[Reproducible-commits] [source-date-epoch-spec] 02/02: explain why it's not feasible to "improve diff"

[Ximin Luo]

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infinity0 pushed a commit to branch master
in repository source-date-epoch-spec.

commit 74e4d95e0b607c68baf30cb583b6751719f3607e
Author: Ximin Luo <infinity0 at debian.org>
Date:   Fri Aug 28 20:29:47 2015 +0200

    explain why it's not feasible to "improve diff"
---
 source-date-epoch-spec.xml | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/source-date-epoch-spec.xml b/source-date-epoch-spec.xml
index d6e8bdf..5a805fb 100644
--- a/source-date-epoch-spec.xml
+++ b/source-date-epoch-spec.xml
@@ -144,6 +144,20 @@
 			formats such as PNG or GZip.
 		</para>
 		<para>
+			One suggestion that is sometimes raised, is to have diff
+			programs detect and ignore embedded timestamps. However,
+			it is not feasible to develop an algorithm to do this for
+			arbitrary data formats, and <emphasis>computationally
+			impossible</emphasis> in the case of Turing-complete data
+			formats such as executables - since the real behaviour of
+			the result can easily change based on a piece of data
+			embedded in the file, even if the data is itself static
+			or immutable. The only way to algorithmically verify
+			identical behaviour in the general case, is to enforce
+			bit-for-bit identical build results, and eliminate build
+			time variations even in data that is static at run time.
+		</para>
+		<para>
 			Historically, there have been several rationales for
 			embedding the build date:
 		</para>

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