#include <Functions/FunctionsDecimalArithmetics.h>
#include <Functions/FunctionFactory.h>

namespace DB
{

namespace
{

struct MultiplyDecimalsImpl
{
    static constexpr auto name = "multiplyDecimal";

    template <typename FirstType, typename SecondType>
    static inline Decimal256
    execute(FirstType a, SecondType b, UInt16 scale_a, UInt16 scale_b, UInt16 result_scale)
    {
        if (a.value == 0 || b.value == 0)
            return Decimal256(0);

        Int256 sign_a = a.value < 0 ? -1 : 1;
        Int256 sign_b = b.value < 0 ? -1 : 1;

        std::vector<UInt8> a_digits = DecimalOpHelpers::toDigits(a.value * sign_a);
        std::vector<UInt8> b_digits = DecimalOpHelpers::toDigits(b.value * sign_b);

        std::vector<UInt8> multiplied = DecimalOpHelpers::multiply(a_digits, b_digits);

        UInt16 product_scale = scale_a + scale_b;
        while (product_scale < result_scale)
        {
            multiplied.push_back(0);
            ++product_scale;
        }

        while (product_scale > result_scale&& !multiplied.empty())
        {
            multiplied.pop_back();
            --product_scale;
        }

        if (multiplied.empty())
            return Decimal256(0);

        if (multiplied.size() > DecimalUtils::max_precision<Decimal256>)
            throw DB::Exception("Numeric overflow: result bigger that Decimal256", ErrorCodes::DECIMAL_OVERFLOW);

        return Decimal256(sign_a * sign_b * DecimalOpHelpers::fromDigits(multiplied));
    }
};

}

REGISTER_FUNCTION(MultiplyDecimals)
{
    factory.registerFunction<FunctionsDecimalArithmetics<MultiplyDecimalsImpl>>(Documentation(
        R"(
Performs multiplication on two decimals. Result value will be of type [Decimal256](../../sql-reference/data-types/decimal.md).
Result scale can be explicitly specified by `result_scale` argument (const Integer in range `[0, 76]`). If not specified, the result scale is the max scale of given arguments.

:::note
These functions work significantly slower than usual `multiply`.
In case you don't really need controlled precision and/or need fast computation, consider using [multiply](#multiply)
:::

**Syntax**

```sql
multiplyDecimal(a, b[, result_scale])
```

**Arguments**

-   `a` — First value: [Decimal](../../sql-reference/data-types/decimal.md).
-   `b` — Second value: [Decimal](../../sql-reference/data-types/decimal.md).
-   `result_scale` — Scale of result: [Int/UInt](../../sql-reference/data-types/int-uint.md).

**Returned value**

-   The result of multiplication with given scale.

Type: [Decimal256](../../sql-reference/data-types/decimal.md).

**Example**

```text
┌─multiplyDecimal(toDecimal256(-12, 0), toDecimal32(-2.1, 1), 1)─┐
│                                                           25.2 │
└────────────────────────────────────────────────────────────────┘
```

**Difference from regular multiplication:**
```sql
SELECT toDecimal64(-12.647, 3) * toDecimal32(2.1239, 4);
SELECT toDecimal64(-12.647, 3) as a, toDecimal32(2.1239, 4) as b, multiplyDecimal(a, b);
```

```text
┌─multiply(toDecimal64(-12.647, 3), toDecimal32(2.1239, 4))─┐
│                                               -26.8609633 │
└───────────────────────────────────────────────────────────┘
┌─multiplyDecimal(toDecimal64(-12.647, 3), toDecimal32(2.1239, 4))─┐
│                                                         -26.8609 │
└──────────────────────────────────────────────────────────────────┘
```

```sql
SELECT
    toDecimal64(-12.647987876, 9) AS a,
    toDecimal64(123.967645643, 9) AS b,
    multiplyDecimal(a, b);
SELECT
    toDecimal64(-12.647987876, 9) AS a,
    toDecimal64(123.967645643, 9) AS b,
    a * b;
```

```text
┌─────────────a─┬─────────────b─┬─multiplyDecimal(toDecimal64(-12.647987876, 9), toDecimal64(123.967645643, 9))─┐
│ -12.647987876 │ 123.967645643 │                                                               -1567.941279108 │
└───────────────┴───────────────┴───────────────────────────────────────────────────────────────────────────────┘
Received exception from server (version 22.11.1):
Code: 407. DB::Exception: Received from localhost:9000. DB::Exception: Decimal math overflow: While processing toDecimal64(-12.647987876, 9) AS a, toDecimal64(123.967645643, 9) AS b, a * b. (DECIMAL_OVERFLOW)
```
)"));

}

}