// Copyright 2019 Ulf Adams
//
// The contents of this file may be used under the terms of the Apache License,
// Version 2.0.
//
//    (See accompanying file LICENSE-Apache or copy at
//     http://www.apache.org/licenses/LICENSE-2.0)
//
// Alternatively, the contents of this file may be used under the terms of
// the Boost Software License, Version 1.0.
//    (See accompanying file LICENSE-Boost or copy at
//     https://www.boost.org/LICENSE_1_0.txt)
//
// Unless required by applicable law or agreed to in writing, this software
// is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
// KIND, either express or implied.

#include <stdbool.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>

#ifdef RYU_DEBUG
#include <inttypes.h>
#include <stdio.h>
#endif

#include "ryu/common.h"
#include "ryu/d2s_intrinsics.h"

#define DOUBLE_MANTISSA_BITS 52
#define DOUBLE_EXPONENT_BITS 11
#define DOUBLE_EXPONENT_BIAS 1023

#if defined(_MSC_VER)
#include <intrin.h>

static inline uint32_t floor_log2(const uint64_t value) {
  long index;
  return _BitScanReverse64(&index, value) ? index : 64;
}

#else

static inline uint32_t floor_log2(const uint64_t value) {
  return 63 - __builtin_clzll(value);
}

#endif

// The max function is already defined on Windows.
static inline int32_t max32(int32_t a, int32_t b) {
  return a < b ? b : a;
}

static inline double int64Bits2Double(uint64_t bits) {
  double f;
  memcpy(&f, &bits, sizeof(double));
  return f;
}

double
__atod_engine(uint64_t m10, int e10)
{
#ifdef RYU_DEBUG
    printf("m10 = %ld\n", m10);
    printf("e10 = %d\n", e10);
    printf("m10 * 10^e10 = %" PRIu64 " * 10^%d\n", m10, e10);
#endif

    // Convert to binary float m2 * 2^e2, while retaining information about whether the conversion
    // was exact (trailingZeros).
    int32_t e2;
    uint64_t m2;
    bool trailingZeros;
    if (e10 >= 0) {
	// The length of m * 10^e in bits is:
	//   log2(m10 * 10^e10) = log2(m10) + e10 log2(10) = log2(m10) + e10 + e10 * log2(5)
	//
	// We want to compute the DOUBLE_MANTISSA_BITS + 1 top-most bits (+1 for the implicit leading
	// one in IEEE format). We therefore choose a binary output exponent of
	//   log2(m10 * 10^e10) - (DOUBLE_MANTISSA_BITS + 1).
	//
	// We use floor(log2(5^e10)) so that we get at least this many bits; better to
	// have an additional bit than to not have enough bits.
	e2 = floor_log2(m10) + e10 + log2pow5(e10) - (DOUBLE_MANTISSA_BITS + 1);

	// We now compute [m10 * 10^e10 / 2^e2] = [m10 * 5^e10 / 2^(e2-e10)].
	// To that end, we use the DOUBLE_POW5_SPLIT table.
	int j = e2 - e10 - ceil_log2pow5(e10) + DOUBLE_POW5_BITCOUNT;
	assert(j >= 0);
	uint64_t pow5[2];
	__double_computePow5(e10, pow5);
	m2 = mulShift64(m10, pow5, j);

	// We also compute if the result is exact, i.e.,
	//   [m10 * 10^e10 / 2^e2] == m10 * 10^e10 / 2^e2.
	// This can only be the case if 2^e2 divides m10 * 10^e10, which in turn requires that the
	// largest power of 2 that divides m10 + e10 is greater than e2. If e2 is less than e10, then
	// the result must be exact. Otherwise we use the existing multipleOfPowerOf2 function.
	trailingZeros = e2 < e10 || (e2 - e10 < 64 && multipleOfPowerOf2(m10, e2 - e10));
    } else {
	e2 = floor_log2(m10) + e10 - ceil_log2pow5(-e10) - (DOUBLE_MANTISSA_BITS + 1);
	int j = e2 - e10 + ceil_log2pow5(-e10) - 1 + DOUBLE_POW5_INV_BITCOUNT;
	uint64_t pow5[2];
	__double_computeInvPow5(-e10, pow5);
	m2 = mulShift64(m10, pow5, j);
	trailingZeros = multipleOfPowerOf5(m10, -e10);
#ifdef RYU_DEBUG
	printf("pow5 %016lx_%016lx j %d trailingZeros %d\n", pow5[0], pow5[1], j, trailingZeros);
#endif
    }

#ifdef RYU_DEBUG
    printf("m2 * 2^e2 = %" PRIu64 " * 2^%d\n", m2, e2);
#endif

    // Compute the final IEEE exponent.
    uint32_t ieee_e2 = (uint32_t) max32(0, e2 + DOUBLE_EXPONENT_BIAS + floor_log2(m2));

    if (ieee_e2 > 0x7fe) {
	// Final IEEE exponent is larger than the maximum representable; return +/-Infinity.
	uint64_t ieee = (0x7ffull << DOUBLE_MANTISSA_BITS);
	return int64Bits2Double(ieee);
    }

    // We need to figure out how much we need to shift m2. The tricky part is that we need to take
    // the final IEEE exponent into account, so we need to reverse the bias and also special-case
    // the value 0.
    int32_t shift = (ieee_e2 == 0 ? 1 : ieee_e2) - e2 - DOUBLE_EXPONENT_BIAS - DOUBLE_MANTISSA_BITS;
    assert(shift >= 0);
#ifdef RYU_DEBUG
    printf("ieee_e2 = %d\n", ieee_e2);
    printf("shift = %d\n", shift);
#endif
  
    // We need to round up if the exact value is more than 0.5 above the value we computed. That's
    // equivalent to checking if the last removed bit was 1 and either the value was not just
    // trailing zeros or the result would otherwise be odd.
    //
    // We need to update trailingZeros given that we have the exact output exponent ieee_e2 now.
    trailingZeros &= (m2 & ((1ull << (shift - 1)) - 1)) == 0;
    uint64_t lastRemovedBit = (m2 >> (shift - 1)) & 1;
    bool roundUp = (lastRemovedBit != 0) && (!trailingZeros || (((m2 >> shift) & 1) != 0));

#ifdef RYU_DEBUG
    printf("roundUp = %d\n", roundUp);
    printf("ieee_m2 = %" PRIu64 "\n", (m2 >> shift) + roundUp);
#endif
    uint64_t ieee_m2 = (m2 >> shift) + roundUp;
    assert(ieee_m2 <= (1ull << (DOUBLE_MANTISSA_BITS + 1)));
    ieee_m2 &= (1ull << DOUBLE_MANTISSA_BITS) - 1;
    if (ieee_m2 == 0 && roundUp) {
	// Due to how the IEEE represents +/-Infinity, we don't need to check for overflow here.
	ieee_e2++;
    }
    uint64_t ieee = (((uint64_t)ieee_e2) << DOUBLE_MANTISSA_BITS) | ieee_m2;
    return int64Bits2Double(ieee);
}

