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= Bitwise arithmetic procedures =

== Abstract ==

This SRFI proposes a coherent and comprehensive set of procedures for
performing bitwise logical operations on integers; it is
accompanied by a reference implementation of the spec in terms of a set of
seven core operators. The sample implementation is portable, as efficient
as practical with pure Scheme arithmetic (it is worthwhile replacing the
core ops with C or assembly language if possible), and open source.
The precise semantics of these operators is almost never an issue. A
consistent, portable set of ''names'' and ''parameter conventions'', however, is.

Hence this SRFI, which is based mainly on [ SRFI 33], with some changes and additions from [ Olin's late revisions to SRFI 33] (which were never consummated).
SRFI 33 was never finalized, but is a reasonably comprehensive proposal.
A few procedures have been added from [ SRFI 60] and
the general vector [ SRFI 133].  
SRFI 60 (based on SLIB) is smaller but has a few procedures of its own;
some of its procedures have both native (often CL) and SRFI 33 names.
[ R6RS]
is a subset of SRFI 60, except that all procedure names begin with a `bitwise-` prefix.

Among the applications of bitwise operations are: hashing,
Galois-field calculations of error-detecting and error-correcting codes,
cryptography and ciphers,
pseudo-random number generation,
register-transfer-level modeling of digital logic designs,
Fast-Fourier transforms,
packing and unpacking numbers in persistant data structures,
space-filling curves with applications to dimension reduction
and sparse multi-dimensional database indexes,
and generating approximate seed values for root-finders
and transcendental function algorithms.

== Procedure index ==

bitwise-and   bitwise-ior 
bitwise-xor   bitwise-eqv
bitwise-nand  bitwise-nor 
bitwise-andc1 bitwise-andc2
bitwise-orc1  bitwise-orc2 

arithmetic-shift bit-count integer-length

bit-set? copy-bit bit-swap
any-bit-set? every-bit-set?

bit-field bit-field-any? bit-field-every?
bit-field-clear bit-field-set
bit-field-replace  bit-field-replace-same
bit-field-rotate bit-field-reverse

integer->list list->integer
integer->vector vector->integer
bitwise-fold bitwise-for-each bitwise-unfold

== Rationale ==

=== General design principles ===

* These operations interpret exact integers using two's-complement representation.

* The associative bitwise ops are required to be n-ary. Programmers can ''reliably'' write `bitwise-and` with 3 arguments, for example.

 * The word `or` is never used by itself, only with modifiers: `xor`, `ior`, `nor`,
   `orc1`, or `orc2`.  This is the same rule as Common Lisp.

 * Extra and redundant functions such as `bitwise-count`, `bitwise-nor` 
   and the bit-field ops have been included.  Settling on a standard
   choice of names makes it easier to read
   code that uses these sorts of operations. It also means computations
   can be clearly expressed using the more powerful ops rather than
   synthesized with a snarled mess of `bitwise-and`s, `bitwise-or`s, and `bitwise-not`s.
   What we gain is having an agreed-upon set of names by which we can refer
   to these functions. If you believe in "small is beautiful," then what is your motivation
   for including anything beyond `bitwise-nand`?

 * The programmer doesn't have to re-implement the redundant functions, and stumble
   over the boundary cases and error checking. The programmer can express
   himself using a full palette of building blocks.

 * Compilers can directly implement many of these ops for great efficiency gains
   without requiring any tricky analysis.

 * Logical right or left shift operations are excluded 
   because they are not well defined on general integers; they are only defined
   on integers in some finite range. Remember that, in this library, integers
   are interpreted as ''semi-infinite'' bit strings that have only a finite
   number of ones or a finite number of zeros. Logical shifting operates on bit
   strings of some fixed size. If we shift left, then leftmost bits "fall off"
   the end (and zeros shift in on the right). If we shift right, then zeros
   shift into the string on the left (and rightmost bits fall off the end). So
   to define a logical shift operation, ''we must specify the size of the
   window''. Typically this is the width of the underlying machine's register
   set (e.g., 32 bits). This is blatantly machine-specific and unportable, and
   clearly not the right thing for Scheme's more machine-independent general

=== Common Lisp ===

The core of this design design mirrors the structure of Common Lisp's pretty closely.  Here are some differences:

      * "load" and "deposit" are the wrong verbs (e.g., Common Lisp's `ldb` and `dpb` ops),
        since they have nothing to do with the store.  
      * `boole` has been removed; it is not one with the Way of Scheme.  Boolean functions
        are directly encoded in Scheme as first-class functions.
      * The name choices are more in tune with Scheme conventions (hyphenation,
        using `?` to mark a predicate, etc.). Common Lisp's name choices were more
        historically motivated, for reasons of backward compatibility with
        Maclisp and Zetalisp.
      * The prefix `log` has been changed to `bitwise-` (e.g, `lognot` to `bitwise-not`),
        as the prefix `bitwise-` more accurately reflects what they do.
      * The six trivial binary boolean ops that return constants, the left or right arguments,
        and the `bitwise-not` of the left or right arguments, do not appear in this SRFI.

=== SRFI 33 ===

This SRFI contains all the procedures of SRFI 33, and retains their original names with these exceptions:

* The name `bitwise-merge` is replaced by `bitwise-if`, the name used in SRFI 60 and R6RS.

* The name `extract-bit-field` (`bit-field-extract` in Olin's revisions) is replaced by `bit-field`, the name used in SRFI 60 and R6RS.

* The names `any-bits-set?` and `all-bits-set?` are replaced by `any-bit-set?` and `every-bit-set?`, in accordance with Olin's revisions.

* The name `test-bit-field?` has been renamed `bit-field-any?` and supplemented with
  `bit-field-every?`, in accordance with Olin's revisions.

* Because  `copy-bit-field` means different things in SRFI 33 and SRFI 60,
  SRFI 33's name `copy-bit-field` (`bit-field-copy` in Olin's revisions)
  has been changed to `bit-field-replace-same`.

=== SRFI 60 ===

SRFI 60 includes six procedures that do not have SRFI 33 equivalents.  They are incorporated into this SRFI as follows:

* The names `rotate-bit-field` and `reverse-bit-field` are replaced by `bit-field-rotate` and `bit-field-reverse`, in parallel with Olin's revisions.

* The procedures `copy-bit`, `integer->list` and `list->integer` are incorporated into this SRFI unchanged.

* The procedure `booleans->integer` is a convenient way to specify a bitwise integer: it accepts an arbitrary number of boolean arguments and returns a non-negative integer.  So in this SRFI it has the short name `bits`, roughly analogous to `list`, `string`, and `vector`.

=== Other sources ===

* The following procedures are inspired by SRFI 133:  `bit-swap`, `bit-field-append`,
`bitwise-fold`, `bitwise-for-each`, `bitwise-unfold`.

* The procedure `bit-field-set` is the counterpart of `bit-field-clear`.

=== Argument ordering and semantics ===

* In general, these procedures place the bitstring arguments to be operated on first.
  Where the operation is not commutative, the "destination" argument that provides
  the background to be operated on is placed before the "source" argument that provides
  the bits to be transferred to it.

* In SRFI 33, `bitwise-nand` and `bitwise-nor` accepted an arbitrary number of arguments
  even though they are not commutative.  Olin's late revisions made them dyadic, and so
  does this SRFI.

* Common Lisp bit-field operations use a ''byte spec'' to encapsulate the position and
  size of the field.  SRFI 33 bit-field operations had leading ''position'' and ''size''
  arguments instead.  These
  have been replaced in this SRFI by trailing ''start'' (inclusive) and ''end'' (exclusive)
  arguments, the convention used not only in SRFI 60 and R6RS but also in most other
  subsequence operations in Scheme standards and SRFIs.

== Specification ==

In the following procedure specifications all parameters and return values
are exact integers unless otherwise indicated (except that procedures with
names ending in `?` are predicates, as usual). It is
an error to pass values of other types as arguments to these functions.

Bitstrings are represented by exact integers, using a two's-complement encoding of
the bitstring. Thus every integer represents a semi-infinite bitstring, having
either a finite number of zeros (negative integers) or a finite number of
ones (non-negative integers). The bits of a bitstring are numbered from the
rightmost/least-significant bit: bit !#0 is the rightmost or 2^0^ bit, bit !#1 is
the next or 2^1^ bit, and so forth.

=== Basic operations ===

`(bitwise-not `''i''`)`

Returns the bitwise complement of ''i''; that is, all 1 bits are changed
to 0 bits and all 0 bits to 1 bits.

  (bitwise-not 10) => -11
  (bitwise-not -37) => 36

The following ten procedures correspond to the useful set
of non-trivial two-argument boolean functions. For each such function,
the corresponding bitwise operator
maps that function across a pair of bitstrings in a bit-wise fashion.
The core idea of this group of functions is this bitwise "lifting"
of the set of dyadic boolean functions to bitstring parameters.

`(bitwise-and `''i'' ...`)`[[BR]]
`(bitwise-ior `''i'' ...`)`[[BR]]
`(bitwise-xor `''i'' ...`)`[[BR]]
`(bitwise-eqv `''i'' ...`)`

These operations are associative.  When passed no arguments, the procedures
return the identity values -1, 0, 0, and -1 respectively.

The `bitwise-eqv` procedure produces the
complement of the `bitwise-xor` procedure.  When applied to three
arguments, it does ''not'' produce a 1 bit
everywhere that a, b and c all agree. That is, it does ''not'' produce

     (bitwise-ior (bitwise-and a b c)
                  (bitwise-and (bitwise-not a)
                               (bitwise-not b)
                               (bitwise-not c)))

Rather, it produces `(bitwise-eqv a (bitwise-eqv b c))` or the equivalent
`(bitwise-eqv (bitwise-eqv a b) c)`.

      (bitwise-ior 3  10)     =>  11
      (bitwise-and 11 26)     =>  10
      (bitwise-xor 3 10)      =>   9
      (bitwise-eqv 37 12)     => -42
      (bitwise-and 37 12)     =>   4

`(bitwise-nand `''i j''`)`[[BR]]
`(bitwise-nor `''i j''`)`[[BR]]
`(bitwise-andc1 `''i j''`)`[[BR]]
`(bitwise-andc2 `''i j''`)`[[BR]]
`(bitwise-orc1 `''i j''`)`[[BR]]
`(bitwise-orc2 `''i j''`)`

These operations are not associative.

      (bitwise-nand 11 26) =>  -11
      (bitwise-nor  11 26) => -28
      (bitwise-andc1 11 26) => 16
      (bitwise-andc2 11 26) => 1
      (bitwise-orc1 11 26) => -2
      (bitwise-orc2 11 26) => -17

=== Integer operations ===

`(arithmetic-shift `''i count''`)`

Returns the arithmetic left shift when ''count''>0; right shift when ''count''<0.

    (arithmetic-shift 8 2) => 32
    (arithmetic-shift 4 0) => 4
    (arithmetic-shift 8 -1) => 4
    (arithmetic-shift -100000000000000000000000000000000 -100) => -79

`(bit-count `''i''`)`

Returns the population count of 1's (''i'' >= 0) or 0's (''i'' < 0).  The result is always non-negative.

    (bit-count 0) =>  0
    (bit-count -1) =>  0
    (bit-count 7) =>  3
    (bit-count  13) =>  3 ;Two's-complement binary: ...0001101
    (bit-count -13) =>  2 ;Two's-complement binary: ...1110011
    (bit-count  30) =>  4 ;Two's-complement binary: ...0011110
    (bit-count -30) =>  4 ;Two's-complement binary: ...1100010
    (bit-count (expt 2 100)) =>  1
    (bit-count (- (expt 2 100))) =>  100
    (bit-count (- (1+ (expt 2 100)))) =>  1

`(integer-length `''i''`)`

The number of bits needed to represent ''i'', i.e.

	(ceiling (/ (log (if (negative? integer)
			     (- integer)
			     (+ 1 integer)))
		    (log 2)))

The result is always non-negative.
For non-negative ''i'', this is the number of bits needed to
represent ''i'' in an unsigned binary representation. For all ''i'',
`(+ 1 (integer-length `''i''`))` is the number of bits needed
to represent ''i'' in a signed twos-complement 
    (integer-length  0) => 0
    (integer-length  1) => 1
    (integer-length -1) => 0
    (integer-length  7) => 3
    (integer-length -7) => 3
    (integer-length  8) => 4
    (integer-length -8) => 3

`(bitwise-if `''mask i j''`)`

Merge the bitstrings ''i'' and ''j'', with bitstring ''mask'' determining
from which string to take each bit. That is, if the ''k''th bit of ''mask''
is 0, then the ''k''th bit of the result is the ''k''th bit of ''i'', otherwise
the ''k''th bit of ''j''.  This is equivalent to:

        (bitwise-ior (bitwise-and (bitwise-not mask) i)
                     (bitwise-and mask j))

=== Single-bit operations ===

`(bit-set? `''index i''`)`

Is bit ''index'' set in bitstring ''i'' (where ''index'' is a non-negative exact
integer)?  As always, the rightmost/least-significant bit in ''i'' is bit 0.

    (bit-set? 1 1) =>  false
    (bit-set? 0 1) =>  true
    (bit-set? 3 10) =>  true
    (bit-set? 1000000 -1) =>  true
    (bit-set? 2 6) =>  true
    (bit-set? 0 6) =>  false

`(copy-bit `''index i boolean''`)`

Returns an integer the same as ''i'' except in the ''index''th bit,
which is 1 if ''boolean'' is `#t` and 0 if ''boolean'' is `#f`.

(copy-bit 0 0 #t) => #b1
(copy-bit 2 0 #t) => #b100
(copy-bit 2 #b1111 #f) => #b1011

`(bit-swap `''index,,1,, index,,2,, i''`)`

Returns an integer the same as ''i'' except that the ''index,,1,,''th bit
and the ''index,,2,,''th bit have been exchanged.

(bit-swap 0 2 4) => #b1

`(any-bit-set? `''test-bits i''`)`[[BR]]
`(every-bit-set? `''test-bits i''`)`

Determines if any/all of the bits set in bitstring ''test-bits'' are set
in bitstring ''i''. I.e.,  returns `(not (zero? (bitwise-and `''test-bits i''`)))`
or `(= `''test-bits''` (bitwise-and ''test-bits i'')))` respectively.

`(first-set-bit `''i''`)`

Return the index of the first (smallest index) 1 bit in bitstring ''i''.
Return -1 if ''i'' contains no 1 bits (i.e., if ''i'' is zero).

    (first-set-bit 1) => 0
    (first-set-bit 2) => 1
    (first-set-bit 0) => -1
    (first-set-bit 40) => 3
    (first-set-bit -28) => 2
    (first-set-bit (expt  2 99)) => 99
    (first-set-bit (expt -2 99)) => 99

=== Bit field operations ===

These functions operate on a contiguous field of bits (a "byte," in
Common-Lisp parlance) in a given bitstring. The ''start'' and ''end''
arguments, which are not optional, are
non-negative exact integers specifying the field: it is the ''end-start'' bits
running from bit ''start'' to bit ''end''-1.

`(bit-field `''i start end''`)`

Returns the field from ''i'', shifted
down to the least-significant position in the result.

`(bit-field-any? `''i start end''`)`

Returns true if any of the field's bits are set in bitstring ''i'', and false otherwise.

`(bit-field-every? `''i start end''`)`

Returns false if any of the field's bits are not set in bitstring ''i'', and true otherwise.

`(bit-field-clear `''i start end''`)`[[BR]]
`(bit-field-set `''i start end''`)`

Returns ''i'' with the field's bits set to all 0s/1s.

`(bit-field-replace `''dst src start end''`)`

Returns ''dst'' with the field replaced
by the least-significant ''end-start'' bits in ''src''.

`(bit-field-replace-same `''dst src start end''`)`

Returns ''dst'' with its field replaced
by the corresponding field in ''src''.

`(bit-field-rotate `''i count start end''`)`

Returns ''i'' with the field cyclically permuted
by ''count'' bits towards high-order.

`(bit-field-reverse `''i start end''`)`

Returns ''i'' with the order of the bits in the field reversed.

`(bit-field-append `''i start end'' ...`)`

It is an error if the number of arguments is not a multiple of three.  The field specified
by each triple of ''i start end'' arguments is extracted, and the
fields are concatenated in left-to-right order and returned as an integer.

=== Bits as booleans ===

`(integer->list `''i'' [ ''len'' ]`)`[[BR]]
`(integer->vector `''i'' [ ''len'' ]`)`

Returns a list/vector of ''len'' booleans corresponding to each bit of the non-negative integer ''i''.
`#t` is returned for each 1; `#f` for 0. The ''len'' argument defaults to `(integer-length `''i''`)`.

`(list->integer `''list''`)`[[BR]]
`(list->integer `''list''`)`

Returns an integer formed from the booleans in ''list/vector'';
it is an error if ''list/vector'' contains non-booleans.
A 1 bit is coded for each `#t`; a 0 bit for `#f`.
Note that the result is never a negative integer.
`integer->list` and `list->integer` are inverses in the sense of `equal?`,
and so are `integer->vector` and `vector->integer`.

`(bits `''bool'' ...`)`

Returns the integer coded by the `bool` arguments.

=== Fold, unfold, and generate ===

It is an error if the arguments named ''proc, stop?, mapper, successor''
are not procedures.
The arguments named ''seed'' may be any Scheme object.

`(bitwise-fold `''proc seed i''`)`

For each bit ''b'' of ''i'' from bit 0 to `(integer-length `''i''`)`, ''proc'' is called as
`(`''proc b r''`)`, where ''r'' is the current accumulated result.  The initial value of ''r''
is ''seed'', and the value returned by ''proc'' becomes the next accumulated result.  When
all bits are exhausted, the final accumulated result becomes the result of `bitwise-fold`.

`(bitwise-for-each `''proc i''`)`

Repeatedly applies ''proc'' to the bits of ''i'' starting with 0 and ending with
`(integer-length `''i''`)`.  The values returned by ''proc'' are discarded.  Returns
an unspecified value.

`(bitwise-unfold `''stop? mapper successor seed''`)`

Generates a non-negative integer bit by bit, starting with bit 0.
If the result of applying ''stop?'' to the current state
(whose initial value is ''seed'') is true, return the
currently accumulated bits as an integer.  Otherwise, apply ''mapper''
to the current state to obtain the next bit of the result by interpreting
a true value as a 1 bit and a false value as a 0 bit.  Then get a new state
by applying ''successor'' to the current state, and repeat this algorithm.

`(make-bitwise-generator `''i''`)`

Returns a [ SRFI 121]
generator that generates all the bits of ''i'' starting
with bit 0.  Note that it is an infinite generator.

== Comparison of proposals ==

The following table compares the names of the bitwise (aka logical) functions of Common Lisp, SRFI 33, Olin's revisions, SRFI 60, R6RS, and this SRFI.

||=Function=||=CL=||=SRFI 33=||=SRFI 33 late revs=||=SRFI 60=||=R6RS=||=This SRFI=||
||Bitwise NOT||`lognot`||`bitwise-not`||`bitwise-not`||`lognot`, `bitwise-not`||`bitwise-not`||`bitwise-not`||
||Bitwise AND||`logand`||`bitwise-and`||`bitwise-and`||`logand`, `bitwise-and`||`bitwise-and`||`bitwise-and`||
||Bitwise IOR||`logior`||`bitwise-ior`||`bitwise-ior`||`logior`, `bitwise-ior`||`bitwise-ior`||`bitwise-ior`||
||Bitwise XOR||`logxor`||`bitwise-xor`||`bitwise-xor`||`logxor`, `bitwise-xor`||`bitwise-xor`||`bitwise-xor`||
||Bitwise EQV||`logeqv`||`bitwise-eqv`||`bitwise-eqv`||---||---||`bitwise-eqv`||
||Bitwise NAND||`lognand`||`bitwise-nand`||`bitwise-nand`||---||---||`bitwise-nand`||
||Bitwise NOR||`lognor`||`bitwise-nor`||`bitwise-nor`||---||---||`bitwise-nor`||
||Bitwise AND with NOT of first arg||`logandc1`||`bitwise-andc1`||`bitwise-andc1`||---||---||`bitwise-andc1`||
||Bitwise AND with NOT of second arg||`logandc2`||`bitwise-andc2`||`bitwise-andc2`||---||---||`bitwise-andc2`||
||Bitwise OR with NOT of first arg||`logorc1`||`bitwise-orc1`||`bitwise-orc1`||---||---||`bitwise-orc1`||
||Bitwise OR with NOT of second arg||`logorc2`||`bitwise-orc2`||`bitwise-orc2`||---||---||`bitwise-orc2`||
||Arithmetic shift||`ash`||`arithmetic-shift`||`arithmetic-shift`||`ash`, `arithmetic-shift`||`bitwise-arithmetic-shift`||`arithmetic-shift`||
||Population count||`logcount`||`bit-count`||`bit-count`||`logcount`, `bit-count`||`bitwise-bit-count`||`bit-count`||
||Integer length||`integer-length`||`integer-length`||`integer-length`||`integer-length`||`bitwise-integer-length`||`integer-length`||
||Mask selects source of bits||---||`bitwise-merge`||`bitwise-merge`||`bitwise-if`, `bitwise-merge`||`bitwise-if`||`bitwise-if`||
||Test single bit||`logbitp`||`bit-set?`||`bit-set?`||`logbit?`, `bit-set?`||`bitwise-bit-set?`||`bit-set?`||
||See if any mask bits set||`logtest`||`any-bits-set?`||`any-bit-set?`||`logtest`, `any-bit-set?`||---||`any-bit-set`||
||See if all mask bits set||---||`all-bits-set?`||`every-bit-set?`||---||---||`every-bit-set?`||
||Replace single bit||---||---||`copy-bit`||`bitwise-copy-bit`||---||`copy-bit`||
||Swap bits||---||---||---||---||---||`bit-swap`||
||Find first bit set||---||`first-bit-set`||`first-set-bit`||`log2-binary-factors`, `first-set-bit`||---||`first-set-bit`||
||Extract bit field||`ldb`||`extract-bit-field`||`extract-bit-field`||`bit-field`||`bitwise-bit-field`||`bit-field`||
||Test bit field (any)||`ldb-test`||`test-bit-field?`||`bit-field-any?`||---||---||`bit-field-any?`||
||Test bit field (every)||---||---||`bit-field-every?`||---||---||`bit-field-every?`||
||Clear bit field||`mask-field`||`clear-bit-field`||`bit-field-clear`||---||---||`bit-field-clear`||
||Set bit field||---||---||---||---||---||`bit-field-set`||
||Replace bit field||`dpb`||`replace-bit-field`||`bit-field-replace`||`copy-bit-field`||`bitwise-copy-bit-field`||`bit-field-replace`||
||Replace corresponding bit field||`deposit-field`||`deposit-field`||`copy-bit-field`||---||---||`bit-field-copy-same`||
||Rotate bit field||---||---||---||`rotate-bit-field`||`bitwise-rotate-bit-field`||`bit-field-rotate`||
||Reverse bit field||---||---||---||`reverse-bit-field`||`bitwise-reverse-bit-field`||`bit-field-reverse`||
||Append bit fields||---||---||---||---||---||`bit-field-append`||
||Integer to boolean list||---||---||---||`integer->list`||---||`integer->list`||
||Integer to boolean vector||---||---||---||---||---||`integer->vector`||
||Boolean list to integer||---||---||---||`list->integer`||---||`list->integer`||
||Boolean vector to integer||---||---||---||---||---||`vector->integer`||
||Booleans to integer||---||---||---||`booleans->integer`||---||`bits`||
||Bitwise fold||---||---||---||---||---||`bitwise-fold`||
||Bitwise for-each||---||---||---||---||---||`bitwise-for-each`||
||Bitwise unfold||---||---||---||---||---||`bitwise-unfold`||


2016-09-15 03:01:41