Resume conversation from an earlier point in history (#2607)

Fixing merge conflict of this: #2588


https://github.com/user-attachments/assets/392c7c37-cf8f-4ed6-952e-8215e8c57bc4

codex-rs/tui/src/backtrack_helpers.rs

@@ -0,0 +1,154 @@
+use ratatui::text::Line;
+
+/// Convenience: compute the highlight range for the Nth last user message.
+pub(crate) fn highlight_range_for_nth_last_user(
+    lines: &[Line<'_>],
+    n: usize,
+) -> Option<(usize, usize)> {
+    let header = find_nth_last_user_header_index(lines, n)?;
+    Some(highlight_range_from_header(lines, header))
+}
+
+/// Compute the wrapped display-line offset before `header_idx`, for a given width.
+pub(crate) fn wrapped_offset_before(lines: &[Line<'_>], header_idx: usize, width: u16) -> usize {
+    let before = &lines[0..header_idx];
+    crate::insert_history::word_wrap_lines(before, width).len()
+}
+
+/// Find the header index for the Nth last user message in the transcript.
+/// Returns `None` if `n == 0` or there are fewer than `n` user messages.
+pub(crate) fn find_nth_last_user_header_index(lines: &[Line<'_>], n: usize) -> Option<usize> {
+    if n == 0 {
+        return None;
+    }
+    let mut found = 0usize;
+    for (idx, line) in lines.iter().enumerate().rev() {
+        let content: String = line
+            .spans
+            .iter()
+            .map(|s| s.content.as_ref())
+            .collect::<Vec<_>>()
+            .join("");
+        if content.trim() == "user" {
+            found += 1;
+            if found == n {
+                return Some(idx);
+            }
+        }
+    }
+    None
+}
+
+/// Normalize a requested backtrack step `n` against the available user messages.
+/// - Returns `0` if there are no user messages.
+/// - Returns `n` if the Nth last user message exists.
+/// - Otherwise wraps to `1` (the most recent user message).
+pub(crate) fn normalize_backtrack_n(lines: &[Line<'_>], n: usize) -> usize {
+    if n == 0 {
+        return 0;
+    }
+    if find_nth_last_user_header_index(lines, n).is_some() {
+        return n;
+    }
+    if find_nth_last_user_header_index(lines, 1).is_some() {
+        1
+    } else {
+        0
+    }
+}
+
+/// Extract the text content of the Nth last user message.
+/// The message body is considered to be the lines following the "user" header
+/// until the first blank line.
+pub(crate) fn nth_last_user_text(lines: &[Line<'_>], n: usize) -> Option<String> {
+    let header_idx = find_nth_last_user_header_index(lines, n)?;
+    extract_message_text_after_header(lines, header_idx)
+}
+
+/// Extract message text starting after `header_idx` until the first blank line.
+fn extract_message_text_after_header(lines: &[Line<'_>], header_idx: usize) -> Option<String> {
+    let start = header_idx + 1;
+    let mut out: Vec<String> = Vec::new();
+    for line in lines.iter().skip(start) {
+        let is_blank = line
+            .spans
+            .iter()
+            .all(|s| s.content.as_ref().trim().is_empty());
+        if is_blank {
+            break;
+        }
+        let text: String = line
+            .spans
+            .iter()
+            .map(|s| s.content.as_ref())
+            .collect::<Vec<_>>()
+            .join("");
+        out.push(text);
+    }
+    if out.is_empty() {
+        None
+    } else {
+        Some(out.join("\n"))
+    }
+}
+
+/// Given a header index, return the inclusive range for the message block
+/// [header_idx, end) where end is the first blank line after the header or the
+/// end of the transcript.
+fn highlight_range_from_header(lines: &[Line<'_>], header_idx: usize) -> (usize, usize) {
+    let mut end = header_idx + 1;
+    while end < lines.len() {
+        let is_blank = lines[end]
+            .spans
+            .iter()
+            .all(|s| s.content.as_ref().trim().is_empty());
+        if is_blank {
+            break;
+        }
+        end += 1;
+    }
+    (header_idx, end)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use ratatui::text::Span;
+
+    fn line(s: &str) -> Line<'static> {
+        Line::from(Span::raw(s.to_string()))
+    }
+
+    fn transcript_with_users(count: usize) -> Vec<Line<'static>> {
+        // Build a transcript with `count` user messages, each followed by one body line and a blank line.
+        let mut v = Vec::new();
+        for i in 0..count {
+            v.push(line("user"));
+            v.push(line(&format!("message {i}")));
+            v.push(line(""));
+        }
+        v
+    }
+
+    #[test]
+    fn normalize_wraps_to_one_when_past_oldest() {
+        let lines = transcript_with_users(2);
+        assert_eq!(normalize_backtrack_n(&lines, 1), 1);
+        assert_eq!(normalize_backtrack_n(&lines, 2), 2);
+        // Requesting 3rd when only 2 exist wraps to 1
+        assert_eq!(normalize_backtrack_n(&lines, 3), 1);
+    }
+
+    #[test]
+    fn normalize_returns_zero_when_no_user_messages() {
+        let lines = transcript_with_users(0);
+        assert_eq!(normalize_backtrack_n(&lines, 1), 0);
+        assert_eq!(normalize_backtrack_n(&lines, 5), 0);
+    }
+
+    #[test]
+    fn normalize_keeps_valid_n() {
+        let lines = transcript_with_users(3);
+        assert_eq!(normalize_backtrack_n(&lines, 2), 2);
+    }
+}