Add the ability to parse conflict markers and diff the two files

6 files changed, 287 insertions, 1 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index beacf1277..68fd2bec1 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,14 @@
 ## 0.50 (unreleased)
 
+### Conflicts
+
+Difftastic now supports parsing files with conflict markers, enabling
+you to diff the two conflicting file states.
+
+```
+$ difft file_with_conflicts.js
+```
+
 ### Parsing
 
 Updated Elixir, Erlang, Go, Kotlin and Racket parsers.
diff --git a/manual/src/usage.md b/manual/src/usage.md
index 11db9cb2d..0824b8b58 100644
--- a/manual/src/usage.md
+++ b/manual/src/usage.md
@@ -28,6 +28,19 @@ You can read a file from stdin by specifying `-` as the file path.
 $ cat sample_files/before.js | difft - sample_files/after.js
 ```
 
+### Files With Conflicts
+
+*(Added in version 0.50.)*
+
+If you have a file with `<<<<<<<` conflict markers, you can pass it as
+a single argument to difftastic. Difftastic will construct the two
+file states and diff those.
+
+```
+$ difft sample_files/conflicts.el
+```
+
+
 ## Language Detection
 
 Difftastic guesses the language used based on the file extension, file
diff --git a/src/conflicts.rs b/src/conflicts.rs
new file mode 100644
index 000000000..f97ba1215
--- /dev/null
+++ b/src/conflicts.rs
@@ -0,0 +1,137 @@
+//! Apply conflict markers to obtain the original file contents.
+//!
+//! https://git-scm.com/docs/git-merge#Documentation/git-merge.txt-mergeconflictStyle
+
+use ConflictState::*;
+
+#[derive(Debug, Clone, Copy)]
+enum ConflictState {
+    NoConflict,
+    Left,
+    Base,
+    Right,
+}
+
+pub const START_LHS_MARKER: &str = "<<<<<<<";
+const START_BASE_MARKER: &str = "|||||||";
+const START_RHS_MARKER: &str = "=======";
+const END_RHS_MARKER: &str = ">>>>>>>";
+
+pub struct ConflictFiles {
+    pub lhs_name: String,
+    pub lhs_content: String,
+    pub rhs_name: String,
+    pub rhs_content: String,
+    pub num_conflicts: usize,
+}
+
+/// Convert a string with conflict markers into the two conflicting
+/// file contents.
+pub fn apply_conflict_markers(s: &str) -> Result<ConflictFiles, String> {
+    let mut lhs_name = String::new();
+    let mut rhs_name = String::new();
+
+    let mut lhs_content = String::with_capacity(s.len());
+    let mut rhs_content = String::with_capacity(s.len());
+    let mut num_conflicts = 0;
+
+    let mut state = NoConflict;
+    let mut conflict_start_line = None;
+    for (i, line) in s.split_inclusive('\n').enumerate() {
+        if let Some(hunk_lhs_name) = line.strip_prefix(START_LHS_MARKER) {
+            state = Left;
+            num_conflicts += 1;
+            conflict_start_line = Some(i);
+
+            let hunk_lhs_name = hunk_lhs_name.trim();
+            if hunk_lhs_name.len() > lhs_name.len() {
+                lhs_name = hunk_lhs_name.to_owned();
+            }
+
+            continue;
+        }
+        if line.starts_with(START_BASE_MARKER) {
+            state = Base;
+            continue;
+        }
+        if line.starts_with(START_RHS_MARKER) {
+            state = Right;
+            continue;
+        }
+        if let Some(hunk_rhs_name) = line.strip_prefix(END_RHS_MARKER) {
+            state = NoConflict;
+
+            let hunk_rhs_name = hunk_rhs_name.trim();
+            if hunk_rhs_name.len() > rhs_name.len() {
+                rhs_name = hunk_rhs_name.to_owned();
+            }
+            continue;
+        }
+
+        match state {
+            NoConflict => {
+                lhs_content.push_str(line);
+                rhs_content.push_str(line);
+            }
+            Left => {
+                lhs_content.push_str(line);
+            }
+            Right => {
+                rhs_content.push_str(line);
+            }
+            Base => {}
+        }
+    }
+
+    if matches!(state, NoConflict) {
+        Ok(ConflictFiles {
+            lhs_name,
+            lhs_content,
+            rhs_name,
+            rhs_content,
+            num_conflicts,
+        })
+    } else {
+        let message = match conflict_start_line {
+            Some(line_i) => format!(
+                "Could not parse conflict markers, line {} has no matching {}.",
+                line_i, END_RHS_MARKER
+            ),
+            None => "Could not parse conflict markers.".to_owned(),
+        };
+        Err(message)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_with_base() {
+        // Deliberately avoid a multiline string literal to avoid
+        // confusing text editors when we open this file.
+        let s = "before\n<<<<<<< Temporary merge branch 1\nnew in left\n||||||| merged common ancestors\noriginal\n=======\nnew in right\n>>>>>>> Temporary merge branch 2\nafter";
+
+        let conflict_files = apply_conflict_markers(s).unwrap();
+        assert_eq!(conflict_files.lhs_content, "before\nnew in left\nafter");
+        assert_eq!(conflict_files.rhs_content, "before\nnew in right\nafter");
+
+        assert_eq!(conflict_files.lhs_name, "Temporary merge branch 1");
+        assert_eq!(conflict_files.rhs_name, "Temporary merge branch 2");
+    }
+
+    #[test]
+    fn test_without_base() {
+        // Deliberately avoid a multiline string literal to avoid
+        // confusing text editors when we open this file.
+        let s = "before\n<<<<<<< Temporary merge branch 1\nnew in left\n=======\nnew in right\n>>>>>>> Temporary merge branch 2\nafter";
+
+        let conflict_files = apply_conflict_markers(s).unwrap();
+        assert_eq!(conflict_files.lhs_content, "before\nnew in left\nafter");
+        assert_eq!(conflict_files.rhs_content, "before\nnew in right\nafter");
+
+        assert_eq!(conflict_files.lhs_name, "Temporary merge branch 1");
+        assert_eq!(conflict_files.rhs_name, "Temporary merge branch 2");
+    }
+}
diff --git a/src/files.rs b/src/files.rs
index 1cad43540..5d4cd099b 100644
--- a/src/files.rs
+++ b/src/files.rs
@@ -13,6 +13,16 @@ use walkdir::WalkDir;
 use crate::exit_codes::EXIT_BAD_ARGUMENTS;
 use crate::options::FileArgument;
 
+pub fn read_file_or_die(path: &FileArgument) -> Vec<u8> {
+    match read_file_arg(path) {
+        Ok(src) => src,
+        Err(e) => {
+            eprint_read_error(path, &e);
+            std::process::exit(EXIT_BAD_ARGUMENTS);
+        }
+    }
+}
+
 pub fn read_files_or_die(
     lhs_path: &FileArgument,
     rhs_path: &FileArgument,
diff --git a/src/main.rs b/src/main.rs
index 8eea8b07d..9503e9172 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -23,6 +23,7 @@
 // correct.
 #![allow(clippy::mutable_key_type)]
 
+mod conflicts;
 mod constants;
 mod diff;
 mod display;
@@ -39,16 +40,20 @@ mod summary;
 #[macro_use]
 extern crate log;
 
+use crate::conflicts::START_LHS_MARKER;
 use crate::diff::{dijkstra, unchanged};
 use crate::display::hunks::{matched_pos_to_hunks, merge_adjacent};
+use crate::exit_codes::EXIT_BAD_ARGUMENTS;
 use crate::parse::guess_language::language_globs;
 use crate::parse::syntax;
+use conflicts::apply_conflict_markers;
 use diff::changes::ChangeMap;
 use diff::dijkstra::ExceededGraphLimit;
 use display::context::opposite_positions;
 use exit_codes::{EXIT_FOUND_CHANGES, EXIT_SUCCESS};
 use files::{
-    guess_content, read_files_or_die, read_or_die, relative_paths_in_either, ProbableFileKind,
+    guess_content, read_file_or_die, read_files_or_die, read_or_die, relative_paths_in_either,
+    ProbableFileKind,
 };
 use log::info;
 use mimalloc::MiMalloc;
@@ -174,6 +179,31 @@ fn main() {
                 println!();
             }
         }
+        Mode::DiffFromConflicts {
+            display_path,
+            path,
+            diff_options,
+            display_options,
+            set_exit_code,
+            language_overrides,
+        } => {
+            let diff_result = diff_conflicts_file(
+                &display_path,
+                &path,
+                &display_options,
+                &diff_options,
+                &language_overrides,
+            );
+
+            print_diff_result(&display_options, &diff_result);
+
+            let exit_code = if set_exit_code && diff_result.has_reportable_change() {
+                EXIT_FOUND_CHANGES
+            } else {
+                EXIT_SUCCESS
+            };
+            std::process::exit(exit_code);
+        }
         Mode::Diff {
             diff_options,
             display_options,
@@ -307,6 +337,56 @@ fn diff_file(
     )
 }
 
+fn diff_conflicts_file(
+    display_path: &str,
+    path: &FileArgument,
+    display_options: &DisplayOptions,
+    diff_options: &DiffOptions,
+    overrides: &[(glob::Pattern, LanguageOverride)],
+) -> DiffResult {
+    let bytes = read_file_or_die(path);
+    let src = match guess_content(&bytes) {
+        ProbableFileKind::Text(src) => src,
+        ProbableFileKind::Binary => {
+            eprintln!("error: Expected a text file with conflict markers, got a binary file.");
+            std::process::exit(EXIT_BAD_ARGUMENTS);
+        }
+    };
+
+    let conflict_files = match apply_conflict_markers(&src) {
+        Ok(cf) => cf,
+        Err(msg) => {
+            eprintln!("error: {}", msg);
+            std::process::exit(EXIT_BAD_ARGUMENTS);
+        }
+    };
+
+    if conflict_files.num_conflicts == 0 {
+        eprintln!(
+            "warning: Expected a file with conflict markers {}, but none were found.",
+            START_LHS_MARKER,
+        );
+        eprintln!("Difftastic parses conflict markers from a single file argument. Did you forget a second file argument?");
+    }
+
+    let extra_info = format!(
+        "Comparing '{}' with '{}'",
+        conflict_files.lhs_name, conflict_files.rhs_name
+    );
+
+    diff_file_content(
+        display_path,
+        Some(extra_info),
+        path,
+        path,
+        &conflict_files.lhs_content,
+        &conflict_files.rhs_content,
+        display_options,
+        diff_options,
+        overrides,
+    )
+}
+
 fn check_only_text(
     file_format: &FileFormat,
     display_path: &str,
diff --git a/src/options.rs b/src/options.rs
index 8752f5549..e0949e286 100644
--- a/src/options.rs
+++ b/src/options.rs
@@ -95,6 +95,10 @@ fn app() -> clap::Command<'static> {
             "$ ",
             env!("CARGO_BIN_NAME"),
             " old/ new/\n\n",
+            "If you have a file with conflict markers, you can pass it as a single argument. Difftastic will diff the two conflicting file states.\n\n",
+            "$ ",
+            env!("CARGO_BIN_NAME"),
+            " file_with_conflicts.js\n\n",
             "Difftastic can also be invoked with 7 arguments in the format that GIT_EXTERNAL_DIFF expects.\n\n",
             "See the full manual at: https://difftastic.wilfred.me.uk/")
         )
@@ -357,6 +361,15 @@ pub enum Mode {
         /// If this file has been renamed, the name it had previously.
         old_path: Option<String>,
     },
+    DiffFromConflicts {
+        diff_options: DiffOptions,
+        display_options: DisplayOptions,
+        set_exit_code: bool,
+        language_overrides: Vec<(glob::Pattern, LanguageOverride)>,
+        path: FileArgument,
+        /// The path that we show to the user.
+        display_path: String,
+    },
     ListLanguages {
         use_color: bool,
         language_overrides: Vec<(glob::Pattern, LanguageOverride)>,
@@ -613,6 +626,30 @@ pub fn parse_args() -> Mode {
                 true,
             )
         }
+        [path] => {
+            let display_options = DisplayOptions {
+                background_color,
+                use_color,
+                print_unchanged,
+                tab_width,
+                display_mode,
+                display_width,
+                num_context_lines,
+                syntax_highlight,
+                in_vcs: true,
+            };
+
+            let display_path = path.to_string_lossy().to_string();
+            let path = FileArgument::from_path_argument(path);
+            return Mode::DiffFromConflicts {
+                display_path,
+                path,
+                diff_options,
+                display_options,
+                set_exit_code,
+                language_overrides,
+            };
+        }
         _ => {
             if !args.is_empty() {
                 eprintln!(