MOHAWK: RIVEN: Generate dome code as in original game

In the original game, the dome combination is not completely random;
only the first number is one that can be seen in the school (range 2-10),
two numbers are chosen from the range 11-15, and two are chosen
from the range 16-24. Presumably, this is done so that the code has
approximately equal difficulty for every game.

Until now, scummvm has generated completely random codes,
which results in some games having very easy codes (when most of
the numbers are below 10). Also, there's a 20% chance that
the last number is 25, which is never used in the original game.

engines/mohawk/riven_vars.cpp

@@ -277,6 +277,22 @@ uint32 &MohawkEngine_Riven::getStackVar(uint32 index) {
 	return _vars[name];
 }
 
+namespace {
+
+uint32 getTwoRandomDomePositionsRng(Common::RandomSource *rnd, uint min, uint max) {
+	uint first = rnd->getRandomNumberRng(min, max);
+	uint second = rnd->getRandomNumberRng(min, max - 1);
+
+	// Avoid overlap of the two positions
+	if (second >= first) {
+		second++;
+	}
+
+	return 1 << (25 - first) | 1 << (25 - second);
+}
+
+} // End of anonymous namespace
+
 void MohawkEngine_Riven::initVars() {
 	// Most variables just start at 0, it's simpler to do this
 	for (uint32 i = 0; i < ARRAYSIZE(variableNames); i++)
@@ -337,17 +353,14 @@ void MohawkEngine_Riven::initVars() {
 
 	// Randomize the dome combination -- each bit represents a slider position,
 	// the highest bit (1 << 24) represents 1, (1 << 23) represents 2, etc.
+	// The dome combination is not completely random:
+	//  - The first slider position is a random number in the range [2, 10].
+	//  - The second and third slider positions are randomly chosen from the interval [11, 15].
+	//  - The fourth and fifth slider positions are randomly chosen from the interval [16, 24].
 	uint32 &domeCombo = _vars["adomecombo"];
-	for (byte bitsSet = 0; bitsSet < 5;) {
-		uint32 randomBit = 1 << (24 - _rnd->getRandomNumber(24));
-
-		// Don't overwrite a bit we already set, and throw out the bottom five bits being set
-		if (domeCombo & randomBit || (domeCombo | randomBit) == 31)
-			continue;
-
-		domeCombo |= randomBit;
-		bitsSet++;
-	}
+	domeCombo |= 1 << (25 - _rnd->getRandomNumberRng(2, 10));
+	domeCombo |= getTwoRandomDomePositionsRng(_rnd, 11, 15);
+	domeCombo |= getTwoRandomDomePositionsRng(_rnd, 16, 24);
 }
 
 } // End of namespace Mohawk