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1 root 1.14 =head1 LIBECB - e-C-Builtins
2 root 1.3
3 root 1.14 =head2 ABOUT LIBECB
4    
5     Libecb is currently a simple header file that doesn't require any
6     configuration to use or include in your project.
7    
8 sf-exg 1.16 It's part of the e-suite of libraries, other members of which include
9 root 1.14 libev and libeio.
10    
11     Its homepage can be found here:
12    
13     http://software.schmorp.de/pkg/libecb
14    
15 root 1.85 It mainly provides a number of wrappers around many compiler built-ins,
16     together with replacement functions for other compilers. In addition
17 root 1.104 to this, it provides a number of other low-level C utilities, such as
18 root 1.85 endianness detection, byte swapping or bit rotations.
19    
20     Or in other words, things that should be built into any standard C
21     system, but aren't, implemented as efficient as possible with GCC (clang,
22 root 1.104 MSVC...), and still correct with other compilers.
23 root 1.17
24 root 1.14 More might come.
25 root 1.3
26     =head2 ABOUT THE HEADER
27    
28 root 1.14 At the moment, all you have to do is copy F<ecb.h> somewhere where your
29     compiler can find it and include it:
30    
31     #include <ecb.h>
32    
33     The header should work fine for both C and C++ compilation, and gives you
34     all of F<inttypes.h> in addition to the ECB symbols.
35    
36 sf-exg 1.16 There are currently no object files to link to - future versions might
37 root 1.14 come with an (optional) object code library to link against, to reduce
38     code size or gain access to additional features.
39    
40     It also currently includes everything from F<inttypes.h>.
41    
42     =head2 ABOUT THIS MANUAL / CONVENTIONS
43    
44     This manual mainly describes each (public) function available after
45     including the F<ecb.h> header. The header might define other symbols than
46     these, but these are not part of the public API, and not supported in any
47     way.
48    
49     When the manual mentions a "function" then this could be defined either as
50     as inline function, a macro, or an external symbol.
51    
52     When functions use a concrete standard type, such as C<int> or
53     C<uint32_t>, then the corresponding function works only with that type. If
54     only a generic name is used (C<expr>, C<cond>, C<value> and so on), then
55     the corresponding function relies on C to implement the correct types, and
56     is usually implemented as a macro. Specifically, a "bool" in this manual
57     refers to any kind of boolean value, not a specific type.
58 root 1.1
59 root 1.40 =head2 TYPES / TYPE SUPPORT
60    
61 root 1.104 F<ecb.h> makes sure that the following types are defined (in the expected way):
62 root 1.40
63 root 1.76 int8_t uint8_
64     int16_t uint16_t
65     int32_t uint32_
66     int64_t uint64_t
67     int_fast8_t uint_fast8_t
68     int_fast16_t uint_fast16_t
69     int_fast32_t uint_fast32_t
70     int_fast64_t uint_fast64_t
71     intptr_t uintptr_t
72 root 1.40
73     The macro C<ECB_PTRSIZE> is defined to the size of a pointer on this
74 root 1.45 platform (currently C<4> or C<8>) and can be used in preprocessor
75     expressions.
76 root 1.40
77 root 1.74 For C<ptrdiff_t> and C<size_t> use C<stddef.h>/C<cstddef>.
78 root 1.49
79 root 1.62 =head2 LANGUAGE/ENVIRONMENT/COMPILER VERSIONS
80 root 1.43
81 sf-exg 1.46 All the following symbols expand to an expression that can be tested in
82 root 1.44 preprocessor instructions as well as treated as a boolean (use C<!!> to
83     ensure it's either C<0> or C<1> if you need that).
84    
85 root 1.88 =over
86 root 1.43
87 root 1.44 =item ECB_C
88    
89     True if the implementation defines the C<__STDC__> macro to a true value,
90 root 1.82 while not claiming to be C++, i..e C, but not C++.
91 root 1.44
92 root 1.43 =item ECB_C99
93    
94 root 1.47 True if the implementation claims to be compliant to C99 (ISO/IEC
95 root 1.55 9899:1999) or any later version, while not claiming to be C++.
96 root 1.47
97     Note that later versions (ECB_C11) remove core features again (for
98     example, variable length arrays).
99 root 1.43
100 root 1.74 =item ECB_C11, ECB_C17
101 root 1.43
102 root 1.74 True if the implementation claims to be compliant to C11/C17 (ISO/IEC
103     9899:2011, :20187) or any later version, while not claiming to be C++.
104 root 1.44
105     =item ECB_CPP
106    
107     True if the implementation defines the C<__cplusplus__> macro to a true
108     value, which is typically true for C++ compilers.
109    
110 root 1.74 =item ECB_CPP11, ECB_CPP14, ECB_CPP17
111 root 1.44
112 root 1.74 True if the implementation claims to be compliant to C++11/C++14/C++17
113     (ISO/IEC 14882:2011, :2014, :2017) or any later version.
114 root 1.43
115 root 1.83 Note that many C++20 features will likely have their own feature test
116     macros (see e.g. L<http://eel.is/c++draft/cpp.predefined#1.8>).
117    
118 root 1.81 =item ECB_OPTIMIZE_SIZE
119    
120     Is C<1> when the compiler optimizes for size, C<0> otherwise. This symbol
121     can also be defined before including F<ecb.h>, in which case it will be
122     unchanged.
123    
124 root 1.57 =item ECB_GCC_VERSION (major, minor)
125 root 1.43
126 root 1.84 Expands to a true value (suitable for testing by the preprocessor) if the
127     compiler used is GNU C and the version is the given version, or higher.
128 root 1.43
129     This macro tries to return false on compilers that claim to be GCC
130     compatible but aren't.
131    
132 root 1.50 =item ECB_EXTERN_C
133    
134     Expands to C<extern "C"> in C++, and a simple C<extern> in C.
135    
136     This can be used to declare a single external C function:
137    
138     ECB_EXTERN_C int printf (const char *format, ...);
139    
140     =item ECB_EXTERN_C_BEG / ECB_EXTERN_C_END
141    
142     These two macros can be used to wrap multiple C<extern "C"> definitions -
143     they expand to nothing in C.
144    
145     They are most useful in header files:
146    
147     ECB_EXTERN_C_BEG
148    
149     int mycfun1 (int x);
150     int mycfun2 (int x);
151    
152     ECB_EXTERN_C_END
153    
154     =item ECB_STDFP
155    
156 root 1.84 If this evaluates to a true value (suitable for testing by the
157 root 1.50 preprocessor), then C<float> and C<double> use IEEE 754 single/binary32
158     and double/binary64 representations internally I<and> the endianness of
159     both types match the endianness of C<uint32_t> and C<uint64_t>.
160    
161     This means you can just copy the bits of a C<float> (or C<double>) to an
162     C<uint32_t> (or C<uint64_t>) and get the raw IEEE 754 bit representation
163     without having to think about format or endianness.
164    
165     This is true for basically all modern platforms, although F<ecb.h> might
166     not be able to deduce this correctly everywhere and might err on the safe
167     side.
168    
169 root 1.87 =item ECB_64BIT_NATIVE
170    
171     Evaluates to a true value (suitable for both preprocessor and C code
172     testing) if 64 bit integer types on this architecture are evaluated
173 sf-exg 1.92 "natively", that is, with similar speeds as 32 bit integers. While 64 bit
174     integer support is very common (and in fact required by libecb), 32 bit
175 root 1.104 CPUs have to emulate operations on them, so you might want to avoid them.
176 root 1.87
177 root 1.54 =item ECB_AMD64, ECB_AMD64_X32
178    
179     These two macros are defined to C<1> on the x86_64/amd64 ABI and the X32
180     ABI, respectively, and undefined elsewhere.
181    
182     The designers of the new X32 ABI for some inexplicable reason decided to
183     make it look exactly like amd64, even though it's completely incompatible
184     to that ABI, breaking about every piece of software that assumed that
185     C<__x86_64> stands for, well, the x86-64 ABI, making these macros
186     necessary.
187    
188 root 1.43 =back
189    
190 root 1.62 =head2 MACRO TRICKERY
191    
192 root 1.88 =over
193 root 1.62
194     =item ECB_CONCAT (a, b)
195    
196     Expands any macros in C<a> and C<b>, then concatenates the result to form
197     a single token. This is mainly useful to form identifiers from components,
198     e.g.:
199    
200     #define S1 str
201     #define S2 cpy
202    
203     ECB_CONCAT (S1, S2)(dst, src); // == strcpy (dst, src);
204    
205     =item ECB_STRINGIFY (arg)
206    
207     Expands any macros in C<arg> and returns the stringified version of
208     it. This is mainly useful to get the contents of a macro in string form,
209     e.g.:
210    
211     #define SQL_LIMIT 100
212     sql_exec ("select * from table limit " ECB_STRINGIFY (SQL_LIMIT));
213    
214 root 1.64 =item ECB_STRINGIFY_EXPR (expr)
215    
216     Like C<ECB_STRINGIFY>, but additionally evaluates C<expr> to make sure it
217     is a valid expression. This is useful to catch typos or cases where the
218     macro isn't available:
219    
220     #include <errno.h>
221    
222     ECB_STRINGIFY (EDOM); // "33" (on my system at least)
223     ECB_STRINGIFY_EXPR (EDOM); // "33"
224    
225     // now imagine we had a typo:
226    
227     ECB_STRINGIFY (EDAM); // "EDAM"
228     ECB_STRINGIFY_EXPR (EDAM); // error: EDAM undefined
229    
230 root 1.62 =back
231    
232 sf-exg 1.60 =head2 ATTRIBUTES
233 root 1.1
234 sf-exg 1.60 A major part of libecb deals with additional attributes that can be
235     assigned to functions, variables and sometimes even types - much like
236     C<const> or C<volatile> in C. They are implemented using either GCC
237     attributes or other compiler/language specific features. Attributes
238     declarations must be put before the whole declaration:
239 root 1.20
240     ecb_const int mysqrt (int a);
241     ecb_unused int i;
242    
243 root 1.88 =over
244 root 1.1
245 root 1.3 =item ecb_unused
246    
247     Marks a function or a variable as "unused", which simply suppresses a
248 root 1.85 warning by the compiler when it detects it as unused. This is useful when
249     you e.g. declare a variable but do not always use it:
250 root 1.3
251 root 1.15 {
252 sf-exg 1.61 ecb_unused int var;
253 root 1.3
254 root 1.15 #ifdef SOMECONDITION
255     var = ...;
256     return var;
257     #else
258     return 0;
259     #endif
260     }
261 root 1.3
262 root 1.56 =item ecb_deprecated
263    
264     Similar to C<ecb_unused>, but marks a function, variable or type as
265     deprecated. This makes some compilers warn when the type is used.
266    
267 root 1.62 =item ecb_deprecated_message (message)
268    
269 root 1.67 Same as C<ecb_deprecated>, but if possible, the specified diagnostic is
270 root 1.62 used instead of a generic depreciation message when the object is being
271     used.
272    
273 root 1.31 =item ecb_inline
274 root 1.29
275 root 1.73 Expands either to (a compiler-specific equivalent of) C<static inline> or
276     to just C<static>, if inline isn't supported. It should be used to declare
277     functions that should be inlined, for code size or speed reasons.
278 root 1.29
279 root 1.104 Example: inline this function, it surely will reduce code size.
280 root 1.29
281 root 1.31 ecb_inline int
282 root 1.29 negmul (int a, int b)
283     {
284     return - (a * b);
285     }
286    
287 root 1.2 =item ecb_noinline
288    
289 sf-exg 1.66 Prevents a function from being inlined - it might be optimised away, but
290 root 1.3 not inlined into other functions. This is useful if you know your function
291     is rarely called and large enough for inlining not to be helpful.
292    
293 root 1.2 =item ecb_noreturn
294    
295 root 1.17 Marks a function as "not returning, ever". Some typical functions that
296     don't return are C<exit> or C<abort> (which really works hard to not
297     return), and now you can make your own:
298    
299     ecb_noreturn void
300     my_abort (const char *errline)
301     {
302     puts (errline);
303     abort ();
304     }
305    
306 sf-exg 1.19 In this case, the compiler would probably be smart enough to deduce it on
307     its own, so this is mainly useful for declarations.
308 root 1.17
309 root 1.53 =item ecb_restrict
310    
311     Expands to the C<restrict> keyword or equivalent on compilers that support
312     them, and to nothing on others. Must be specified on a pointer type or
313     an array index to indicate that the memory doesn't alias with any other
314     restricted pointer in the same scope.
315    
316     Example: multiply a vector, and allow the compiler to parallelise the
317     loop, because it knows it doesn't overwrite input values.
318    
319     void
320 sf-exg 1.61 multiply (ecb_restrict float *src,
321     ecb_restrict float *dst,
322 root 1.53 int len, float factor)
323     {
324     int i;
325    
326     for (i = 0; i < len; ++i)
327     dst [i] = src [i] * factor;
328     }
329    
330 root 1.2 =item ecb_const
331    
332 sf-exg 1.19 Declares that the function only depends on the values of its arguments,
333 root 1.17 much like a mathematical function. It specifically does not read or write
334     any memory any arguments might point to, global variables, or call any
335     non-const functions. It also must not have any side effects.
336    
337     Such a function can be optimised much more aggressively by the compiler -
338     for example, multiple calls with the same arguments can be optimised into
339     a single call, which wouldn't be possible if the compiler would have to
340     expect any side effects.
341    
342     It is best suited for functions in the sense of mathematical functions,
343 sf-exg 1.19 such as a function returning the square root of its input argument.
344 root 1.17
345     Not suited would be a function that calculates the hash of some memory
346     area you pass in, prints some messages or looks at a global variable to
347     decide on rounding.
348    
349     See C<ecb_pure> for a slightly less restrictive class of functions.
350    
351 root 1.2 =item ecb_pure
352    
353 root 1.17 Similar to C<ecb_const>, declares a function that has no side
354     effects. Unlike C<ecb_const>, the function is allowed to examine global
355     variables and any other memory areas (such as the ones passed to it via
356     pointers).
357    
358     While these functions cannot be optimised as aggressively as C<ecb_const>
359     functions, they can still be optimised away in many occasions, and the
360     compiler has more freedom in moving calls to them around.
361    
362     Typical examples for such functions would be C<strlen> or C<memcmp>. A
363     function that calculates the MD5 sum of some input and updates some MD5
364     state passed as argument would I<NOT> be pure, however, as it would modify
365     some memory area that is not the return value.
366    
367 root 1.2 =item ecb_hot
368    
369 root 1.17 This declares a function as "hot" with regards to the cache - the function
370     is used so often, that it is very beneficial to keep it in the cache if
371     possible.
372    
373     The compiler reacts by trying to place hot functions near to each other in
374     memory.
375    
376 sf-exg 1.19 Whether a function is hot or not often depends on the whole program,
377 root 1.17 and less on the function itself. C<ecb_cold> is likely more useful in
378     practise.
379    
380 root 1.2 =item ecb_cold
381    
382 root 1.17 The opposite of C<ecb_hot> - declares a function as "cold" with regards to
383     the cache, or in other words, this function is not called often, or not at
384     speed-critical times, and keeping it in the cache might be a waste of said
385     cache.
386    
387     In addition to placing cold functions together (or at least away from hot
388     functions), this knowledge can be used in other ways, for example, the
389 root 1.104 function will be optimised for size, as opposed to speed, and code paths
390 root 1.17 leading to calls to those functions can automatically be marked as if
391 root 1.27 C<ecb_expect_false> had been used to reach them.
392 root 1.17
393     Good examples for such functions would be error reporting functions, or
394     functions only called in exceptional or rare cases.
395    
396 root 1.2 =item ecb_artificial
397    
398 root 1.17 Declares the function as "artificial", in this case meaning that this
399 root 1.52 function is not really meant to be a function, but more like an accessor
400 root 1.17 - many methods in C++ classes are mere accessor functions, and having a
401     crash reported in such a method, or single-stepping through them, is not
402     usually so helpful, especially when it's inlined to just a few instructions.
403    
404     Marking them as artificial will instruct the debugger about just this,
405     leading to happier debugging and thus happier lives.
406    
407     Example: in some kind of smart-pointer class, mark the pointer accessor as
408     artificial, so that the whole class acts more like a pointer and less like
409     some C++ abstraction monster.
410    
411     template<typename T>
412     struct my_smart_ptr
413     {
414     T *value;
415    
416     ecb_artificial
417     operator T *()
418     {
419     return value;
420     }
421     };
422    
423 root 1.2 =back
424 root 1.1
425     =head2 OPTIMISATION HINTS
426    
427 root 1.88 =over
428 root 1.1
429 root 1.58 =item bool ecb_is_constant (expr)
430 root 1.1
431 root 1.3 Returns true iff the expression can be deduced to be a compile-time
432     constant, and false otherwise.
433    
434     For example, when you have a C<rndm16> function that returns a 16 bit
435     random number, and you have a function that maps this to a range from
436 root 1.5 0..n-1, then you could use this inline function in a header file:
437 root 1.3
438     ecb_inline uint32_t
439     rndm (uint32_t n)
440     {
441 root 1.6 return (n * (uint32_t)rndm16 ()) >> 16;
442 root 1.3 }
443    
444     However, for powers of two, you could use a normal mask, but that is only
445     worth it if, at compile time, you can detect this case. This is the case
446     when the passed number is a constant and also a power of two (C<n & (n -
447     1) == 0>):
448    
449     ecb_inline uint32_t
450     rndm (uint32_t n)
451     {
452     return is_constant (n) && !(n & (n - 1))
453     ? rndm16 () & (num - 1)
454 root 1.6 : (n * (uint32_t)rndm16 ()) >> 16;
455 root 1.3 }
456    
457 root 1.62 =item ecb_expect (expr, value)
458 root 1.1
459 root 1.7 Evaluates C<expr> and returns it. In addition, it tells the compiler that
460     the C<expr> evaluates to C<value> a lot, which can be used for static
461     branch optimisations.
462 root 1.1
463 root 1.27 Usually, you want to use the more intuitive C<ecb_expect_true> and
464     C<ecb_expect_false> functions instead.
465 root 1.1
466 root 1.27 =item bool ecb_expect_true (cond)
467 root 1.1
468 root 1.27 =item bool ecb_expect_false (cond)
469 root 1.1
470 root 1.7 These two functions expect a expression that is true or false and return
471     C<1> or C<0>, respectively, so when used in the condition of an C<if> or
472     other conditional statement, it will not change the program:
473    
474     /* these two do the same thing */
475     if (some_condition) ...;
476 root 1.27 if (ecb_expect_true (some_condition)) ...;
477 root 1.7
478 root 1.27 However, by using C<ecb_expect_true>, you tell the compiler that the
479     condition is likely to be true (and for C<ecb_expect_false>, that it is
480     unlikely to be true).
481 root 1.7
482 root 1.9 For example, when you check for a null pointer and expect this to be a
483 root 1.27 rare, exceptional, case, then use C<ecb_expect_false>:
484 root 1.7
485     void my_free (void *ptr)
486     {
487 root 1.27 if (ecb_expect_false (ptr == 0))
488 root 1.7 return;
489     }
490    
491     Consequent use of these functions to mark away exceptional cases or to
492     tell the compiler what the hot path through a function is can increase
493     performance considerably.
494    
495 root 1.27 You might know these functions under the name C<likely> and C<unlikely>
496     - while these are common aliases, we find that the expect name is easier
497     to understand when quickly skimming code. If you wish, you can use
498     C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of
499     C<ecb_expect_false> - these are simply aliases.
500    
501 root 1.7 A very good example is in a function that reserves more space for some
502     memory block (for example, inside an implementation of a string stream) -
503 root 1.9 each time something is added, you have to check for a buffer overrun, but
504 root 1.7 you expect that most checks will turn out to be false:
505    
506     /* make sure we have "size" extra room in our buffer */
507     ecb_inline void
508     reserve (int size)
509     {
510 root 1.27 if (ecb_expect_false (current + size > end))
511 root 1.7 real_reserve_method (size); /* presumably noinline */
512     }
513    
514 root 1.62 =item ecb_assume (cond)
515 root 1.7
516 sf-exg 1.66 Tries to tell the compiler that some condition is true, even if it's not
517 root 1.65 obvious. This is not a function, but a statement: it cannot be used in
518     another expression.
519 root 1.7
520     This can be used to teach the compiler about invariants or other
521     conditions that might improve code generation, but which are impossible to
522     deduce form the code itself.
523    
524 root 1.27 For example, the example reservation function from the C<ecb_expect_false>
525 root 1.7 description could be written thus (only C<ecb_assume> was added):
526    
527     ecb_inline void
528     reserve (int size)
529     {
530 root 1.27 if (ecb_expect_false (current + size > end))
531 root 1.7 real_reserve_method (size); /* presumably noinline */
532    
533     ecb_assume (current + size <= end);
534     }
535    
536     If you then call this function twice, like this:
537    
538     reserve (10);
539     reserve (1);
540    
541     Then the compiler I<might> be able to optimise out the second call
542     completely, as it knows that C<< current + 1 > end >> is false and the
543     call will never be executed.
544    
545 root 1.62 =item ecb_unreachable ()
546 root 1.7
547     This function does nothing itself, except tell the compiler that it will
548 root 1.9 never be executed. Apart from suppressing a warning in some cases, this
549 root 1.65 function can be used to implement C<ecb_assume> or similar functionality.
550 root 1.7
551 root 1.62 =item ecb_prefetch (addr, rw, locality)
552 root 1.7
553 root 1.104 Tells the compiler to try to prefetch memory at the given I<addr>ess
554     for either reading (I<rw> = 0) or writing (I<rw> = 1). A I<locality> of
555 root 1.7 C<0> means that there will only be one access later, C<3> means that
556     the data will likely be accessed very often, and values in between mean
557     something... in between. The memory pointed to by the address does not
558     need to be accessible (it could be a null pointer for example), but C<rw>
559     and C<locality> must be compile-time constants.
560    
561 root 1.65 This is a statement, not a function: you cannot use it as part of an
562     expression.
563    
564 root 1.7 An obvious way to use this is to prefetch some data far away, in a big
565 root 1.9 array you loop over. This prefetches memory some 128 array elements later,
566 root 1.7 in the hope that it will be ready when the CPU arrives at that location.
567    
568     int sum = 0;
569    
570     for (i = 0; i < N; ++i)
571     {
572     sum += arr [i]
573     ecb_prefetch (arr + i + 128, 0, 0);
574     }
575    
576     It's hard to predict how far to prefetch, and most CPUs that can prefetch
577     are often good enough to predict this kind of behaviour themselves. It
578     gets more interesting with linked lists, especially when you do some fair
579     processing on each list element:
580    
581     for (node *n = start; n; n = n->next)
582     {
583     ecb_prefetch (n->next, 0, 0);
584     ... do medium amount of work with *n
585     }
586    
587     After processing the node, (part of) the next node might already be in
588     cache.
589 root 1.1
590 root 1.2 =back
591 root 1.1
592 root 1.36 =head2 BIT FIDDLING / BIT WIZARDRY
593 root 1.1
594 root 1.88 =over
595 root 1.4
596 root 1.3 =item bool ecb_big_endian ()
597    
598     =item bool ecb_little_endian ()
599    
600 sf-exg 1.11 These two functions return true if the byte order is big endian
601     (most-significant byte first) or little endian (least-significant byte
602     first) respectively.
603    
604 root 1.24 On systems that are neither, their return values are unspecified.
605    
606 root 1.3 =item int ecb_ctz32 (uint32_t x)
607    
608 root 1.35 =item int ecb_ctz64 (uint64_t x)
609    
610 root 1.77 =item int ecb_ctz (T x) [C++]
611    
612 sf-exg 1.11 Returns the index of the least significant bit set in C<x> (or
613 root 1.24 equivalently the number of bits set to 0 before the least significant bit
614 root 1.35 set), starting from 0. If C<x> is 0 the result is undefined.
615    
616 root 1.36 For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>.
617    
618 root 1.77 The overloaded C++ C<ecb_ctz> function supports C<uint8_t>, C<uint16_t>,
619     C<uint32_t> and C<uint64_t> types.
620    
621 root 1.35 For example:
622 sf-exg 1.11
623 root 1.15 ecb_ctz32 (3) = 0
624     ecb_ctz32 (6) = 1
625 sf-exg 1.11
626 root 1.105 =item int ecb_clz32 (uint32_t x)
627    
628     =item int ecb_clz64 (uint64_t x)
629    
630     =item int ecb_clz (T x) [C++]
631    
632     Counts the number of leading zero bits in C<x>. If C<x> is 0 the result is
633     undefined.
634    
635     The overloaded C++ C<ecb_clz> function supports C<uint32_t> and
636     C<uint64_t> types only.
637    
638     It is often simpler to use one of the C<ecb_ld*> functions instead, whoise
639     result only depends on the value and not the size of the type.
640    
641     For example:
642    
643     ecb_clz32 (3) = 30
644     ecb_clz32 (6) = 29
645    
646 root 1.41 =item bool ecb_is_pot32 (uint32_t x)
647    
648     =item bool ecb_is_pot64 (uint32_t x)
649    
650 root 1.77 =item bool ecb_is_pot (T x) [C++]
651    
652 sf-exg 1.66 Returns true iff C<x> is a power of two or C<x == 0>.
653 root 1.41
654 sf-exg 1.66 For smaller types than C<uint32_t> you can safely use C<ecb_is_pot32>.
655 root 1.41
656 root 1.77 The overloaded C++ C<ecb_is_pot> function supports C<uint8_t>, C<uint16_t>,
657     C<uint32_t> and C<uint64_t> types.
658    
659 root 1.35 =item int ecb_ld32 (uint32_t x)
660    
661     =item int ecb_ld64 (uint64_t x)
662    
663 root 1.77 =item int ecb_ld64 (T x) [C++]
664    
665 root 1.35 Returns the index of the most significant bit set in C<x>, or the number
666     of digits the number requires in binary (so that C<< 2**ld <= x <
667     2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is
668     to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for
669     example to see how many bits a certain number requires to be encoded.
670    
671     This function is similar to the "count leading zero bits" function, except
672     that that one returns how many zero bits are "in front" of the number (in
673     the given data type), while C<ecb_ld> returns how many bits the number
674     itself requires.
675    
676 root 1.36 For smaller types than C<uint32_t> you can safely use C<ecb_ld32>.
677    
678 root 1.77 The overloaded C++ C<ecb_ld> function supports C<uint8_t>, C<uint16_t>,
679     C<uint32_t> and C<uint64_t> types.
680    
681 root 1.3 =item int ecb_popcount32 (uint32_t x)
682    
683 root 1.35 =item int ecb_popcount64 (uint64_t x)
684    
685 root 1.77 =item int ecb_popcount (T x) [C++]
686    
687 root 1.36 Returns the number of bits set to 1 in C<x>.
688    
689     For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>.
690    
691 root 1.77 The overloaded C++ C<ecb_popcount> function supports C<uint8_t>, C<uint16_t>,
692     C<uint32_t> and C<uint64_t> types.
693    
694 root 1.36 For example:
695 sf-exg 1.11
696 root 1.15 ecb_popcount32 (7) = 3
697     ecb_popcount32 (255) = 8
698 sf-exg 1.11
699 root 1.39 =item uint8_t ecb_bitrev8 (uint8_t x)
700    
701     =item uint16_t ecb_bitrev16 (uint16_t x)
702    
703     =item uint32_t ecb_bitrev32 (uint32_t x)
704    
705 root 1.77 =item T ecb_bitrev (T x) [C++]
706    
707 root 1.39 Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1
708     and so on.
709    
710 root 1.77 The overloaded C++ C<ecb_bitrev> function supports C<uint8_t>, C<uint16_t> and C<uint32_t> types.
711    
712 root 1.39 Example:
713    
714     ecb_bitrev8 (0xa7) = 0xea
715     ecb_bitrev32 (0xffcc4411) = 0x882233ff
716    
717 root 1.77 =item T ecb_bitrev (T x) [C++]
718    
719     Overloaded C++ bitrev function.
720    
721     C<T> must be one of C<uint8_t>, C<uint16_t> or C<uint32_t>.
722    
723 root 1.8 =item uint32_t ecb_bswap16 (uint32_t x)
724    
725 root 1.3 =item uint32_t ecb_bswap32 (uint32_t x)
726    
727 root 1.34 =item uint64_t ecb_bswap64 (uint64_t x)
728 sf-exg 1.13
729 root 1.78 =item T ecb_bswap (T x)
730    
731 root 1.34 These functions return the value of the 16-bit (32-bit, 64-bit) value
732     C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in
733     C<ecb_bswap32>).
734    
735 root 1.77 The overloaded C++ C<ecb_bswap> function supports C<uint8_t>, C<uint16_t>,
736     C<uint32_t> and C<uint64_t> types.
737 root 1.76
738 root 1.34 =item uint8_t ecb_rotl8 (uint8_t x, unsigned int count)
739    
740     =item uint16_t ecb_rotl16 (uint16_t x, unsigned int count)
741 root 1.3
742     =item uint32_t ecb_rotl32 (uint32_t x, unsigned int count)
743    
744 root 1.34 =item uint64_t ecb_rotl64 (uint64_t x, unsigned int count)
745    
746     =item uint8_t ecb_rotr8 (uint8_t x, unsigned int count)
747    
748     =item uint16_t ecb_rotr16 (uint16_t x, unsigned int count)
749    
750     =item uint32_t ecb_rotr32 (uint32_t x, unsigned int count)
751    
752 root 1.33 =item uint64_t ecb_rotr64 (uint64_t x, unsigned int count)
753    
754 root 1.34 These two families of functions return the value of C<x> after rotating
755     all the bits by C<count> positions to the right (C<ecb_rotr>) or left
756 root 1.94 (C<ecb_rotl>). There are no restrictions on the value C<count>, i.e. both
757 root 1.95 zero and values equal or larger than the word width work correctly. Also,
758     notwithstanding C<count> being unsigned, negative numbers work and shift
759     to the opposite direction.
760 root 1.93
761 root 1.85 Current GCC/clang versions understand these functions and usually compile
762     them to "optimal" code (e.g. a single C<rol> or a combination of C<shld>
763     on x86).
764 root 1.20
765 root 1.77 =item T ecb_rotl (T x, unsigned int count) [C++]
766    
767     =item T ecb_rotr (T x, unsigned int count) [C++]
768    
769     Overloaded C++ rotl/rotr functions.
770    
771     C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t>.
772    
773 root 1.102 =item uint_fast8_t ecb_gray8_encode (uint_fast8_t b)
774    
775     =item uint_fast16_t ecb_gray16_encode (uint_fast16_t b)
776    
777     =item uint_fast32_t ecb_gray32_encode (uint_fast32_t b)
778    
779     =item uint_fast64_t ecb_gray64_encode (uint_fast64_t b)
780    
781     Encode an unsigned into its corresponding (reflective) gray code - the
782     kind of gray code meant when just talking about "gray code". These
783     functions are very fast and all have identical implementation, so there is
784 root 1.103 no need to use a smaller type, as long as your CPU can handle it natively.
785 root 1.102
786     =item T ecb_gray_encode (T b) [C++]
787    
788     Overloaded C++ version of the above, for C<uint{8,16,32,64}_t>.
789    
790     =item uint_fast8_t ecb_gray8_decode (uint_fast8_t b)
791    
792     =item uint_fast16_t ecb_gray16_decode (uint_fast16_t b)
793    
794     =item uint_fast32_t ecb_gray32_decode (uint_fast32_t b)
795    
796     =item uint_fast64_t ecb_gray64_decode (uint_fast64_t b)
797    
798 root 1.103 Decode a gray code back into linear index form (the reverse of
799     C<ecb_gray*_encode>. Unlike the encode functions, the decode functions
800     have higher time complexity for larger types, so it can pay off to use a
801 root 1.102 smaller type here.
802    
803     =item T ecb_gray_decode (T b) [C++]
804    
805     Overloaded C++ version of the above, for C<uint{8,16,32,64}_t>.
806    
807 root 1.3 =back
808 root 1.1
809 root 1.104 =head2 HILBERT CURVES
810    
811     These functions deal with (square, pseudo) Hilbert curves. The parameter
812     I<order> indicates the size of the square and is specified in bits, that
813     means for order C<8>, the coordinates range from C<0>..C<255>, and the
814     curve index ranges from C<0>..C<65535>.
815    
816     The 32 bit variants of these functions map a 32 bit index to two 16 bit
817     coordinates, stored in a 32 bit variable, where the high order bits are
818     the x-coordinate, and the low order bits are the y-coordinate, thus,
819     these functions map 32 bit linear index on the curve to a 32 bit packed
820     coordinate pair, and vice versa.
821    
822     The 64 bit variants work similarly.
823    
824     The I<order> can go from C<1> to C<16> for the 32 bit curve, and C<1> to
825     C<32> for the 64 bit curve.
826    
827     When going from one order to the next higher order, these functions
828     replace the curve segments by smaller versions of the generating shape,
829     while doubling the size (since they use integer coordinates), which is
830     what you would expect mathematically. This means that the curve will be
831     mirrored at the diagonal. If your goal is to simply cover more area while
832     retaining existing point coordinates you should increase or decrease the
833     I<order> by C<2> or, in the case of C<ecb_hilbert2d_index_to_coord>,
834     simply specify the maximum I<order> of C<16> or C<32>, respectively, as
835     these are constant-time.
836    
837     =over
838    
839     =item uint32_t ecb_hilbert2d_index_to_coord32 (int order, uint32_t index)
840    
841     =item uint64_t ecb_hilbert2d_index_to_coord64 (int order, uint64_t index)
842    
843     Map a point on a pseudo Hilbert curve from its linear distance from the
844     origin on the curve to a x|y coordinate pair. The result is a packed
845     coordinate pair, to get the actual x and < coordinates, you could do
846     something like this:
847    
848     uint32_t xy = ecb_hilbert2d_index_to_coord32 (16, 255);
849     uint16_t x = xy >> 16;
850     uint16_t y = xy & 0xffffU;
851    
852     uint64_t xy = ecb_hilbert2d_index_to_coord64 (32, 255);
853     uint32_t x = xy >> 32;
854     uint32_t y = xy & 0xffffffffU;
855    
856     These functions work in constant time, so for many applications it is
857     preferable to simply hard-code the order to the maximum (C<16> or C<32>).
858    
859     This (production-ready, i.e. never run) example generates an SVG image of
860     an order 8 pseudo Hilbert curve:
861    
862     printf ("<svg xmlns='http://www.w3.org/2000/svg' width='%d' height='%d'>\n", 64 * 8, 64 * 8);
863     printf ("<g transform='translate(4) scale(8)' stroke-width='0.25' stroke='black'>\n");
864     for (uint32_t i = 0; i < 64*64 - 1; ++i)
865     {
866     uint32_t p1 = ecb_hilbert2d_index_to_coord32 (6, i );
867     uint32_t p2 = ecb_hilbert2d_index_to_coord32 (6, i + 1);
868     printf ("<line x1='%d' y1='%d' x2='%d' y2='%d'/>\n",
869     p1 >> 16, p1 & 0xffff,
870     p2 >> 16, p2 & 0xffff);
871     }
872     printf ("</g>\n");
873     printf ("</svg>\n");
874    
875     =item uint32_t ecb_hilbert2d_coord_to_index32 (int order, uint32_t xy)
876    
877     =item uint64_t ecb_hilbert2d_coord_to_index64 (int order, uint64_t xy)
878    
879     The reverse of C<ecb_hilbert2d_index_to_coord> - map a packed pair of
880     coordinates to their linear index on the pseudo Hilbert curve of order
881     I<order>.
882    
883     They are an exact inverse of the C<ecb_hilbert2d_coord_to_index> functions
884     for the same I<order>:
885    
886     assert (
887     u == ecb_hilbert2d_coord_to_index (32,
888     ecb_hilbert2d_index_to_coord32 (32,
889     u)));
890    
891     Packing coordinates is done the same way, as well, from I<x> and I<y>:
892    
893     uint32_t xy = ((uint32_t)x << 16) | y; // for ecb_hilbert2d_coord_to_index32
894     uint64_t xy = ((uint64_t)x << 32) | y; // for ecb_hilbert2d_coord_to_index64
895    
896     Unlike C<ecb_hilbert2d_coord_to_index>, these functions are O(I<order>),
897     so it is preferable to use the lowest possible order.
898    
899     =back
900    
901 root 1.96 =head2 BIT MIXING, HASHING
902    
903     Sometimes you have an integer and want to distribute its bits well, for
904 root 1.104 example, to use it as a hash in a hash table. A common example is pointer
905 root 1.96 values, which often only have a limited range (e.g. low and high bits are
906     often zero).
907    
908     The following functions try to mix the bits to get a good bias-free
909     distribution. They were mainly made for pointers, but the underlying
910     integer functions are exposed as well.
911    
912     As an added benefit, the functions are reversible, so if you find it
913     convenient to store only the hash value, you can recover the original
914 root 1.104 pointer from the hash ("unmix"), as long as your pointers are 32 or 64 bit
915 root 1.96 (if this isn't the case on your platform, drop us a note and we will add
916     functions for other bit widths).
917    
918     The unmix functions are very slightly slower than the mix functions, so
919     it is equally very slightly preferable to store the original values wehen
920     convenient.
921    
922     The underlying algorithm if subject to change, so currently these
923     functions are not suitable for persistent hash tables, as their result
924 root 1.104 value can change between different versions of libecb.
925 root 1.96
926     =over
927    
928     =item uintptr_t ecb_ptrmix (void *ptr)
929    
930     Mixes the bits of a pointer so the result is suitable for hash table
931     lookups. In other words, this hashes the pointer value.
932    
933     =item uintptr_t ecb_ptrmix (T *ptr) [C++]
934    
935     Overload the C<ecb_ptrmix> function to work for any pointer in C++.
936    
937     =item void *ecb_ptrunmix (uintptr_t v)
938    
939     Unmix the hash value into the original pointer. This only works as long
940     as the hash value is not truncated, i.e. you used C<uintptr_t> (or
941     equivalent) throughout to store it.
942    
943     =item T *ecb_ptrunmix<T> (uintptr_t v) [C++]
944    
945     The somewhat less useful template version of C<ecb_ptrunmix> for
946     C++. Example:
947    
948     sometype *myptr;
949     uintptr_t hash = ecb_ptrmix (myptr);
950     sometype *orig = ecb_ptrunmix<sometype> (hash);
951    
952     =item uint32_t ecb_mix32 (uint32_t v)
953    
954     =item uint64_t ecb_mix64 (uint64_t v)
955    
956     Sometimes you don't have a pointer but an integer whose values are very
957 root 1.104 badly distributed. In this case you can use these integer versions of the
958 root 1.96 mixing function. No C++ template is provided currently.
959    
960     =item uint32_t ecb_unmix32 (uint32_t v)
961    
962     =item uint64_t ecb_unmix64 (uint64_t v)
963    
964     The reverse of the C<ecb_mix> functions - they take a mixed/hashed value
965     and recover the original value.
966    
967     =back
968    
969 root 1.76 =head2 HOST ENDIANNESS CONVERSION
970    
971 root 1.88 =over
972 root 1.76
973     =item uint_fast16_t ecb_be_u16_to_host (uint_fast16_t v)
974    
975     =item uint_fast32_t ecb_be_u32_to_host (uint_fast32_t v)
976    
977     =item uint_fast64_t ecb_be_u64_to_host (uint_fast64_t v)
978    
979     =item uint_fast16_t ecb_le_u16_to_host (uint_fast16_t v)
980    
981     =item uint_fast32_t ecb_le_u32_to_host (uint_fast32_t v)
982    
983     =item uint_fast64_t ecb_le_u64_to_host (uint_fast64_t v)
984    
985     Convert an unsigned 16, 32 or 64 bit value from big or little endian to host byte order.
986    
987     The naming convention is C<ecb_>(C<be>|C<le>)C<_u>C<16|32|64>C<_to_host>,
988 root 1.79 where C<be> and C<le> stand for big endian and little endian, respectively.
989 root 1.76
990     =item uint_fast16_t ecb_host_to_be_u16 (uint_fast16_t v)
991    
992     =item uint_fast32_t ecb_host_to_be_u32 (uint_fast32_t v)
993    
994     =item uint_fast64_t ecb_host_to_be_u64 (uint_fast64_t v)
995    
996     =item uint_fast16_t ecb_host_to_le_u16 (uint_fast16_t v)
997    
998     =item uint_fast32_t ecb_host_to_le_u32 (uint_fast32_t v)
999    
1000     =item uint_fast64_t ecb_host_to_le_u64 (uint_fast64_t v)
1001    
1002     Like above, but converts I<from> host byte order to the specified
1003     endianness.
1004    
1005     =back
1006    
1007 root 1.77 In C++ the following additional template functions are supported:
1008 root 1.76
1009 root 1.88 =over
1010 root 1.76
1011     =item T ecb_be_to_host (T v)
1012    
1013     =item T ecb_le_to_host (T v)
1014    
1015     =item T ecb_host_to_be (T v)
1016    
1017     =item T ecb_host_to_le (T v)
1018    
1019 root 1.86 =back
1020    
1021 root 1.77 These functions work like their C counterparts, above, but use templates,
1022     which make them useful in generic code.
1023 root 1.76
1024     C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t>
1025     (so unlike their C counterparts, there is a version for C<uint8_t>, which
1026     again can be useful in generic code).
1027    
1028     =head2 UNALIGNED LOAD/STORE
1029    
1030     These function load or store unaligned multi-byte values.
1031    
1032 root 1.88 =over
1033 root 1.76
1034     =item uint_fast16_t ecb_peek_u16_u (const void *ptr)
1035    
1036     =item uint_fast32_t ecb_peek_u32_u (const void *ptr)
1037    
1038     =item uint_fast64_t ecb_peek_u64_u (const void *ptr)
1039    
1040     These functions load an unaligned, unsigned 16, 32 or 64 bit value from
1041     memory.
1042    
1043     =item uint_fast16_t ecb_peek_be_u16_u (const void *ptr)
1044    
1045     =item uint_fast32_t ecb_peek_be_u32_u (const void *ptr)
1046    
1047     =item uint_fast64_t ecb_peek_be_u64_u (const void *ptr)
1048    
1049     =item uint_fast16_t ecb_peek_le_u16_u (const void *ptr)
1050    
1051     =item uint_fast32_t ecb_peek_le_u32_u (const void *ptr)
1052    
1053     =item uint_fast64_t ecb_peek_le_u64_u (const void *ptr)
1054    
1055     Like above, but additionally convert from big endian (C<be>) or little
1056     endian (C<le>) byte order to host byte order while doing so.
1057    
1058     =item ecb_poke_u16_u (void *ptr, uint16_t v)
1059    
1060     =item ecb_poke_u32_u (void *ptr, uint32_t v)
1061    
1062     =item ecb_poke_u64_u (void *ptr, uint64_t v)
1063    
1064     These functions store an unaligned, unsigned 16, 32 or 64 bit value to
1065     memory.
1066    
1067     =item ecb_poke_be_u16_u (void *ptr, uint_fast16_t v)
1068    
1069     =item ecb_poke_be_u32_u (void *ptr, uint_fast32_t v)
1070    
1071     =item ecb_poke_be_u64_u (void *ptr, uint_fast64_t v)
1072    
1073     =item ecb_poke_le_u16_u (void *ptr, uint_fast16_t v)
1074    
1075     =item ecb_poke_le_u32_u (void *ptr, uint_fast32_t v)
1076    
1077     =item ecb_poke_le_u64_u (void *ptr, uint_fast64_t v)
1078    
1079     Like above, but additionally convert from host byte order to big endian
1080     (C<be>) or little endian (C<le>) byte order while doing so.
1081    
1082     =back
1083    
1084 root 1.77 In C++ the following additional template functions are supported:
1085 root 1.76
1086 root 1.88 =over
1087 root 1.76
1088 root 1.80 =item T ecb_peek<T> (const void *ptr)
1089 root 1.76
1090 root 1.80 =item T ecb_peek_be<T> (const void *ptr)
1091 root 1.76
1092 root 1.80 =item T ecb_peek_le<T> (const void *ptr)
1093 root 1.76
1094 root 1.80 =item T ecb_peek_u<T> (const void *ptr)
1095 root 1.76
1096 root 1.80 =item T ecb_peek_be_u<T> (const void *ptr)
1097 root 1.76
1098 root 1.80 =item T ecb_peek_le_u<T> (const void *ptr)
1099 root 1.76
1100     Similarly to their C counterparts, these functions load an unsigned 8, 16,
1101     32 or 64 bit value from memory, with optional conversion from big/little
1102     endian.
1103    
1104 root 1.80 Since the type cannot be deduced, it has to be specified explicitly, e.g.
1105 root 1.76
1106     uint_fast16_t v = ecb_peek<uint16_t> (ptr);
1107    
1108     C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t>.
1109    
1110     Unlike their C counterparts, these functions support 8 bit quantities
1111     (C<uint8_t>) and also have an aligned version (without the C<_u> prefix),
1112     all of which hopefully makes them more useful in generic code.
1113    
1114     =item ecb_poke (void *ptr, T v)
1115    
1116     =item ecb_poke_be (void *ptr, T v)
1117    
1118     =item ecb_poke_le (void *ptr, T v)
1119    
1120     =item ecb_poke_u (void *ptr, T v)
1121    
1122     =item ecb_poke_be_u (void *ptr, T v)
1123    
1124     =item ecb_poke_le_u (void *ptr, T v)
1125    
1126     Again, similarly to their C counterparts, these functions store an
1127 root 1.104 unsigned 8, 16, 32 or 64 bit value to memory, with optional conversion to
1128 root 1.76 big/little endian.
1129    
1130     C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t>.
1131    
1132     Unlike their C counterparts, these functions support 8 bit quantities
1133     (C<uint8_t>) and also have an aligned version (without the C<_u> prefix),
1134     all of which hopefully makes them more useful in generic code.
1135    
1136     =back
1137    
1138 root 1.89 =head2 FAST INTEGER TO STRING
1139    
1140     Libecb defines a set of very fast integer to decimal string (or integer
1141 root 1.104 to ASCII, short C<i2a>) functions. These work by converting the integer
1142 root 1.89 to a fixed point representation and then successively multiplying out
1143     the topmost digits. Unlike some other, also very fast, libraries, ecb's
1144     algorithm should be completely branchless per digit, and does not rely on
1145 root 1.104 the presence of special CPU functions (such as C<clz>).
1146 root 1.89
1147     There is a high level API that takes an C<int32_t>, C<uint32_t>,
1148     C<int64_t> or C<uint64_t> as argument, and a low-level API, which is
1149     harder to use but supports slightly more formatting options.
1150    
1151     =head3 HIGH LEVEL API
1152    
1153     The high level API consists of four functions, one each for C<int32_t>,
1154     C<uint32_t>, C<int64_t> and C<uint64_t>:
1155    
1156 root 1.91 Example:
1157    
1158     char buf[ECB_I2A_MAX_DIGITS + 1];
1159     char *end = ecb_i2a_i32 (buf, 17262);
1160     *end = 0;
1161     // buf now contains "17262"
1162    
1163 root 1.89 =over
1164    
1165     =item ECB_I2A_I32_DIGITS (=11)
1166    
1167     =item char *ecb_i2a_u32 (char *ptr, uint32_t value)
1168    
1169     Takes an C<uint32_t> I<value> and formats it as a decimal number starting
1170     at I<ptr>, using at most C<ECB_I2A_I32_DIGITS> characters. Returns a
1171     pointer to just after the generated string, where you would normally put
1172 sf-exg 1.92 the terminating C<0> character. This function outputs the minimum number
1173 root 1.89 of digits.
1174    
1175     =item ECB_I2A_U32_DIGITS (=10)
1176    
1177     =item char *ecb_i2a_i32 (char *ptr, int32_t value)
1178    
1179     Same as C<ecb_i2a_u32>, but formats a C<int32_t> value, including a minus
1180     sign if needed.
1181    
1182     =item ECB_I2A_I64_DIGITS (=20)
1183    
1184     =item char *ecb_i2a_u64 (char *ptr, uint64_t value)
1185    
1186     =item ECB_I2A_U64_DIGITS (=21)
1187    
1188     =item char *ecb_i2a_i64 (char *ptr, int64_t value)
1189    
1190     Similar to their 32 bit counterparts, these take a 64 bit argument.
1191    
1192 root 1.90 =item ECB_I2A_MAX_DIGITS (=21)
1193 root 1.89
1194 root 1.97 Instead of using a type specific length macro, you can just use
1195 root 1.90 C<ECB_I2A_MAX_DIGITS>, which is good enough for any C<ecb_i2a> function.
1196 root 1.89
1197     =back
1198    
1199     =head3 LOW-LEVEL API
1200    
1201     The functions above use a number of low-level APIs which have some strict
1202 root 1.98 limitations, but can be used as building blocks (studying C<ecb_i2a_i32>
1203 sf-exg 1.92 and related functions is recommended).
1204 root 1.89
1205     There are three families of functions: functions that convert a number
1206     to a fixed number of digits with leading zeroes (C<ecb_i2a_0N>, C<0>
1207     for "leading zeroes"), functions that generate up to N digits, skipping
1208     leading zeroes (C<_N>), and functions that can generate more digits, but
1209     the leading digit has limited range (C<_xN>).
1210    
1211 sf-exg 1.92 None of the functions deal with negative numbers.
1212 root 1.89
1213 root 1.104 Example: convert an IP address in an C<uint32_t> into dotted-quad:
1214 root 1.91
1215     uint32_t ip = 0x0a000164; // 10.0.1.100
1216     char ips[3 * 4 + 3 + 1];
1217     char *ptr = ips;
1218     ptr = ecb_i2a_3 (ptr, ip >> 24 ); *ptr++ = '.';
1219     ptr = ecb_i2a_3 (ptr, (ip >> 16) & 0xff); *ptr++ = '.';
1220     ptr = ecb_i2a_3 (ptr, (ip >> 8) & 0xff); *ptr++ = '.';
1221     ptr = ecb_i2a_3 (ptr, ip & 0xff); *ptr++ = 0;
1222     printf ("ip: %s\n", ips); // prints "ip: 10.0.1.100"
1223    
1224 root 1.89 =over
1225    
1226     =item char *ecb_i2a_02 (char *ptr, uint32_t value) // 32 bit
1227    
1228     =item char *ecb_i2a_03 (char *ptr, uint32_t value) // 32 bit
1229    
1230     =item char *ecb_i2a_04 (char *ptr, uint32_t value) // 32 bit
1231    
1232     =item char *ecb_i2a_05 (char *ptr, uint32_t value) // 64 bit
1233    
1234     =item char *ecb_i2a_06 (char *ptr, uint32_t value) // 64 bit
1235    
1236     =item char *ecb_i2a_07 (char *ptr, uint32_t value) // 64 bit
1237    
1238     =item char *ecb_i2a_08 (char *ptr, uint32_t value) // 64 bit
1239    
1240     =item char *ecb_i2a_09 (char *ptr, uint32_t value) // 64 bit
1241    
1242 root 1.101 The C<< ecb_i2a_0I<N> >> functions take an unsigned I<value> and convert
1243 root 1.89 them to exactly I<N> digits, returning a pointer to the first character
1244     after the digits. The I<value> must be in range. The functions marked with
1245     I<32 bit> do their calculations internally in 32 bit, the ones marked with
1246     I<64 bit> internally use 64 bit integers, which might be slow on 32 bit
1247     architectures (the high level API decides on 32 vs. 64 bit versions using
1248     C<ECB_64BIT_NATIVE>).
1249    
1250     =item char *ecb_i2a_2 (char *ptr, uint32_t value) // 32 bit
1251    
1252     =item char *ecb_i2a_3 (char *ptr, uint32_t value) // 32 bit
1253    
1254     =item char *ecb_i2a_4 (char *ptr, uint32_t value) // 32 bit
1255    
1256     =item char *ecb_i2a_5 (char *ptr, uint32_t value) // 64 bit
1257    
1258     =item char *ecb_i2a_6 (char *ptr, uint32_t value) // 64 bit
1259    
1260     =item char *ecb_i2a_7 (char *ptr, uint32_t value) // 64 bit
1261    
1262     =item char *ecb_i2a_8 (char *ptr, uint32_t value) // 64 bit
1263    
1264     =item char *ecb_i2a_9 (char *ptr, uint32_t value) // 64 bit
1265    
1266 root 1.101 Similarly, the C<< ecb_i2a_I<N> >> functions take an unsigned I<value>
1267 root 1.89 and convert them to at most I<N> digits, suppressing leading zeroes, and
1268     returning a pointer to the first character after the digits.
1269    
1270     =item ECB_I2A_MAX_X5 (=59074)
1271    
1272     =item char *ecb_i2a_x5 (char *ptr, uint32_t value) // 32 bit
1273    
1274     =item ECB_I2A_MAX_X10 (=2932500665)
1275    
1276     =item char *ecb_i2a_x10 (char *ptr, uint32_t value) // 64 bit
1277    
1278 root 1.101 The C<< ecb_i2a_xI<N> >> functions are similar to the C<< ecb_i2a_I<N> >>
1279 root 1.89 functions, but they can generate one digit more, as long as the number
1280     is within range, which is given by the symbols C<ECB_I2A_MAX_X5> (almost
1281     16 bit range) and C<ECB_I2A_MAX_X10> (a bit more than 31 bit range),
1282     respectively.
1283    
1284 sf-exg 1.92 For example, the digit part of a 32 bit signed integer just fits into the
1285 root 1.89 C<ECB_I2A_MAX_X10> range, so while C<ecb_i2a_x10> cannot convert a 10
1286     digit number, it can convert all 32 bit signed numbers. Sadly, it's not
1287     good enough for 32 bit unsigned numbers.
1288    
1289     =back
1290    
1291 root 1.50 =head2 FLOATING POINT FIDDLING
1292    
1293 root 1.88 =over
1294 root 1.50
1295 root 1.71 =item ECB_INFINITY [-UECB_NO_LIBM]
1296 root 1.62
1297     Evaluates to positive infinity if supported by the platform, otherwise to
1298     a truly huge number.
1299    
1300 root 1.71 =item ECB_NAN [-UECB_NO_LIBM]
1301 root 1.62
1302     Evaluates to a quiet NAN if supported by the platform, otherwise to
1303     C<ECB_INFINITY>.
1304    
1305 root 1.71 =item float ecb_ldexpf (float x, int exp) [-UECB_NO_LIBM]
1306 root 1.62
1307     Same as C<ldexpf>, but always available.
1308    
1309 root 1.71 =item uint32_t ecb_float_to_binary16 (float x) [-UECB_NO_LIBM]
1310    
1311 root 1.50 =item uint32_t ecb_float_to_binary32 (float x) [-UECB_NO_LIBM]
1312    
1313     =item uint64_t ecb_double_to_binary64 (double x) [-UECB_NO_LIBM]
1314    
1315     These functions each take an argument in the native C<float> or C<double>
1316 root 1.71 type and return the IEEE 754 bit representation of it (binary16/half,
1317     binary32/single or binary64/double precision).
1318 root 1.50
1319     The bit representation is just as IEEE 754 defines it, i.e. the sign bit
1320     will be the most significant bit, followed by exponent and mantissa.
1321    
1322     This function should work even when the native floating point format isn't
1323     IEEE compliant, of course at a speed and code size penalty, and of course
1324     also within reasonable limits (it tries to convert NaNs, infinities and
1325     denormals, but will likely convert negative zero to positive zero).
1326    
1327     On all modern platforms (where C<ECB_STDFP> is true), the compiler should
1328 root 1.99 be able to completely optimise away the 32 and 64 bit functions.
1329 root 1.50
1330     These functions can be helpful when serialising floats to the network - you
1331 root 1.71 can serialise the return value like a normal uint16_t/uint32_t/uint64_t.
1332 root 1.50
1333     Another use for these functions is to manipulate floating point values
1334     directly.
1335    
1336     Silly example: toggle the sign bit of a float.
1337    
1338     /* On gcc-4.7 on amd64, */
1339     /* this results in a single add instruction to toggle the bit, and 4 extra */
1340     /* instructions to move the float value to an integer register and back. */
1341    
1342     x = ecb_binary32_to_float (ecb_float_to_binary32 (x) ^ 0x80000000U)
1343    
1344 root 1.58 =item float ecb_binary16_to_float (uint16_t x) [-UECB_NO_LIBM]
1345    
1346 root 1.50 =item float ecb_binary32_to_float (uint32_t x) [-UECB_NO_LIBM]
1347    
1348 root 1.70 =item double ecb_binary64_to_double (uint64_t x) [-UECB_NO_LIBM]
1349 root 1.50
1350 sf-exg 1.59 The reverse operation of the previous function - takes the bit
1351 root 1.71 representation of an IEEE binary16, binary32 or binary64 number (half,
1352     single or double precision) and converts it to the native C<float> or
1353     C<double> format.
1354 root 1.50
1355     This function should work even when the native floating point format isn't
1356     IEEE compliant, of course at a speed and code size penalty, and of course
1357     also within reasonable limits (it tries to convert normals and denormals,
1358     and might be lucky for infinities, and with extraordinary luck, also for
1359     negative zero).
1360    
1361     On all modern platforms (where C<ECB_STDFP> is true), the compiler should
1362     be able to optimise away this function completely.
1363    
1364 root 1.71 =item uint16_t ecb_binary32_to_binary16 (uint32_t x)
1365    
1366     =item uint32_t ecb_binary16_to_binary32 (uint16_t x)
1367    
1368     Convert a IEEE binary32/single precision to binary16/half format, and vice
1369 root 1.72 versa, handling all details (round-to-nearest-even, subnormals, infinity
1370     and NaNs) correctly.
1371 root 1.71
1372     These are functions are available under C<-DECB_NO_LIBM>, since
1373     they do not rely on the platform floating point format. The
1374     C<ecb_float_to_binary16> and C<ecb_binary16_to_float> functions are
1375     usually what you want.
1376    
1377 root 1.50 =back
1378    
1379 root 1.1 =head2 ARITHMETIC
1380    
1381 root 1.88 =over
1382 root 1.3
1383 root 1.14 =item x = ecb_mod (m, n)
1384 root 1.3
1385 root 1.25 Returns C<m> modulo C<n>, which is the same as the positive remainder
1386     of the division operation between C<m> and C<n>, using floored
1387     division. Unlike the C remainder operator C<%>, this function ensures that
1388     the return value is always positive and that the two numbers I<m> and
1389     I<m' = m + i * n> result in the same value modulo I<n> - in other words,
1390     C<ecb_mod> implements the mathematical modulo operation, which is missing
1391     in the language.
1392 root 1.14
1393 sf-exg 1.23 C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be
1394 root 1.14 negatable, that is, both C<m> and C<-m> must be representable in its
1395 root 1.30 type (this typically excludes the minimum signed integer value, the same
1396 root 1.25 limitation as for C</> and C<%> in C).
1397 sf-exg 1.11
1398 root 1.85 Current GCC/clang versions compile this into an efficient branchless
1399     sequence on almost all CPUs.
1400 root 1.24
1401     For example, when you want to rotate forward through the members of an
1402     array for increasing C<m> (which might be negative), then you should use
1403     C<ecb_mod>, as the C<%> operator might give either negative results, or
1404     change direction for negative values:
1405    
1406     for (m = -100; m <= 100; ++m)
1407     int elem = myarray [ecb_mod (m, ecb_array_length (myarray))];
1408    
1409 sf-exg 1.37 =item x = ecb_div_rd (val, div)
1410    
1411     =item x = ecb_div_ru (val, div)
1412    
1413     Returns C<val> divided by C<div> rounded down or up, respectively.
1414     C<val> and C<div> must have integer types and C<div> must be strictly
1415 sf-exg 1.38 positive. Note that these functions are implemented with macros in C
1416     and with function templates in C++.
1417 sf-exg 1.37
1418 root 1.3 =back
1419 root 1.1
1420     =head2 UTILITY
1421    
1422 root 1.88 =over
1423 root 1.3
1424 sf-exg 1.23 =item element_count = ecb_array_length (name)
1425 root 1.3
1426 sf-exg 1.13 Returns the number of elements in the array C<name>. For example:
1427    
1428     int primes[] = { 2, 3, 5, 7, 11 };
1429     int sum = 0;
1430    
1431     for (i = 0; i < ecb_array_length (primes); i++)
1432     sum += primes [i];
1433    
1434 root 1.3 =back
1435 root 1.1
1436 root 1.43 =head2 SYMBOLS GOVERNING COMPILATION OF ECB.H ITSELF
1437    
1438     These symbols need to be defined before including F<ecb.h> the first time.
1439    
1440 root 1.88 =over
1441 root 1.43
1442 root 1.51 =item ECB_NO_THREADS
1443 root 1.43
1444     If F<ecb.h> is never used from multiple threads, then this symbol can
1445     be defined, in which case memory fences (and similar constructs) are
1446     completely removed, leading to more efficient code and fewer dependencies.
1447    
1448     Setting this symbol to a true value implies C<ECB_NO_SMP>.
1449    
1450     =item ECB_NO_SMP
1451    
1452     The weaker version of C<ECB_NO_THREADS> - if F<ecb.h> is used from
1453     multiple threads, but never concurrently (e.g. if the system the program
1454 root 1.104 runs on has only a single CPU with a single core, no hyper-threading and so
1455 root 1.43 on), then this symbol can be defined, leading to more efficient code and
1456     fewer dependencies.
1457    
1458 root 1.50 =item ECB_NO_LIBM
1459    
1460     When defined to C<1>, do not export any functions that might introduce
1461     dependencies on the math library (usually called F<-lm>) - these are
1462     marked with [-UECB_NO_LIBM].
1463    
1464 sf-exg 1.69 =back
1465    
1466 root 1.68 =head1 UNDOCUMENTED FUNCTIONALITY
1467    
1468     F<ecb.h> is full of undocumented functionality as well, some of which is
1469     intended to be internal-use only, some of which we forgot to document, and
1470     some of which we hide because we are not sure we will keep the interface
1471     stable.
1472    
1473     While you are welcome to rummage around and use whatever you find useful
1474 root 1.100 (we don't want to stop you), keep in mind that we will change undocumented
1475 root 1.68 functionality in incompatible ways without thinking twice, while we are
1476     considerably more conservative with documented things.
1477    
1478     =head1 AUTHORS
1479    
1480     C<libecb> is designed and maintained by:
1481    
1482     Emanuele Giaquinta <e.giaquinta@glauco.it>
1483     Marc Alexander Lehmann <schmorp@schmorp.de>