Make alignment when free space is negative configurable (#241)

Untested, but probably fixes
#240

---------

Signed-off-by: Nico Burns <[email protected]>

examples/swash_render/src/main.rs

@@ -168,7 +168,11 @@ fn main() {
 
     // Perform layout (including bidi resolution and shaping) with start alignment
     layout.break_all_lines(max_advance);
-    layout.align(max_advance, Alignment::Start);
+    layout.align(
+        max_advance,
+        Alignment::Start,
+        false, /* align_when_overflowing */
+    );
 
     // Create image to render into
     let width = layout.width().ceil() as u32 + (padding * 2);

examples/tiny_skia_render/src/main.rs

@@ -99,7 +99,11 @@ fn main() {
 
     // Perform layout (including bidi resolution and shaping) with start alignment
     layout.break_all_lines(max_advance);
-    layout.align(max_advance, Alignment::Start);
+    layout.align(
+        max_advance,
+        Alignment::Start,
+        false, /* align_when_overflowing */
+    );
     let width = layout.width().ceil() as u32;
     let height = layout.height().ceil() as u32;
     let padded_width = width + padding * 2;

parley/src/layout/alignment.rs

@@ -8,6 +8,7 @@ pub(crate) fn align<B: Brush>(
     layout: &mut LayoutData<B>,
     alignment_width: Option<f32>,
     alignment: Alignment,
+    align_when_overflowing: bool,
 ) {
     let alignment_width = alignment_width.unwrap_or_else(|| {
         let max_line_length = layout
@@ -27,8 +28,7 @@ pub(crate) fn align<B: Brush>(
         // Compute free space.
         let free_space = alignment_width - line.metrics.advance + line.metrics.trailing_whitespace;
 
-        // Alignment only applies if free_space > 0
-        if free_space <= 0. {
+        if !align_when_overflowing && free_space <= 0.0 {
             continue;
         }
 
@@ -43,6 +43,13 @@ pub(crate) fn align<B: Brush>(
                 line.metrics.offset = free_space * 0.5;
             }
             Alignment::Justified => {
+                // Justified alignment doesn't have any effect if free_space is negative or zero
+                if free_space <= 0.0 {
+                    continue;
+                }
+
+                // Justified alignment doesn't apply to the last line of a paragraph (`BreakReason::None`)
+                // or if there are no whitespace gaps to adjust
                 if line.break_reason == BreakReason::None || line.num_spaces == 0 {
                     continue;
                 }

parley/src/layout/cluster.rs

@@ -487,7 +487,11 @@ mod tests {
     fn cluster_from_position_with_alignment(alignment: Alignment) {
         let mut layout = create_unaligned_layout();
         let width = layout.full_width();
-        layout.align(Some(width + 100.), alignment);
+        layout.align(
+            Some(width + 100.),
+            alignment,
+            false, /* align_when_overflowing */
+        );
         assert_eq!(
             layout.len(),
             1,

parley/src/layout/editor.rs

@@ -1038,7 +1038,11 @@ where
         }
         self.layout = builder.build(&self.buffer);
         self.layout.break_all_lines(self.width);
-        self.layout.align(self.width, self.alignment);
+        self.layout.align(
+            self.width,
+            self.alignment,
+            false, /* align_when_overflowing */
+        );
         self.selection = self.selection.refresh(&self.layout);
         self.layout_dirty = false;
         self.generation.nudge();

parley/src/layout/mod.rs

@@ -144,8 +144,22 @@ impl<B: Brush> Layout<B> {
 
     // Apply to alignment to layout relative to the specified container width. If container_width is not
     // specified then the max line length is used.
-    pub fn align(&mut self, container_width: Option<f32>, alignment: Alignment) {
-        align(&mut self.data, container_width, alignment);
+    //
+    // If `align_when_overflowing` is set to `true` then `Center` and `End` alignment will apply even if
+    // the line contents are wider than the `container_width`. If it is set to `false` then all overflowing
+    // lines will be `Start` aligned.
+    pub fn align(
+        &mut self,
+        container_width: Option<f32>,
+        alignment: Alignment,
+        align_when_overflowing: bool,
+    ) {
+        align(
+            &mut self.data,
+            container_width,
+            alignment,
+            align_when_overflowing,
+        );
     }
 
     /// Returns the index and `Line` object for the line containing the

parley/src/lib.rs

@@ -53,7 +53,7 @@
 //! // Run line-breaking and alignment on the Layout
 //! const MAX_WIDTH : Option<f32> = Some(100.0);
 //! layout.break_all_lines(MAX_WIDTH);
-//! layout.align(MAX_WIDTH, Alignment::Start);
+//! layout.align(MAX_WIDTH, Alignment::Start, false /* align_when_overflowing */);
 //!
 //! // Inspect computed layout (see examples for more details)
 //! let width = layout.width();

parley/src/tests/test_basic.rs

@@ -11,7 +11,11 @@ fn plain_multiline_text() {
     let mut builder = env.builder(text);
     let mut layout = builder.build(text);
     layout.break_all_lines(None);
-    layout.align(None, Alignment::Start);
+    layout.align(
+        None,
+        Alignment::Start,
+        false, /* align_when_overflowing */
+    );
 
     env.check_layout_snapshot(&layout);
 }
@@ -36,7 +40,11 @@ fn placing_inboxes() {
         });
         let mut layout = builder.build(text);
         layout.break_all_lines(None);
-        layout.align(None, Alignment::Start);
+        layout.align(
+            None,
+            Alignment::Start,
+            false, /* align_when_overflowing */
+        );
         env.with_name(test_case_name).check_layout_snapshot(&layout);
     }
 }
@@ -57,7 +65,11 @@ fn only_inboxes_wrap() {
     }
     let mut layout = builder.build(text);
     layout.break_all_lines(Some(40.0));
-    layout.align(None, Alignment::Middle);
+    layout.align(
+        None,
+        Alignment::Middle,
+        false, /* align_when_overflowing */
+    );
 
     env.check_layout_snapshot(&layout);
 }