From 8f49898c1760db200505e3fe0ff416ffe10f4886 Mon Sep 17 00:00:00 2001
From: "Bernhard M. Wiedemann" <bwiedemann@suse.de>
Date: Fri, 17 Jul 2026 23:42:36 +0200
Subject: [PATCH 1/2] epub: Honor SOURCE_DATE_EPOCH for dcterms:modified and
 derive dc:identifier

The EPUB content.opf embedded the build time as dcterms:modified and a
random UUID as dc:identifier, making the .epub files differ between
two builds of the same docs.

Derive the identifier from the project name and version instead of
random data, and use SOURCE_DATE_EPOCH
(https://reproducible-builds.org/docs/source-date-epoch/) as the
modification time when it is set.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
---
 lib/ex_doc/formatter/epub.ex | 42 ++++++++++++++++++++----------------
 1 file changed, 24 insertions(+), 18 deletions(-)

diff --git a/lib/ex_doc/formatter/epub.ex b/lib/ex_doc/formatter/epub.ex
index 64cafbf54..d308836af 100644
--- a/lib/ex_doc/formatter/epub.ex
+++ b/lib/ex_doc/formatter/epub.ex
@@ -59,7 +59,7 @@ defmodule ExDoc.Formatter.EPUB do
   end
 
   defp generate_content(config, modules, tasks, extras, static_files) do
-    uuid = "urn:uuid:#{uuid4()}"
+    uuid = "urn:uuid:#{uuid(config)}"
     datetime = format_datetime()
 
     static_files =
@@ -137,7 +137,22 @@ defmodule ExDoc.Formatter.EPUB do
 
   # Helper to format Erlang datetime tuple
   defp format_datetime do
-    {{year, month, day}, {hour, min, sec}} = :calendar.universal_time()
+    # Use SOURCE_DATE_EPOCH instead of the current time when set, for
+    # reproducible builds
+    # (https://reproducible-builds.org/docs/source-date-epoch/).
+    time =
+      case System.get_env("SOURCE_DATE_EPOCH") do
+        nil ->
+          :calendar.universal_time()
+
+        epoch ->
+          case Integer.parse(epoch) do
+            {seconds, ""} -> :calendar.system_time_to_universal_time(seconds, :second)
+            _ -> :calendar.universal_time()
+          end
+      end
+
+    {{year, month, day}, {hour, min, sec}} = time
     list = [year, month, day, hour, min, sec]
 
     "~4..0B-~2..0B-~2..0BT~2..0B:~2..0B:~2..0BZ"
@@ -145,21 +160,12 @@ defmodule ExDoc.Formatter.EPUB do
     |> IO.iodata_to_binary()
   end
 
-  @two_power_16 65536
-  @two_power_32 4_294_967_296
-  @two_power_48 281_474_976_710_656
-
-  defp uuid4 do
-    Enum.map_join(
-      [
-        <<:rand.uniform(@two_power_32) - 1::32>>,
-        <<:rand.uniform(@two_power_16) - 1::16>>,
-        <<:rand.uniform(@two_power_16) - 1::16>>,
-        <<:rand.uniform(@two_power_16) - 1::16>>,
-        <<:rand.uniform(@two_power_48) - 1::48>>
-      ],
-      <<45>>,
-      &Base.encode16(&1, case: :lower)
-    )
+  # For reproducible builds, derive the identifier from the project name
+  # and version instead of random data
+  defp uuid(config) do
+    <<a::binary-size(4), b::binary-size(2), c::binary-size(2), d::binary-size(2),
+      e::binary-size(6)>> = :erlang.md5("#{config.project} #{config.version}")
+
+    Enum.map_join([a, b, c, d, e], <<45>>, &Base.encode16(&1, case: :lower))
   end
 end

From 1e99af417e68cbcc5a05bf9fa0011894450945b1 Mon Sep 17 00:00:00 2001
From: "Bernhard M. Wiedemann" <bwiedemann@suse.de>
Date: Mon, 27 Jul 2026 22:56:09 +0200
Subject: [PATCH 2/2] Apply suggestion from @josevalim

---
 lib/ex_doc/formatter/epub.ex | 17 ++++++-----------
 1 file changed, 6 insertions(+), 11 deletions(-)

diff --git a/lib/ex_doc/formatter/epub.ex b/lib/ex_doc/formatter/epub.ex
index d308836af..5fdd9257b 100644
--- a/lib/ex_doc/formatter/epub.ex
+++ b/lib/ex_doc/formatter/epub.ex
@@ -140,19 +140,14 @@ defmodule ExDoc.Formatter.EPUB do
     # Use SOURCE_DATE_EPOCH instead of the current time when set, for
     # reproducible builds
     # (https://reproducible-builds.org/docs/source-date-epoch/).
-    time =
-      case System.get_env("SOURCE_DATE_EPOCH") do
-        nil ->
-          :calendar.universal_time()
-
-        epoch ->
-          case Integer.parse(epoch) do
-            {seconds, ""} -> :calendar.system_time_to_universal_time(seconds, :second)
-            _ -> :calendar.universal_time()
-          end
+    {{year, month, day}, {hour, min, sec}} =
+      with epoch when is_binary(epoch) <- System.get_env("SOURCE_DATE_EPOCH"),
+           {seconds, ""} <- Integer.parse(epoch) do
+        :calendar.system_time_to_universal_time(seconds, :second)
+      else
+        _ -> :calendar.universal_time()
       end
 
-    {{year, month, day}, {hour, min, sec}} = time
     list = [year, month, day, hour, min, sec]
 
     "~4..0B-~2..0B-~2..0BT~2..0B:~2..0B:~2..0BZ"