == ascendingNumbers.get(i + 1).mPageNum) {
+                return result | PAGE_NUM_CONSECUTIVE_MASK;
+            }
+        }
+
+        // Otherwise, there's no pair of consecutive values.
+        return result;
+    }
+
+    /**
+     * Determines if the list of PageLinkInfo's form a linear formula:
+     *   pageParamValue = coefficient * pageNum + delta (delta == -coefficient or delta == 0).
+     *
+     * The coefficient and delta are calculated from the page parameter values and page numbers of 2
+     * PageLinkInfo's, and then validated against the remaining PageLinkInfo's.
+     * The order of page numbers doesn't matter.
+     *
+     * Returns LinearFormula, containing the coefficient and delta, if the page
+     * parameter forumla could be determined.  Otherwise, returns null.
+     *
+     * @param allLinkInfo the list of PageLinkInfo's to evaluate
+     */
+    // TODO(kuan): As this gets rolled out, reassesss the necessity of non-1 coefficient support.
+    private static LinearFormula getLinearFormula(List<PageLinkInfo> allLinkInfo) {
+        if (allLinkInfo.size() < MIN_LINKS_TO_JUSTIFY_LINEAR_MAP) return null;
+
+        final PageLinkInfo firstLink = allLinkInfo.get(0);
+        final PageLinkInfo secondLink = allLinkInfo.get(1);
+
+        if (allLinkInfo.size() == 2 && Math.max(firstLink.mPageNum, secondLink.mPageNum) > 4) {
+            return null;
+        }
+
+        int deltaX = secondLink.mPageNum - firstLink.mPageNum;
+        if (deltaX == 0) return null;
+
+        int deltaY = secondLink.mPageParamValue - firstLink.mPageParamValue;
+        int coefficient = deltaY / deltaX;
+        if (coefficient == 0) return null;
+
+        int delta = firstLink.mPageParamValue - coefficient * firstLink.mPageNum;
+        if (delta != 0 && delta != -coefficient) return null;
+
+        // Check if the remaining elements are on the same linear map.
+        for (int i = 2; i < allLinkInfo.size(); i++) {
+            final PageLinkInfo link = allLinkInfo.get(i);
+            if (link.mPageParamValue != coefficient * link.mPageNum + delta) return null;
+        }
+
+        return new LinearFormula(coefficient, delta);
+    }
+
+    /**
+     * Returns true if page numbers in list of PageInfo's form a sequence, based on a pipeline of
+     * rules:
+     * - first PageInfo must have a URL unless it is the first page
+     * - there's only one plain number without URL in list
+     * - if only two pages, they must be siblings - 2nd page number must follow 1st
+     * - page numbers must be adjacent and consecutive; otherwise, 2 non-consecutive numbers must be
+     *   head/tail or have URLs.
+     *
+     * @param ascendingNumbers list of PageInfo's with ascending mPageNum's
+     */
+    private static boolean isPageNumberSequence(List<PageInfo> ascendingNumbers) {
+        if (ascendingNumbers.size() <= 1) return false;
+
+        // The first one must have a URL unless it is the first page.
+        final PageInfo firstPage = ascendingNumbers.get(0);
+        if (firstPage.mPageNum != 1 && firstPage.mUrl.isEmpty()) return false;
+
+        // There's only one plain number without URL in ascending numbers group.
+        boolean hasPlainNum = false;
+        for (PageInfo page : ascendingNumbers) {
+            if (page.mUrl.isEmpty()) {
+                if (hasPlainNum) return false;
+                hasPlainNum = true;
+            }
+        }
+
+        // If there are only two pages, they must be siblings.
+        if (ascendingNumbers.size() == 2) {
+            return firstPage.mPageNum + 1 == ascendingNumbers.get(1).mPageNum;
+        }
+
+        // Check if page numbers in ascendingNumbers are adjacent and consecutive.
+        for (int i = 1; i < ascendingNumbers.size(); i++) {
+            // If two adjacent numbers are not consecutive, we accept them only when:
+            // 1) one of them is head/tail, like [1],[n-i][n-i+1]..[n] or [1],[2], [3]...[i], [n].
+            // 2) both of them have URLs.
+            final PageInfo currPage = ascendingNumbers.get(i);
+            final PageInfo prevPage = ascendingNumbers.get(i - 1);
+            if (currPage.mPageNum - prevPage.mPageNum != 1) {
+                if (i != 1 && i != ascendingNumbers.size() - 1) return false;
+                if (currPage.mUrl.isEmpty() || prevPage.mUrl.isEmpty()) return false;
+            }
+        }
+
+        return true;
+    }
+
 }