Add combination iterator

.gitignore

@@ -1,6 +1,7 @@
 # Licensed under the MIT License.
 
 a.out
+a.py
 *.exe
 *.a
 *.lib

Makefile

@@ -32,7 +32,7 @@ all: \
 	id0068$(E) id0069$(E) id0070$(E) id0071$(E) id0072$(E) id0073$(E) \
 	id0074$(E) id0075$(E) id0076$(E) id0077$(E) id0078$(E) id0079$(E) \
 	id0080$(E) id0081$(E) id0082$(E) id0083$(E) id0084$(E) id0085$(E) \
-	id0086$(E) id0087$(E) id0089$(E) id0091$(E) id0092$(E)
+	id0086$(E) id0087$(E) id0089$(E) id0091$(E) id0092$(E) id0093$(E) \
 
 libeuler$(A): $(call RECURSE,lib,*.c)
 	$(RM) *.o
@@ -312,5 +312,8 @@ id0091$(E): src/id0091.c libeuler$(A)
 id0092$(E): src/id0092.c libeuler$(A)
 	$(CC) $(CFLAGS) $< -o $@ $(LEULER) $(LM)
 
+id0093$(E): src/id0093.c libeuler$(A)
+	$(CC) $(CFLAGS) $< -o $@ $(LEULER) $(LM)
+
 clean:
 	$(RM) *$(A) *.o

cstyle.md

@@ -49,10 +49,10 @@ is impractical, use numeric suffixes instead.
 |  `b`   | blue color component, boundary points in Pick\'s Theorem                                     |
 |  `cb`  | cube                                                                                         |
 |  `d`   | divisor                                                                                      |
+|  `dx`  | change in an arbitrary dimension                                                             |
 |  `e`   | arbitrary edge                                                                               |
 |  `ef`  | even Fibonacci number                                                                        |
 |  `f`   | Fibonacci number                                                                             |
-|  `dx`  | change in an arbitrary dimension                                                             |
 |  `g`   | green color component, arbitrary graph                                                       |
 |  `h`   | row offset, horizontal offset                                                                |
 |  `hi`  | larger pointer, point above, upper buffer                                                    |

lib/combination_iterator.c

@@ -0,0 +1,49 @@
+// Licensed under the MIT License.
+
+#include <stdint.h>
+#include <string.h>
+#include "combination_iterator.h"
+
+void combination_begin(
+    CombinationIterator iterator,
+    size_t subset[],
+    size_t n,
+    size_t k)
+{
+    iterator->subset = subset;
+    iterator->i = 0;
+    iterator->k = k;
+    iterator->n = n;
+    iterator->end = false;
+
+    for (size_t i = 0; i < iterator->k; i++)
+    {
+        iterator->subset[i] = 0;
+    }
+
+    combination_next(iterator);
+}
+
+void combination_next(CombinationIterator iterator)
+{
+    while (iterator->i != SIZE_MAX)
+    {
+        iterator->subset[iterator->i]++;
+
+        if (iterator->subset[iterator->i] > iterator->n)
+        {
+            iterator->i--;
+        }
+        else if (iterator->i == iterator->k - 1)
+        {
+            return;
+        }
+        else
+        {
+            iterator->i++;
+            iterator->subset[iterator->i] = iterator->subset[iterator->i - 1];
+        }
+    }
+
+    iterator->end = true;
+}

lib/combination_iterator.h

@@ -0,0 +1,24 @@
+// Licensed under the MIT License.
+
+#include "object.h"
+#include <stdbool.h>
+#include <stddef.h>
+
+struct CombinationIterator
+{
+    size_t* subset;
+    size_t i;
+    size_t k;
+    size_t n;
+    bool end;
+};
+
+typedef struct CombinationIterator* CombinationIterator;
+
+void combination_begin(
+    CombinationIterator iterator,
+    size_t subset[],
+    size_t n,
+    size_t k);
+
+void combination_next(CombinationIterator iterator);

src/id0092.c

@@ -41,7 +41,7 @@ int main(void)
     int results[568] = { 0 };
     clock_t start = clock();
 
-    for (long m = 1; m < 568; m++)
+    for (int m = 1; m < 568; m++)
     {
         results[m] = math_square_digit_chain(m, results);
 
@@ -51,7 +51,7 @@ int main(void)
         }
     }
 
-    for (long m = 1; m < 10000000l; m++)
+    for (long m = 568; m < 10000000l; m++)
     {
         long n = math_square_digit_chain(m, results);
 

src/id0093.c

@@ -0,0 +1,22 @@
+#include "../lib/combination_iterator.h"
+#include "../lib/euler.h"
+
+int main(void)
+{
+    struct CombinationIterator it;
+    size_t subset[2];
+
+    for (combination_begin(&it, subset, 4, 2); !it.end; combination_next(&it))
+    {
+        printf("=> ");
+
+        for (int i = 0; i < 2; i++)
+        {
+            printf("%zu ", subset[i]);
+        }
+
+        printf("\n");
+    }
+
+    return 0;
+}

tools/test.ps1

@@ -69,3 +69,10 @@ for ($i = 84; $i -le 88; $i++)
 {
     Invoke-Expression "./../id00${i}.exe"
 }
+
+./../id0089.exe
+
+for ($i = 91; $i -le 92; $i++)
+{
+    Invoke-Expression "./../id00${i}.exe"
+}

tools/test.sh

@@ -76,3 +76,10 @@ for i in {0084..0088};
 do
     $run ./../id${i}
 done
+
+$run ./../id0089
+
+for i in {0091..0092};
+do
+    $run ./../id${i}
+done