lib: add footer module for commit footers

To be used for parsing `Change-Id`s from commits, in service of Gerrit
support.

Signed-off-by: Austin Seipp <[email protected]>
Change-Id: I434d76b1229b36b815622ad7409ced3a405cbe22

lib/src/footer.rs

@@ -0,0 +1,132 @@
+// Copyright 2024 The Jujutsu Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! Parsing footer lines from commit messages.
+
+/// A key-value pair representing a footer line in a commit message, of the
+/// form `Key: Value`.
+#[derive(Debug, PartialEq, Clone)]
+pub struct FooterEntry(pub String, pub String);
+
+/// Parse the footer lines from a commit message; these are simple key-value
+/// pairs, separated by a colon, describing extra information in a commit
+/// message; an example is the following:
+///
+/// ```text
+/// chore: fix bug 1234
+///
+/// Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod
+/// tempor incididunt ut labore et dolore magna aliqua.
+///
+/// Co-authored-by: Alice <[email protected]>
+/// Co-authored-by: Bob <[email protected]>
+/// Reviewed-by: Charlie <[email protected]>
+/// Change-Id: I1234567890abcdef1234567890abcdef12345678
+/// ```
+///
+/// In this case, there are four footer lines: two `Co-authored-by` lines, one
+/// `Reviewed-by` line, and one `Change-Id` line.
+pub fn get_footer_lines(body: &str) -> Vec<FooterEntry> {
+    // a footer always comes at the end of a message; we can split the message
+    // by newline, but we need to immediately reverse the order of the lines
+    // to ensure we parse the footer in an unambiguous manner; this avoids cases
+    // where a colon in the body of the message is mistaken for a footer line
+
+    let lines = body.trim().lines().rev().collect::<Vec<&str>>();
+
+    // short-circuit if there is only 1 line; this avoids a case where a commit
+    // with a single-line description like 'cli: fix bug' does not have a
+    // footer, but would otherwise be mistaken for a footer line
+    if lines.len() <= 1 {
+        return vec![];
+    }
+
+    let mut footer: Vec<FooterEntry> = Vec::new();
+    for line in lines {
+        if line.is_empty() {
+            break;
+        }
+        if let Some((key, value)) = line.split_once(": ") {
+            let key = key.trim();
+            let value = value.trim();
+            footer.push(FooterEntry(key.to_string(), value.to_string()));
+        } else {
+            break;
+        }
+    }
+
+    // reverse the insert order, since we parsed the footer in reverse
+    footer.reverse();
+
+    if footer.is_empty() {
+        vec![]
+    } else {
+        footer
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_simple_footer_lines() {
+        let body = r#"chore: fix bug 1234
+
+Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed
+do eiusmod tempor incididunt ut labore et dolore magna aliqua.
+
+Acked-by: Austin Seipp <[email protected]>
+Reviewed-by: Yuya Nishihara <[email protected]>
+Reviewed-by: Martin von Zweigbergk <[email protected]>
+Change-Id: I1234567890abcdef1234567890abcdef12345678"#;
+
+        let footer = get_footer_lines(body);
+        assert_eq!(footer.len(), 4);
+
+        assert_eq!(footer.first().unwrap().1, "Austin Seipp <[email protected]>");
+        assert_eq!(footer.get(1).unwrap().1, "Yuya Nishihara <[email protected]>");
+        assert_eq!(
+            footer.get(2).unwrap().1,
+            "Martin von Zweigbergk <[email protected]>"
+        );
+        assert_eq!(
+            footer.get(3).unwrap().1,
+            "I1234567890abcdef1234567890abcdef12345678"
+        );
+    }
+
+    #[test]
+    fn test_footer_lines_with_colon_in_body() {
+        let body = r#"chore: fix bug 1234
+
+Summary: Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod
+tempor incididunt ut labore et dolore magna aliqua.
+
+Change-Id: I1234567890abcdef1234567890abcdef12345678"#;
+
+        let footer = get_footer_lines(body);
+
+        // should only have Change-Id
+        assert_eq!(footer.len(), 1);
+        assert_eq!(footer.first().unwrap().0, "Change-Id");
+    }
+
+    #[test]
+    fn test_footer_lines_with_single_line_description() {
+        let body = r#"chore: fix bug 1234"#;
+        let footer = get_footer_lines(body);
+        assert_eq!(footer.len(), 0);
+    }
+}

lib/src/lib.rs

@@ -40,6 +40,7 @@ pub mod extensions_map;
 pub mod file_util;
 pub mod files;
 pub mod fmt_util;
+pub mod footer;
 pub mod fsmonitor;
 pub mod git;
 pub mod git_backend;