src/Grid.cpp

@@ -75,9 +75,15 @@ void Grid::Random(int8 minMoves)
 	for (int8 index = 0; index < numButtons; index++)
 		buttonIndices[index] = index;
 
-	const int8 begin = ChooseRandom(buttonIndices, numButtons, minMoves);
+	int8 begin;
 	const int8 n = fDimension;
 
+	if (n == 5) {
+		ChooseRandom5x5(buttonIndices, minMoves);
+		begin = 0;
+	} else
+		begin = ChooseRandom(buttonIndices, numButtons, minMoves);
+
 	for (int8 i = 0; i < minMoves; i++) {
 		const int8 index = buttonIndices[begin + i];
 

src/Random.cpp

@@ -10,23 +10,34 @@
 
 //#define NDEBUG
 #include <assert.h>	// assert
-
 #include <stdlib.h>	// random
 
 
 static void
 Swap(int& a, int& b)
 {
-	assert (a != b);
+	if (a == b)
+		return;
 
 	const int tmp = a;
 	a = b;
 	b = tmp;
 }
 
 
-// array size >= n
-int ChooseRandom(int array[], int n, int k)
+static bool
+find(int m, int array[], int size)
+{
+	for (int index = 0; index < size; index++)
+		if (m == array[index])
+			return true;
+
+	return false;
+}
+
+
+int
+ChooseRandom(int array[], int n, int k)	// array size >= n
 {
 	assert(k > 0 && k < n);
 
@@ -35,12 +46,78 @@ int ChooseRandom(int array[], int n, int k)
 	if (chooseComplement)
 		k = n - k;
 
-	for (int i = 0; i < k; i++) {
-		const int offset = random() % (n - i);
-
-		if (offset > 0)
-			Swap(array[i], array[i + offset]);
+	for (int index = 0; index < k; index++) {
+		assert(n - index > 1);
+		Swap(array[index], array[index + random() % (n - index)]);
 	}
 
 	return chooseComplement ? k : 0;
 }
+
+
+void
+ChooseRandom5x5(int array[], int k)	// array size >= 25
+{
+	assert(k > 0 && k <= 15);
+
+	// partition the 5x5 grid
+	int c[] = { 0, 4, 20, 24 };					// corners
+	int h[] = { 1, 3, 10, 11, 13, 14, 21, 23 };	// horizontal
+	int v[] = { 2, 5, 7, 9, 15, 17, 19, 22 };	// vertical
+//	int d[] = { 6, 8, 12, 16, 18 };				// diagonals
+
+	// initialize the corresponding counters
+	int C = 0;
+	int H = 0;
+	int V = 0;
+
+	// set the upper limits of the counters
+	int CMax, HMax, VMax;
+	switch (k) {
+		case 14:
+			CMax = 3;
+			HMax = VMax = 5;
+			break;
+		case 15:
+			CMax = 2;
+			HMax = VMax = 4;
+			break;
+		default:
+			CMax = 4;
+			HMax = VMax = 8;
+	}
+
+	int n = 25;	// 5x5
+
+	for (int index = 0; index < k;) {
+		assert(n - index > 0);
+
+		int nextIndex = index;
+		if (n - index > 1)
+			nextIndex += random() % (n - index);
+
+		const int m = array[nextIndex];
+		bool skip = false;
+
+		if (find(m, c, 4))
+			if (C == CMax || C + H == 6 || C + V == 6)
+				skip = true;
+			else
+				C++;
+		else if (find(m, h, 8))
+			if (H == HMax || C + H == 6 || H + V == 8)
+				skip = true;
+			else
+				H++;
+		else if (find(m, v, 8))
+			if (V == VMax || C + V == 6 || H + V == 8)
+				skip = true;
+			else
+				V++;
+
+		if (skip)
+			array[nextIndex] = array[--n];
+		else
+			Swap(array[index++], array[nextIndex]);
+	}
+}

src/Random.h

@@ -1,6 +1,7 @@
 #ifndef RANDOM_H
 #define RANDOM_H
 
-int ChooseRandom(int array[], int n, int k);
+int		ChooseRandom(int array[], int n, int k);
+void	ChooseRandom5x5(int array[], int k);
 
 #endif