Fix emitting :: for centered column in table.

Emitting `::` in a table to indicate alignment is `Center` seems to
break some parsers. Emitting `:-:` appears to work fine everywhere.

This commit also removes an allocation when encountering an empty header
cell. Rather than allocating a non-empty string, let the string be empty
and add a minimum width for emitting alignments.

src/lib.rs

@@ -530,13 +530,9 @@ where
                 Ok(())
             }
             TagEnd::TableCell => {
-                state.table_headers.push(
-                    state
-                        .text_for_header
-                        .take()
-                        .filter(|s| !s.is_empty())
-                        .unwrap_or_else(|| "  ".into()),
-                );
+                state
+                    .table_headers
+                    .push(state.text_for_header.take().unwrap_or_default());
                 Ok(())
             }
             ref t @ TagEnd::TableRow | ref t @ TagEnd::TableHead => {
@@ -551,8 +547,19 @@ where
                         formatter.write_char('|')?;
                         // NOTE: For perfect counting, count grapheme clusters.
                         // The reason this is not done is to avoid the dependency.
-                        let last_minus_one = name.chars().count().saturating_sub(1);
-                        for c in 0..name.len() {
+
+                        // The minimum width of the column so that we can represent its alignment.
+                        let min_width = match alignment {
+                            // Must at least represent `-`.
+                            Alignment::None => 1,
+                            // Must at least represent `:-` or `-:`
+                            Alignment::Left | Alignment::Right => 2,
+                            // Must at least represent `:-:`
+                            Alignment::Center => 3,
+                        };
+                        let length = name.chars().count().max(min_width);
+                        let last_minus_one = length.saturating_sub(1);
+                        for c in 0..length {
                             formatter.write_char(
                                 if (c == 0 && (alignment == &Alignment::Center || alignment == &Alignment::Left))
                                     || (c == last_minus_one

tests/fmt.rs

@@ -909,7 +909,42 @@ mod table {
             indoc!(
                 "
             |Tables|Are|Cool|yo||
-            |------|:-:|---:|:-|--|
+            |------|:-:|---:|:-|-|
+            |col 3 is|right-aligned|$1600|x|01|
+            |col 2 is|centered|$12|y|02|
+            |zebra stripes|are neat|$1|z|03|"
+            )
+        );
+
+        let p = Parser::new_ext(&generated_markdown, Options::all());
+        let generated_events: Vec<_> = p.into_iter().collect();
+
+        assert_eq!(original_events, generated_events);
+    }
+
+    #[test]
+    fn it_generates_equivalent_table_markdown_with_empty_headers() {
+        use pulldown_cmark::{Options, Parser};
+
+        let original_table_markdown = indoc!(
+            "
+            ||||||
+            |:-------------:|:--------------|------:|:--:|:-:|
+            | col 3 is      | right-aligned | $1600 | x  |01|
+            | col 2 is      | centered      |   $12 | y  |02|
+            | zebra stripes | are neat      |    $1 | z  |03|"
+        );
+        let p = Parser::new_ext(original_table_markdown, Options::all());
+        let original_events: Vec<_> = p.into_iter().collect();
+
+        let (generated_markdown, _) = fmte(&original_events);
+
+        assert_eq!(
+            generated_markdown,
+            indoc!(
+                "
+            ||||||
+            |:-:|:-|-:|:-:|:-:|
             |col 3 is|right-aligned|$1600|x|01|
             |col 2 is|centered|$12|y|02|
             |zebra stripes|are neat|$1|z|03|"