From 6b76c382117f5551b90507cea9121c9cbd500e60 Mon Sep 17 00:00:00 2001
From: Ben Bridle <ben@derelict.engineering>
Date: Sat, 1 Feb 2025 17:25:42 +1300
Subject: Only copy modified files

When copying a file, if a file already exists at the destination path
with the same last-modified date and the same size, the file is not
copied.
---
 src/entry.rs         |  8 +++++++-
 src/operations/cp.rs | 33 +++++++++++++++++++++++++++++++--
 2 files changed, 38 insertions(+), 3 deletions(-)

diff --git a/src/entry.rs b/src/entry.rs
index 72eeb7e..1b6b577 100644
--- a/src/entry.rs
+++ b/src/entry.rs
@@ -1,6 +1,9 @@
-use std::path::{Path, PathBuf};
 use crate::*;
 
+use std::path::{Path, PathBuf};
+use std::time::SystemTime;
+
+
 #[derive(PartialEq)]
 pub enum EntryType {
     File,
@@ -17,6 +20,8 @@ pub struct Entry {
     pub path: PathBuf,
     /// The file path as originally presented to the [Entry] constructor.
     pub original_path: PathBuf,
+    /// Not available on all platforms.
+    pub last_modified: Option<SystemTime>,
 }
 
 impl Entry {
@@ -51,6 +56,7 @@ impl Entry {
             path: canonical_path,
             original_path: path.to_path_buf(),
             is_symlink: metadata.is_symlink(),
+            last_modified: metadata.modified().ok(),
         })
     }
 
diff --git a/src/operations/cp.rs b/src/operations/cp.rs
index 33d4d04..6037b67 100644
--- a/src/operations/cp.rs
+++ b/src/operations/cp.rs
@@ -1,5 +1,8 @@
 use crate::*;
 
+use std::time::SystemTime;
+
+
 #[must_use]
 pub fn copy(source_path: impl AsRef<Path>, destination_path: impl AsRef<Path>) -> WriteResult<()> {
     let source = get_entry(&source_path)?;
@@ -40,8 +43,16 @@ pub fn copy(source_path: impl AsRef<Path>, destination_path: impl AsRef<Path>) -
 
 #[must_use]
 fn copy_file(source_path: impl AsRef<Path>, destination_path: impl AsRef<Path>) -> WriteResult<()> {
-    let copy_result = std::fs::copy(&source_path, &destination_path);
-    io_result_to_write_result(copy_result, &destination_path.as_ref())?;
+    // Only copy file if size or last modified time is different to file at destination.
+    if let Comparison::Different(last_modified) = compare_files(&source_path, &destination_path) {
+        let copy_result = std::fs::copy(&source_path, &destination_path);
+        io_result_to_write_result(copy_result, &destination_path.as_ref())?;
+        if let Some(time) = last_modified {
+            if let Ok(dest) = std::fs::File::open(&destination_path) {
+                let _ = dest.set_modified(time);
+            }
+        }
+    }
     Ok(())
 }
 
@@ -53,3 +64,21 @@ fn copy_directory(source_path: impl AsRef<Path>, destination_path: impl AsRef<Pa
     }
     Ok(())
 }
+
+enum Comparison {
+    Same,
+    Different(Option<SystemTime>),
+}
+
+fn compare_files(source_path: impl AsRef<Path>, destination_path: impl AsRef<Path>) -> Comparison {
+    let Ok(source) = std::fs::File::open(&source_path)    else { return Comparison::Different(None) };
+    let Ok(source_metadata) = source.metadata()           else { return Comparison::Different(None) };
+    let Ok(source_modified) = source_metadata.modified()  else { return Comparison::Different(None) };
+    let Ok(dest) = std::fs::File::open(&destination_path) else { return Comparison::Different(Some(source_modified)) };
+    let Ok(dest_metadata) = dest.metadata()               else { return Comparison::Different(Some(source_modified)) };
+    let Ok(dest_modified) = dest_metadata.modified()      else { return Comparison::Different(Some(source_modified)) };
+    match source_modified == dest_modified && source_metadata.len() == dest_metadata.len() {
+        true => Comparison::Same,
+        false => Comparison::Different(Some(source_modified)),
+    }
+}
-- 
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