IREPEAT

C++ preprocessor macros for indexed repetitions

Copyright © 2024 The Lemuriad. Distributed under the Boost Software License, V1.0

Boost Software License - Version 1.0 - August 17th, 2003

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GitHub Actions meson build: linux gcc10 & 12, clang 12 & 14, macos 12, windows latest

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IREPEAT, XREPEAT and VREPEAT

Preprocessor utilities to generate indexed repetitions, for codegen & etc.
The maximum repetition count is 2^16 = 65536; a 4 hex-digit index limit.

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// IREPEAT_example.hpp
#include "IREPEAT.hpp"
IREPEAT((f)(f),HEXLIT,COMMA) // Generate 256 values 0-255
#include "IREPEAT_UNDEF.hpp"

Preprocess: e.g. cpp -P -E IREPEAT_example.hpp ...

The provided HEXLIT macro is expanded 256 times to a comma-separated list
of hexadecimal literals, ranging from 0x00 to 0xff inclusive. The double-digit
output reflects the number of digits in the specified end index (f)(f):

0x00,0x01,0x02,0x03,0x04,0x05,0x06,0x07,0x08,0x09,0x0a,0x0b,...
 ... 0xf5,0xf6,0xf7,0xf8,0xf9,0xfa,0xfb,0xfc,0xfd,0xfe,0xff

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IREPEAT.hpp: Horizontal repetition

Macro expansion is 'horizontal'; replacements are made within a single line.

Indexed-repeat macros expand to a single long line of separated repetitions:

IREPEAT(N,M,S) -> M((0)) S() M((1)) S() ... M(N-1) S() M(N)
XREPEAT(N,M,S) -> M((0)) S() ... M(N-2) S() M(N-1)

I for Inclusive: M(H) is repeated N+1 times for H in range [0,N]
X for eXclusive: M(H) is repeated N times for H in range [0,N)

• N is the number of XREPEATs. The number of IREPEATs is N + 1.
• M is a macro, or tokens, parameterized by HEXS repeat-index H.
• S is a separator-generator argument, i.e. S() expands to a separator.

Number N (the provided 'cardinal') and its generated repeat-indices H... ('ordinals')
are represented as 'HEXS'; sequences of (adjacent parenthesized) hex digits:

 XREPEAT((0)(3),X,COLON); -> X((0)(0));X((0)(1));X((0)(2));
 XREPEAT((0)(0),X,COLON); -> ; // caution; 0-length XREPEAT

Note that generated indices have the same number of digits as their cardinal N.
Beware of vanishing zero-length XREPEAT; use IREPEAT, I repeat, use IREPEAT.

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VREPEAT.hpp: Vertical repetition

'Vertical' repetition expands each rep on its own line by repeated #include

First, #define VREPEAT_MACRO and VREPEAT_COUNT (inclusive),
and optionally #define VREPEAT_SEPARATOR to add separators,
then #include "VREPEAT.hpp" to process the repetitions:

#include "IREPEAT.hpp"

#define VREPEAT_COUNT (f)(f) // 0xff for 256 incl. reps
#define VREPEAT_MACRO Iteration HEXLIT
#define VREPEAT_SEPARATOR COMMA
#include "VREPEAT.hpp"

#include "IREPEAT_UNDEF.hpp"

Preprocesses to:

Iteration 0x00,
Iteration 0x01,
  ...
Iteration 0xfe,
Iteration 0xff

VREPEAT_MACRO expands either (1) as a macro or (2) by #include:

1. VREPEAT_MACRO(VREPEAT_INDEX)

2. #include STR(VREPEAT_MACRO)

#include expansion (2) is used if __has_include__(STR(VREPEAT_MACRO))
i.e. the stringized VREPEAT_MACRO is the name of a file in the include path.

Otherwise, macro expansion (1) is used with VREPEAT_SEPARATOR() appended,
if defined, on all but the final repeat (when NREPEAT == NREPEATS).
A separator is not added after #include expansion (2); add it in the file if needed.

The following preprocessor names are set automatically:

• VREPEAT_INDEX: The repeat index digits in HEXS format
• C3,C2,C1,C0: Individual hex digits of VREPEAT_COUNT
• D3,D2,D1,D0: Individual hex digits of VREPEAT_INDEX
• NREPEAT: The repeat index as an integer literal
• NREPEATS: VREPEAT_COUNT as an integer literal

#include "VREPEAT.hpp" undef's these symbols and all others it defines
so can be used repeatedly.

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HEXS: hexadecimal digit sequences

Numbers are represented as hex digit sequences, e.g. (1)(f) == 0x1f == 31

The HEXS representation here has up to 4 parenthesized hexadecimal digits:

HEXS = (h0) | (h1)(h0) | (h2)(h1)(h0) | (h3)(h2)(h1)(h0)

The hex letter-digits are required to be lowercase only.

Four digits is a practical repeat-count limit; 2^16 reps take ~1s and ~1Gbyte.

Documentation page

// Don't forget to #undef your #define's
#include <IREPEAT_UNDEF.hpp>

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Credits

Props to pre-pro pioneers, particularly Paul Mensonides.