Move outer_backticks to a util module

This is because it shall be used by other functions later on.

It was made more generic by accepting raw bytes instead of strictly
UTF-8 strings. This allows future functions to call it without having to
convert the contents to UTF-8 strings first.

2 files changed, 79 insertions, 0 deletions

diff --git a/src/util/fence.rs b/src/util/fence.rs
new file mode 100644
index 0000000..16f398a
--- /dev/null
+++ b/src/util/fence.rs
@@ -0,0 +1,78 @@
+pub fn generate_outer_backticks(contents: impl AsRef<[u8]>) -> String {
+    let bytes = contents.as_ref();
+    let mut max_ticks = 0;
+    let mut current_count = 0;
+    for &byte in bytes {
+        if byte == b'`' {
+            current_count += 1;
+            if current_count > max_ticks {
+                max_ticks = current_count;
+            }
+        } else {
+            current_count = 0;
+        }
+    }
+    let fence_len = std::cmp::max(3, max_ticks + 1);
+    "`".repeat(fence_len)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::generate_outer_backticks;
+
+    #[test]
+    fn empty_content_returns_three_backticks() {
+        assert_eq!(generate_outer_backticks(b""), "```");
+        assert_eq!(generate_outer_backticks(""), "```");
+    }
+
+    #[test]
+    fn content_without_backticks_returns_three_backticks() {
+        assert_eq!(generate_outer_backticks(b"no backticks here"), "```");
+        assert_eq!(generate_outer_backticks("hello world"), "```");
+    }
+
+    #[test]
+    fn single_backtick_returns_three_backticks() {
+        assert_eq!(generate_outer_backticks(b"`"), "```");
+        assert_eq!(generate_outer_backticks("a ` b"), "```");
+    }
+
+    #[test]
+    fn two_consecutive_backticks_returns_three_backticks() {
+        assert_eq!(generate_outer_backticks(b"``"), "```");
+        assert_eq!(generate_outer_backticks("a `` b"), "```");
+    }
+
+    #[test]
+    fn three_consecutive_backticks_returns_four_backticks() {
+        assert_eq!(generate_outer_backticks(b"```"), "````");
+        assert_eq!(generate_outer_backticks("a ``` b"), "````");
+    }
+
+    #[test]
+    fn four_consecutive_backticks_returns_five_backticks() {
+        assert_eq!(generate_outer_backticks(b"````"), "`````");
+        assert_eq!(generate_outer_backticks("a ```` b"), "`````");
+    }
+
+    #[test]
+    fn long_run_of_backticks_returns_run_plus_one() {
+        let run = 10;
+        let input = "`".repeat(run);
+        let expected = "`".repeat(run + 1);
+        assert_eq!(generate_outer_backticks(&input), expected);
+    }
+
+    #[test]
+    fn maximum_run_across_multiple_groups() {
+        let input = "``` and then ````` and then ``"; // longest run is 5
+        assert_eq!(generate_outer_backticks(input), "``````"); // 5+1 = 6
+    }
+
+    #[test]
+    fn backticks_at_start_or_end_handled_correctly() {
+        assert_eq!(generate_outer_backticks("````start"), "`````");
+        assert_eq!(generate_outer_backticks("end`````"), "``````");
+    }
+}
diff --git a/src/util/mod.rs b/src/util/mod.rs
index 5595a45..24ba6fc 100644
--- a/src/util/mod.rs
+++ b/src/util/mod.rs
@@ -1,2 +1,3 @@
+pub mod fence;
 pub mod language;
 pub mod path_display;