bit
Synopsis
| cheia.load “bit” |
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| r = bit.bnot(x) |
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| r = bit.band(a, b, ...) |
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| r = bit.bor(a, b, ...) |
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| r = bit.bxor(a, b, ...) |
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| r = bit.lshift(x, places) |
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| r = bit.rshift(x, places) |
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| r = bit.arshift(x, places) |
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| r = bit.mod(numerator, denominator) |
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Description
This module provides a selection of bitwise operations.
Warning: Beware of precision. Lua stores all numbers in double-precision
floating-point format (by default). This means that large numbers are
not stored accurately. Therefore, the functions in this module should
not be expected to behave correctly for numbers exceeding 32 bits
(although 53 bits are probably safe at least on i386). This is of
particular consequence for the bit.lshift operation: the most
significant bits are not discarded, but shifted with the rest, which
may cause the number to grow large enough that precision is lost for
the least significant bits. For this reason, a bit.lshift should
always be preceded by a bit.mod to discard the unwanted bits when
there is any danger of the number growing too large.
Note: The fractional part of any operand is discarded. The right shift
operations do not move bits into the fractional part.
Examples
To clear bit n (0-based) of number x:
| x = bit.band(bit.bnot(bit.lshift(1, n)), x) |
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To retrieve the high and low bytes of unsigned 16-bit number x:
| high = bit.rshift(x, 8) |
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| low = bit.mod(x, 256) |
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Reference
All operations are directly implemented using their C equivalents operating on long longs.
- bit.bnot(x)
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Perforns a bitwise NOT of its operand. Equivalent to ~ in C.
| Parameters: |
| x : number
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Operand.
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| Returns: |
| number
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Result.
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- bit.band(...)
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Performs a bitwise AND of its operands. Equivalent to & in C.
| Parameters: |
| ... : numbers
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Two or more operands.
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| Returns: |
| number
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Result.
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- bit.bor(...)
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Performs a bitwise OR of its operands. Equivalent to | in C.
| Parameters: |
| ... : numbers
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Two or more operands.
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| Returns: |
| number
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Result.
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- bit.bxor(...)
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Performs a bitwise XOR of its operands. Equivalent to ^ in C.
| Parameters: |
| ... : numbers
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Two or more operands.
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| Returns: |
| number
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Result.
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- bit.lshift(x, n)
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Performs a bitwise left shift of its operand. Equivalent to << in C. See the warning above.
| Parameters: |
| x : number
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Operand.
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| n : number
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Number of places to shift.
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- bit.rshift(x, n)
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Performs an unsigned bitwise right shift of its operand. The result
is always positive. Equivalent to >> in C.
| Parameters: |
| x : number
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Operand.
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| n : number
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Number of places to shift.
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- bit.arshift(x, n)
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Performs a bitwise right shift of its operand, but the sign of the
operand is preserved. Equivalent to >> in C.
| Parameters: |
| x : number
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Operand.
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| n : number
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Number of places to shift.
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- bit.mod(n, d)
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Returns the remainder of an integer division of n by d. Equivalent to % in C.
| Parameters: |
| n : number
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Numerator.
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| d : number
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Denominator.
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Issues
Should the module automatically clip numbers to within the ‘safe’ range?
Since all ‘safe’ numbers fit within a signed long long, there
seems no need to distinguish bit.rshift and bit.arshift.
See also
None.
Revision history
Added in LuaCheia 5.0.
Credits
Lua module by Reuben Thomas.
Modifications for LuaCheia by Thatcher Ulrich.
Documentation for LuaCheia by Jamie Webb.
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