Center images when they are the only contents in a paragraph.

src/context/paragraph.rs

@@ -12,6 +12,7 @@ use super::RenderObject;
 #[serde(rename = "paragraph")]
 pub(crate) struct RenderParagraph {
     children: Vec<RenderObject>,
+    is_single_image: bool,
     post_blank: organic::types::PostBlank,
 }
 
@@ -26,6 +27,7 @@ render!(RenderParagraph, IParagraph, original, render_context, {
 
     Ok(RenderParagraph {
         children,
+        is_single_image: original.is_single_image(),
         post_blank: original.post_blank,
     })
 });

src/intermediate/paragraph.rs

@@ -1,3 +1,5 @@
+use std::any::Any;
+
 use super::macros::intermediate;
 use super::IObject;
 use crate::error::CustomError;
@@ -41,4 +43,32 @@ impl IParagraph {
             post_blank,
         })
     }
+
+    /// Checks if the paragraph contains nothing but a single image.
+    ///
+    /// When this happens, we want to center the image.
+    pub(crate) fn is_single_image(&self) -> bool {
+        let num_images = self
+            .children
+            .iter()
+            .filter(|c| match c {
+                IObject::RegularLink(iregular_link) => match &iregular_link.target {
+                    super::LinkTarget::Image { src, alt } => true,
+                    _ => false,
+                },
+                _ => false,
+            })
+            .count();
+        num_images == 1
+            && self.children.iter().all(|c| match c {
+                IObject::RegularLink(iregular_link) => match &iregular_link.target {
+                    super::LinkTarget::Image { src, alt } => true,
+                    _ => false,
+                },
+                IObject::PlainText(iplain_text) => {
+                    iplain_text.source.chars().all(|c| c.is_ascii_whitespace())
+                }
+                _ => false,
+            })
+    }
 }