unit Myc.Data.Decimal;

interface

{$H+}

type
    TScale = 0..7;

    TDecimal = record
    strict private
        FValue: Int64;
    public
        constructor Create(AValue: Int64; AScale: TScale); overload;
        constructor Create(const AValue: TDecimal; ANewScale: TScale); overload;

        class operator Add(const A, B: TDecimal): TDecimal;
        class operator Subtract(const A, B: TDecimal): TDecimal;
        class operator Multiply(const A, B: TDecimal): TDecimal;
        class operator Divide(const A, B: TDecimal): TDecimal;

        class operator Equal(const A, B: TDecimal): Boolean;
        class operator NotEqual(const A, B: TDecimal): Boolean;

        class operator Implicit(const A: Int64): TDecimal;
        class operator Explicit(const A: TDecimal): Int64;
        class operator Explicit(const A: TDecimal): Double;

        class function MinValue(AScale: TScale): TDecimal; static;
        class function MaxValue(AScale: TScale): TDecimal; static;

        function GetValue: Int64;
        function GetScale: TScale;
        function GetRawValue: Int64; inline;
    end;

implementation

uses
    System.SysUtils,
    System.Math;

const
    SCALE_BITS = 3;
    VALUE_BITS = 61;
    SCALE_MASK = $07;

    // Range of the 61-bit signed value part
    MIN_VALUE_61BIT = -(1 shl 60);
    MAX_VALUE_61BIT = (1 shl 60) - 1;

    // Use a lookup table for powers of 10 for performance
    PowersOf10: array[TScale] of Int64 = (1, 10, 100, 1000, 10000, 100000, 1000000, 10000000);

{ TDecimal }

constructor TDecimal.Create(AValue: Int64; AScale: TScale);
begin
    FValue := (Int64(AScale) shl VALUE_BITS) or (AValue and ((1 shl VALUE_BITS) - 1));
end;

constructor TDecimal.Create(const AValue: TDecimal; ANewScale: TScale);
var
    oldScale: TScale;
    newValue: Int64;
    scaleDiff: Integer;
    i: Integer;
begin
    // Create a new decimal by changing the scale of an existing one.
    oldScale := AValue.GetScale;
    newValue := AValue.GetValue;

    if oldScale = ANewScale then
    begin
        FValue := AValue.FValue;
        exit;
    end;

    scaleDiff := ANewScale - oldScale;
    if scaleDiff > 0 then
    begin
        // Increasing scale: multiply by 10^scaleDiff, checking for overflow at each step.
        for i := 1 to scaleDiff do
        begin
            if newValue > (MAX_VALUE_61BIT div 10) then
                raise EOverflow.Create('Decimal scale up resulted in an overflow.');
            if newValue < (MIN_VALUE_61BIT div 10) then
                raise EOverflow.Create('Decimal scale up resulted in an overflow.');
            newValue := newValue * 10;
        end;
    end
    else
    begin
        // Decreasing scale: divide by 10^(-scaleDiff). Overflow is not an issue here.
        newValue := newValue div PowersOf10[-scaleDiff];
    end;

    // Pack the new value and scale
    FValue := (Int64(ANewScale) shl VALUE_BITS) or (newValue and ((1 shl VALUE_BITS) - 1));
end;

class operator TDecimal.Add(const A, B: TDecimal): TDecimal;
var
    scale: TScale;
    resultValue: Int64;
begin
    scale := A.GetScale;
    if scale <> B.GetScale then
        raise EArgumentException.Create('Operands must have the same scale.');

    resultValue := A.GetValue + B.GetValue;
    Result := TDecimal.Create(resultValue, A.GetScale);
end;

class operator TDecimal.Subtract(const A, B: TDecimal): TDecimal;
var
    scale: TScale;
    resultValue: Int64;
begin
    scale := A.GetScale;
    if scale <> B.GetScale then
        raise EArgumentException.Create('Operands must have the same scale.');

    resultValue := A.GetValue - B.GetValue;
    Result := TDecimal.Create(resultValue, scale);
end;

class operator TDecimal.Multiply(const A, B: TDecimal): TDecimal;
var
    scale: TScale;
    resultValue: Int64;
begin
    scale := A.GetScale;
    if scale <> B.GetScale then
        raise EArgumentException.Create('Operands must have the same scale.');

    resultValue := Trunc(A.GetValue * B.GetValue / PowersOf10[scale]);
    Result := TDecimal.Create(resultValue, scale);
end;

class operator TDecimal.Divide(const A, B: TDecimal): TDecimal;
var
    scale: TScale;
    resultValue: Int64;
begin
    scale := A.GetScale;
    if scale <> B.GetScale then
        raise EArgumentException.Create('Operands must have the same scale.');

    if B.GetValue = 0 then
        raise EDivByZero.Create('Division by zero');

    // Correctly scale the dividend to preserve precision
    resultValue := Trunc(A.GetValue * PowersOf10[scale] / B.GetValue);
    Result := TDecimal.Create(resultValue, scale);
end;

class operator TDecimal.Equal(const A, B: TDecimal): Boolean;
begin
    if A.GetScale <> B.GetScale then
        Result := False
    else
        Result := (A.GetValue = B.GetValue);
end;

class operator TDecimal.NotEqual(const A, B: TDecimal): Boolean;
begin
    Result := not (A = B);
end;

class operator TDecimal.Implicit(const A: Int64): TDecimal;
begin
    // Correctly create a TDecimal with scale 0
    Result := TDecimal.Create(A, 0);
end;

class operator TDecimal.Explicit(const A: TDecimal): Int64;
begin
    Result := A.GetValue;
end;

class operator TDecimal.Explicit(const A: TDecimal): Double;
begin
    Result := A.GetValue / PowersOf10[A.GetScale];
end;

class function TDecimal.MaxValue(AScale: TScale): TDecimal;
begin
    Result := TDecimal.Create(MAX_VALUE_61BIT, AScale);
end;

class function TDecimal.MinValue(AScale: TScale): TDecimal;
begin
    Result := TDecimal.Create(MIN_VALUE_61BIT, AScale);
end;

function TDecimal.GetValue: Int64;
var
    value: Int64;
begin
    value := FValue and ((1 shl VALUE_BITS) - 1);
    // Perform sign extension from 61 to 64 bits
    if (value and (1 shl (VALUE_BITS - 1))) <> 0 then
        Result := value or (not ((1 shl VALUE_BITS) - 1))
    else
        Result := value;
end;

function TDecimal.GetScale: TScale;
begin
    Result := TScale((FValue shr VALUE_BITS) and SCALE_MASK);
end;

function TDecimal.GetRawValue: Int64;
begin
    Result := FValue;
end;

end.