From: Lars Magne Ingebrigtsen <larsi@gnus.org>
Date: Mon, 28 Mar 2016 17:07:39 +0000 (+0200)
Subject: Add a new function `buffer-hash'
X-Git-Tag: emacs-26.0.90~2280
X-Git-Url: http://git.eshelyaron.com/gitweb/?a=commitdiff_plain;h=15357f6d1f90b03719f650823ac6531a305a9818;p=emacs.git

Add a new function `buffer-hash'

* doc/lispref/text.texi (Checksum/Hash): Document `buffer-hash'.

* src/fns.c (Fbuffer_hash): New function.
(make_digest_string): Refactored out into its own function.
(secure_hash): Use it.

* test/src/fns-tests.el (fns-tests-hash-buffer): New tests.
---

diff --git a/doc/lispref/text.texi b/doc/lispref/text.texi
index 4c3a1a01e7d..5e473166d1d 100644
--- a/doc/lispref/text.texi
+++ b/doc/lispref/text.texi
@@ -4468,6 +4468,20 @@ using the specified or chosen coding system.  However, if
 coding instead.
 @end defun
 
+@defun buffer-hash &optional buffer-or-name
+Return a hash of @var{buffer-or-name}.  If @code{nil}, this defaults
+to the current buffer.  As opposed to @code{secure-hash}, this
+function computes the hash based on the internal representation of the
+buffer, disregarding any coding systems.  It's therefore only useful
+when comparing two buffers running in the same Emacs, and is not
+guaranteed to return the same hash between different Emacs versions.
+It should be somewhat more efficient on larger buffers than
+@code{secure-hash} is, and should not allocate more memory.
+@c Note that we do not document what hashing function we're using, or
+@c even whether it's a cryptographic hash, since that may change
+@c according to what we find useful.
+@end defun
+
 @node Parsing HTML/XML
 @section Parsing HTML and XML
 @cindex parsing html
diff --git a/etc/NEWS b/etc/NEWS
index ce21532b68d..0a363711626 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -53,6 +53,10 @@ has been added.  They are: 'file-attribute-type',
 'file-attribute-modes', 'file-attribute-inode-number', and
 'file-attribute-device-number'
 
++++
+** The new function `buffer-hash' has been added, and can be used to
+compute a fash, non-consing hash of the contents of a buffer.
+
 ---
 ** The locale language name 'ca' is now mapped to the language
 environment 'Catalan', which has been added.
diff --git a/src/fns.c b/src/fns.c
index 0e3fc2765b4..9513387f93e 100644
--- a/src/fns.c
+++ b/src/fns.c
@@ -4737,6 +4737,22 @@ returns nil, then (funcall TEST x1 x2) also returns nil.  */)
 #include "sha256.h"
 #include "sha512.h"
 
+Lisp_Object
+make_digest_string (Lisp_Object digest, int digest_size)
+{
+  unsigned char *p = SDATA (digest);
+  int i;
+
+  for (i = digest_size - 1; i >= 0; i--)
+    {
+      static char const hexdigit[16] = "0123456789abcdef";
+      int p_i = p[i];
+      p[2 * i] = hexdigit[p_i >> 4];
+      p[2 * i + 1] = hexdigit[p_i & 0xf];
+    }
+  return digest;
+}
+
 /* ALGORITHM is a symbol: md5, sha1, sha224 and so on. */
 
 static Lisp_Object
@@ -4936,17 +4952,7 @@ secure_hash (Lisp_Object algorithm, Lisp_Object object, Lisp_Object start,
 	     SSDATA (digest));
 
   if (NILP (binary))
-    {
-      unsigned char *p = SDATA (digest);
-      for (i = digest_size - 1; i >= 0; i--)
-	{
-	  static char const hexdigit[16] = "0123456789abcdef";
-	  int p_i = p[i];
-	  p[2 * i] = hexdigit[p_i >> 4];
-	  p[2 * i + 1] = hexdigit[p_i & 0xf];
-	}
-      return digest;
-    }
+    return make_digest_string (digest, digest_size);
   else
     return make_unibyte_string (SSDATA (digest), digest_size);
 }
@@ -4997,6 +5003,45 @@ If BINARY is non-nil, returns a string in binary form.  */)
 {
   return secure_hash (algorithm, object, start, end, Qnil, Qnil, binary);
 }
+
+DEFUN ("buffer-hash", Fbuffer_hash, Sbuffer_hash, 0, 1, 0,
+       doc: /* Return a hash of the contents of BUFFER-OR-NAME.
+This hash is performed on the raw internal format of the buffer,
+disregarding any coding systems.
+If nil, use the current buffer." */ )
+  (Lisp_Object buffer_or_name)
+{
+  Lisp_Object buffer;
+  struct buffer *b;
+  struct sha1_ctx ctx;
+  Lisp_Object digest = make_uninit_string (SHA1_DIGEST_SIZE * 2);
+
+  if (NILP (buffer_or_name))
+    buffer = Fcurrent_buffer ();
+  else
+    buffer = Fget_buffer (buffer_or_name);
+  if (NILP (buffer))
+    nsberror (buffer_or_name);
+
+  b = XBUFFER (buffer);
+  sha1_init_ctx (&ctx);
+
+  /* Process the first part of the buffer. */
+  sha1_process_bytes (BUF_BEG_ADDR (b),
+		      BUF_GPT_BYTE (b) - BUF_BEG_BYTE (b),
+		      &ctx);
+
+  /* If the gap is before the end of the buffer, process the last half
+     of the buffer. */
+  if (BUF_GPT_BYTE (b) < BUF_Z_BYTE (b))
+    sha1_process_bytes (BUF_GAP_END_ADDR (b),
+			BUF_Z_ADDR (b) - BUF_GAP_END_ADDR (b),
+			&ctx);
+
+  sha1_finish_ctx (&ctx, SSDATA (digest));
+  return make_digest_string (digest, SHA1_DIGEST_SIZE);
+}
+
 
 void
 syms_of_fns (void)
@@ -5156,6 +5201,7 @@ this variable.  */);
   defsubr (&Sbase64_decode_string);
   defsubr (&Smd5);
   defsubr (&Ssecure_hash);
+  defsubr (&Sbuffer_hash);
   defsubr (&Slocale_info);
 
   hashtest_eq.name = Qeq;
diff --git a/test/src/fns-tests.el b/test/src/fns-tests.el
index 688ff1f6bd9..848589692ea 100644
--- a/test/src/fns-tests.el
+++ b/test/src/fns-tests.el
@@ -219,3 +219,19 @@
   (should (equal (func-arity (eval (lambda (x &optional y)) nil)) '(1 . 2)))
   (should (equal (func-arity (eval (lambda (x &optional y)) t)) '(1 . 2)))
   (should (equal (func-arity 'let) '(1 . unevalled))))
+
+(ert-deftest fns-tests-hash-buffer ()
+  (should (equal (sha1 "foo") "0beec7b5ea3f0fdbc95d0dd47f3c5bc275da8a33"))
+  (should (equal (with-temp-buffer
+                   (insert "foo")
+                   (buffer-hash))
+                 (sha1 "foo")))
+  ;; This tests whether the presence of a gap in the middle of the
+  ;; buffer is handled correctly.
+  (should (equal (with-temp-buffer
+                   (insert "foo")
+                   (goto-char 2)
+                   (insert " ")
+                   (backward-delete-char 1)
+                   (buffer-hash))
+                 (sha1 "foo"))))