fdl_rounding.h

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// SPDX-FileCopyrightText: 2024-present American Society Of Cinematographers
// SPDX-License-Identifier: Apache-2.0
#ifndef FDL_ROUNDING_INTERNAL_H
#define FDL_ROUNDING_INTERNAL_H
 
#include <cmath>
#include <cstdint>
#include <limits>
 
#include "fdl_constants.h"
 
namespace fdl::detail {
 
inline int64_t safe_to_int64(double value) {
    // Doubles beyond int64_t range would cause UB in static_cast.
    constexpr auto kMax = static_cast<double>(std::numeric_limits<int64_t>::max());
    constexpr auto kMin = static_cast<double>(std::numeric_limits<int64_t>::min());
    if (value >= kMax) {
        return std::numeric_limits<int64_t>::max();
    }
    if (value <= kMin) {
        return std::numeric_limits<int64_t>::min();
    }
    return static_cast<int64_t>(value);
}
 
inline int64_t bankers_round(double value) {
    double rounded = std::round(value);
    // Check if we're exactly at the halfway point
    double const remainder = value - std::floor(value);
    if (std::abs(remainder - constants::kHalfway) < constants::kFpHalfwayTolerance) {
        // Halfway case: round to even
        auto r = safe_to_int64(rounded);
        if (r % constants::kEvenDivisor != 0) {
            // rounded is odd, go the other way
            rounded = std::floor(value + constants::kHalfway);
            if (safe_to_int64(rounded) % constants::kEvenDivisor != 0) {
                rounded = std::ceil(value - constants::kHalfway);
            }
        }
    }
    return safe_to_int64(rounded);
}
 
} // namespace fdl::detail
 
#endif // FDL_ROUNDING_INTERNAL_H