#include <stdint.h>
#include <stdio.h>
#include <time.h>
#include <stdlib.h>
#include <time.h>
#include <stdbool.h>

// Следующее сочетание k элементов диапазона 1...n в массив a[]
// Возвращает true если оно есть
bool nexn_combinations(int* a, int k, int n) {
    for (int i = k - 1, max = n; i >= 0; i--, max--) {
	if (a[i] < 1 || a[i] > max) {  // за границей диапазона
	    printf("ERROR\n");
	    return false;
	}
	if (a[i] < max) {
	    a[i]++;
	    return true;
	}
	if (i > 0) {
	    a[i] = a[i - 1] + 2;
	    if (a[i - 1] == max - 1) {
		a[i]--;
	    }
	}
	for (int j = i + 1; j < k; j++) a[j] = a[j - 1] + 1;
    }
    return false;
}



//**********************************************

int main(int argc, char**argv)
{
    if (argc < 3) {
      fprintf(stderr, "\n  Usage: perm-k-n K N [s]\n\n");
      return 1;
    }
    clock_t start = clock();
    int K = atoi(argv[1]);
    int N = atoi(argv[2]);
    bool silent = argc > 3 ? true : false;
    fprintf(stderr, "C(%d, %d)\n", K,N);
    int a[K];
    for (int i = 0; i < K; i++) a[i] = i + 1;
    int cnt = 0;
    if (silent) {
     fprintf(stderr, "silent_mode = on\n");
     do {
	cnt++;
     } while (nexn_combinations(a, K, N));
    } else {
     do {
	cnt++;
	for(int& i : a) printf("%d ", i);
	printf("\n");
     } while (nexn_combinations(a, K, N));
    }
    fprintf(stderr, "Total: %d Time %.2f s\n", cnt, (float)(clock() - start) / CLOCKS_PER_SEC);
    return 0;

$ ./perm-k-n.exe 2 100000 s
C(2, 100000)
silent_mode = on
Total: 704982704 Time 22.79 s

$ /probe-permutation/anatoly-moskovsky$ ./make-run
Time 10.05 s
k=2, n=100000, print=1
total: 4999950000

$ gcc version 9.3.0 (Ubuntu 9.3.0-17ubuntu1~20.04) 
$ uname -a Linux ryzen-ssd 5.8.0-40-generic #45~20.04.1-Ubuntu SMP Fri Jan 15 11:35:04 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux

#!/bin/bash

rm -f perm-k-n.exe

gcc \
 -O3 -DNDEBUG \
 perm-k-n.cpp \
 -o perm-k-n.exe \
 -lstdc++ && ./perm-k-n.exe 2 100000

 long int cnt = 0;
    if (silent) {
     fprintf(stderr, "silent_mode = on\n");
     do {
	cnt++;
     } while (nexn_combinations(a, K, N));
    } else {
     do {
	cnt++;
	for(int& i : a) printf("%d ", i);
	printf("\n");
     } while (nexn_combinations(a, K, N));
    }
 fprintf(stderr, "Total: %ld Time %.2f s\n", cnt, (float)(clock() - start) / CLOCKS_PER_SEC);

1 2 3 4
1 2 3 5
1 2 3 6
1 2 4 5
1 2 4 6
1 2 5 6
1 3 4 5
1 3 4 6
1 3 5 6
1 4 5 6
2 3 4 5
2 3 4 6
2 3 5 6
2 4 5 6
3 4 5 6

#!/bin/bash

rm -f perm-k-n.exe

fpc -CX -O3 -XX -vewnhi -Fi. -Fu. -FU. perm-k-n.lpr \
  && mv perm-k-n perm-k-n.exe

Hint: Start of reading config file /etc/fpc.cfg
Hint: End of reading config file /etc/fpc.cfg
Free Pascal Compiler version 3.0.4+dfsg-23 [2019/11/25] for x86_64
Copyright (c) 1993-2017 by Florian Klaempfl and others
Target OS: Linux for x86-64
Compiling perm-k-n.lpr
perm-k-n.lpr(7,3) Error: Identifier not found "Result"
perm-k-n.lpr(13,13) Error: Incompatible types: got "SmallInt" expected "IntegerArray"
perm-k-n.lpr(18,13) Error: Incompatible types: got "SmallInt" expected "IntegerArray"
perm-k-n.lpr(19,11) Error: Identifier not found "Result"
perm-k-n.lpr(24,11) Error: Incompatible types: got "IntegerArray" expected "SmallInt"
perm-k-n.lpr(34,46) Error: Operator is not overloaded: "Byte" + "IntegerArray"
perm-k-n.lpr(43,13) Error: Identifier not found "c"
perm-k-n.lpr(44,3) Error: Identifier not found "cnt"
perm-k-n.lpr(44,33) Error: Identifier not found "c"
perm-k-n.lpr(47,1) Fatal: There were 9 errors compiling module, stopping
Fatal: Compilation aborted
Error: /usr/bin/ppcx64 returned an error exitcode

Hint: Start of reading config file /etc/fpc.cfg
Hint: End of reading config file /etc/fpc.cfg
Free Pascal Compiler version 3.0.4+dfsg-23 [2019/11/25] for x86_64
Copyright (c) 1993-2017 by Florian Klaempfl and others
Target OS: Linux for x86-64
Compiling perm-k-n.lpr
perm-k-n.lpr(3,2) Warning: APPTYPE is not supported by the target OS
perm-k-n.lpr(13,46) Error: Operator is not overloaded: "Byte" + "IntegerArray"
perm-k-n.lpr(21,3) Error: Identifier not found "Result"
perm-k-n.lpr(27,13) Error: Incompatible types: got "SmallInt" expected "IntegerArray"
perm-k-n.lpr(32,13) Error: Incompatible types: got "SmallInt" expected "IntegerArray"
perm-k-n.lpr(33,11) Error: Identifier not found "Result"
perm-k-n.lpr(38,11) Error: Incompatible types: got "IntegerArray" expected "SmallInt"
perm-k-n.lpr(54) Fatal: There were 6 errors compiling module, stopping
Fatal: Compilation aborted
Error: /usr/bin/ppcx64 returned an error exitcode