From fd08f3ac6b8fe61e6d4931d693d3c67fe7e00215 Mon Sep 17 00:00:00 2001
From: knu <knu@b2dd03c8-39d4-4d8f-98ff-823fe69b080e>
Date: Tue, 20 Mar 2007 02:29:09 +0000
Subject: [PATCH] * lib/matrix.rb (Matrix::inverse_from): adding partial
 pivoting to   the Gauss-Jordan algorithm, making it stable.  a patch from  
 Peter Vanbroekhoven.  [ruby-core:10641]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_8@12105 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
---
 ChangeLog     |  6 ++++++
 lib/matrix.rb | 18 ++++++++++++------
 2 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index e6abf606ec..82a43b44d6 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Tue Mar 20 11:28:41 2007  Akinori MUSHA  <knu@iDaemons.org>
+
+	* lib/matrix.rb (Matrix::inverse_from): adding partial pivoting to
+	  the Gauss-Jordan algorithm, making it stable.  a patch from
+	  Peter Vanbroekhoven.  [ruby-core:10641]
+
 Mon Mar 19 11:39:29 2007  Minero Aoki  <aamine@loveruby.net>
 
 	* lib/net/protocol.rb (rbuf_read): extend buffer size for speed.
diff --git a/lib/matrix.rb b/lib/matrix.rb
index c62acdf9aa..470db19b05 100644
--- a/lib/matrix.rb
+++ b/lib/matrix.rb
@@ -590,15 +590,21 @@ class Matrix
     a = src.to_a
     
     for k in 0..size
-      if (akk = a[k][k]) == 0
-        i = k
-        begin
-          Matrix.Raise ErrNotRegular if (i += 1) > size
-        end while a[i][k] == 0
+      i = k
+      akk = a[k][k].abs
+      for j in (k+1)..size
+        v = a[j][k].abs
+        if v > akk
+          i = j
+          akk = v
+        end
+      end
+      Matrix.Raise ErrNotRegular if akk == 0
+      if i != k
         a[i], a[k] = a[k], a[i]
         @rows[i], @rows[k] = @rows[k], @rows[i]
-        akk = a[k][k]
       end
+      akk = a[k][k]
       
       for i in 0 .. size
         next if i == k