76 lines
2.6 KiB
Dart
76 lines
2.6 KiB
Dart
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// lib/editor/layout/double_page_layout.dart
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//
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// Two-up (spread) layout foundation for book-like reading (F2/F3 — the user's
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// explicit "two-page spread" ask). Pages are grouped into spread ROWS; each row
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// then stacks vertically exactly like continuous-single, so windowing reuses
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// PageStackMetrics on the row heights.
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//
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// Pure geometry (no widgets / pdfrx): the row-mounting widget is device-gated;
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// the pairing + row-height math is here, fully unit-tested. Building it ahead of
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// its phase is zero-rework-risk (it is not rendering and the P0.5 perf gate
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// can't invalidate pure geometry).
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import 'page_viewport.dart';
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import '../pdf/pdf_document_source.dart';
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/// Groups page indices into two-up spread rows. With [coverAlone], page 0 sits
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/// on its own row (a book cover / title page) and the rest pair 1-2, 3-4, …;
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/// otherwise pages pair from 0. A trailing odd page occupies a single-page row.
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List<List<int>> pairIntoRows(int pageCount, {bool coverAlone = false}) {
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assert(pageCount >= 0);
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final rows = <List<int>>[];
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var i = 0;
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if (coverAlone && pageCount > 0) {
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rows.add(<int>[0]);
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i = 1;
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}
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while (i < pageCount) {
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if (i + 1 < pageCount) {
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rows.add(<int>[i, i + 1]);
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i += 2;
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} else {
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rows.add(<int>[i]);
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i += 1;
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}
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}
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return rows;
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}
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/// Height of each spread row when each page is fit to HALF of [columnWidth]
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/// (two pages share the column). A row's height is the tallest of its pages so
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/// both pages sit on a common baseline. Non-positive page widths contribute 0.
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List<double> spreadRowHeights(
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PageDocumentSource source,
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double columnWidth,
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List<List<int>> rows,
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) {
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assert(columnWidth >= 0);
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final half = columnWidth / 2;
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return rows.map((row) {
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var tallest = 0.0;
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for (final pageIndex in row) {
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final size = source.pageSize(pageIndex);
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if (size.width <= 0) continue;
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final fit = half * (size.height / size.width);
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if (fit > tallest) tallest = fit;
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}
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return tallest;
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}).toList(growable: false);
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}
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/// Continuous-DOUBLE stacking metrics: pair pages into spread rows, then stack
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/// the rows vertically. [PageStackMetrics.visibleRange] over the result yields
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/// the visible ROW range; map rows back to pages via the [pairIntoRows] result.
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PageStackMetrics spreadStackMetrics(
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PageDocumentSource source,
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double columnWidth, {
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bool coverAlone = false,
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double gap = 0.0,
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}) {
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final rows = pairIntoRows(source.pageCount, coverAlone: coverAlone);
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return PageStackMetrics(
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pageHeights: spreadRowHeights(source, columnWidth, rows),
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gap: gap,
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);
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}
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