Guard against int64 overflow when building instances with huge profits#13
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Instances with profits near INT64_MAX could silently corrupt total_item_profit_ (signed overflow wraps to a negative bound) or overflow inside upper_bound_reverse's (capacity - weight) * profit term, since a state's weight can exceed the capacity by up to two item weights. Both now throw std::overflow_error at build time instead of corrupting state or crashing mid-algorithm. InstanceFromFloatProfitsBuilder's multiplier selection also trusted a double-precision estimate that isn't accurate enough at this magnitude; it now verifies the actual rounded integer profits and backs off the multiplier if they would overflow.
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Instances with profits near INT64_MAX could silently corrupt total_item_profit_ (signed overflow wraps to a negative bound) or overflow inside upper_bound_reverse's (capacity - weight) * profit term, since a state's weight can exceed the capacity by up to two item weights. Both now throw std::overflow_error at build time instead of corrupting state or crashing mid-algorithm.
InstanceFromFloatProfitsBuilder's multiplier selection also trusted a double-precision estimate that isn't accurate enough at this magnitude; it now verifies the actual rounded integer profits and backs off the multiplier if they would overflow.