/// efficient double to text conversion using the GRISU-1 algorithm
// - as a complement to SynCommons, which tended to increase too much
// - licensed under a MPL/GPL/LGPL tri-license; version 1.18

{
    Implement 64-bit floating point (double) to ASCII conversion using the
    GRISU-1 efficient algorithm.

    Original Code in flt_core.inc flt_conv.inc flt_pack.inc from FPC RTL.
    Copyright (C) 2013 by Max Nazhalov
    Licenced with LGPL 2 with the linking exception.
    If you don't agree with these License terms, disable this feature
    by undefining DOUBLETOSHORT_USEGRISU in Synopse.inc

    GRISU Original Algorithm
    Copyright (c) 2009 Florian Loitsch

    We extracted a double-to-ascii only cut-down version of those files,
    and made a huge refactoring to reach the best performance, especially
    tuning the Intel target with some dedicated asm and code rewrite.

  With Delphi 10.3 on Win32: (no benefit)
   100000 FloatToText    in 38.11ms i.e. 2,623,570/s, aver. 0us, 47.5 MB/s
   100000 str            in 43.19ms i.e. 2,315,082/s, aver. 0us, 50.7 MB/s
   100000 DoubleToShort  in 45.50ms i.e. 2,197,367/s, aver. 0us, 43.8 MB/s
   100000 DoubleToAscii  in 42.44ms i.e. 2,356,045/s, aver. 0us, 47.8 MB/s

  With Delphi 10.3 on Win64:
   100000 FloatToText    in 61.83ms i.e. 1,617,233/s, aver. 0us, 29.3 MB/s
   100000 str            in 53.20ms i.e. 1,879,663/s, aver. 0us, 41.2 MB/s
   100000 DoubleToShort  in 18.45ms i.e. 5,417,998/s, aver. 0us, 108 MB/s
   100000 DoubleToAscii  in 18.19ms i.e. 5,496,921/s, aver. 0us, 111.5 MB/s

  With FPC on Win32:
   100000 FloatToText    in 115.62ms i.e.  864,842/s, aver. 1us, 15.6 MB/s
   100000 str            in 57.30ms i.e. 1,745,109/s, aver. 0us, 39.9 MB/s
   100000 DoubleToShort  in 23.88ms i.e. 4,187,078/s, aver. 0us, 83.5 MB/s
   100000 DoubleToAscii  in 23.34ms i.e. 4,284,490/s, aver. 0us, 86.9 MB/s

  With FPC on Win64:
   100000 FloatToText    in 76.92ms i.e. 1,300,052/s, aver. 0us, 23.5 MB/s
   100000 str            in 27.70ms i.e. 3,609,456/s, aver. 0us, 82.6 MB/s
   100000 DoubleToShort  in 14.73ms i.e. 6,787,944/s, aver. 0us, 135.4 MB/s
   100000 DoubleToAscii  in 13.78ms i.e. 7,253,735/s, aver. 0us, 147.2 MB/s

  With FPC on Linux x86_64:
   100000 FloatToText    in 81.48ms i.e. 1,227,249/s, aver. 0us, 22.2 MB/s
   100000 str            in 36.98ms i.e. 2,703,871/s, aver. 0us, 61.8 MB/s
   100000 DoubleToShort  in 13.11ms i.e. 7,626,601/s, aver. 0us, 152.1 MB/s
   100000 DoubleToAscii  in 12.59ms i.e. 7,942,180/s, aver. 0us, 161.2 MB/s

  - Our rewrite is twice faster than original flt_conv.inc from FPC RTL (str)
  - Delphi Win32 has trouble making 64-bit computation - no benefit since it
    has good optimized i87 asm (but slower than our code with FPC/Win32)
  - FPC is more efficient when compiling integer arithmetic; we avoided slow
    division by calling our Div100(), but Delphi Win64 is still far behind
  - Delphi Win64 has very slow FloatToText and str()

}


// Controls printing of NaN-sign.
// Undefine to print NaN sign during float->ASCII conversion.
// IEEE does not interpret the sign of a NaN, so leave it defined.
{$define GRISU1_F2A_NAN_SIGNLESS}

// Controls rounding of generated digits when formatting with narrowed
// width (either fixed or exponential notation).
// Traditionally, FPC and BP7/Delphi use "roundTiesToAway" mode.
// Undefine to use "roundTiesToEven" approach.
{$define GRISU1_F2A_HALF_ROUNDUP}

// This one is a hack against Grusu sub-optimality.
// It may be used only strictly together with GRISU1_F2A_HALF_ROUNDUP.
// It does not violate most general rules due to the fact that it is
// applicable only when formatting with narrowed width, where the fine
// view is more desirable, and the precision is already lost, so it can
// be used in general-purpose applications.
// Refer to its implementation.
{$define GRISU1_F2A_AGRESSIVE_ROUNDUP} // Defining this fixes several tests.

// Undefine to enable SNaN support.
// Note: IEEE [754-2008, page 31] requires (1) to recognize "SNaN" during
// ASCII->float, and (2) to generate the "invalid FP operation" exception
// either when SNaN is printed as "NaN", or "SNaN" is evaluated to QNaN,
// so it would be preferable to undefine these settings,
// but the FPC RTL is not ready for this right now..
{$define GRISU1_F2A_NO_SNAN}

/// If Value=0 would just store '0', whatever frac_digits is supplied.
{$define GRISU1_F2A_ZERONOFRACT}


{$ifndef FPC}

// those functions are intrinsics with FPC :)

function BSRdword(c: cardinal): cardinal;
asm
    {$ifdef CPU64}
    .noframe
    mov eax, c
    {$endif}
    bsr eax, eax
end; // in our code below, we are sure that c<>0

function BSRqword(const q: qword): cardinal;
asm
    {$ifdef CPU32}
    bsr eax, [esp + 8]
    jz  @1
    add eax, 32
    ret
@1: bsr eax, [esp + 4]
@2: {$else}
    .noframe
    mov rax, q
    bsr rax, rax
    {$endif}
end; // in our code below, we are sure that q<>0


{$endif FPC}

const
  // TFloatFormatProfile for double
  nDig_mantissa = 17;
  nDig_exp10 = 3;

type
    // "Do-It-Yourself Floating Point" structures
  TDIY_FP = record
    f: qword;
    e: integer;
  end;

  TDIY_FP_Power_of_10 = record
    c: TDIY_FP;
    e10: integer;
  end;
  PDIY_FP_Power_of_10 = ^TDIY_FP_Power_of_10;

const
  ROUNDER = $80000000;

{$ifdef CPUINTEL} // our faster version using 128-bit x86_64 multiplication

procedure d2a_diy_fp_multiply(var x, y: TDIY_FP; normalize: boolean;
  out result: TDIY_FP); {$ifdef HASINLINE} inline; {$endif}
var
  p: THash128Rec;
begin
  mul64x64(x.f, y.f, p); // fast x86_64 / i386 asm
  if (p.c1 and ROUNDER) <>  0 then
    inc(p.h);
  result.f := p.h;
  result.e := PtrInt(x.e) + PtrInt(y.e) + 64;
  if normalize then
    if (PQWordRec(@result.f)^.h and ROUNDER) = 0 then
    begin
      result.f := result.f * 2;
      dec(result.e);
    end;
end;

{$else} // regular Grisu method - optimized for 32-bit CPUs

procedure d2a_diy_fp_multiply(var x, y: TDIY_FP; normalize: boolean; out result: TDIY_FP);
var
  _x: TQWordRec absolute x;
  _y: TQWordRec absolute y;
  r: TQWordRec absolute result;
  ac, bc, ad, bd, t1: TQWordRec;
begin
  ac.v := qword(_x.h) * _y.h;
  bc.v := qword(_x.l) * _y.h;
  ad.v := qword(_x.h) * _y.l;
  bd.v := qword(_x.l) * _y.l;
  t1.v := qword(ROUNDER) + bd.h + bc.l + ad.l;
  result.f := ac.v + ad.h + bc.h + t1.h;
  result.e := x.e + y.e + 64;
  if normalize then
    if (r.h and ROUNDER) = 0 then
    begin
      inc(result.f, result.f);
      dec(result.e);
    end;
end;

{$endif CPUINTEL}

const
  // alpha =-61; gamma = 0
  // full cache: 1E-450 .. 1E+432, step = 1E+18
  // sparse = 1/10
  C_PWR10_DELTA = 18;
  C_PWR10_COUNT = 50;

type
  TDIY_FP_Cached_Power10 = record
    base:         array [ 0 .. 9 ] of TDIY_FP_Power_of_10;
    factor_plus:  array [ 0 .. 1 ] of TDIY_FP_Power_of_10;
    factor_minus: array [ 0 .. 1 ] of TDIY_FP_Power_of_10;
    // extra mantissa correction [ulp; signed]
    corrector:    array [ 0 .. C_PWR10_COUNT - 1 ] of shortint;
  end;

const
  CACHED_POWER10: TDIY_FP_Cached_Power10 = (
    base: (
        ( c: ( f: qword($825ECC24C8737830); e: -362 ); e10:  -90 ),
        ( c: ( f: qword($E2280B6C20DD5232); e: -303 ); e10:  -72 ),
        ( c: ( f: qword($C428D05AA4751E4D); e: -243 ); e10:  -54 ),
        ( c: ( f: qword($AA242499697392D3); e: -183 ); e10:  -36 ),
        ( c: ( f: qword($9392EE8E921D5D07); e: -123 ); e10:  -18 ),
        ( c: ( f: qword($8000000000000000); e:  -63 ); e10:    0 ),
        ( c: ( f: qword($DE0B6B3A76400000); e:   -4 ); e10:   18 ),
        ( c: ( f: qword($C097CE7BC90715B3); e:   56 ); e10:   36 ),
        ( c: ( f: qword($A70C3C40A64E6C52); e:  116 ); e10:   54 ),
        ( c: ( f: qword($90E40FBEEA1D3A4B); e:  176 ); e10:   72 )
    );
    factor_plus: (
        ( c: ( f: qword($F6C69A72A3989F5C); e:   534 ); e10:  180 ),
        ( c: ( f: qword($EDE24AE798EC8284); e:  1132 ); e10:  360 )
    );
    factor_minus: (
        ( c: ( f: qword($84C8D4DFD2C63F3B); e:  -661 ); e10: -180 ),
        ( c: ( f: qword($89BF722840327F82); e: -1259 ); e10: -360 )
    );
    corrector: (
        0,  0,  0,  0,  1,  0,  0,  0,  1, -1,
        0,  1,  1,  1, -1,  0,  0,  1,  0, -1,
        0,  0,  0,  0,  0,  0,  0,  0,  0,  0,
       -1,  0,  0, -1,  0,  0,  0,  0,  0, -1,
        0,  0,  0,  0,  1,  0,  0,  0, -1,  0
    ));
  CACHED_POWER10_MIN10 = -90 -360;
  // = ref.base[low(ref.base)].e10 + ref.factor_minus[high(ref.factor_minus)].e10

// return normalized correctly rounded approximation of the power of 10
// scaling factor, intended to shift a binary exponent of the original number
// into selected [ alpha .. gamma ] range
procedure d2a_diy_fp_cached_power10(exp10: integer; out factor: TDIY_FP_Power_of_10);
var
  i, xmul: integer;
  A, B: PDIY_FP_Power_of_10;
  cx: PtrInt;
  ref: ^TDIY_FP_Cached_Power10;
begin
  ref := @CACHED_POWER10; // much better code generation on PIC/x86_64
  // find non-sparse index
  if exp10 <= CACHED_POWER10_MIN10 then
    i := 0
  else
  begin
    i := (exp10 - CACHED_POWER10_MIN10) div C_PWR10_DELTA;
    if i * C_PWR10_DELTA + CACHED_POWER10_MIN10 <> exp10 then
      inc(i); // round-up
    if i > C_PWR10_COUNT - 1 then
      i := C_PWR10_COUNT - 1;
  end;
  // generate result
  xmul := i div length(ref.base);
  A := @ref.base[i - (xmul * length(ref.base))]; // fast mod
  dec(xmul, length(ref.factor_minus));
  if xmul = 0 then
  begin
    // base
    factor := A^;
    exit;
  end;
  // surrogate
  if xmul > 0 then
  begin
    dec(xmul);
    B := @ref.factor_plus[xmul];
  end
  else
  begin
    xmul := -(xmul + 1);
    B := @ref.factor_minus[xmul];
  end;
  factor.e10 := A.e10 + B.e10;
  if A.e10 <> 0 then
  begin
    d2a_diy_fp_multiply(A.c, B.c, true, factor.c);
    // adjust mantissa
    cx := ref.corrector[i];
    if cx <> 0 then
      inc(int64(factor.c.f), int64(cx));
  end
  else
    // exact
    factor.c := B^.c;
end;


procedure d2a_unpack_float(const f: double; out minus: boolean; out result: TDIY_FP);
  {$ifdef HASINLINE} inline;{$endif}
type
  TSplitFloat = packed record
    case byte of
      0: (f: double);
      1: (b: array[0..7] of byte);
      2: (w: array[0..3] of word);
      3: (d: array[0..1] of cardinal);
      4: (l: qword);
  end;
var
  doublebits: TSplitFloat;
begin
{$ifdef FPC_DOUBLE_HILO_SWAPPED}
  // high and low cardinal are swapped when using the arm fpa
  doublebits.d[0] := TSplitFloat(f).d[1];
  doublebits.d[1] := TSplitFloat(f).d[0];
{$else not FPC_DOUBLE_HILO_SWAPPED}
  doublebits.f := f;
{$endif FPC_DOUBLE_HILO_SWAPPED}
{$ifdef endian_big}
  minus := (doublebits.b[0] and $80 <> 0);
  result.e := (doublebits.w[0] shr 4) and $7FF;
{$else endian_little}
  minus := (doublebits.b[7] and $80 <> 0);
  result.e := (doublebits.w[3] shr 4) and $7FF;
{$endif endian}
  result.f := doublebits.l and $000FFFFFFFFFFFFF;
end;

const
  C_FRAC2_BITS = 52;
  C_EXP2_BIAS = 1023;
  C_DIY_FP_Q = 64;
  C_GRISU_ALPHA = -61;
  C_GRISU_GAMMA = 0;

  C_EXP2_SPECIAL = C_EXP2_BIAS * 2 + 1;
  C_MANT2_INTEGER = qword(1) shl C_FRAC2_BITS;

type
  TAsciiDigits = array[0..47] of byte;
  PAsciiDigits = ^TAsciiDigits;

// convert unsigned integers into decimal digits

{$ifdef FPC_64} // leverage efficient FPC 64-bit division as mul reciprocal

function d2a_gen_digits_64(buf: PAsciiDigits; x: qword): PtrInt;
var
  tab: PWordArray;
  P: PAnsiChar;
  c100: qword;
begin
  tab := @TwoDigitByteLookupW; // 0..99 value -> two byte digits (0..9)
  P := PAnsiChar(@buf[24]); // append backwards
  repeat
    if x >= 100 then
    begin
      dec(P, 2);
      c100 := x div 100;
      dec(x, c100 * 100);
      PWord(P)^ := tab[x];
      if c100 = 0 then
        break;
      x := c100;
      continue;
    end;
    if x < 10 then
    begin
      dec(P);
      P^ := AnsiChar(x);
      break;
    end;
    dec(P, 2);
    PWord(P)^ := tab[x];
    break;
  until false;
  PQWordArray(buf)[0] := PQWordArray(P)[0]; // faster than MoveSmall(P,buf,result)
  PQWordArray(buf)[1] := PQWordArray(P)[1];
  PQWordArray(buf)[2] := PQWordArray(P)[2];
  result := PAnsiChar(@buf[24]) - P;
end;

{$else not FPC_64} // use three 32-bit groups of digit

function d2a_gen_digits_32(buf: PAsciiDigits; x: dword; pad_9zero: boolean): PtrInt;
const
  digits: array[0..9] of cardinal = (
    0, 10, 100, 1000, 10000, 100000, 1000000, 10000000, 100000000, 1000000000);
var
  n: PtrInt;
  m: cardinal;
  {$ifdef FPC}
  z: cardinal;
  {$else}
  d100: TDiv100Rec;
  {$endif FPC}
  tab: PWordArray;
begin
  // Calculate amount of digits
  if x = 0 then
    n := 0  // emit nothing if padding is not required
  else
  begin
    n := integer((BSRdword(x) + 1) * 1233) shr 12;
    if x >= digits[n] then
      inc(n);
  end;
  if pad_9zero and (n < 9) then
    n := 9;
  result := n;
  if n = 0 then
    exit;
  // Emit digits
  dec(PByte(buf));
  tab := @TwoDigitByteLookupW;
  m := x;
  while (n >= 2) and (m <> 0) do
  begin
    dec(n);
    {$ifdef FPC} // FPC will use fast mul reciprocal
    z := m div 100; // compute two 0..9 digits
    PWord(@buf[n])^ := tab^[m - z * 100];
    m := z;
    {$else}
    Div100(m, d100); // our asm is faster than Delphi div operation
    PWord(@buf[n])^ := tab^[d100.M];
    m := d100.D;
    {$endif FPC}
    dec(n);
  end;
  if n = 0 then
    exit;
  if m <> 0 then
  begin
    if m > 9 then
      m := m mod 10; // compute last 0..9 digit
    buf[n] := m; 
    dec(n);
    if n = 0 then
      exit;
  end;
  repeat
    buf[n] := 0; // padding with 0
    dec(n);
  until n = 0;
end;

function d2a_gen_digits_64(buf: PAsciiDigits; const x: qword): PtrInt;
var
  n_digits: PtrInt;
  temp: qword;
  splitl, splitm, splith: cardinal;
begin
  // Split X into 3 unsigned 32-bit integers; lower two should be < 10 digits long
  n_digits := 0;
  if x < 1000000000 then
    splitl := x
  else
  begin
    temp := x div 1000000000;
    splitl := x - temp * 1000000000;
    if temp < 1000000000 then
      splitm := temp
    else
    begin
      splith := temp div 1000000000;
      splitm := cardinal(temp) - splith * 1000000000;
      n_digits := d2a_gen_digits_32(buf, splith, false); // Generate hi digits
    end;
    inc(n_digits, d2a_gen_digits_32(@buf[n_digits], splitm, n_digits <> 0));
  end;
  // Generate digits
  inc(n_digits, d2a_gen_digits_32(@buf[n_digits], splitl, n_digits <> 0));
  result := n_digits;
end;

{$endif FPC_64}

// Performs digit sequence rounding, returns decimal point correction
function d2a_round_digits(var buf: TAsciiDigits; var n_current: integer;
  n_max: PtrInt; half_round_to_even: boolean = true): PtrInt;
var
  n: PtrInt;
  dig_round, dig_sticky: byte;
  {$ifdef GRISU1_F2A_AGRESSIVE_ROUNDUP}
  i: PtrInt;
  {$endif}
begin
  result := 0;
  n := n_current;
  n_current := n_max;
  // Get round digit
  dig_round := buf[n_max];
{$ifdef GRISU1_F2A_AGRESSIVE_ROUNDUP}
  // Detect if rounding-up the second last digit turns the "dig_round"
  // into "5"; also make sure we have at least 1 digit between "dig_round"
  // and the second last.
  if not half_round_to_even then
    if (dig_round = 4) and (n_max < n - 3) then
      if buf[n - 2] >= 8 then // somewhat arbitrary...
      begin
        // check for only "9" are in between
        i := n - 2;
        repeat
          dec(i);
        until (i = n_max) or (buf[i] <> 9);
        if i = n_max then
          // force round-up
          dig_round := 9; // any value ">=5"
      end;
{$endif GRISU1_F2A_AGRESSIVE_ROUNDUP}
  if dig_round < 5 then
    exit;
  // Handle "round half to even" case
  if (dig_round = 5) and half_round_to_even and
     ((n_max = 0) or (buf[n_max - 1] and 1 = 0)) then
  begin
    // even and a half: check if exactly the half
    dig_sticky := 0;
    while (n > n_max + 1) and (dig_sticky = 0) do
    begin
      dec(n);
      dig_sticky := buf[n];
    end;
    if dig_sticky = 0 then
      exit; // exactly a half -> no rounding is required
  end;
  // Round-up
  while n_max > 0 do
  begin
    dec(n_max);
    inc(buf[n_max]);
    if buf[n_max] < 10 then
    begin
      // no more overflow: stop now
      n_current := n_max + 1;
      exit;
    end;
    // continue rounding
  end;
  // Overflow out of the 1st digit, all n_max digits became 0
  buf[0] := 1;
  n_current := 1;
  result := 1;
end;


// format the number in the fixed-point representation
procedure d2a_return_fixed(str: PAnsiChar; minus: boolean; var digits: TAsciiDigits;
  n_digits_have, fixed_dot_pos, frac_digits: integer);
var
  p: PAnsiChar;
  d: PByte;
  cut_digits_at, n_before_dot, n_before_dot_pad0, n_after_dot_pad0,
  n_after_dot, n_tail_pad0: integer;
begin
  // Round digits if necessary
  cut_digits_at := fixed_dot_pos + frac_digits;
  if cut_digits_at < 0 then
    // zero
    n_digits_have := 0
  else if cut_digits_at < n_digits_have then
    // round digits
    inc(fixed_dot_pos, d2a_round_digits(digits, n_digits_have, cut_digits_at
      {$ifdef GRISU1_F2A_HALF_ROUNDUP}, false {$endif} ));
  // Before dot: digits, pad0
  if (fixed_dot_pos <= 0) or (n_digits_have = 0) then
  begin
    n_before_dot := 0;
    n_before_dot_pad0 := 1;
  end
  else if fixed_dot_pos > n_digits_have then
  begin
    n_before_dot := n_digits_have;
    n_before_dot_pad0 := fixed_dot_pos - n_digits_have;
  end
  else
  begin
    n_before_dot := fixed_dot_pos;
    n_before_dot_pad0 := 0;
  end;
  // After dot: pad0, digits, pad0
  if fixed_dot_pos < 0 then
    n_after_dot_pad0 := -fixed_dot_pos
  else
    n_after_dot_pad0 := 0;
  if n_after_dot_pad0 > frac_digits then
    n_after_dot_pad0 := frac_digits;
  n_after_dot := n_digits_have - n_before_dot;
  n_tail_pad0 := frac_digits - n_after_dot - n_after_dot_pad0;
  p := str + 1;
  // Sign
  if minus then
  begin
    p^ := '-';
    inc(p);
  end;
  // Integer significant digits
  d := @digits;
  if n_before_dot > 0 then
    repeat
      p^ := AnsiChar(d^ + ord('0'));
      inc(p);
      inc(d);
      dec(n_before_dot);
    until n_before_dot = 0;
  // Integer 0-padding
  if n_before_dot_pad0 > 0 then
    repeat
      p^ := '0';
      inc(p);
      dec(n_before_dot_pad0);
    until n_before_dot_pad0 = 0;
  //
  if frac_digits <> 0 then
  begin
    // Dot
    p^ := '.';
    inc(p);
    // Pre-fraction 0-padding
    if n_after_dot_pad0 > 0 then
      repeat
        p^ := '0';
        inc(p);
        dec(n_after_dot_pad0);
      until n_after_dot_pad0 = 0;
    // Fraction significant digits
    if n_after_dot > 0 then
      repeat
        p^ := AnsiChar(d^ + ord('0'));
        inc(p);
        inc(d);
        dec(n_after_dot);
      until n_after_dot = 0;
    // Tail 0-padding
    if n_tail_pad0 > 0 then
      repeat
        p^ := '0';
        inc(p);
        dec(n_tail_pad0);
      until n_tail_pad0 = 0;
  end;
  // Store length
  str[0] := AnsiChar(p - str - 1);
end;

// formats the number as exponential representation
procedure d2a_return_exponential(str: PAnsiChar; minus: boolean;
  digits: PByte; n_digits_have, n_digits_req, d_exp: PtrInt);
var
  p, exp: PAnsiChar;
begin
  p := str + 1;
  // Sign
  if minus then
  begin
    p^ := '-';
    inc(p);
  end;
  // Integer part
  if n_digits_have > 0 then
  begin
    p^ := AnsiChar(digits^ + ord('0'));
    dec(n_digits_have);
  end
  else
    p^ := '0';
  inc(p);
  // Dot
  if n_digits_req > 1 then
  begin
    p^ := '.';
    inc(p);
  end;
  // Fraction significant digits
  if n_digits_req < n_digits_have then
    n_digits_have := n_digits_req;
  if n_digits_have > 0 then
  begin
    repeat
      inc(digits);
      p^ := AnsiChar(digits^ + ord('0'));
      inc(p);
      dec(n_digits_have);
    until n_digits_have = 0;
    while p[-1] = '0' do
      dec(p); // trim #.###00000 -> #.###
    if p[-1] = '.' then
      dec(p); // #.0 -> #
  end;
  // Exponent designator
  p^ := 'E';
  inc(p);
  // Exponent sign (+ is not stored, as in Delphi)
  if d_exp < 0 then
  begin
    p^ := '-';
    d_exp := -d_exp;
    inc(p);
  end;
  // Exponent digits
  exp := pointer(SmallUInt32UTF8[d_exp]); // 0..999 range is fine
  PCardinal(p)^ := PCardinal(exp)^;
  inc(p, PStrLen(exp - _STRLEN)^);
  // Store length
  str[0] := AnsiChar(p - str - 1);
end;

/// set one of special results with proper sign
procedure d2a_return_special(str: PAnsiChar; sign: integer; const spec: shortstring);
begin
  // Compute length
  str[0] := spec[0];
  if sign <> 0 then
    inc(str[0]);
  inc(str);
  // Sign
  if sign <> 0 then
  begin
    if sign > 0 then
      str^ := '+'
    else
      str^ := '-';
    inc(str);
  end;
  // Special text (3 chars)
  PCardinal(str)^ := PCardinal(@spec[1])^;
end;


// Calculates the exp10 of a factor required to bring the binary exponent
// of the original number into selected [ alpha .. gamma ] range:
// result := ceiling[ ( alpha - e ) * log10(2) ]
function d2a_k_comp(e, alpha{, gamma}: integer): integer;
var
  dexp: double;
const
  D_LOG10_2: double = 0.301029995663981195213738894724493027; // log10(2)
var
  x, n: integer;
begin
  x := alpha - e;
  dexp := x * D_LOG10_2;
  // ceil( dexp )
  n := trunc(dexp);
  if x > 0 then
    if dexp <> n then
      inc(n); // round-up
  result := n;
end;


/// raw function to convert a 64-bit double into a shortstring, stored in str
// - implements Fabian Loitsch's Grisu algorithm dedicated to double values
// - currently, SynCommnons only set min_width=0 (for DoubleToShortNoExp to avoid
// any scientific notation ) or min_width=C_NO_MIN_WIDTH (for DoubleToShort to
// force the scientific notation when the double cannot be represented as
// a simple fractinal number)
procedure DoubleToAscii(min_width, frac_digits: integer; const v: double; str: PAnsiChar);
var
  w, D: TDIY_FP;
  c_mk: TDIY_FP_Power_of_10;
  n, mk, dot_pos, n_digits_need, n_digits_have: integer;
  n_digits_req, n_digits_sci: integer;
  minus: boolean;
  fl, one_maskl: qword;
  one_e: integer;
  {$ifdef CPU32}
  one_mask, f: cardinal; // run a 2nd loop with 32-bit range
  {$endif CPU32}
  buf: TAsciiDigits;
begin
  // Limit parameters
  if frac_digits > 216 then
    frac_digits := 216; // Delphi compatible
  if min_width <= C_NO_MIN_WIDTH then
    min_width := -1 // no minimal width
  else if min_width < 0 then
    min_width := 0; // minimal width is as short as possible
  // Format profile: select "n_digits_need" (and "n_digits_exp")
  n_digits_req := nDig_mantissa;
  // number of digits to be calculated by Grisu
  n_digits_need := nDig_mantissa;
  if n_digits_req < n_digits_need then
    n_digits_need := n_digits_req;
  // number of mantissa digits to be printed in exponential notation
  if min_width < 0 then
    n_digits_sci := n_digits_req
  else
  begin
    n_digits_sci := min_width -1 {sign} -1 {dot} -1 {E} -1 {E-sign} - nDig_exp10;
    if n_digits_sci < 2 then
      n_digits_sci := 2; // at least 2 digits
    if n_digits_sci > n_digits_req then
      n_digits_sci := n_digits_req; // at most requested by real_type
  end;
  // Float -> DIY_FP
  d2a_unpack_float(v, minus, w);
  // Handle Zero
  if (w.e = 0) and (w.f = 0) then
  begin
    {$ifdef GRISU1_F2A_ZERONOFRACT}
    PWord(str)^ := 1 + ord('0') shl 8; // just return '0'
    {$else}
    if frac_digits >= 0 then
      d2a_return_fixed(str, minus, buf, 0, 1, frac_digits)
    else
      d2a_return_exponential(str, minus, @buf, 0, n_digits_sci, 0);
    {$endif GRISU1_F2A_ZERONOFRACT}
    exit;
  end;
  // Handle specials
  if w.e = C_EXP2_SPECIAL then
  begin
    n := 1 - ord(minus) * 2; // default special sign [-1|+1]
    if w.f = 0 then
      d2a_return_special(str, n, C_STR_INF)
    else
    begin
      // NaN [also pseudo-NaN, pseudo-Inf, non-normal for floatx80]
      {$ifdef GRISU1_F2A_NAN_SIGNLESS}
      n := 0;
      {$endif}
      {$ifndef GRISU1_F2A_NO_SNAN}
      if (w.f and (C_MANT2_INTEGER shr 1)) = 0 then
        return_special(str, n, C_STR_SNAN)
      else
      {$endif GRISU1_F2A_NO_SNAN}
        d2a_return_special(str, n, C_STR_QNAN);
    end;
    exit;
  end;
  // Handle denormals
  if w.e <> 0 then
  begin
    // normal
    w.f := w.f or C_MANT2_INTEGER;
    n := C_DIY_FP_Q - C_FRAC2_BITS - 1;
  end
  else
  begin
    // denormal (w.e=0)
    n := 63 - BSRqword(w.f); // we are sure that w.f<>0 - see Handle Zero above
    inc(w.e);
  end;
  // Final normalization
  w.f := w.f shl n;
  dec(w.e, C_EXP2_BIAS + n + C_FRAC2_BITS);
  // 1. Find the normalized "c_mk = f_c * 2^e_c" such that
  //    "alpha <= e_c + e_w + q <= gamma"
  // 2. Define "V = D * 10^k": multiply the input number by "c_mk", do not
  //    normalize to land into [ alpha .. gamma ]
  // 3. Generate digits ( n_digits_need + "round" )
  if (C_GRISU_ALPHA <= w.e) and (w.e <= C_GRISU_GAMMA) then
  begin
    // no scaling required
    D := w;
    c_mk.e10 := 0;
  end
  else
  begin
    mk := d2a_k_comp(w.e, C_GRISU_ALPHA{, C_GRISU_GAMMA} );
    d2a_diy_fp_cached_power10(mk, c_mk);
    // Let "D = f_D * 2^e_D := w (*) c_mk"
    if c_mk.e10 = 0 then
      D := w
    else
      d2a_diy_fp_multiply(w, c_mk.c, false, D);
  end;
  // Generate digits: integer part
  n_digits_have := d2a_gen_digits_64(@buf, D.f shr (-D.e));
  dot_pos := n_digits_have;
  // Generate digits: fractional part
  {$ifdef CPU32}
  f := 0; // "sticky" digit
  {$endif CPU32}
  if D.e < 0 then
    repeat
      // MOD by ONE
      one_e := D.e;
      one_maskl := qword(1) shl (-D.e) - 1;
      fl := D.f and one_maskl;
      // 64-bit loop (very efficient on x86_64, slower on i386)
      while {$ifdef CPU32} (one_e < -29) and {$endif}
            (n_digits_have < n_digits_need + 1) and (fl <> 0) do
      begin
        // f := f * 5;
        inc(fl, fl shl 2);
        // one := one / 2
        one_maskl := one_maskl shr 1;
        inc(one_e);
        // DIV by one
        buf[n_digits_have] := fl shr (-one_e);
        // MOD by one
        fl := fl and one_maskl;
        // next
        inc(n_digits_have);
      end;
      {$ifdef CPU32}
      if n_digits_have >= n_digits_need + 1 then
      begin
        // only "sticky" digit remains
        f := ord(fl <> 0);
        break;
      end;
      one_mask := cardinal(one_maskl);
      f := cardinal(fl);
      // 32-bit loop
      while (n_digits_have < n_digits_need + 1) and (f <> 0) do
      begin
        // f := f * 5;
        inc(f, f shl 2);
        // one := one / 2
        one_mask := one_mask shr 1;
        inc(one_e);
        // DIV by one
        buf[n_digits_have] := f shr (-one_e);
        // MOD by one
        f := f and one_mask;
        // next
        inc(n_digits_have);
      end;
      {$endif CPU32}
    until true;
  {$ifdef CPU32}
  // Append "sticky" digit if any
  if (f <> 0) and (n_digits_have >= n_digits_need + 1) then
  begin
    // single "<>0" digit is enough
    n_digits_have := n_digits_need + 2;
    buf[n_digits_need + 1] := 1;
  end;
  {$endif CPU32}
  // Round to n_digits_need using "roundTiesToEven"
  if n_digits_have > n_digits_need then
    inc(dot_pos, d2a_round_digits(buf, n_digits_have, n_digits_need));
  // Generate output
  if frac_digits >= 0 then
  begin
    d2a_return_fixed(str, minus, buf, n_digits_have, dot_pos - c_mk.e10,
      frac_digits);
    exit;
  end;
  if n_digits_have > n_digits_sci then
    inc(dot_pos, d2a_round_digits(buf, n_digits_have, n_digits_sci
      {$ifdef GRISU1_F2A_HALF_ROUNDUP}, false {$endif} ));
  d2a_return_exponential(str, minus, @buf, n_digits_have, n_digits_sci,
    dot_pos - c_mk.e10 - 1);
end;