Use std::prev and std::next where applicable

aegisub/libaegisub/common/mru.cpp

@@ -85,9 +85,7 @@ std::string const& MRUManager::GetEntry(std::string const& key, const size_t ent
 	if (entry >= map->size())
 		throw MRUErrorIndexOutOfRange("Requested element index is out of range.");
 
-	MRUListMap::const_iterator index = map->begin();
-	advance(index, entry);
-	return *index;
+	return *next(map->begin(), entry);
 }
 
 void MRUManager::Flush() {

aegisub/src/command/edit.cpp

@@ -610,8 +610,7 @@ static void duplicate_lines(agi::Context *c, bool shift) {
 
 		// And the last line in this contiguous selection
 		entryIter insert_pos = find_if_not(start, end, sel);
-		entryIter last = insert_pos;
-		--last;
+		entryIter last = std::prev(insert_pos);
 
 		// Duplicate each of the selected lines, inserting them in a block
 		// after the selected block

aegisub/src/spline.cpp

@@ -164,10 +164,8 @@ void Spline::DecodeFromAss(wxString str) {
 }
 
 void Spline::MovePoint(iterator curve,int point,Vector2D pos) {
-	iterator prev = curve;
-	if (curve != begin()) --prev;
-	iterator next = curve;
-	++next;
+	iterator prev = std::prev(curve, curve != begin());
+	iterator next = std::next(curve);
 	if (next != end() && next->type == SplineCurve::POINT)
 		next = end();
 
@@ -279,14 +277,12 @@ void Spline::Smooth(float smooth) {
 	if (size() < 3) return;
 
 	// Smooth curve
-	iterator cur_curve = end();
-	--cur_curve;
-	for (iterator cur = begin(); cur != end(); ) {
-		iterator prev_curve = cur_curve;
-		cur_curve = cur;
-		++cur;
-		iterator next_curve = cur == end() ? begin() : cur;
+	for (iterator cur = begin(); cur != end(); ++cur) {
+		iterator prev_curve = prev(cur == begin() ? cur : end());
+		iterator next_curve = next(cur);
+		if (next_curve == end())
+			next_curve = begin();
 
-		cur_curve->Smooth(prev_curve->p1, next_curve->EndPoint(), smooth);
+		cur->Smooth(prev_curve->p1, next_curve->EndPoint(), smooth);
 	}
 }

aegisub/src/visual_tool_drag.cpp

@@ -235,7 +235,7 @@ void VisualToolDrag::MakeFeatures(AssDialogue *diag, feature_iterator pos) {
 	feat.line = diag;
 	feat.parent = features.end();
 	features.insert(pos, feat);
-	feature_iterator cur = pos; --cur;
+	feature_iterator cur = prev(pos);
 	feat.parent = cur;
 	if (selection.count(diag))
 		sel_features.insert(cur);
@@ -252,7 +252,7 @@ void VisualToolDrag::MakeFeatures(AssDialogue *diag, feature_iterator pos) {
 		feat.time = t2;
 		feat.line = diag;
 		features.insert(pos, feat);
-		feat.parent->parent = --pos; ++pos;
+		feat.parent->parent = prev(pos);
 	}
 
 	// Create org feature

aegisub/src/visual_tool_vector_clip.cpp

@@ -223,8 +223,7 @@ bool VisualToolVectorClip::InitializeDrag(feature_iterator feature) {
 		feature->curve->p2 = feature->curve->p4;
 	}
 	else {
-		Spline::iterator next = feature->curve;
-		next++;
+		Spline::iterator next = std::next(feature->curve);
 		if (next != spline.end()) {
 			if (feature->curve->type == SplineCurve::POINT) {
 				next->p1 = next->EndPoint();