//! 内置数字类型的数字 traits 和函数。

#![stable(feature = "rust1", since = "1.0.0")]

use crate::ascii;
use crate::convert::TryInto;
use crate::intrinsics;
use crate::mem;
use crate::ops::{Add, Mul, Sub};
use crate::str::FromStr;

// 之所以使用它，是因为 const 上下文中不允许使用 `?` 运算符。
macro_rules! try_opt {
    ($e:expr) => {
        match $e {
            Some(x) => x,
            None => return None,
        }
    };
}

#[allow_internal_unstable(const_likely)]
macro_rules! unlikely {
    ($e: expr) => {
        intrinsics::unlikely($e)
    };
}

// 所有这些模块在技术上都是私有的，仅针对核心测试公开：
#[cfg(not(no_fp_fmt_parse))]
pub mod bignum;
#[cfg(not(no_fp_fmt_parse))]
pub mod dec2flt;
#[cfg(not(no_fp_fmt_parse))]
pub mod diy_float;
#[cfg(not(no_fp_fmt_parse))]
pub mod flt2dec;
pub mod fmt;

#[macro_use]
mod int_macros; // 导入 int_impl!
#[macro_use]
mod uint_macros; // 导入 uint_impl!

mod error;
mod int_log10;
mod nonzero;
#[unstable(feature = "saturating_int_impl", issue = "87920")]
mod saturating;
mod wrapping;

#[unstable(feature = "saturating_int_impl", issue = "87920")]
pub use saturating::Saturating;
#[stable(feature = "rust1", since = "1.0.0")]
pub use wrapping::Wrapping;

#[stable(feature = "rust1", since = "1.0.0")]
#[cfg(not(no_fp_fmt_parse))]
pub use dec2flt::ParseFloatError;

#[stable(feature = "rust1", since = "1.0.0")]
pub use error::ParseIntError;

#[stable(feature = "nonzero", since = "1.28.0")]
pub use nonzero::{NonZeroU128, NonZeroU16, NonZeroU32, NonZeroU64, NonZeroU8, NonZeroUsize};

#[stable(feature = "signed_nonzero", since = "1.34.0")]
pub use nonzero::{NonZeroI128, NonZeroI16, NonZeroI32, NonZeroI64, NonZeroI8, NonZeroIsize};

#[stable(feature = "try_from", since = "1.34.0")]
pub use error::TryFromIntError;

#[stable(feature = "int_error_matching", since = "1.55.0")]
pub use error::IntErrorKind;

macro_rules! usize_isize_to_xe_bytes_doc {
    () => {
        "

**Note**: This function returns an array of length 2, 4 or 8 bytes
depending on the target pointer size.

"
    };
}

macro_rules! usize_isize_from_xe_bytes_doc {
    () => {
        "

**Note**: This function takes an array of length 2, 4 or 8 bytes
depending on the target pointer size.

"
    };
}

macro_rules! midpoint_impl {
    ($SelfT:ty, unsigned) => {
        /// 计算 `self` 和 `rhs` 的中点。
        ///
        /// `midpoint(a, b)` 是 `(a + b) >> 1`，就好像它是在足够大的带符号整数类型中执行的一样。
        /// 这意味着结果总是向 negative 无穷大舍入，并且永远不会发生溢出。
        ///
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(num_midpoint)]
        #[doc = concat!("assert_eq!(0", stringify!($SelfT), ".midpoint(4), 2);")]
        #[doc = concat!("assert_eq!(1", stringify!($SelfT), ".midpoint(4), 2);")]
        /// ```
        #[unstable(feature = "num_midpoint", issue = "110840")]
        #[rustc_const_unstable(feature = "const_num_midpoint", issue = "110840")]
        #[must_use = "this returns the result of the operation, \
                      without modifying the original"]
        #[inline]
        pub const fn midpoint(self, rhs: $SelfT) -> $SelfT {
            // 使用 Hacker's Delight 中众所周知的无分支算法来计算 `(a + b) / 2` 而不会溢出: `((a ^ b) >> 1) + (a & b)`。
            //
            ((self ^ rhs) >> 1) + (self & rhs)
        }
    };
    ($SelfT:ty, $WideT:ty, unsigned) => {
        /// 计算 `self` 和 `rhs` 的中点。
        ///
        /// `midpoint(a, b)` 是 `(a + b) >> 1`，就好像它是在足够大的带符号整数类型中执行的一样。
        /// 这意味着结果总是向 negative 无穷大舍入，并且永远不会发生溢出。
        ///
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(num_midpoint)]
        #[doc = concat!("assert_eq!(0", stringify!($SelfT), ".midpoint(4), 2);")]
        #[doc = concat!("assert_eq!(1", stringify!($SelfT), ".midpoint(4), 2);")]
        /// ```
        #[unstable(feature = "num_midpoint", issue = "110840")]
        #[rustc_const_unstable(feature = "const_num_midpoint", issue = "110840")]
        #[must_use = "this returns the result of the operation, \
                      without modifying the original"]
        #[inline]
        pub const fn midpoint(self, rhs: $SelfT) -> $SelfT {
            ((self as $WideT + rhs as $WideT) / 2) as $SelfT
        }
    };
}

macro_rules! widening_impl {
    ($SelfT:ty, $WideT:ty, $BITS:literal, unsigned) => {
        /// 计算完整的产品 `self * rhs`，没有溢出的可能性。
        ///
        /// 这将返回结果的低位 (wrapping) 位和高位 (overflow) 位作为两个单独的值，按该顺序。
        ///
        ///
        /// 如果您还需要在宽结果中添加进位，那么您需要 [`Self::carrying_mul`]。
        ///
        /// # Examples
        ///
        /// 基本用法：
        ///
        /// 请注意，此示例在整数类型之间共享。
        /// 这就解释了为什么在这里使用 `u32`。
        ///
        /// ```
        /// #![feature(bigint_helper_methods)]
        /// assert_eq!(5u32.widening_mul(2), (10, 0));
        /// assert_eq!(1_000_000_000u32.widening_mul(10), (1410065408, 2));
        /// ```
        ///
        #[unstable(feature = "bigint_helper_methods", issue = "85532")]
        #[rustc_const_unstable(feature = "const_bigint_helper_methods", issue = "85532")]
        #[must_use = "this returns the result of the operation, \
                      without modifying the original"]
        #[inline]
        pub const fn widening_mul(self, rhs: Self) -> (Self, Self) {
            // note: 从长远来看，这应该通过内部函数来实现，但目前我们可以不使用 impl 来处理 u128/i128
            //
            // SAFETY: 溢出将包含在更广泛的类型中
            let wide = unsafe { (self as $WideT).unchecked_mul(rhs as $WideT) };
            (wide as $SelfT, (wide >> $BITS) as $SelfT)
        }

        /// 计算 "full multiplication" `self * rhs + carry` 而不可能溢出。
        ///
        /// 这将返回结果的低位 (wrapping) 位和高位 (overflow) 位作为两个单独的值，按该顺序。
        ///
        ///
        /// 执行 "long multiplication"，它需要添加额外的量，并且可能返回额外的溢出量。
        /// 这允许将多个乘法链接在一起以创建代表更大值的 "大整数"。
        ///
        /// 如果您不需要 `carry`，那么您可以使用 [`Self::widening_mul`]。
        ///
        /// # Examples
        ///
        /// 基本用法：
        ///
        /// 请注意，此示例在整数类型之间共享。
        /// 这就解释了为什么在这里使用 `u32`。
        ///
        /// ```
        /// #![feature(bigint_helper_methods)]
        /// assert_eq!(5u32.carrying_mul(2, 0), (10, 0));
        /// assert_eq!(5u32.carrying_mul(2, 10), (20, 0));
        /// assert_eq!(1_000_000_000u32.carrying_mul(10, 0), (1410065408, 2));
        /// assert_eq!(1_000_000_000u32.carrying_mul(10, 10), (1410065418, 2));
        ///
        ///
        #[doc = concat!("assert_eq!(",
            stringify!($SelfT), "::MAX.carrying_mul(", stringify!($SelfT), "::MAX, ", stringify!($SelfT), "::MAX), ",
            "(0, ", stringify!($SelfT), "::MAX));"
        )]
        /// ```
        ///
        /// This is the core operation needed for scalar multiplication when
        /// implementing it for wider-than-native types.
        ///
        /// ```
        ///
        /// #![feature(bigint_helper_methods)]
        /// fn scalar_mul_eq(little_endian_digits: &mut Vec<u16>, multiplicand: u16) {
        ///     let mut carry = 0;
        ///     对于 little_endian_digits.iter_mut() { (*d, carry) = d.carrying_mul(multiplicand, carry); } 中的 d 如果进位 != 0 {
        ///
        ///         little_endian_digits.push(carry);
        ///     }
        /// }
        ///
        /// let mut v = vec![10, 20];
        /// scalar_mul_eq(&mut v, 3);
        /// assert_eq!(v, [30, 60]);
        ///
        /// assert_eq!(0x87654321_u64 * 0xFEED, 0x86D3D159E38D);
        /// let mut v = vec![0x4321, 0x8765];
        /// scalar_mul_eq(&mut v, 0xFEED);
        /// assert_eq!(v, [0xE38D, 0xD159, 0x86D3]);
        ///
        /// ```
        ///
        /// If `carry` is zero, this is similar to [`overflowing_mul`](Self::overflowing_mul),
        /// except that it gives the value of the overflow instead of just whether one happened:
        ///
        /// ```
        /// #![feature(bigint_helper_methods)]
        /// let r = u8::carrying_mul(7, 13, 0);
        /// assert_eq!((r.0, r.1 != 0), u8::overflowing_mul(7, 13));
        /// let r = u8::carrying_mul(13, 42, 0);
        /// assert_eq!((r.0, r.1 != 0), u8::overflowing_mul(13, 42));
        ///
        /// ```
        ///
        /// The value of the first field in the returned tuple matches what you'd get
        /// by combining the [`wrapping_mul`](Self::wrapping_mul) and
        /// [`wrapping_add`](Self::wrapping_add) methods:
        ///
        /// ```
        /// #![feature(bigint_helper_methods)]
        /// assert_eq!(
        ///     789_u16.carrying_mul(456, 123).0, 789_u16.wrapping_mul(456).wrapping_add(123), );
        /// ```
        ///
        #[unstable(feature = "bigint_helper_methods", issue = "85532")]
        #[rustc_const_unstable(feature = "bigint_helper_methods", issue = "85532")]
        #[must_use = "this returns the result of the operation, \
                      without modifying the original"]
        #[inline]
        pub const fn carrying_mul(self, rhs: Self, carry: Self) -> (Self, Self) {
            // note: 从长远来看，这应该通过内部函数来实现，但目前我们可以不使用 impl 来处理 u128/i128
            //
            // SAFETY: 溢出将包含在更广泛的类型中
            let wide = unsafe {
                (self as $WideT).unchecked_mul(rhs as $WideT).unchecked_add(carry as $WideT)
            };
            (wide as $SelfT, (wide >> $BITS) as $SelfT)
        }
    };
}

macro_rules! conv_rhs_for_unchecked_shift {
    ($SelfT:ty, $x:expr) => {{
        #[inline]
        fn conv(x: u32) -> $SelfT {
            // FIXME(const-hack) 替换为 `.try_into().ok().unwrap_unchecked()`。
            // SAFETY: 任何合法的转移量都必须在自身类型中无损地表示。
            unsafe { x.try_into().ok().unwrap_unchecked() }
        }
        #[inline]
        const fn const_conv(x: u32) -> $SelfT {
            x as _
        }

        intrinsics::const_eval_select(($x,), const_conv, conv)
    }};
}

impl i8 {
    int_impl! {
        Self = i8,
        ActualT = i8,
        UnsignedT = u8,
        BITS = 8,
        BITS_MINUS_ONE = 7,
        Min = -128,
        Max = 127,
        rot = 2,
        rot_op = "-0x7e",
        rot_result = "0xa",
        swap_op = "0x12",
        swapped = "0x12",
        reversed = "0x48",
        le_bytes = "[0x12]",
        be_bytes = "[0x12]",
        to_xe_bytes_doc = "",
        from_xe_bytes_doc = "",
        bound_condition = "",
    }
}

impl i16 {
    int_impl! {
        Self = i16,
        ActualT = i16,
        UnsignedT = u16,
        BITS = 16,
        BITS_MINUS_ONE = 15,
        Min = -32768,
        Max = 32767,
        rot = 4,
        rot_op = "-0x5ffd",
        rot_result = "0x3a",
        swap_op = "0x1234",
        swapped = "0x3412",
        reversed = "0x2c48",
        le_bytes = "[0x34, 0x12]",
        be_bytes = "[0x12, 0x34]",
        to_xe_bytes_doc = "",
        from_xe_bytes_doc = "",
        bound_condition = "",
    }
}

impl i32 {
    int_impl! {
        Self = i32,
        ActualT = i32,
        UnsignedT = u32,
        BITS = 32,
        BITS_MINUS_ONE = 31,
        Min = -2147483648,
        Max = 2147483647,
        rot = 8,
        rot_op = "0x10000b3",
        rot_result = "0xb301",
        swap_op = "0x12345678",
        swapped = "0x78563412",
        reversed = "0x1e6a2c48",
        le_bytes = "[0x78, 0x56, 0x34, 0x12]",
        be_bytes = "[0x12, 0x34, 0x56, 0x78]",
        to_xe_bytes_doc = "",
        from_xe_bytes_doc = "",
        bound_condition = "",
    }
}

impl i64 {
    int_impl! {
        Self = i64,
        ActualT = i64,
        UnsignedT = u64,
        BITS = 64,
        BITS_MINUS_ONE = 63,
        Min = -9223372036854775808,
        Max = 9223372036854775807,
        rot = 12,
        rot_op = "0xaa00000000006e1",
        rot_result = "0x6e10aa",
        swap_op = "0x1234567890123456",
        swapped = "0x5634129078563412",
        reversed = "0x6a2c48091e6a2c48",
        le_bytes = "[0x56, 0x34, 0x12, 0x90, 0x78, 0x56, 0x34, 0x12]",
        be_bytes = "[0x12, 0x34, 0x56, 0x78, 0x90, 0x12, 0x34, 0x56]",
        to_xe_bytes_doc = "",
        from_xe_bytes_doc = "",
        bound_condition = "",
    }
}

impl i128 {
    int_impl! {
        Self = i128,
        ActualT = i128,
        UnsignedT = u128,
        BITS = 128,
        BITS_MINUS_ONE = 127,
        Min = -170141183460469231731687303715884105728,
        Max = 170141183460469231731687303715884105727,
        rot = 16,
        rot_op = "0x13f40000000000000000000000004f76",
        rot_result = "0x4f7613f4",
        swap_op = "0x12345678901234567890123456789012",
        swapped = "0x12907856341290785634129078563412",
        reversed = "0x48091e6a2c48091e6a2c48091e6a2c48",
        le_bytes = "[0x12, 0x90, 0x78, 0x56, 0x34, 0x12, 0x90, 0x78, \
            0x56, 0x34, 0x12, 0x90, 0x78, 0x56, 0x34, 0x12]",
        be_bytes = "[0x12, 0x34, 0x56, 0x78, 0x90, 0x12, 0x34, 0x56, \
            0x78, 0x90, 0x12, 0x34, 0x56, 0x78, 0x90, 0x12]",
        to_xe_bytes_doc = "",
        from_xe_bytes_doc = "",
        bound_condition = "",
    }
}

#[cfg(target_pointer_width = "16")]
impl isize {
    int_impl! {
        Self = isize,
        ActualT = i16,
        UnsignedT = usize,
        BITS = 16,
        BITS_MINUS_ONE = 15,
        Min = -32768,
        Max = 32767,
        rot = 4,
        rot_op = "-0x5ffd",
        rot_result = "0x3a",
        swap_op = "0x1234",
        swapped = "0x3412",
        reversed = "0x2c48",
        le_bytes = "[0x34, 0x12]",
        be_bytes = "[0x12, 0x34]",
        to_xe_bytes_doc = usize_isize_to_xe_bytes_doc!(),
        from_xe_bytes_doc = usize_isize_from_xe_bytes_doc!(),
        bound_condition = " on 16-bit targets",
    }
}

#[cfg(target_pointer_width = "32")]
impl isize {
    int_impl! {
        Self = isize,
        ActualT = i32,
        UnsignedT = usize,
        BITS = 32,
        BITS_MINUS_ONE = 31,
        Min = -2147483648,
        Max = 2147483647,
        rot = 8,
        rot_op = "0x10000b3",
        rot_result = "0xb301",
        swap_op = "0x12345678",
        swapped = "0x78563412",
        reversed = "0x1e6a2c48",
        le_bytes = "[0x78, 0x56, 0x34, 0x12]",
        be_bytes = "[0x12, 0x34, 0x56, 0x78]",
        to_xe_bytes_doc = usize_isize_to_xe_bytes_doc!(),
        from_xe_bytes_doc = usize_isize_from_xe_bytes_doc!(),
        bound_condition = " on 32-bit targets",
    }
}

#[cfg(target_pointer_width = "64")]
impl isize {
    int_impl! {
        Self = isize,
        ActualT = i64,
        UnsignedT = usize,
        BITS = 64,
        BITS_MINUS_ONE = 63,
        Min = -9223372036854775808,
        Max = 9223372036854775807,
        rot = 12,
        rot_op = "0xaa00000000006e1",
        rot_result = "0x6e10aa",
        swap_op = "0x1234567890123456",
        swapped = "0x5634129078563412",
        reversed = "0x6a2c48091e6a2c48",
        le_bytes = "[0x56, 0x34, 0x12, 0x90, 0x78, 0x56, 0x34, 0x12]",
        be_bytes = "[0x12, 0x34, 0x56, 0x78, 0x90, 0x12, 0x34, 0x56]",
        to_xe_bytes_doc = usize_isize_to_xe_bytes_doc!(),
        from_xe_bytes_doc = usize_isize_from_xe_bytes_doc!(),
        bound_condition = " on 64-bit targets",
    }
}

/// 如果设置第 6 位，则 ascii 为小写。
const ASCII_CASE_MASK: u8 = 0b0010_0000;

impl u8 {
    uint_impl! {
        Self = u8,
        ActualT = u8,
        SignedT = i8,
        NonZeroT = NonZeroU8,
        BITS = 8,
        MAX = 255,
        rot = 2,
        rot_op = "0x82",
        rot_result = "0xa",
        swap_op = "0x12",
        swapped = "0x12",
        reversed = "0x48",
        le_bytes = "[0x12]",
        be_bytes = "[0x12]",
        to_xe_bytes_doc = "",
        from_xe_bytes_doc = "",
        bound_condition = "",
    }
    widening_impl! { u8, u16, 8, unsigned }
    midpoint_impl! { u8, u16, unsigned }

    /// 检查该值是否在 ASCII 范围内。
    ///
    /// # Examples
    ///
    /// ```
    /// let ascii = 97u8;
    /// let non_ascii = 150u8;
    ///
    /// assert!(ascii.is_ascii());
    /// assert!(!non_ascii.is_ascii());
    /// ```
    #[must_use]
    #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
    #[rustc_const_stable(feature = "const_u8_is_ascii", since = "1.43.0")]
    #[inline]
    pub const fn is_ascii(&self) -> bool {
        *self <= 127
    }

    /// 如果此字节的值在 ASCII 范围内，则将其作为 [ASCII character](ascii::Char) 返回。
    /// 否则，返回 `None`。
    #[must_use]
    #[unstable(feature = "ascii_char", issue = "110998")]
    #[inline]
    pub const fn as_ascii(&self) -> Option<ascii::Char> {
        ascii::Char::from_u8(*self)
    }

    /// 使值的副本等效于其 ASCII 大写字母。
    ///
    /// ASCII 字母 'a' 到 'z' 映射到 'A' 到 'Z'，但是非 ASCII 字母不变。
    ///
    ///
    /// 要就地将值大写，请使用 [`make_ascii_uppercase`]。
    ///
    /// # Examples
    ///
    /// ```
    /// let lowercase_a = 97u8;
    ///
    /// assert_eq!(65, lowercase_a.to_ascii_uppercase());
    /// ```
    ///
    /// [`make_ascii_uppercase`]: Self::make_ascii_uppercase
    #[must_use = "to uppercase the value in-place, use `make_ascii_uppercase()`"]
    #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
    #[rustc_const_stable(feature = "const_ascii_methods_on_intrinsics", since = "1.52.0")]
    #[inline]
    pub const fn to_ascii_uppercase(&self) -> u8 {
        // 如果这是一个小写字母，则切换第五位
        *self ^ ((self.is_ascii_lowercase() as u8) * ASCII_CASE_MASK)
    }

    /// 以等效的 ASCII 小写形式复制值。
    ///
    /// ASCII 字母 'A' 到 'Z' 映射到 'a' 到 'z'，但是非 ASCII 字母不变。
    ///
    ///
    /// 要就地小写该值，请使用 [`make_ascii_lowercase`]。
    ///
    /// # Examples
    ///
    /// ```
    /// let uppercase_a = 65u8;
    ///
    /// assert_eq!(97, uppercase_a.to_ascii_lowercase());
    /// ```
    ///
    /// [`make_ascii_lowercase`]: Self::make_ascii_lowercase
    #[must_use = "to lowercase the value in-place, use `make_ascii_lowercase()`"]
    #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
    #[rustc_const_stable(feature = "const_ascii_methods_on_intrinsics", since = "1.52.0")]
    #[inline]
    pub const fn to_ascii_lowercase(&self) -> u8 {
        // 如果这是一个大写字母，则设置第五个位
        *self | (self.is_ascii_uppercase() as u8 * ASCII_CASE_MASK)
    }

    /// 假设自己是 ascii
    #[inline]
    pub(crate) const fn ascii_change_case_unchecked(&self) -> u8 {
        *self ^ ASCII_CASE_MASK
    }

    /// 检查两个值是否为 ASCII 不区分大小写的匹配。
    ///
    /// 这等效于 `to_ascii_lowercase(a) == to_ascii_lowercase(b)`。
    ///
    /// # Examples
    ///
    /// ```
    /// let lowercase_a = 97u8;
    /// let uppercase_a = 65u8;
    ///
    /// assert!(lowercase_a.eq_ignore_ascii_case(&uppercase_a));
    /// ```
    #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
    #[rustc_const_stable(feature = "const_ascii_methods_on_intrinsics", since = "1.52.0")]
    #[inline]
    pub const fn eq_ignore_ascii_case(&self, other: &u8) -> bool {
        self.to_ascii_lowercase() == other.to_ascii_lowercase()
    }

    /// 将此值就地转换为其 ASCII 大写等效项。
    ///
    /// ASCII 字母 'a' 到 'z' 映射到 'A' 到 'Z'，但是非 ASCII 字母不变。
    ///
    /// 要返回新的大写值而不修改现有值，请使用 [`to_ascii_uppercase`]。
    ///
    ///
    /// # Examples
    ///
    /// ```
    /// let mut byte = b'a';
    ///
    /// byte.make_ascii_uppercase();
    ///
    /// assert_eq!(b'A', byte);
    /// ```
    ///
    /// [`to_ascii_uppercase`]: Self::to_ascii_uppercase
    ///
    #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
    #[inline]
    pub fn make_ascii_uppercase(&mut self) {
        *self = self.to_ascii_uppercase();
    }

    /// 将此值就地转换为其 ASCII 小写等效项。
    ///
    /// ASCII 字母 'A' 到 'Z' 映射到 'a' 到 'z'，但是非 ASCII 字母不变。
    ///
    /// 要返回新的小写值而不修改现有值，请使用 [`to_ascii_lowercase`]。
    ///
    ///
    /// # Examples
    ///
    /// ```
    /// let mut byte = b'A';
    ///
    /// byte.make_ascii_lowercase();
    ///
    /// assert_eq!(b'a', byte);
    /// ```
    ///
    /// [`to_ascii_lowercase`]: Self::to_ascii_lowercase
    ///
    #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
    #[inline]
    pub fn make_ascii_lowercase(&mut self) {
        *self = self.to_ascii_lowercase();
    }

    /// 检查值是否为 ASCII 字母字符：
    ///
    /// - U+0041 'A' ..= U+005A 'Z', or
    /// - U+0061 'a' ..= U+007A 'z'.
    ///
    /// # Examples
    ///
    /// ```
    /// let uppercase_a = b'A';
    /// let uppercase_g = b'G';
    /// let a = b'a';
    /// let g = b'g';
    /// let zero = b'0';
    /// let percent = b'%';
    /// let space = b' ';
    /// let lf = b'\n';
    /// let esc = b'\x1b';
    ///
    /// assert!(uppercase_a.is_ascii_alphabetic());
    /// assert!(uppercase_g.is_ascii_alphabetic());
    /// assert!(a.is_ascii_alphabetic());
    /// assert!(g.is_ascii_alphabetic());
    /// assert!(!zero.is_ascii_alphabetic());
    /// assert!(!percent.is_ascii_alphabetic());
    /// assert!(!space.is_ascii_alphabetic());
    /// assert!(!lf.is_ascii_alphabetic());
    /// assert!(!esc.is_ascii_alphabetic());
    /// ```
    #[must_use]
    #[stable(feature = "ascii_ctype_on_intrinsics", since = "1.24.0")]
    #[rustc_const_stable(feature = "const_ascii_ctype_on_intrinsics", since = "1.47.0")]
    #[inline]
    pub const fn is_ascii_alphabetic(&self) -> bool {
        matches!(*self, b'A'..=b'Z' | b'a'..=b'z')
    }

    /// 检查值是否为 ASCII 大写字符：
    /// U+0041 'A' ..= U+005A 'Z'.
    ///
    /// # Examples
    ///
    /// ```
    /// let uppercase_a = b'A';
    /// let uppercase_g = b'G';
    /// let a = b'a';
    /// let g = b'g';
    /// let zero = b'0';
    /// let percent = b'%';
    /// let space = b' ';
    /// let lf = b'\n';
    /// let esc = b'\x1b';
    ///
    /// assert!(uppercase_a.is_ascii_uppercase());
    /// assert!(uppercase_g.is_ascii_uppercase());
    /// assert!(!a.is_ascii_uppercase());
    /// assert!(!g.is_ascii_uppercase());
    /// assert!(!zero.is_ascii_uppercase());
    /// assert!(!percent.is_ascii_uppercase());
    /// assert!(!space.is_ascii_uppercase());
    /// assert!(!lf.is_ascii_uppercase());
    /// assert!(!esc.is_ascii_uppercase());
    /// ```
    #[must_use]
    #[stable(feature = "ascii_ctype_on_intrinsics", since = "1.24.0")]
    #[rustc_const_stable(feature = "const_ascii_ctype_on_intrinsics", since = "1.47.0")]
    #[inline]
    pub const fn is_ascii_uppercase(&self) -> bool {
        matches!(*self, b'A'..=b'Z')
    }

    /// 检查值是否为 ASCII 小写字符：
    /// U+0061 'a' ..= U+007A 'z'.
    ///
    /// # Examples
    ///
    /// ```
    /// let uppercase_a = b'A';
    /// let uppercase_g = b'G';
    /// let a = b'a';
    /// let g = b'g';
    /// let zero = b'0';
    /// let percent = b'%';
    /// let space = b' ';
    /// let lf = b'\n';
    /// let esc = b'\x1b';
    ///
    /// assert!(!uppercase_a.is_ascii_lowercase());
    /// assert!(!uppercase_g.is_ascii_lowercase());
    /// assert!(a.is_ascii_lowercase());
    /// assert!(g.is_ascii_lowercase());
    /// assert!(!zero.is_ascii_lowercase());
    /// assert!(!percent.is_ascii_lowercase());
    /// assert!(!space.is_ascii_lowercase());
    /// assert!(!lf.is_ascii_lowercase());
    /// assert!(!esc.is_ascii_lowercase());
    /// ```
    #[must_use]
    #[stable(feature = "ascii_ctype_on_intrinsics", since = "1.24.0")]
    #[rustc_const_stable(feature = "const_ascii_ctype_on_intrinsics", since = "1.47.0")]
    #[inline]
    pub const fn is_ascii_lowercase(&self) -> bool {
        matches!(*self, b'a'..=b'z')
    }

    /// 检查值是否为 ASCII 字母数字字符：
    ///
    /// - U+0041 'A' ..= U+005A 'Z', or
    /// - U+0061 'a' ..= U+007A 'z', or
    /// - U+0030 '0' ..= U+0039 '9'.
    ///
    /// # Examples
    ///
    /// ```
    /// let uppercase_a = b'A';
    /// let uppercase_g = b'G';
    /// let a = b'a';
    /// let g = b'g';
    /// let zero = b'0';
    /// let percent = b'%';
    /// let space = b' ';
    /// let lf = b'\n';
    /// let esc = b'\x1b';
    ///
    /// assert!(uppercase_a.is_ascii_alphanumeric());
    /// assert!(uppercase_g.is_ascii_alphanumeric());
    /// assert!(a.is_ascii_alphanumeric());
    /// assert!(g.is_ascii_alphanumeric());
    /// assert!(zero.is_ascii_alphanumeric());
    /// assert!(!percent.is_ascii_alphanumeric());
    /// assert!(!space.is_ascii_alphanumeric());
    /// assert!(!lf.is_ascii_alphanumeric());
    /// assert!(!esc.is_ascii_alphanumeric());
    /// ```
    #[must_use]
    #[stable(feature = "ascii_ctype_on_intrinsics", since = "1.24.0")]
    #[rustc_const_stable(feature = "const_ascii_ctype_on_intrinsics", since = "1.47.0")]
    #[inline]
    pub const fn is_ascii_alphanumeric(&self) -> bool {
        matches!(*self, b'0'..=b'9' | b'A'..=b'Z' | b'a'..=b'z')
    }

    /// 检查值是否为 ASCII 十进制数字：
    /// U+0030 '0' ..= U+0039 '9'.
    ///
    /// # Examples
    ///
    /// ```
    /// let uppercase_a = b'A';
    /// let uppercase_g = b'G';
    /// let a = b'a';
    /// let g = b'g';
    /// let zero = b'0';
    /// let percent = b'%';
    /// let space = b' ';
    /// let lf = b'\n';
    /// let esc = b'\x1b';
    ///
    /// assert!(!uppercase_a.is_ascii_digit());
    /// assert!(!uppercase_g.is_ascii_digit());
    /// assert!(!a.is_ascii_digit());
    /// assert!(!g.is_ascii_digit());
    /// assert!(zero.is_ascii_digit());
    /// assert!(!percent.is_ascii_digit());
    /// assert!(!space.is_ascii_digit());
    /// assert!(!lf.is_ascii_digit());
    /// assert!(!esc.is_ascii_digit());
    /// ```
    #[must_use]
    #[stable(feature = "ascii_ctype_on_intrinsics", since = "1.24.0")]
    #[rustc_const_stable(feature = "const_ascii_ctype_on_intrinsics", since = "1.47.0")]
    #[inline]
    pub const fn is_ascii_digit(&self) -> bool {
        matches!(*self, b'0'..=b'9')
    }

    /// 检查值是否为 ASCII 八进制数字:
    /// U+0030 '0' ..= U+0037 '7'.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(is_ascii_octdigit)]
    ///
    /// let uppercase_a = b'A';
    /// let a = b'a';
    /// let zero = b'0';
    /// let seven = b'7';
    /// let nine = b'9';
    /// let percent = b'%';
    /// let lf = b'\n';
    ///
    /// assert!(!uppercase_a.is_ascii_octdigit());
    /// assert!(!a.is_ascii_octdigit());
    /// assert!(zero.is_ascii_octdigit());
    /// assert!(seven.is_ascii_octdigit());
    /// assert!(!nine.is_ascii_octdigit());
    /// assert!(!percent.is_ascii_octdigit());
    /// assert!(!lf.is_ascii_octdigit());
    /// ```
    #[must_use]
    #[unstable(feature = "is_ascii_octdigit", issue = "101288")]
    #[rustc_const_unstable(feature = "is_ascii_octdigit", issue = "101288")]
    #[inline]
    pub const fn is_ascii_octdigit(&self) -> bool {
        matches!(*self, b'0'..=b'7')
    }

    /// 检查值是否为 ASCII 十六进制数字：
    ///
    /// - U+0030 '0' ..= U+0039 '9', or
    /// - U+0041 'A' ..= U+0046 'F', or
    /// - U+0061 'a' ..= U+0066 'f'.
    ///
    /// # Examples
    ///
    /// ```
    /// let uppercase_a = b'A';
    /// let uppercase_g = b'G';
    /// let a = b'a';
    /// let g = b'g';
    /// let zero = b'0';
    /// let percent = b'%';
    /// let space = b' ';
    /// let lf = b'\n';
    /// let esc = b'\x1b';
    ///
    /// assert!(uppercase_a.is_ascii_hexdigit());
    /// assert!(!uppercase_g.is_ascii_hexdigit());
    /// assert!(a.is_ascii_hexdigit());
    /// assert!(!g.is_ascii_hexdigit());
    /// assert!(zero.is_ascii_hexdigit());
    /// assert!(!percent.is_ascii_hexdigit());
    /// assert!(!space.is_ascii_hexdigit());
    /// assert!(!lf.is_ascii_hexdigit());
    /// assert!(!esc.is_ascii_hexdigit());
    /// ```
    #[must_use]
    #[stable(feature = "ascii_ctype_on_intrinsics", since = "1.24.0")]
    #[rustc_const_stable(feature = "const_ascii_ctype_on_intrinsics", since = "1.47.0")]
    #[inline]
    pub const fn is_ascii_hexdigit(&self) -> bool {
        matches!(*self, b'0'..=b'9' | b'A'..=b'F' | b'a'..=b'f')
    }

    /// 检查值是否为 ASCII 标点符号：
    ///
    /// - U+0021 ..= U+002F `! " # $ % & ' ( ) * + , - . /`, or
    /// - U+003A ..= U+0040 `: ; < = > ? @`, or
    /// - U+005B ..= U+0060 `` [ \ ] ^ _ ` ``，或
    /// - U+007B ..= U+007E `{ | } ~`
    ///
    /// # Examples
    ///
    /// ```
    /// let uppercase_a = b'A';
    /// let uppercase_g = b'G';
    /// let a = b'a';
    /// let g = b'g';
    /// let zero = b'0';
    /// let percent = b'%';
    /// let space = b' ';
    /// let lf = b'\n';
    /// let esc = b'\x1b';
    ///
    /// assert!(!uppercase_a.is_ascii_punctuation());
    /// assert!(!uppercase_g.is_ascii_punctuation());
    /// assert!(!a.is_ascii_punctuation());
    /// assert!(!g.is_ascii_punctuation());
    /// assert!(!zero.is_ascii_punctuation());
    /// assert!(percent.is_ascii_punctuation());
    /// assert!(!space.is_ascii_punctuation());
    /// assert!(!lf.is_ascii_punctuation());
    /// assert!(!esc.is_ascii_punctuation());
    /// ```
    #[must_use]
    #[stable(feature = "ascii_ctype_on_intrinsics", since = "1.24.0")]
    #[rustc_const_stable(feature = "const_ascii_ctype_on_intrinsics", since = "1.47.0")]
    #[inline]
    pub const fn is_ascii_punctuation(&self) -> bool {
        matches!(*self, b'!'..=b'/' | b':'..=b'@' | b'['..=b'`' | b'{'..=b'~')
    }

    /// 检查值是否为 ASCII 图形字符：
    /// U+0021 '!' ..= U+007E '~'.
    ///
    /// # Examples
    ///
    /// ```
    /// let uppercase_a = b'A';
    /// let uppercase_g = b'G';
    /// let a = b'a';
    /// let g = b'g';
    /// let zero = b'0';
    /// let percent = b'%';
    /// let space = b' ';
    /// let lf = b'\n';
    /// let esc = b'\x1b';
    ///
    /// assert!(uppercase_a.is_ascii_graphic());
    /// assert!(uppercase_g.is_ascii_graphic());
    /// assert!(a.is_ascii_graphic());
    /// assert!(g.is_ascii_graphic());
    /// assert!(zero.is_ascii_graphic());
    /// assert!(percent.is_ascii_graphic());
    /// assert!(!space.is_ascii_graphic());
    /// assert!(!lf.is_ascii_graphic());
    /// assert!(!esc.is_ascii_graphic());
    /// ```
    #[must_use]
    #[stable(feature = "ascii_ctype_on_intrinsics", since = "1.24.0")]
    #[rustc_const_stable(feature = "const_ascii_ctype_on_intrinsics", since = "1.47.0")]
    #[inline]
    pub const fn is_ascii_graphic(&self) -> bool {
        matches!(*self, b'!'..=b'~')
    }

    /// 检查值是否为 ASCII 空格字符：
    /// U+0020 空格、U+0009 水平制表符、U+000A 换行、U+000C 换页或 U+000D 回车。
    ///
    /// Rust 使用 WhatWG 基础标准的 [ASCII 空格的定义][infra-aw]。还有其他几种广泛使用的定义。
    /// 例如，[POSIX 语言环境][pct] 包括 U+000B 垂直标签以及所有上述字符，但是 - 从相同的规格来看 -[Bourne shell 中 "field splitting" 的默认规则][bfs] 仅考虑空格，水平标签和 LINE FEED 作为空白。
    ///
    ///
    /// 如果要编写将处理现有文件格式的程序，请在使用此函数之前检查该格式的空格定义。
    ///
    /// [infra-aw]: https://infra.spec.whatwg.org/#ascii-whitespace
    /// [pct]: https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap07.html#tag_07_03_01
    /// [bfs]: https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_05
    ///
    /// # Examples
    ///
    /// ```
    /// let uppercase_a = b'A';
    /// let uppercase_g = b'G';
    /// let a = b'a';
    /// let g = b'g';
    /// let zero = b'0';
    /// let percent = b'%';
    /// let space = b' ';
    /// let lf = b'\n';
    /// let esc = b'\x1b';
    ///
    /// assert!(!uppercase_a.is_ascii_whitespace());
    /// assert!(!uppercase_g.is_ascii_whitespace());
    /// assert!(!a.is_ascii_whitespace());
    /// assert!(!g.is_ascii_whitespace());
    /// assert!(!zero.is_ascii_whitespace());
    /// assert!(!percent.is_ascii_whitespace());
    /// assert!(space.is_ascii_whitespace());
    /// assert!(lf.is_ascii_whitespace());
    /// assert!(!esc.is_ascii_whitespace());
    /// ```
    ///
    ///
    ///
    ///
    ///
    ///
    ///
    #[must_use]
    #[stable(feature = "ascii_ctype_on_intrinsics", since = "1.24.0")]
    #[rustc_const_stable(feature = "const_ascii_ctype_on_intrinsics", since = "1.47.0")]
    #[inline]
    pub const fn is_ascii_whitespace(&self) -> bool {
        matches!(*self, b'\t' | b'\n' | b'\x0C' | b'\r' | b' ')
    }

    /// 检查值是否为 ASCII 控制字符：
    /// U+0000 NUL ..= U+001F 单元分隔符，或 U+007F 删除。
    /// 请注意，大多数 ASCII 空格字符是控制字符，而 SPACE 不是。
    ///
    ///
    /// # Examples
    ///
    /// ```
    /// let uppercase_a = b'A';
    /// let uppercase_g = b'G';
    /// let a = b'a';
    /// let g = b'g';
    /// let zero = b'0';
    /// let percent = b'%';
    /// let space = b' ';
    /// let lf = b'\n';
    /// let esc = b'\x1b';
    ///
    /// assert!(!uppercase_a.is_ascii_control());
    /// assert!(!uppercase_g.is_ascii_control());
    /// assert!(!a.is_ascii_control());
    /// assert!(!g.is_ascii_control());
    /// assert!(!zero.is_ascii_control());
    /// assert!(!percent.is_ascii_control());
    /// assert!(!space.is_ascii_control());
    /// assert!(lf.is_ascii_control());
    /// assert!(esc.is_ascii_control());
    /// ```
    #[must_use]
    #[stable(feature = "ascii_ctype_on_intrinsics", since = "1.24.0")]
    #[rustc_const_stable(feature = "const_ascii_ctype_on_intrinsics", since = "1.47.0")]
    #[inline]
    pub const fn is_ascii_control(&self) -> bool {
        matches!(*self, b'\0'..=b'\x1F' | b'\x7F')
    }

    /// 返回生成 `u8` 的转义版本的迭代器，将其视为 ASCII 字符。
    ///
    ///
    /// 行为与 [`ascii::escape_default`] 相同。
    ///
    /// # Examples
    ///
    /// ```
    ///
    /// assert_eq!("0", b'0'.escape_ascii().to_string());
    /// assert_eq!("\\t", b'\t'.escape_ascii().to_string());
    /// assert_eq!("\\r", b'\r'.escape_ascii().to_string());
    /// assert_eq!("\\n", b'\n'.escape_ascii().to_string());
    /// assert_eq!("\\'", b'\''.escape_ascii().to_string());
    /// assert_eq!("\\\"", b'"'.escape_ascii().to_string());
    /// assert_eq!("\\\\", b'\\'.escape_ascii().to_string());
    /// assert_eq!("\\x9d", b'\x9d'.escape_ascii().to_string());
    /// ```
    #[must_use = "this returns the escaped byte as an iterator, \
                  without modifying the original"]
    #[stable(feature = "inherent_ascii_escape", since = "1.60.0")]
    #[inline]
    pub fn escape_ascii(self) -> ascii::EscapeDefault {
        ascii::escape_default(self)
    }

    #[inline]
    pub(crate) const fn is_utf8_char_boundary(self) -> bool {
        // 这有点神奇，等于: b <128 ||b>=192
        (self as i8) >= -0x40
    }
}

impl u16 {
    uint_impl! {
        Self = u16,
        ActualT = u16,
        SignedT = i16,
        NonZeroT = NonZeroU16,
        BITS = 16,
        MAX = 65535,
        rot = 4,
        rot_op = "0xa003",
        rot_result = "0x3a",
        swap_op = "0x1234",
        swapped = "0x3412",
        reversed = "0x2c48",
        le_bytes = "[0x34, 0x12]",
        be_bytes = "[0x12, 0x34]",
        to_xe_bytes_doc = "",
        from_xe_bytes_doc = "",
        bound_condition = "",
    }
    widening_impl! { u16, u32, 16, unsigned }
    midpoint_impl! { u16, u32, unsigned }

    /// 检查该值是否是 Unicode 代理代码点，这是 [`char`] 的不允许值。
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(utf16_extra)]
    ///
    /// let low_non_surrogate = 0xA000u16;
    /// let low_surrogate = 0xD800u16;
    /// let high_surrogate = 0xDC00u16;
    /// let high_non_surrogate = 0xE000u16;
    ///
    /// assert!(!low_non_surrogate.is_utf16_surrogate());
    /// assert!(low_surrogate.is_utf16_surrogate());
    /// assert!(high_surrogate.is_utf16_surrogate());
    /// assert!(!high_non_surrogate.is_utf16_surrogate());
    /// ```
    #[must_use]
    #[unstable(feature = "utf16_extra", issue = "94919")]
    #[rustc_const_unstable(feature = "utf16_extra_const", issue = "94919")]
    #[inline]
    pub const fn is_utf16_surrogate(self) -> bool {
        matches!(self, 0xD800..=0xDFFF)
    }
}

impl u32 {
    uint_impl! {
        Self = u32,
        ActualT = u32,
        SignedT = i32,
        NonZeroT = NonZeroU32,
        BITS = 32,
        MAX = 4294967295,
        rot = 8,
        rot_op = "0x10000b3",
        rot_result = "0xb301",
        swap_op = "0x12345678",
        swapped = "0x78563412",
        reversed = "0x1e6a2c48",
        le_bytes = "[0x78, 0x56, 0x34, 0x12]",
        be_bytes = "[0x12, 0x34, 0x56, 0x78]",
        to_xe_bytes_doc = "",
        from_xe_bytes_doc = "",
        bound_condition = "",
    }
    widening_impl! { u32, u64, 32, unsigned }
    midpoint_impl! { u32, u64, unsigned }
}

impl u64 {
    uint_impl! {
        Self = u64,
        ActualT = u64,
        SignedT = i64,
        NonZeroT = NonZeroU64,
        BITS = 64,
        MAX = 18446744073709551615,
        rot = 12,
        rot_op = "0xaa00000000006e1",
        rot_result = "0x6e10aa",
        swap_op = "0x1234567890123456",
        swapped = "0x5634129078563412",
        reversed = "0x6a2c48091e6a2c48",
        le_bytes = "[0x56, 0x34, 0x12, 0x90, 0x78, 0x56, 0x34, 0x12]",
        be_bytes = "[0x12, 0x34, 0x56, 0x78, 0x90, 0x12, 0x34, 0x56]",
        to_xe_bytes_doc = "",
        from_xe_bytes_doc = "",
        bound_condition = "",
    }
    widening_impl! { u64, u128, 64, unsigned }
    midpoint_impl! { u64, u128, unsigned }
}

impl u128 {
    uint_impl! {
        Self = u128,
        ActualT = u128,
        SignedT = i128,
        NonZeroT = NonZeroU128,
        BITS = 128,
        MAX = 340282366920938463463374607431768211455,
        rot = 16,
        rot_op = "0x13f40000000000000000000000004f76",
        rot_result = "0x4f7613f4",
        swap_op = "0x12345678901234567890123456789012",
        swapped = "0x12907856341290785634129078563412",
        reversed = "0x48091e6a2c48091e6a2c48091e6a2c48",
        le_bytes = "[0x12, 0x90, 0x78, 0x56, 0x34, 0x12, 0x90, 0x78, \
            0x56, 0x34, 0x12, 0x90, 0x78, 0x56, 0x34, 0x12]",
        be_bytes = "[0x12, 0x34, 0x56, 0x78, 0x90, 0x12, 0x34, 0x56, \
            0x78, 0x90, 0x12, 0x34, 0x56, 0x78, 0x90, 0x12]",
        to_xe_bytes_doc = "",
        from_xe_bytes_doc = "",
        bound_condition = "",
    }
    midpoint_impl! { u128, unsigned }
}

#[cfg(target_pointer_width = "16")]
impl usize {
    uint_impl! {
        Self = usize,
        ActualT = u16,
        SignedT = isize,
        NonZeroT = NonZeroUsize,
        BITS = 16,
        MAX = 65535,
        rot = 4,
        rot_op = "0xa003",
        rot_result = "0x3a",
        swap_op = "0x1234",
        swapped = "0x3412",
        reversed = "0x2c48",
        le_bytes = "[0x34, 0x12]",
        be_bytes = "[0x12, 0x34]",
        to_xe_bytes_doc = usize_isize_to_xe_bytes_doc!(),
        from_xe_bytes_doc = usize_isize_from_xe_bytes_doc!(),
        bound_condition = " on 16-bit targets",
    }
    widening_impl! { usize, u32, 16, unsigned }
    midpoint_impl! { usize, u32, unsigned }
}

#[cfg(target_pointer_width = "32")]
impl usize {
    uint_impl! {
        Self = usize,
        ActualT = u32,
        SignedT = isize,
        NonZeroT = NonZeroUsize,
        BITS = 32,
        MAX = 4294967295,
        rot = 8,
        rot_op = "0x10000b3",
        rot_result = "0xb301",
        swap_op = "0x12345678",
        swapped = "0x78563412",
        reversed = "0x1e6a2c48",
        le_bytes = "[0x78, 0x56, 0x34, 0x12]",
        be_bytes = "[0x12, 0x34, 0x56, 0x78]",
        to_xe_bytes_doc = usize_isize_to_xe_bytes_doc!(),
        from_xe_bytes_doc = usize_isize_from_xe_bytes_doc!(),
        bound_condition = " on 32-bit targets",
    }
    widening_impl! { usize, u64, 32, unsigned }
    midpoint_impl! { usize, u64, unsigned }
}

#[cfg(target_pointer_width = "64")]
impl usize {
    uint_impl! {
        Self = usize,
        ActualT = u64,
        SignedT = isize,
        NonZeroT = NonZeroUsize,
        BITS = 64,
        MAX = 18446744073709551615,
        rot = 12,
        rot_op = "0xaa00000000006e1",
        rot_result = "0x6e10aa",
        swap_op = "0x1234567890123456",
        swapped = "0x5634129078563412",
        reversed = "0x6a2c48091e6a2c48",
        le_bytes = "[0x56, 0x34, 0x12, 0x90, 0x78, 0x56, 0x34, 0x12]",
        be_bytes = "[0x12, 0x34, 0x56, 0x78, 0x90, 0x12, 0x34, 0x56]",
        to_xe_bytes_doc = usize_isize_to_xe_bytes_doc!(),
        from_xe_bytes_doc = usize_isize_from_xe_bytes_doc!(),
        bound_condition = " on 64-bit targets",
    }
    widening_impl! { usize, u128, 64, unsigned }
    midpoint_impl! { usize, u128, unsigned }
}

impl usize {
    /// 返回一个 `usize`，其中每个字节都等于 `x`。
    #[inline]
    pub(crate) const fn repeat_u8(x: u8) -> usize {
        usize::from_ne_bytes([x; mem::size_of::<usize>()])
    }

    /// 返回一个 `usize`，其中每个字节对都等于 `x`。
    #[inline]
    pub(crate) const fn repeat_u16(x: u16) -> usize {
        let mut r = 0usize;
        let mut i = 0;
        while i < mem::size_of::<usize>() {
            // 使用 `wrapping_shl` 使其适用于具有 16 位 `usize` 的目标
            r = r.wrapping_shl(16) | (x as usize);
            i += 2;
        }
        r
    }
}

/// 浮点数的分类。
///
/// 该 `enum` 用作 [`f32::classify`] 和 [`f64::classify`] 的返回类型。
/// 有关更多信息，请参见他们的文档。
///
/// # Examples
///
/// ```
/// use std::num::FpCategory;
///
/// let num = 12.4_f32;
/// let inf = f32::INFINITY;
/// let zero = 0f32;
/// let sub: f32 = 1.1754942e-38;
/// let nan = f32::NAN;
///
/// assert_eq!(num.classify(), FpCategory::Normal);
/// assert_eq!(inf.classify(), FpCategory::Infinite);
/// assert_eq!(zero.classify(), FpCategory::Zero);
/// assert_eq!(sub.classify(), FpCategory::Subnormal);
/// assert_eq!(nan.classify(), FpCategory::Nan);
/// ```
#[derive(Copy, Clone, PartialEq, Eq, Debug)]
#[stable(feature = "rust1", since = "1.0.0")]
pub enum FpCategory {
    /// NaN (不是数字) : 这个值来自于类似于 `(-1.0).sqrt()` 等的计算得出。
    ///
    /// 有关 NaN 的独特属性的更多信息，请参见 [`f32` 的文档](f32)。
    ///
    #[stable(feature = "rust1", since = "1.0.0")]
    Nan,

    /// 正无穷大或负无穷大，通常由非零数除以零产生。
    ///
    #[stable(feature = "rust1", since = "1.0.0")]
    Infinite,

    /// 正零或负零。
    ///
    /// 有关零的符号性的更多信息，请参见 [`f32` 的文档](f32)。
    #[stable(feature = "rust1", since = "1.0.0")]
    Zero,

    /// `Subnormal` 或 `denormal` 浮点表示 (相对于它们的大小，不如 [`Normal`] 精确)。
    ///
    ///
    /// subnormal 数在数量级上比 [`Zero`] 大，但在数量级上小于所有 [`Normal`] 数。
    ///
    /// [`Normal`]: Self::Normal
    /// [`Zero`]: Self::Zero
    ///
    #[stable(feature = "rust1", since = "1.0.0")]
    Subnormal,

    /// 常规浮点数，不是任何特殊类别。
    ///
    /// 最小的 positive normal 数是 [`f32::MIN_POSITIVE`] 和 [`f64::MIN_POSITIVE`]，最大的 positive normal 数是 [`f32::MAX`] 和 [`f64::MAX`]。
    /// (与有符号整数不同，浮点数在其范围内是对称的，因此否定这些常量中的任何一个都会产生它们的负对应项。)
    ///
    ///
    #[stable(feature = "rust1", since = "1.0.0")]
    Normal,
}

#[doc(hidden)]
trait FromStrRadixHelper:
    PartialOrd + Copy + Add<Output = Self> + Sub<Output = Self> + Mul<Output = Self>
{
    const MIN: Self;
    fn from_u32(u: u32) -> Self;
    fn checked_mul(&self, other: u32) -> Option<Self>;
    fn checked_sub(&self, other: u32) -> Option<Self>;
    fn checked_add(&self, other: u32) -> Option<Self>;
}

macro_rules! from_str_radix_int_impl {
    ($($t:ty)*) => {$(
        #[stable(feature = "rust1", since = "1.0.0")]
        impl FromStr for $t {
            type Err = ParseIntError;
            fn from_str(src: &str) -> Result<Self, ParseIntError> {
                from_str_radix(src, 10)
            }
        }
    )*}
}
from_str_radix_int_impl! { isize i8 i16 i32 i64 i128 usize u8 u16 u32 u64 u128 }

macro_rules! impl_helper_for {
    ($($t:ty)*) => ($(impl FromStrRadixHelper for $t {
        const MIN: Self = Self::MIN;
        #[inline]
        fn from_u32(u: u32) -> Self { u as Self }
        #[inline]
        fn checked_mul(&self, other: u32) -> Option<Self> {
            Self::checked_mul(*self, other as Self)
        }
        #[inline]
        fn checked_sub(&self, other: u32) -> Option<Self> {
            Self::checked_sub(*self, other as Self)
        }
        #[inline]
        fn checked_add(&self, other: u32) -> Option<Self> {
            Self::checked_add(*self, other as Self)
        }
    })*)
}
impl_helper_for! { i8 i16 i32 i64 i128 isize u8 u16 u32 u64 u128 usize }

/// 确定是否可以保证以给定类型 T 存储该基数长度的文本字符串。
///
/// 请注意，如果编译器知道基数，则只是在运行时检查 digits.len。
///
#[doc(hidden)]
#[inline(always)]
#[unstable(issue = "none", feature = "std_internals")]
pub fn can_not_overflow<T>(radix: u32, is_signed_ty: bool, digits: &[u8]) -> bool {
    radix <= 16 && digits.len() <= mem::size_of::<T>() * 2 - is_signed_ty as usize
}

fn from_str_radix<T: FromStrRadixHelper>(src: &str, radix: u32) -> Result<T, ParseIntError> {
    use self::IntErrorKind::*;
    use self::ParseIntError as PIE;

    assert!(
        (2..=36).contains(&radix),
        "from_str_radix_int: must lie in the range `[2, 36]` - found {}",
        radix
    );

    if src.is_empty() {
        return Err(PIE { kind: Empty });
    }

    let is_signed_ty = T::from_u32(0) > T::MIN;

    // 所有有效数字均为 ascii，因此我们将遍历 utf8 字节并将其转换为 char。
    // 除了给定基数的有效 ascii 数字之外，.to_digit() 将安全地返回 None，包括多字节序列的第一个字节
    //
    //
    let src = src.as_bytes();

    let (is_positive, digits) = match src[0] {
        b'+' | b'-' if src[1..].is_empty() => {
            return Err(PIE { kind: InvalidDigit });
        }
        b'+' => (true, &src[1..]),
        b'-' if is_signed_ty => (false, &src[1..]),
        _ => (true, src),
    };

    let mut result = T::from_u32(0);

    if can_not_overflow::<T>(radix, is_signed_ty, digits) {
        // 如果 str 的 len 与我们要解析的类型的范围相比很短，那么我们可以确定不会发生溢出。
        //
        // 这个界限是当 `radix.pow(digits.len()) - 1 <= T::MAX` 但上面的条件是一个更快的 (conservative) 近似值。
        //
        // 考虑基数 16，因为它的每个数字的信息密度最高，因此最早会溢出:
        // `u8::MAX` 是 `ff`-len 2 的任何 str 都保证不会溢出。
        // `i8::MAX` 是 `7f` - 只有 len 1 的 str 保证不会溢出。
        //
        macro_rules! run_unchecked_loop {
            ($unchecked_additive_op:expr) => {
                for &c in digits {
                    result = result * T::from_u32(radix);
                    let x = (c as char).to_digit(radix).ok_or(PIE { kind: InvalidDigit })?;
                    result = $unchecked_additive_op(result, T::from_u32(x));
                }
            };
        }
        if is_positive {
            run_unchecked_loop!(<T as core::ops::Add>::add)
        } else {
            run_unchecked_loop!(<T as core::ops::Sub>::sub)
        };
    } else {
        macro_rules! run_checked_loop {
            ($checked_additive_op:ident, $overflow_err:expr) => {
                for &c in digits {
                    // 当 `radix` 作为字面量传入时，如果 `radix` 可以表示为 2 的幂和 (x*10 可以写为 x*8 + x*2)，编译器可以使用移位，而不是执行缓慢的 `imul`。
                    // 当编译器不能使用这些优化时，乘法的延迟可以通过在需要结果之前发出来隐藏，以提高现代无序 CPU 的性能，因为这里的乘法比其他指令慢，我们可以得到最终结果更快地先进行乘法运算，让 CPU 花其他周期进行其他计算，然后再得到乘法结果。
                    //
                    //
                    //
                    //
                    //
                    //
                    //
                    //
                    let mul = result.checked_mul(radix);
                    let x = (c as char).to_digit(radix).ok_or(PIE { kind: InvalidDigit })?;
                    result = mul.ok_or_else($overflow_err)?;
                    result = T::$checked_additive_op(&result, x).ok_or_else($overflow_err)?;
                }
            };
        }
        if is_positive {
            run_checked_loop!(checked_add, || PIE { kind: PosOverflow })
        } else {
            run_checked_loop!(checked_sub, || PIE { kind: NegOverflow })
        };
    }
    Ok(result)
}