Core Timing: Rework Core Timing to run all cores evenly.

src/core/core_timing.h

@@ -7,6 +7,7 @@
 #include <chrono>
 #include <functional>
 #include <mutex>
+#include <optional>
 #include <string>
 #include <unordered_map>
 #include <vector>
@@ -104,7 +105,19 @@ public:
 
     std::chrono::microseconds GetGlobalTimeUs() const;
 
-    int GetDowncount() const;
+    void ResetRun();
+
+    s64 GetDowncount() const;
+
+    void SwitchContext(u64 new_context) {
+        current_context = new_context;
+    }
+
+    bool CurrentContextCanRun() const {
+        return time_slice[current_context] > 0;
+    }
+
+    std::optional<u64> NextAvailableCore(const s64 needed_ticks) const;
 
 private:
     struct Event;
@@ -112,10 +125,15 @@ private:
     /// Clear all pending events. This should ONLY be done on exit.
     void ClearPendingEvents();
 
+    static constexpr u64 num_cpu_cores = 4;
+
     s64 global_timer = 0;
     s64 idled_cycles = 0;
-    int slice_length = 0;
-    int downcount = 0;
+    s64 slice_length = 0;
+    std::array<s64, num_cpu_cores> downcounts{};
+    // Slice of time assigned to each core per run.
+    std::array<s64, num_cpu_cores> time_slice{};
+    u64 current_context = 0;
 
     // Are we in a function that has been called from Advance()
     // If events are scheduled from a function that gets called from Advance(),