libdpf/include/grotto/dyadic_lut.hpp

/// @file grotto/dyadic_lut.hpp
/// @brief Exact dyadic step functions from the Grotto gadget list.
/// @details Integer results and boolean 1s are raw fixed-point values:
///          an integer n is stored as `n << fractional_bits`. `ilogb(0)` and
///          `ilog10(0)` return `ilog_of_zero`.
///
///          `make_msb_lut(i)` is bit `i` counting from the most significant
///          bit. It is constant on `2^{i+1}` intervals, so `i` must be less
///          than `msb_bit_limit`.
/// @copyright Copyright (c) 2019-2026 Ryan Henry and [others](@ref authors)
/// @license Released under a GNU General Public v2.0 (GPLv2) license.

#ifndef LIBDPF_INCLUDE_GROTTO_DYADIC_LUT_HPP__
#define LIBDPF_INCLUDE_GROTTO_DYADIC_LUT_HPP__

#include "hedley/hedley.h"

#include "grotto/easy_lut.hpp"

#include <cstdint>
#include <limits>
#include <stdexcept>
#include <type_traits>
#include <vector>

namespace grotto
{

/// Sentinel raw value for `ilogb(0)` and `ilog10(0)`.
inline constexpr std::int64_t ilog_of_zero =
    std::numeric_limits<std::int64_t>::min();

/// `make_msb_lut(i)` allows `i` in `[0, msb_bit_limit)`.
/// Bit 0 is two intervals; bit 7 is 256.
inline constexpr unsigned msb_bit_limit = 8;

namespace detail
{

using u128 = unsigned __int128;

template <typename Raw>
HEDLEY_CONST
HEDLEY_NO_THROW
constexpr int raw_width() noexcept
{
    return std::numeric_limits<Raw>::digits + 1;
}

inline std::int64_t encode_units(std::int64_t units, unsigned fractional_bits)
{
    if (fractional_bits >= 63)
        throw std::invalid_argument("dyadic lut: fractional width does not fit");
    const __int128 scaled = static_cast<__int128>(units) << fractional_bits;
    if (scaled > std::numeric_limits<std::int64_t>::max()
        || scaled < std::numeric_limits<std::int64_t>::min())
        throw std::overflow_error("dyadic lut: encoded value does not fit int64");
    return static_cast<std::int64_t>(scaled);
}

inline easy_poly unit_poly(std::int64_t units, unsigned fractional_bits)
{
    return easy_poly{encode_units(units, fractional_bits), 0, 0, 1};
}

inline easy_poly indicator_poly(bool on, unsigned fractional_bits)
{
    return unit_poly(on ? 1 : 0, fractional_bits);
}

template <typename Raw>
HEDLEY_CONST
HEDLEY_NO_THROW
constexpr std::uint64_t raw_bits(std::int64_t raw) noexcept
{
    constexpr int width = raw_width<Raw>();
    const auto masked = static_cast<std::uint64_t>(raw);
    if constexpr (width >= 64)
        return masked;
    else
        return masked & ((std::uint64_t{1} << width) - 1);
}

inline int countl_zero_width(std::uint64_t bits, int width)
{
    if (width <= 0 || width > 64)
        throw std::invalid_argument("dyadic lut: width must be 1..64");
    if (width < 64)
        bits &= (std::uint64_t{1} << width) - 1;
    if (bits == 0)
        return width;
    return __builtin_clzll(bits) - (64 - width);
}

inline int countl_one_width(std::uint64_t bits, int width)
{
    const std::uint64_t flipped = width >= 64 ? ~bits : (~bits & ((std::uint64_t{1} << width) - 1));
    return countl_zero_width(flipped, width);
}

template <typename Raw>
int clz_of(std::int64_t raw)
{
    return countl_zero_width(raw_bits<Raw>(raw), raw_width<Raw>());
}

template <typename Raw>
int clrsb_of(std::int64_t raw)
{
    constexpr int width = raw_width<Raw>();
    const std::uint64_t bits = raw_bits<Raw>(raw);
    const bool neg = ((bits >> (width - 1)) & 1u) != 0;
    const int matched = neg ? countl_one_width(bits, width)
                            : countl_zero_width(bits, width);
    return matched - 1;
}

HEDLEY_CONST
HEDLEY_NO_THROW
constexpr int floor_log2_u128(u128 mag) noexcept
{
    if (mag == 0)
        return -1;
    int n = 0;
    while (mag > 1)
    {
        mag >>= 1;
        ++n;
    }
    return n;
}

template <typename Raw>
HEDLEY_CONST
HEDLEY_NO_THROW
constexpr u128 magnitude(std::int64_t raw) noexcept
{
    using lim = std::numeric_limits<Raw>;
    if (raw == static_cast<std::int64_t>(lim::min()))
        return u128{1} << (raw_width<Raw>() - 1);
    const auto abs = raw < 0 ? -raw : raw;
    return static_cast<u128>(abs);
}

template <typename Raw>
HEDLEY_CONST
HEDLEY_NO_THROW
constexpr int ilogb_units(std::int64_t raw, unsigned fractional_bits) noexcept
{
    if (raw == 0)
        return 0;
    return floor_log2_u128(magnitude<Raw>(raw)) - static_cast<int>(fractional_bits);
}

inline u128 pow10_u128(int exponent)
{
    if (exponent < 0 || exponent > 38)
        throw std::invalid_argument("dyadic lut: power of ten is out of range");
    u128 p = 1;
    for (int i = 0; i < exponent; ++i)
        p *= 10;
    return p;
}

/// `mag / 2^k >= 10^e`.
inline bool magnitude_ge_pow10(u128 mag, unsigned fractional_bits, int exponent)
{
    if (mag == 0)
        return false;
    if (exponent >= 0)
    {
        const u128 decade = pow10_u128(exponent);
        if (fractional_bits > 0 && decade > (~u128{0} >> fractional_bits))
            return false;
        return mag >= (decade << fractional_bits);
    }
    const u128 decade = pow10_u128(-exponent);
    const u128 scale = u128{1} << fractional_bits;
    u128 threshold = scale / decade;
    if (scale % decade != 0)
        ++threshold;
    return mag >= threshold;
}

template <typename Raw>
int ilog10_units(std::int64_t raw, unsigned fractional_bits)
{
    if (raw == 0)
        return 0;
    const u128 mag = magnitude<Raw>(raw);
    int lo = -static_cast<int>(fractional_bits) - 2;
    int hi = raw_width<Raw>();
    while (lo < hi)
    {
        const int mid = lo + (hi - lo + 1) / 2;
        if (magnitude_ge_pow10(mag, fractional_bits, mid))
            lo = mid;
        else
            hi = mid - 1;
    }
    return lo;
}

template <typename Raw, typename At>
HEDLEY_WARN_UNUSED_RESULT
easy_lut<Raw> steps_from_cuts(std::vector<std::int64_t> cuts, At && at)
{
    return assemble_easy<Raw>(std::move(cuts), [&](std::int64_t raw) {
        return at(raw);
    });
}

template <typename Raw>
HEDLEY_WARN_UNUSED_RESULT
easy_lut<Raw> predicate_lut(unsigned fractional_bits, bool at_or_below_zero,
    bool at_zero, bool above_zero)
{
    std::vector<std::int64_t> cuts{0, 1};
    return steps_from_cuts<Raw>(std::move(cuts), [=](std::int64_t raw) {
        const bool on = raw < 0 ? at_or_below_zero : (raw == 0 ? at_zero : above_zero);
        return indicator_poly(on, fractional_bits);
    });
}

}  // namespace detail

template <typename Raw>
HEDLEY_WARN_UNUSED_RESULT
easy_lut<Raw> make_positive_lut(unsigned fractional_bits = 0)
{
    return detail::predicate_lut<Raw>(fractional_bits, false, false, true);
}

template <typename Raw>
HEDLEY_WARN_UNUSED_RESULT
easy_lut<Raw> make_negative_lut(unsigned fractional_bits = 0)
{
    return detail::predicate_lut<Raw>(fractional_bits, true, false, false);
}

template <typename Raw>
HEDLEY_WARN_UNUSED_RESULT
easy_lut<Raw> make_nonnegative_lut(unsigned fractional_bits = 0)
{
    return detail::predicate_lut<Raw>(fractional_bits, false, true, true);
}

template <typename Raw>
HEDLEY_WARN_UNUSED_RESULT
easy_lut<Raw> make_nonpositive_lut(unsigned fractional_bits = 0)
{
    return detail::predicate_lut<Raw>(fractional_bits, true, true, false);
}

template <typename Raw>
HEDLEY_WARN_UNUSED_RESULT
easy_lut<Raw> make_zero_lut(unsigned fractional_bits = 0)
{
    return detail::predicate_lut<Raw>(fractional_bits, false, true, false);
}

template <typename Raw>
HEDLEY_WARN_UNUSED_RESULT
easy_lut<Raw> make_nonzero_lut(unsigned fractional_bits = 0)
{
    return detail::predicate_lut<Raw>(fractional_bits, true, false, true);
}

template <typename Raw>
HEDLEY_WARN_UNUSED_RESULT
easy_lut<Raw> make_signum_lut(unsigned fractional_bits = 0)
{
    const std::int64_t one = detail::encode_units(1, fractional_bits);
    return detail::steps_from_cuts<Raw>({0, 1}, [=](std::int64_t raw) {
        if (raw < 0)
            return detail::easy_poly{-one, 0, 0, 1};
        if (raw == 0)
            return detail::kZero;
        return detail::easy_poly{one, 0, 0, 1};
    });
}

template <typename Raw>
HEDLEY_WARN_UNUSED_RESULT
easy_lut<Raw> make_clz_lut(unsigned fractional_bits = 0)
{
    constexpr int width = detail::raw_width<Raw>();
    std::vector<std::int64_t> cuts{0, 1};
    for (int b = 1; b <= width - 2; ++b)
        cuts.push_back(std::int64_t{1} << b);
    return detail::steps_from_cuts<Raw>(std::move(cuts), [=](std::int64_t raw) {
        return detail::unit_poly(detail::clz_of<Raw>(raw), fractional_bits);
    });
}

template <typename Raw>
HEDLEY_WARN_UNUSED_RESULT
easy_lut<Raw> make_clrsb_lut(unsigned fractional_bits = 0)
{
    constexpr int width = detail::raw_width<Raw>();
    std::vector<std::int64_t> cuts{0, -1, -2};
    for (int exp = 0; exp <= width - 2; ++exp)
        cuts.push_back(std::int64_t{1} << exp);
    for (int exp = 2; exp <= width - 1; ++exp)
    {
        if (exp >= 63)
            cuts.push_back(static_cast<std::int64_t>(std::numeric_limits<Raw>::min()));
        else
            cuts.push_back(-(std::int64_t{1} << exp));
    }
    return detail::steps_from_cuts<Raw>(std::move(cuts), [=](std::int64_t raw) {
        return detail::unit_poly(detail::clrsb_of<Raw>(raw), fractional_bits);
    });
}

template <typename Raw>
HEDLEY_WARN_UNUSED_RESULT
easy_lut<Raw> make_ilogb_lut(unsigned fractional_bits = 0)
{
    constexpr int width = detail::raw_width<Raw>();
    std::vector<std::int64_t> cuts{0, 1};
    for (int b = 1; b <= width - 2; ++b)
        cuts.push_back(std::int64_t{1} << b);
    for (int exp = 1; exp <= width - 1; ++exp)
    {
        if (exp >= width - 1)
            cuts.push_back(static_cast<std::int64_t>(std::numeric_limits<Raw>::min()) + 1);
        else
            cuts.push_back(-(std::int64_t{1} << exp) + 1);
    }
    return detail::steps_from_cuts<Raw>(std::move(cuts), [=](std::int64_t raw) {
        if (raw == 0)
            return detail::easy_poly{ilog_of_zero, 0, 0, 1};
        return detail::unit_poly(detail::ilogb_units<Raw>(raw, fractional_bits),
            fractional_bits);
    });
}

template <typename Raw>
HEDLEY_WARN_UNUSED_RESULT
easy_lut<Raw> make_ilog10_lut(unsigned fractional_bits = 0)
{
    using lim = std::numeric_limits<Raw>;
    const std::int64_t maxv = static_cast<std::int64_t>(lim::max());
    const int lo = detail::ilog10_units<Raw>(1, fractional_bits);
    const int hi_pos = detail::ilog10_units<Raw>(maxv, fractional_bits);
    const int hi_neg = detail::ilog10_units<Raw>(static_cast<std::int64_t>(lim::min()),
        fractional_bits);
    const int hi = hi_pos > hi_neg ? hi_pos : hi_neg;

    std::vector<std::int64_t> cuts{0, 1};
    std::int64_t prev = 0;
    for (int e = lo; e <= hi; ++e)
    {
        std::int64_t left = 1;
        std::int64_t right = maxv;
        while (left < right)
        {
            const std::int64_t mid = left + (right - left) / 2;
            if (detail::ilog10_units<Raw>(mid, fractional_bits) >= e)
                right = mid;
            else
                left = mid + 1;
        }
        if (left == prev)
            continue;
        prev = left;
        cuts.push_back(left);
        if (left < maxv)
        {
            std::int64_t end = left;
            std::int64_t scan_left = left;
            std::int64_t scan_right = maxv;
            while (scan_left < scan_right)
            {
                const std::int64_t mid = scan_left + (scan_right - scan_left + 1) / 2;
                if (detail::ilog10_units<Raw>(mid, fractional_bits) == e)
                    scan_left = mid;
                else
                    scan_right = mid - 1;
            }
            end = scan_left;
            const std::int64_t neg = -end;
            if (neg > static_cast<std::int64_t>(lim::min()))
                cuts.push_back(neg);
        }
    }
    return detail::steps_from_cuts<Raw>(std::move(cuts), [=](std::int64_t raw) {
        if (raw == 0)
            return detail::easy_poly{ilog_of_zero, 0, 0, 1};
        return detail::unit_poly(detail::ilog10_units<Raw>(raw, fractional_bits),
            fractional_bits);
    });
}

/// Bit `index` counting down from the most significant bit of `Raw`.
/// Index 0 is the sign bit. Larger indexes are refused: the bit is constant
/// on `2^{index+1}` intervals.
template <typename Raw>
HEDLEY_WARN_UNUSED_RESULT
easy_lut<Raw> make_msb_lut(unsigned index, unsigned fractional_bits = 0)
{
    constexpr int width = detail::raw_width<Raw>();
    if (index >= msb_bit_limit || static_cast<int>(index) >= width)
        throw std::invalid_argument(
            "msb lut: only the most significant bits are piecewise-cheap");
    const int shift = width - 1 - static_cast<int>(index);
    if (shift >= 63)
    {
        return detail::steps_from_cuts<Raw>({0}, [=](std::int64_t raw) {
            const bool on = ((detail::raw_bits<Raw>(raw) >> shift) & 1u) != 0;
            return detail::indicator_poly(on, fractional_bits);
        });
    }
    const std::int64_t step = std::int64_t{1} << shift;
    std::vector<std::int64_t> cuts;
    const auto minv = static_cast<std::int64_t>(std::numeric_limits<Raw>::min());
    for (std::int64_t boundary = minv; ; )
    {
        cuts.push_back(boundary);
        if (boundary > static_cast<std::int64_t>(std::numeric_limits<Raw>::max()) - step)
            break;
        boundary += step;
    }
    return detail::steps_from_cuts<Raw>(std::move(cuts), [=](std::int64_t raw) {
        const bool on = ((detail::raw_bits<Raw>(raw) >> shift) & 1u) != 0;
        return detail::indicator_poly(on, fractional_bits);
    });
}

}  // namespace grotto

#endif  // LIBDPF_INCLUDE_GROTTO_DYADIC_LUT_HPP__