From eb693cad8b866f6b9436f6ea06921912e7bbcb0a Mon Sep 17 00:00:00 2001
From: Jean Boussier <jean.boussier@gmail.com>
Date: Wed, 20 May 2026 09:13:17 +0200
Subject: [PATCH] Use IO#seek when possible

Saves on reading a potentially large string.

Also noop of the offset is `0` (which it very often is).
---
 lib/marcel/magic.rb | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/lib/marcel/magic.rb b/lib/marcel/magic.rb
index c79eeab..9d4ea86 100644
--- a/lib/marcel/magic.rb
+++ b/lib/marcel/magic.rb
@@ -128,11 +128,11 @@ def self.magic_match_io(io, matches, buffer)
             if value.is_a?(Regexp)
               match_regex(io, offset, value, buffer)
             elsif Range === offset
-              io.read(offset.begin, buffer)
+              io_seek(io, offset.begin, buffer)
               x = io.read(offset.end - offset.begin + value.bytesize, buffer)
               x && x.include?(value)
             else
-              io.read(offset, buffer)
+              io_seek(io, offset, buffer)
               io.read(value.bytesize, buffer) == value
             end
           end
@@ -151,6 +151,17 @@ def self.match_regex(io, offset, regexp, buffer)
       data.match?(regexp)
     end
 
-    private_class_method :magic_match, :magic_match_io, :match_regex
+    def self.io_seek(io, offset, buffer)
+      return if offset == 0
+
+      if io.respond_to?(:seek)
+        io.seek(offset, IO::SEEK_CUR)
+      else
+        # Some IOs don't support `seek`. e.g. Rack::RewindableInput
+        io.read(offset, buffer)
+      end
+    end
+
+    private_class_method :magic_match, :magic_match_io, :match_regex, :io_seek
   end
 end