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<sect1 id="functions-bitstring">
<title>Bit String Functions and Operators</title>
<indexterm zone="functions-bitstring">
<primary>bit strings</primary>
<secondary>functions</secondary>
</indexterm>
<para>
This section describes functions and operators for examining and
manipulating bit strings, that is values of the types
<type>bit</type> and <type>bit varying</type>. (While only
type <type>bit</type> is mentioned in these tables, values of
type <type>bit varying</type> can be used interchangeably.)
Bit strings support the usual comparison operators shown in
<xref linkend="functions-comparison-op-table"/>, as well as the
operators shown in <xref linkend="functions-bit-string-op-table"/>.
</para>
<table id="functions-bit-string-op-table">
<title>Bit String Operators</title>
<tgroup cols="1">
<thead>
<row>
<entry role="func_table_entry"><para role="func_signature">
Operator
</para>
<para>
Description
</para>
<para>
Example(s)
</para></entry>
</row>
</thead>
<tbody>
<row>
<entry role="func_table_entry"><para role="func_signature">
<type>bit</type> <literal>||</literal> <type>bit</type>
<returnvalue>bit</returnvalue>
</para>
<para>
Concatenation
</para>
<para>
<literal>B'10001' || B'011'</literal>
<returnvalue>10001011</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<type>bit</type> <literal>&</literal> <type>bit</type>
<returnvalue>bit</returnvalue>
</para>
<para>
Bitwise AND (inputs must be of equal length)
</para>
<para>
<literal>B'10001' & B'01101'</literal>
<returnvalue>00001</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<type>bit</type> <literal>|</literal> <type>bit</type>
<returnvalue>bit</returnvalue>
</para>
<para>
Bitwise OR (inputs must be of equal length)
</para>
<para>
<literal>B'10001' | B'01101'</literal>
<returnvalue>11101</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<type>bit</type> <literal>#</literal> <type>bit</type>
<returnvalue>bit</returnvalue>
</para>
<para>
Bitwise exclusive OR (inputs must be of equal length)
</para>
<para>
<literal>B'10001' # B'01101'</literal>
<returnvalue>11100</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<literal>~</literal> <type>bit</type>
<returnvalue>bit</returnvalue>
</para>
<para>
Bitwise NOT
</para>
<para>
<literal>~ B'10001'</literal>
<returnvalue>01110</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<type>bit</type> <literal><<</literal> <type>integer</type>
<returnvalue>bit</returnvalue>
</para>
<para>
Bitwise shift left
(string length is preserved)
</para>
<para>
<literal>B'10001' << 3</literal>
<returnvalue>01000</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<type>bit</type> <literal>>></literal> <type>integer</type>
<returnvalue>bit</returnvalue>
</para>
<para>
Bitwise shift right
(string length is preserved)
</para>
<para>
<literal>B'10001' >> 2</literal>
<returnvalue>00100</returnvalue>
</para></entry>
</row>
</tbody>
</tgroup>
</table>
<para>
Some of the functions available for binary strings are also available
for bit strings, as shown in <xref linkend="functions-bit-string-table"/>.
</para>
<table id="functions-bit-string-table">
<title>Bit String Functions</title>
<tgroup cols="1">
<thead>
<row>
<entry role="func_table_entry"><para role="func_signature">
Function
</para>
<para>
Description
</para>
<para>
Example(s)
</para></entry>
</row>
</thead>
<tbody>
<row>
<entry role="func_table_entry"><para role="func_signature">
<indexterm>
<primary>bit_count</primary>
</indexterm>
<function>bit_count</function> ( <type>bit</type> )
<returnvalue>bigint</returnvalue>
</para>
<para>
Returns the number of bits set in the bit string (also known as
<quote>popcount</quote>).
</para>
<para>
<literal>bit_count(B'10111')</literal>
<returnvalue>4</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<indexterm>
<primary>bit_length</primary>
</indexterm>
<function>bit_length</function> ( <type>bit</type> )
<returnvalue>integer</returnvalue>
</para>
<para>
Returns number of bits in the bit string.
</para>
<para>
<literal>bit_length(B'10111')</literal>
<returnvalue>5</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<indexterm>
<primary>length</primary>
</indexterm>
<indexterm>
<primary>bit string</primary>
<secondary>length</secondary>
</indexterm>
<function>length</function> ( <type>bit</type> )
<returnvalue>integer</returnvalue>
</para>
<para>
Returns number of bits in the bit string.
</para>
<para>
<literal>length(B'10111')</literal>
<returnvalue>5</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<indexterm>
<primary>octet_length</primary>
</indexterm>
<function>octet_length</function> ( <type>bit</type> )
<returnvalue>integer</returnvalue>
</para>
<para>
Returns number of bytes in the bit string.
</para>
<para>
<literal>octet_length(B'1011111011')</literal>
<returnvalue>2</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<indexterm>
<primary>overlay</primary>
</indexterm>
<function>overlay</function> ( <parameter>bits</parameter> <type>bit</type> <literal>PLACING</literal> <parameter>newsubstring</parameter> <type>bit</type> <literal>FROM</literal> <parameter>start</parameter> <type>integer</type> <optional> <literal>FOR</literal> <parameter>count</parameter> <type>integer</type> </optional> )
<returnvalue>bit</returnvalue>
</para>
<para>
Replaces the substring of <parameter>bits</parameter> that starts at
the <parameter>start</parameter>'th bit and extends
for <parameter>count</parameter> bits
with <parameter>newsubstring</parameter>.
If <parameter>count</parameter> is omitted, it defaults to the length
of <parameter>newsubstring</parameter>.
</para>
<para>
<literal>overlay(B'01010101010101010' placing B'11111' from 2 for 3)</literal>
<returnvalue>0111110101010101010</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<indexterm>
<primary>position</primary>
</indexterm>
<function>position</function> ( <parameter>substring</parameter> <type>bit</type> <literal>IN</literal> <parameter>bits</parameter> <type>bit</type> )
<returnvalue>integer</returnvalue>
</para>
<para>
Returns first starting index of the specified <parameter>substring</parameter>
within <parameter>bits</parameter>, or zero if it's not present.
</para>
<para>
<literal>position(B'010' in B'000001101011')</literal>
<returnvalue>8</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<indexterm>
<primary>substring</primary>
</indexterm>
<function>substring</function> ( <parameter>bits</parameter> <type>bit</type> <optional> <literal>FROM</literal> <parameter>start</parameter> <type>integer</type> </optional> <optional> <literal>FOR</literal> <parameter>count</parameter> <type>integer</type> </optional> )
<returnvalue>bit</returnvalue>
</para>
<para>
Extracts the substring of <parameter>bits</parameter> starting at
the <parameter>start</parameter>'th bit if that is specified,
and stopping after <parameter>count</parameter> bits if that is
specified. Provide at least one of <parameter>start</parameter>
and <parameter>count</parameter>.
</para>
<para>
<literal>substring(B'110010111111' from 3 for 2)</literal>
<returnvalue>00</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<indexterm>
<primary>get_bit</primary>
</indexterm>
<function>get_bit</function> ( <parameter>bits</parameter> <type>bit</type>,
<parameter>n</parameter> <type>integer</type> )
<returnvalue>integer</returnvalue>
</para>
<para>
Extracts <parameter>n</parameter>'th bit
from bit string; the first (leftmost) bit is bit 0.
</para>
<para>
<literal>get_bit(B'101010101010101010', 6)</literal>
<returnvalue>1</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<indexterm>
<primary>set_bit</primary>
</indexterm>
<function>set_bit</function> ( <parameter>bits</parameter> <type>bit</type>,
<parameter>n</parameter> <type>integer</type>,
<parameter>newvalue</parameter> <type>integer</type> )
<returnvalue>bit</returnvalue>
</para>
<para>
Sets <parameter>n</parameter>'th bit in
bit string to <parameter>newvalue</parameter>;
the first (leftmost) bit is bit 0.
</para>
<para>
<literal>set_bit(B'101010101010101010', 6, 0)</literal>
<returnvalue>101010001010101010</returnvalue>
</para></entry>
</row>
</tbody>
</tgroup>
</table>
<para>
In addition, it is possible to cast integral values to and from type
<type>bit</type>.
Casting an integer to <type>bit(n)</type> copies the rightmost
<literal>n</literal> bits. Casting an integer to a bit string width wider
than the integer itself will sign-extend on the left.
Some examples:
<programlisting>
44::bit(10) <lineannotation>0000101100</lineannotation>
44::bit(3) <lineannotation>100</lineannotation>
cast(-44 as bit(12)) <lineannotation>111111010100</lineannotation>
'1110'::bit(4)::integer <lineannotation>14</lineannotation>
</programlisting>
Note that casting to just <quote>bit</quote> means casting to
<literal>bit(1)</literal>, and so will deliver only the least significant
bit of the integer.
</para>
</sect1>
|
