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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=ANSI_X3.4-1968" /><title>bitmap_onto</title><meta name="generator" content="DocBook XSL Stylesheets V1.75.2" /><link rel="home" href="index.html" title="The Linux Kernel API" /><link rel="up" href="ch03.html#id2962812" title="Bitmap Operations" /><link rel="prev" href="re111.html" title="bitmap_bitremap" /><link rel="next" href="re113.html" title="bitmap_fold" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center"><span>bitmap_onto</span></th></tr><tr><td width="20%" align="left"><a accesskey="p" href="re111.html">Prev</a> </td><th width="60%" align="center">Bitmap Operations</th><td width="20%" align="right"> <a accesskey="n" href="re113.html">Next</a></td></tr></table><hr /></div><div class="refentry" title="bitmap_onto"><a id="API-bitmap-onto"></a><div class="titlepage"></div><div class="refnamediv"><h2>Name</h2><p>bitmap_onto —
translate one bitmap relative to another
</p></div><div class="refsynopsisdiv" title="Synopsis"><h2>Synopsis</h2><div class="funcsynopsis"><table border="0" summary="Function synopsis" cellspacing="0" cellpadding="0" class="funcprototype-table"><tr><td><code class="funcdef">void <b class="fsfunc">bitmap_onto </b>(</code></td><td>unsigned long * <var class="pdparam">dst</var>, </td></tr><tr><td> </td><td>const unsigned long * <var class="pdparam">orig</var>, </td></tr><tr><td> </td><td>const unsigned long * <var class="pdparam">relmap</var>, </td></tr><tr><td> </td><td>int <var class="pdparam">bits</var><code>)</code>;</td></tr></table><div class="funcprototype-spacer"> </div></div></div><div class="refsect1" title="Arguments"><a id="id2964979"></a><h2>Arguments</h2><div class="variablelist"><dl><dt><span class="term"><em class="parameter"><code>dst</code></em></span></dt><dd><p>
resulting translated bitmap
</p></dd><dt><span class="term"><em class="parameter"><code>orig</code></em></span></dt><dd><p>
original untranslated bitmap
</p></dd><dt><span class="term"><em class="parameter"><code>relmap</code></em></span></dt><dd><p>
bitmap relative to which translated
</p></dd><dt><span class="term"><em class="parameter"><code>bits</code></em></span></dt><dd><p>
number of bits in each of these bitmaps
</p></dd></dl></div></div><div class="refsect1" title="Description"><a id="id2965058"></a><h2>Description</h2><p>
Set the n-th bit of <em class="parameter"><code>dst</code></em> iff there exists some m such that the
n-th bit of <em class="parameter"><code>relmap</code></em> is set, the m-th bit of <em class="parameter"><code>orig</code></em> is set, and
the n-th bit of <em class="parameter"><code>relmap</code></em> is also the m-th _set_ bit of <em class="parameter"><code>relmap</code></em>.
(If you understood the previous sentence the first time your
read it, you're overqualified for your current job.)
</p><p>
In other words, <em class="parameter"><code>orig</code></em> is mapped onto (surjectively) <em class="parameter"><code>dst</code></em>,
using the the map { <n, m> | the n-th bit of <em class="parameter"><code>relmap</code></em> is the
m-th set bit of <em class="parameter"><code>relmap</code></em> }.
</p><p>
Any set bits in <em class="parameter"><code>orig</code></em> above bit number W, where W is the
weight of (number of set bits in) <em class="parameter"><code>relmap</code></em> are mapped nowhere.
In particular, if for all bits m set in <em class="parameter"><code>orig</code></em>, m >= W, then
<em class="parameter"><code>dst</code></em> will end up empty. In situations where the possibility
of such an empty result is not desired, one way to avoid it is
to use the <code class="function">bitmap_fold</code> operator, below, to first fold the
<em class="parameter"><code>orig</code></em> bitmap over itself so that all its set bits x are in the
range 0 <= x < W. The <code class="function">bitmap_fold</code> operator does this by
setting the bit (m % W) in <em class="parameter"><code>dst</code></em>, for each bit (m) set in <em class="parameter"><code>orig</code></em>.
</p><p>
Example [1] for <code class="function">bitmap_onto</code>:
Let's say <em class="parameter"><code>relmap</code></em> has bits 30-39 set, and <em class="parameter"><code>orig</code></em> has bits
1, 3, 5, 7, 9 and 11 set. Then on return from this routine,
<em class="parameter"><code>dst</code></em> will have bits 31, 33, 35, 37 and 39 set.
</p><p>
When bit 0 is set in <em class="parameter"><code>orig</code></em>, it means turn on the bit in
<em class="parameter"><code>dst</code></em> corresponding to whatever is the first bit (if any)
that is turned on in <em class="parameter"><code>relmap</code></em>. Since bit 0 was off in the
above example, we leave off that bit (bit 30) in <em class="parameter"><code>dst</code></em>.
</p><p>
When bit 1 is set in <em class="parameter"><code>orig</code></em> (as in the above example), it
means turn on the bit in <em class="parameter"><code>dst</code></em> corresponding to whatever
is the second bit that is turned on in <em class="parameter"><code>relmap</code></em>. The second
bit in <em class="parameter"><code>relmap</code></em> that was turned on in the above example was
bit 31, so we turned on bit 31 in <em class="parameter"><code>dst</code></em>.
</p><p>
Similarly, we turned on bits 33, 35, 37 and 39 in <em class="parameter"><code>dst</code></em>,
because they were the 4th, 6th, 8th and 10th set bits
set in <em class="parameter"><code>relmap</code></em>, and the 4th, 6th, 8th and 10th bits of
<em class="parameter"><code>orig</code></em> (i.e. bits 3, 5, 7 and 9) were also set.
</p><p>
When bit 11 is set in <em class="parameter"><code>orig</code></em>, it means turn on the bit in
<em class="parameter"><code>dst</code></em> corresponding to whatever is the twelth bit that is
turned on in <em class="parameter"><code>relmap</code></em>. In the above example, there were
only ten bits turned on in <em class="parameter"><code>relmap</code></em> (30..39), so that bit
11 was set in <em class="parameter"><code>orig</code></em> had no affect on <em class="parameter"><code>dst</code></em>.
</p><p>
Example [2] for <code class="function">bitmap_fold</code> + <code class="function">bitmap_onto</code>:
Let's say <em class="parameter"><code>relmap</code></em> has these ten bits set:
40 41 42 43 45 48 53 61 74 95
(for the curious, that's 40 plus the first ten terms of the
Fibonacci sequence.)
</p><p>
Further lets say we use the following code, invoking
<code class="function">bitmap_fold</code> then bitmap_onto, as suggested above to
avoid the possitility of an empty <em class="parameter"><code>dst</code></em> result:
</p><p>
unsigned long *tmp; // a temporary bitmap's bits
</p><p>
bitmap_fold(tmp, orig, bitmap_weight(relmap, bits), bits);
bitmap_onto(dst, tmp, relmap, bits);
</p><p>
Then this table shows what various values of <em class="parameter"><code>dst</code></em> would be, for
various <em class="parameter"><code>orig</code></em>'s. I list the zero-based positions of each set bit.
The tmp column shows the intermediate result, as computed by
using <code class="function">bitmap_fold</code> to fold the <em class="parameter"><code>orig</code></em> bitmap modulo ten
(the weight of <em class="parameter"><code>relmap</code></em>).
</p><p>
<em class="parameter"><code>orig</code></em> tmp <em class="parameter"><code>dst</code></em>
0 0 40
1 1 41
9 9 95
10 0 40 (*)
1 3 5 7 1 3 5 7 41 43 48 61
0 1 2 3 4 0 1 2 3 4 40 41 42 43 45
0 9 18 27 0 9 8 7 40 61 74 95
0 10 20 30 0 40
0 11 22 33 0 1 2 3 40 41 42 43
0 12 24 36 0 2 4 6 40 42 45 53
78 102 211 1 2 8 41 42 74 (*)
</p><p>
(*) For these marked lines, if we hadn't first done <code class="function">bitmap_fold</code>
into tmp, then the <em class="parameter"><code>dst</code></em> result would have been empty.
</p><p>
If either of <em class="parameter"><code>orig</code></em> or <em class="parameter"><code>relmap</code></em> is empty (no set bits), then <em class="parameter"><code>dst</code></em>
will be returned empty.
</p><p>
If (as explained above) the only set bits in <em class="parameter"><code>orig</code></em> are in positions
m where m >= W, (where W is the weight of <em class="parameter"><code>relmap</code></em>) then <em class="parameter"><code>dst</code></em> will
once again be returned empty.
</p><p>
All bits in <em class="parameter"><code>dst</code></em> not set by the above rule are cleared.
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