Calculate content width based on visible lines

CHANGELOG.md

@@ -12,6 +12,11 @@ Difftastic will now prefer matching up comments that are similar
 Contiguous syntax logic now considers close delimiter positions, so
 `[ \n ];` now treats the `;` atom as contiguous.
 
+### Display
+
+Display width calculations are now based on the longest line visible
+in the diff, not the longest line in the file.
+
 ## 0.6
 
 ### Parsing

src/lines.rs

@@ -333,8 +333,45 @@ pub fn format_line_num_padded(line_num: usize, column_width: usize) -> String {
     format!("{:width$} ", line_num + 1, width = column_width - 1)
 }
 
-pub fn lhs_printable_width(lhs: &str, lhs_column_width: usize, terminal_width: usize) -> usize {
-    let longest_line_length = lhs.lines().map(str::len).max().unwrap_or(1);
+fn longest_visible_line_lhs(s: &str, groups: &[LineGroup]) -> usize {
+    let lines: Vec<_> = s.lines().collect();
+    let mut longest = 0;
+
+    for group in groups {
+        if let Some(lhs_lines) = &group.lhs_lines {
+            for line_num in lhs_lines.start.0..lhs_lines.end.0 {
+                let current_len = lines[line_num].len();
+                longest = max(longest, current_len);
+            }
+        }
+    }
+
+    longest
+}
+
+fn longest_visible_line_rhs(s: &str, groups: &[LineGroup]) -> usize {
+    let lines: Vec<_> = s.lines().collect();
+    let mut longest = 1;
+
+    for group in groups {
+        if let Some(rhs_lines) = &group.rhs_lines {
+            for line_num in rhs_lines.start.0..rhs_lines.end.0 {
+                let current_len = lines[line_num].len();
+                longest = max(longest, current_len);
+            }
+        }
+    }
+
+    longest
+}
+
+pub fn lhs_printable_width(
+    lhs: &str,
+    groups: &[LineGroup],
+    lhs_column_width: usize,
+    terminal_width: usize,
+) -> usize {
+    let longest_line_length = longest_visible_line_lhs(lhs, groups);
     let longest_line = longest_line_length + lhs_column_width;
 
     let space_available = terminal_width / 2 - SPACER.len();
@@ -343,11 +380,12 @@ pub fn lhs_printable_width(lhs: &str, lhs_column_width: usize, terminal_width: u
 
 pub fn rhs_printable_width(
     rhs: &str,
+    groups: &[LineGroup],
     lhs_width: usize,
     rhs_column_width: usize,
     terminal_width: usize,
 ) -> usize {
-    let longest_line_length = rhs.lines().map(str::len).max().unwrap_or(1);
+    let longest_line_length = longest_visible_line_rhs(rhs, groups);
     let longest_line = longest_line_length + rhs_column_width;
 
     let space_available = (terminal_width - SPACER.len()) - lhs_width;

src/main.rs

@@ -161,9 +161,11 @@ fn main() {
     let lhs_column_width = format_line_num(groups.last().unwrap().max_visible_lhs().0).len();
     let rhs_column_width = format_line_num(groups.last().unwrap().max_visible_rhs().0).len();
 
-    let lhs_formatted_length = lhs_printable_width(&lhs_src, lhs_column_width, terminal_width);
+    let lhs_formatted_length =
+        lhs_printable_width(&lhs_src, &groups, lhs_column_width, terminal_width);
     let rhs_formatted_length = rhs_printable_width(
         &rhs_src,
+        &groups,
         lhs_formatted_length,
         rhs_column_width,
         terminal_width,