NSDecimal 精确数值

• 数据结构
• 舍入
• 操作
• 计算

NSDecimal

数据结构

• 真是难以理解，水平不够，下面只好用 NSDecimalNumber 来初始化 NSDecimal 简单使用以下 NSDecimal 的相关内容；

typedef struct {
    signed   int _exponent:8;
    unsigned int _length:4;     // length == 0 && isNegative -> NaN
    unsigned int _isNegative:1;
    unsigned int _isCompact:1;
    unsigned int _reserved:18;
    unsigned short _mantissa[NSDecimalMaxSize];
} NSDecimal;

舍入

• NSRoundingMode 舍入规则

typedef NS_ENUM(NSUInteger, NSRoundingMode) {
    NSRoundPlain,   // Round up on a tie
    NSRoundDown,    // Always down == truncate
    NSRoundUp,      // Always up
    NSRoundBankers  // on a tie round so last digit is even
};

• 举例数据

// Original
//    value 1.2  1.21  1.25  1.35  1.27 原始值 
// Plain    1.2  1.2   1.3   1.4   1.3  四舍五入
// Down     1.2  1.2   1.2   1.3   1.2  向下取整
// Up       1.2  1.3   1.3   1.4   1.3  向上取整
// Bankers  1.2  1.2   1.2   1.4   1.3  四舍六入五留双（大学学的是不是还给老师了）

• 举例代码

void NSDecimalRound(NSDecimal *result, const NSDecimal *number, NSInteger scale, NSRoundingMode roundingMode);

    NSDecimalNumber *decimalNumber1 = [NSDecimalNumber decimalNumberWithString:@"1.25"];
    NSDecimal decimal1 = [decimalNumber1 decimalValue];
    
    NSDecimal resDecimal;
    NSDecimalRound(&resDecimal, &decimal1, 1, NSRoundBankers);// 结果，操作数，保留小数位数，舍入规则
    NSDecimalNumber *resDecimalNumber = [NSDecimalNumber decimalNumberWithDecimal:resDecimal];
    
    NSLog(@"%@", resDecimalNumber);

操作

• 是否是个数

    BOOL isNotANumber = NSDecimalIsNotANumber(&decimal);

• 复制

    NSDecimal copyDecimal;
    NSDecimalCopy(&copyDecimal, &decimal);

• 压缩：按文档说法，计算前先压缩，可以提高效率；

    NSDecimalCompact(&decimal);

• 描述

    NSString *decimalString = NSDecimalString(&decimal, nil);
    NSLog(@"%@", decimalString);

计算

• NSCalculationError 计算结果

    NSCalculationNoError = 0,
    NSCalculationLossOfPrecision, // Result lost precision
    NSCalculationUnderflow,       // Result became 0
    NSCalculationOverflow,        // Result exceeds possible representation
    NSCalculationDivideByZero

• 准备 2 个 Decimal，下面都用这 2 个（只好 DecimalNumber 来初始化了）

    NSDecimalNumber *decimalNumber2_3 = [NSDecimalNumber decimalNumberWithString:@"2.3"];
    NSDecimal decimal2_3 = [decimalNumber2_3 decimalValue];
    NSDecimalNumber *decimalNumber3_4 = [NSDecimalNumber decimalNumberWithString:@"3.4"];
    NSDecimal decimal3_4 = [decimalNumber3_4 decimalValue];

    NSDecimal resultDecimal;

• 规则化，相同的数值有很多表示方法，此函数即为了统一格式；

    NSCalculationError normalizeError = NSDecimalNormalize(&decimal2_3, &decimal3_4, NSRoundBankers);
    if (normalizeError == NSCalculationNoError) {
        NSLog(@"NSCalculationNoError");
    }

• decimalA + decimalB

    NSCalculationError addError = NSDecimalAdd(&resultDecimal, &decimal2_3, &decimal3_4, NSRoundBankers);
    if (addError == NSCalculationNoError) {
        NSLog(@"add:%@", NSDecimalString(&resultDecimal,nil));
    }

• decimalA - decimalB

    NSCalculationError subtractError = NSDecimalSubtract(&resultDecimal, &decimal2_3, &decimal3_4, NSRoundBankers);
    if (subtractError == NSCalculationNoError) {
        NSLog(@"subtract:%@", NSDecimalString(&resultDecimal,nil));
    }

• decimalA * decimalB

    NSCalculationError multiplyError = NSDecimalMultiply(&resultDecimal, &decimal2_3, &decimal3_4, NSRoundBankers);
    if (multiplyError == NSCalculationNoError) {
        NSLog(@"multiply:%@", NSDecimalString(&resultDecimal,nil));
    }

• decimalA / decimalB

    NSCalculationError divideError = NSDecimalDivide(&resultDecimal, &decimal2_3, &decimal3_4, NSRoundBankers);
    if (divideError == NSCalculationNoError) {
        NSLog(@"divide:%@", NSDecimalString(&resultDecimal,nil));
    }

• decimalA ^ uintB

    NSCalculationError powerError = NSDecimalPower(&resultDecimal, &decimal2_3, 2, NSRoundBankers);
    if (powerError == NSCalculationNoError) {
        NSLog(@"power:%@", NSDecimalString(&resultDecimal,nil));
    }

• decimalA * 10 ^ shortB

    NSCalculationError power10Error = NSDecimalMultiplyByPowerOf10(&resultDecimal, &decimal2_3, 2, NSRoundBankers);
    if (power10Error == NSCalculationNoError) {
        NSLog(@"power10:%@", NSDecimalString(&resultDecimal,nil));
    }