refactor(engine): extract composite bleed-box geometry into CompositeDecoration

src/main/java/com/demcha/compose/document/layout/CompositeDecoration.java

@@ -30,6 +30,62 @@ final class CompositeDecoration {
     private CompositeDecoration() {
     }
 
+    /**
+     * Resolves the decoration (fill / border) box for a composite. Without bleed
+     * the box is the in-margin content geometry; each declared bleed edge instead
+     * snaps that side of the box to the trimmed page edge (left/right adjust the x
+     * span, top/bottom the y span) while the children stay in the content region.
+     *
+     * @param bleed   the node's bleed edges
+     * @param x       content-box left (in-margin)
+     * @param width   content-box width (in-margin)
+     * @param topY    content-box top y (in-margin)
+     * @param bottomY content-box bottom y (in-margin)
+     * @param canvas  the page canvas, for the trimmed page-edge coordinates
+     * @return the resolved decoration box
+     */
+    static BleedBox resolveBleedBox(DocumentBleed bleed,
+                                    double x,
+                                    double width,
+                                    double topY,
+                                    double bottomY,
+                                    LayoutCanvas canvas) {
+        double decorX = x;
+        double decorWidth = width;
+        double decorTopY = topY;
+        double decorBottomY = bottomY;
+        if (bleed.any()) {
+            double pageWidth = canvas.width();
+            double pageHeight = canvas.height();
+            if (bleed.bleeds(DocumentEdge.LEFT)) {
+                decorWidth += decorX;
+                decorX = 0.0;
+            }
+            if (bleed.bleeds(DocumentEdge.RIGHT)) {
+                decorWidth = Math.max(0.0, pageWidth - decorX);
+            }
+            if (bleed.bleeds(DocumentEdge.TOP)) {
+                decorTopY = pageHeight;
+            }
+            if (bleed.bleeds(DocumentEdge.BOTTOM)) {
+                decorBottomY = 0.0;
+            }
+        }
+        return new BleedBox(decorX, decorWidth, decorTopY, decorBottomY);
+    }
+
+    /**
+     * A resolved composite decoration box — the fill / border extent after content
+     * bleed, in page space.
+     *
+     * @param x       decoration box left
+     * @param width   decoration box width
+     * @param topY    decoration box top y
+     * @param bottomY decoration box bottom y
+     */
+    record BleedBox(double x, double width, double topY, double bottomY) {
+    }
+
     static List<PlacedFragment> fill(PreparedNode<DocumentNode> prepared,
                                      NodeDefinition<DocumentNode> definition,
                                      String path,

src/main/java/com/demcha/compose/document/layout/LayoutCompiler.java

@@ -8,7 +8,6 @@
 import com.demcha.compose.document.node.RowNode;
 import com.demcha.compose.document.node.RowVerticalAlign;
 import com.demcha.compose.document.style.DocumentBleed;
-import com.demcha.compose.document.style.DocumentEdge;
 import com.demcha.compose.engine.components.style.Margin;
 import com.demcha.compose.engine.components.style.Padding;
 import org.slf4j.Logger;
@@ -320,43 +319,23 @@ private void compileComposite(PreparedNode<DocumentNode> prepared,
         int endPage = state.pageIndex;
         double endPageBottomY = state.pageTop() - state.usedHeight + margin.bottom();
 
-        // Content bleed: the decoration box (fill/border) extends to the trimmed
-        // page edge on the declared edges, while children stay in the content
-        // region (so text never runs off the page). Byte-identical when the node
-        // does not bleed — every value below collapses to the in-margin geometry.
+        // Content bleed extends the decoration box to the trimmed page edge on the
+        // declared edges while children stay in the content region; without bleed the
+        // box is the in-margin geometry (see CompositeDecoration#resolveBleedBox).
         DocumentBleed bleed = node.bleed();
-        double decorX = placementX;
-        double decorWidth = naturalMeasure.width();
-        double decorTopY = placementTopY;
-        double decorBottomY = endPageBottomY;
-        if (bleed.any()) {
-            double pageWidth = state.canvas.width();
-            double pageHeight = state.canvas.height();
-            if (bleed.bleeds(DocumentEdge.LEFT)) {
-                decorWidth += decorX;
-                decorX = 0.0;
-            }
-            if (bleed.bleeds(DocumentEdge.RIGHT)) {
-                decorWidth = Math.max(0.0, pageWidth - decorX);
-            }
-            if (bleed.bleeds(DocumentEdge.TOP)) {
-                decorTopY = pageHeight;
-            }
-            if (bleed.bleeds(DocumentEdge.BOTTOM)) {
-                decorBottomY = 0.0;
-            }
-        }
+        CompositeDecoration.BleedBox decor = CompositeDecoration.resolveBleedBox(
+                bleed, placementX, naturalMeasure.width(), placementTopY, endPageBottomY, state.canvas);
         List<PlacedFragment> decorationFragments = CompositeDecoration.fill(
                 prepared,
                 definition,
                 path,
                 parentPath,
                 childIndex,
                 depth,
-                decorX,
-                decorTopY,
-                decorBottomY,
-                decorWidth,
+                decor.x(),
+                decor.topY(),
+                decor.bottomY(),
+                decor.width(),
                 startPage,
                 endPage,
                 margin,

src/test/java/com/demcha/compose/document/layout/CompositeDecorationTest.java

@@ -174,4 +174,56 @@ void toPlacedFragmentsReindexesFragmentIndexPerPlacement() {
             assertThat(f.pageIndex()).isEqualTo(3);
         });
     }
+
+    // resolveBleedBox: content box x=20 w=100, top=80 bottom=30, on the 200×100 canvas.
+
+    @Test
+    void resolveBleedBoxWithoutBleedReturnsTheInMarginBox() {
+        CompositeDecoration.BleedBox box = CompositeDecoration.resolveBleedBox(
+                DocumentBleed.none(), 20, 100, 80, 30, canvas);
+        assertThat(box.x()).isCloseTo(20, within(EPS));
+        assertThat(box.width()).isCloseTo(100, within(EPS));
+        assertThat(box.topY()).isCloseTo(80, within(EPS));
+        assertThat(box.bottomY()).isCloseTo(30, within(EPS));
+    }
+
+    @Test
+    void resolveBleedBoxLeftSnapsOriginToZeroAndAbsorbsTheLeftGap() {
+        CompositeDecoration.BleedBox box = CompositeDecoration.resolveBleedBox(
+                DocumentBleed.of(DocumentEdge.LEFT), 20, 100, 80, 30, canvas);
+        assertThat(box.x()).isCloseTo(0, within(EPS));
+        assertThat(box.width()).isCloseTo(120, within(EPS)); // 100 + 20
+        assertThat(box.topY()).isCloseTo(80, within(EPS));
+        assertThat(box.bottomY()).isCloseTo(30, within(EPS));
+    }
+
+    @Test
+    void resolveBleedBoxRightStretchesWidthToThePageEdge() {
+        CompositeDecoration.BleedBox box = CompositeDecoration.resolveBleedBox(
+                DocumentBleed.of(DocumentEdge.RIGHT), 20, 100, 80, 30, canvas);
+        assertThat(box.x()).isCloseTo(20, within(EPS));
+        assertThat(box.width()).isCloseTo(180, within(EPS)); // pageWidth 200 - x 20
+        assertThat(box.topY()).isCloseTo(80, within(EPS));
+        assertThat(box.bottomY()).isCloseTo(30, within(EPS));
+    }
+
+    @Test
+    void resolveBleedBoxTopAndBottomSnapToThePageEdges() {
+        CompositeDecoration.BleedBox box = CompositeDecoration.resolveBleedBox(
+                DocumentBleed.of(DocumentEdge.TOP, DocumentEdge.BOTTOM), 20, 100, 80, 30, canvas);
+        assertThat(box.x()).isCloseTo(20, within(EPS));
+        assertThat(box.width()).isCloseTo(100, within(EPS));
+        assertThat(box.topY()).isCloseTo(100, within(EPS)); // pageHeight
+        assertThat(box.bottomY()).isCloseTo(0, within(EPS));
+    }
+
+    @Test
+    void resolveBleedBoxAllSnapsToTheFullPage() {
+        CompositeDecoration.BleedBox box = CompositeDecoration.resolveBleedBox(
+                DocumentBleed.all(), 20, 100, 80, 30, canvas);
+        assertThat(box.x()).isCloseTo(0, within(EPS));
+        assertThat(box.width()).isCloseTo(200, within(EPS)); // full page width
+        assertThat(box.topY()).isCloseTo(100, within(EPS));
+        assertThat(box.bottomY()).isCloseTo(0, within(EPS));
+    }
 }