From ec37c5bef1f43cf8dfaacf07e2946d490c690a1b Mon Sep 17 00:00:00 2001
From: Wilfred Hughes <me@wilfred.me.uk>
Date: Sun, 26 Sep 2021 21:23:22 -0700
Subject: [PATCH] Add full Mercurial extdiff support

---
 CHANGELOG.md                  |  6 ++++++
 Cargo.lock                    |  1 +
 Cargo.toml                    |  1 +
 manual/src/getting_started.md |  3 +--
 src/bin/difftastic.rs         | 31 ++++++++++++++++++++++++++++++-
 5 files changed, 39 insertions(+), 3 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4a54b32cc..22933e54d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,12 @@
 
 Improved handling of paired delimiters, particularly in C, C++ and C#.
 
+### Integration
+
+Added support for Mercuial, see [this section in the
+manual](http://difftastic.wilfred.me.uk/getting_started.html#mercurial-external-diffs)
+for instructions.
+
 ### Build
 
 Fix compilation for systems where the C++ compiler defaulted to a
diff --git a/Cargo.lock b/Cargo.lock
index 01ca5dc72..02541367b 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -257,6 +257,7 @@ dependencies = [
  "term_size",
  "tree-sitter",
  "typed-arena",
+ "walkdir",
 ]
 
 [[package]]
diff --git a/Cargo.toml b/Cargo.toml
index 13e1cf63a..f05e2837e 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -36,6 +36,7 @@ log = "0.4.14"
 pretty_env_logger = "0.4.0"
 mimalloc = { version = "0.1.26", default-features = false }
 radix-heap = "0.3.8"
+walkdir = "2.3.2"
 
 [dev-dependencies]
 pretty_assertions = "0.6.1"
diff --git a/manual/src/getting_started.md b/manual/src/getting_started.md
index eaf1a1284..88c0b0d28 100644
--- a/manual/src/getting_started.md
+++ b/manual/src/getting_started.md
@@ -89,8 +89,7 @@ extdiff =
 ```
 
 You can then run `hg extdiff -p difftastic` (assumes difftastic is on
-your `$PATH`). **This currently only supports diffs where a single file
-is modified.**
+your `$PATH`).
 
 You can also define an alias to run difftastic with hg. Add the
 following to your `.hgrc` to run difftastic with `hg dft`.
diff --git a/src/bin/difftastic.rs b/src/bin/difftastic.rs
index 158477e47..3a925f69b 100644
--- a/src/bin/difftastic.rs
+++ b/src/bin/difftastic.rs
@@ -7,6 +7,7 @@ use atty::Stream;
 use clap::{crate_version, App, AppSettings, Arg};
 use std::{env, ffi::OsStr, path::Path};
 use typed_arena::Arena;
+use walkdir::WalkDir;
 
 use difftastic::*;
 use difftastic::{
@@ -191,9 +192,14 @@ fn main() {
         } => (display_path, lhs_path, rhs_path),
     };
 
-    diff_file(&display_path, &lhs_path, &rhs_path);
+    if Path::new(&lhs_path).is_dir() && Path::new(&rhs_path).is_dir() {
+        diff_directories(&lhs_path, &rhs_path);
+    } else {
+        diff_file(&display_path, &lhs_path, &rhs_path);
+    }
 }
 
+// TODO: prefer PathBuf to &str for paths.
 fn diff_file(display_path: &str, lhs_path: &str, rhs_path: &str) {
     let lhs_bytes = read_or_die(lhs_path);
     let rhs_bytes = read_or_die(rhs_path);
@@ -276,3 +282,26 @@ fn diff_file(display_path: &str, lhs_path: &str, rhs_path: &str) {
     }
     println!();
 }
+
+/// Given two directories that contain the files, compare them
+/// pairwise.
+///
+/// This is how the hg extdiff extension calls a diff tool when there
+/// is more than one changed file.
+fn diff_directories(lhs_dir: &str, rhs_dir: &str) {
+    for entry in WalkDir::new(lhs_dir).into_iter().filter_map(Result::ok) {
+        let lhs_path = entry.path();
+        if lhs_path.is_dir() {
+            continue;
+        }
+
+        let rel_path = lhs_path.strip_prefix(lhs_dir).unwrap();
+        let rhs_path = Path::new(rhs_dir).join(rel_path);
+
+        diff_file(
+            &rel_path.to_string_lossy(),
+            &lhs_path.to_string_lossy(),
+            &rhs_path.to_string_lossy(),
+        );
+    }
+}