Bug 1661868 part 1: Broaden PrintedSheetFrame's "layout-time" page-range handling to support complex page ranges. r=TYLin

This patch does the following:

* It adds better documentation for the meanings of the page range member-vars
in nsSharedPageData.

* It copies some logic (with minor tweaks) from the legacy codepath that
handles page-range support (nsPageSequenceFrame::DetermineWhetherToPrintPage())
into to the new codepath for page-range support (PrintedSheetFrame.cpp's helper
function "TagIfSkippedByCustomRange()").

At this point in the patch stack, the legacy codepath is unchanged & still
handles all print operations; and the new PrintedSheetFrame.cpp codepath only
handles page-range-skipping for the print-preview visualization in the
Tab-Modal print dialog. So, this patch effectively gives that print-preview
visualization the platform-support that it would need for complex page ranges
(though that's not available in the UI at this point, so you can't really test
that).

Spoiler alert: the next patch in this series will remove the legacy codepath
entirely, and at *that* point, the platform-native print dialog's
complex-page-range feature will be backed by the PrintedSheetFrame.cpp code
that's added here.

Differential Revision: https://phabricator.services.mozilla.com/D92696

layout/generic/PrintedSheetFrame.cpp

@@ -58,11 +58,45 @@ static bool TagIfSkippedByCustomRange(nsPageFrame* aPageFrame, int32_t aPageNum,
     return false;
   }
 
-  if (!aPD->mDoingPageRange ||
-      (aPD->mFromPageNum <= aPageNum && aPD->mToPageNum >= aPageNum)) {
+  if (!aPD->mDoingPageRange) {
     MOZ_ASSERT(!aPageFrame->HasAnyStateBits(NS_PAGE_SKIPPED_BY_CUSTOM_RANGE),
-               "page frames in print range shouldn't be tagged with the"
-               "NS_PAGE_SKIPPED_BY_CUSTOM_RANGE state bit");
+               "page frames shouldn't be tagged as skipped if we're not "
+               "printing with a custom page range");
+    return false;
+  }
+
+  bool isPageSkipped = false;
+
+  // As described in nsSharedPageData documentation: we can use mFromPageNum
+  // and mToPageNum as a quick way to reject entirely-out-of-range pages. For
+  // page numbers inside these bounds, we need to check mRanges to find out if
+  // they're included in our subranges (if there are any).
+  if (aPageNum < aPD->mFromPageNum || aPageNum > aPD->mToPageNum) {
+    // Page is out-of-bounds; it's skipped.
+    isPageSkipped = true;
+  } else {
+    // Check subranges (if there are any).
+    const auto& ranges = aPD->mPageRanges;
+    int32_t length = ranges.Length();
+
+    // Page ranges are pairs (start, end) of included pages.
+    if (length && (length % 2 == 0)) {
+      isPageSkipped = true;
+      for (int32_t i = 0; i < length; i += 2) {
+        if (ranges[i] <= aPageNum && aPageNum <= ranges[i + 1]) {
+          // The page is included in this piece of the custom range,
+          // so it's not skipped.
+          isPageSkipped = false;
+          break;
+        }
+      }
+    }
+  }
+
+  if (!isPageSkipped) {
+    MOZ_ASSERT(!aPageFrame->HasAnyStateBits(NS_PAGE_SKIPPED_BY_CUSTOM_RANGE),
+               "page frames NS_PAGE_SKIPPED_BY_CUSTOM_RANGE state should "
+               "only be set if we actually want to skip the page");
     return false;
   }
 

layout/generic/nsPageSequenceFrame.h

@@ -40,9 +40,19 @@ class nsSharedPageData {
   // that it's reflowed the final page):
   int32_t mRawNumPages = 0;
 
-  // Page range info (only relevant if mDoingPageRange is true):
+  // Smallest included page num. 1-based. Only used if mDoingPageRange is true.
   int32_t mFromPageNum = 0;
+
+  // Largest included page num. 1-based. Only used if mDoingPageRange is true.
   int32_t mToPageNum = 0;
+
+  // If there's more than one page-range, then its components are stored here
+  // as pairs of (start,end).  They're stored in the order provided (not
+  // necessarily in ascending order). The most extreme included values will
+  // still be stored in mFromPageNum and mToPageNum, so that entirely
+  // out-of-bounds pages can be easily filtered out without needing to inspect
+  // this array. As above, the values are 1-based, and this member is only used
+  // if mDoingPageRange is true.
   nsTArray<int32_t> mPageRanges;
 
   // Margin for headers and footers; it defaults to 4/100 of an inch on UNIX