Ensure files with no common content are aligned

CHANGELOG.md

@@ -1,5 +1,10 @@
 ## 0.60 (unreleased)
 
+### Display
+
+Fixed an issue where files with no common content would show duplicate
+hunks.
+
 ## 0.59 (released 20th July 2024)
 
 ### Diffing

sample_files/all_changed_1.js

@@ -0,0 +1 @@
+a

sample_files/all_changed_2.js

@@ -0,0 +1 @@
+b

sample_files/compare.expected

@@ -10,6 +10,9 @@ sample_files/added_line_1.txt sample_files/added_line_2.txt
 sample_files/align_footer_1.txt sample_files/align_footer_2.txt
 d640bd2de31e56a39f0efb92aff0f379  -
 
+sample_files/all_changed_1.js sample_files/all_changed_2.js
+40eb536d8b626e27ae227a090fb05f25  -
+
 sample_files/apex_1.cls sample_files/apex_2.cls
 8e477350907734ac4d5201752523dff3  -
 
@@ -134,7 +137,7 @@ sample_files/load_1.js sample_files/load_2.js
 22365043359164f717d495336d86b996  -
 
 sample_files/long_line_1.txt sample_files/long_line_2.txt
-40e22d364bb714421eec24b8ef808f01  -
+7fc50bd547f0c20fda89a1931e5eb61e  -
 
 sample_files/lua_1.lua sample_files/lua_2.lua
 8318115b642361955346cc620bc3ad86  -
@@ -227,7 +230,7 @@ sample_files/simple_1.scss sample_files/simple_2.scss
 265261e79df78abfc09392d72a0273d8  -
 
 sample_files/simple_1.txt sample_files/simple_2.txt
-416d05b8a02e41cb70b78d28cd3153d9  -
+6fd97a544afd0a3be7eebba75f82f696  -
 
 sample_files/slider_1.rs sample_files/slider_2.rs
 d10128f3d9ffc4a8670f417a9371bacc  -

src/line_parser.rs

@@ -1,7 +1,7 @@
 //! A fallback "parser" for plain text.
 
 use lazy_static::lazy_static;
-use line_numbers::LinePositions;
+use line_numbers::{LinePositions, SingleLineSpan};
 use regex::Regex;
 
 use crate::words::split_words;
@@ -118,9 +118,12 @@ pub(crate) fn change_positions(lhs_src: &str, rhs_src: &str) -> Vec<MatchedPos>
     let mut rhs_offset = 0;
 
     let mut mps = vec![];
+
+    let mut seen_unchanged = false;
     for (kind, lhs_lines, rhs_lines) in changed_parts(lhs_src, rhs_src) {
         match kind {
             TextChangeKind::Unchanged => {
+                seen_unchanged = true;
                 for (lhs_line, rhs_line) in lhs_lines.iter().zip(rhs_lines) {
                     let lhs_pos =
                         lhs_lp.from_region(lhs_offset, lhs_offset + line_len_in_bytes(lhs_line));
@@ -210,6 +213,33 @@ pub(crate) fn change_positions(lhs_src: &str, rhs_src: &str) -> Vec<MatchedPos>
         }
     }
 
+    // If there are no unchanged items, insert a dummy item at the
+    // beginning of both files with a width of zero. This gives
+    // display something to use when aligning.
+    if !seen_unchanged {
+        let lhs_pos = SingleLineSpan {
+            line: 0.into(),
+            start_col: 0,
+            end_col: 0,
+        };
+        let rhs_pos = SingleLineSpan {
+            line: 0.into(),
+            start_col: 0,
+            end_col: 0,
+        };
+        mps.insert(
+            0,
+            MatchedPos {
+                kind: MatchKind::UnchangedToken {
+                    highlight: TokenKind::Atom(AtomKind::Normal),
+                    self_pos: vec![lhs_pos],
+                    opposite_pos: vec![rhs_pos],
+                },
+                pos: lhs_pos,
+            },
+        );
+    }
+
     mps
 }
 

src/parse/syntax.rs

@@ -875,20 +875,55 @@ pub(crate) fn change_positions<'a>(
     change_map: &ChangeMap<'a>,
 ) -> Vec<MatchedPos> {
     let mut positions = Vec::new();
-    change_positions_(nodes, change_map, &mut positions);
+    let mut seen_unchanged = false;
+
+    change_positions_(nodes, change_map, &mut positions, &mut seen_unchanged);
+
+    // If there are no unchanged items, insert a dummy item at the
+    // beginning of both files with a width of zero. This gives
+    // display something to use when aligning.
+    if !seen_unchanged {
+        let lhs_pos = SingleLineSpan {
+            line: 0.into(),
+            start_col: 0,
+            end_col: 0,
+        };
+        let rhs_pos = SingleLineSpan {
+            line: 0.into(),
+            start_col: 0,
+            end_col: 0,
+        };
+        positions.insert(
+            0,
+            MatchedPos {
+                kind: MatchKind::UnchangedToken {
+                    highlight: TokenKind::Atom(AtomKind::Normal),
+                    self_pos: vec![lhs_pos],
+                    opposite_pos: vec![rhs_pos],
+                },
+                pos: lhs_pos,
+            },
+        );
+    }
+
     positions
 }
 
 fn change_positions_<'a>(
     nodes: &[&'a Syntax<'a>],
     change_map: &ChangeMap<'a>,
     positions: &mut Vec<MatchedPos>,
+    seen_unchanged: &mut bool,
 ) {
     for node in nodes {
         let change = change_map
             .get(node)
             .unwrap_or_else(|| panic!("Should have changes set in all nodes: {:#?}", node));
 
+        if matches!(change, ChangeKind::Unchanged(_)) {
+            *seen_unchanged = true;
+        }
+
         match node {
             List {
                 open_position,
@@ -903,7 +938,7 @@ fn change_positions_<'a>(
                     false,
                 ));
 
-                change_positions_(children, change_map, positions);
+                change_positions_(children, change_map, positions, seen_unchanged);
 
                 positions.extend(MatchedPos::new(
                     change,