rust: str: add radix prefixed integer parsing functions

Add the trait `ParseInt` for parsing string representations of integers
where the string representations are optionally prefixed by a radix
specifier. Implement the trait for the primitive integer types.

Tested-by: Daniel Almeida <daniel.almeida@collabora.com>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>

Author: Andreas Hindborg

rust/kernel/str.rs

@@ -945,3 +945,158 @@ impl fmt::Debug for CString {
 macro_rules! fmt {
     ($($f:tt)*) => ( core::format_args!($($f)*) )
 }
+
+pub mod parse_int {
+    //! Integer parsing functions for parsing signed and unsigned integers
+    //! potentially prefixed with `0x`, `0o`, or `0b`.
+
+    use crate::prelude::*;
+    use crate::str::BStr;
+    use core::ops::Deref;
+
+    // Make `FromStrRadix` a public type with a private name. This seals
+    // `ParseInt`, that is, prevents downstream users from implementing the
+    // trait.
+    mod private {
+        use crate::str::BStr;
+
+        /// Trait that allows parsing a [`&BStr`] to an integer with a radix.
+        ///
+        /// [`&BStr`]: kernel::str::BStr
+        // This is required because the `from_str_radix` function on the primitive
+        // integer types is not part of any trait.
+        pub trait FromStrRadix: Sized {
+            /// Parse `src` to `Self` using radix `radix`.
+            fn from_str_radix(src: &BStr, radix: u32) -> Result<Self, crate::error::Error>;
+
+            /// Return the absolute value of Self::MIN.
+            fn abs_min() -> u64;
+
+            /// Perform bitwise 2's complement on `self`.
+            ///
+            /// Note: This function does not make sense for unsigned integers.
+            fn complement(self) -> Self;
+        }
+    }
+
+    /// Extract the radix from an integer literal optionally prefixed with
+    /// one of `0x`, `0X`, `0o`, `0O`, `0b`, `0B`, `0`.
+    fn strip_radix(src: &BStr) -> (u32, &BStr) {
+        match src.deref() {
+            [b'0', b'x' | b'X', rest @ ..] => (16, rest.as_ref()),
+            [b'0', b'o' | b'O', rest @ ..] => (8, rest.as_ref()),
+            [b'0', b'b' | b'B', rest @ ..] => (2, rest.as_ref()),
+            // NOTE: We are including the leading zero to be able to parse
+            // literal 0 here. If we removed it as a radix prefix, we would not
+            // be able to parse `0`.
+            [b'0', ..] => (8, src),
+            _ => (10, src),
+        }
+    }
+
+    /// Trait for parsing string representations of integers.
+    ///
+    /// Strings beginning with `0x`, `0o`, or `0b` are parsed as hex, octal, or
+    /// binary respectively. Strings beginning with `0` otherwise are parsed as
+    /// octal. Anything else is parsed as decimal. A leading `+` or `-` is also
+    /// permitted. Any string parsed by [`kstrtol()`] or [`kstrtoul()`] will be
+    /// successfully parsed.
+    ///
+    /// [`kstrtol()`]: https://www.kernel.org/doc/html/latest/core-api/kernel-api.html#c.kstrtol
+    /// [`kstrtoul()`]: https://www.kernel.org/doc/html/latest/core-api/kernel-api.html#c.kstrtoul
+    ///
+    /// # Example
+    /// ```
+    /// use kernel::str::parse_int::ParseInt;
+    /// use kernel::b_str;
+    ///
+    /// assert_eq!(Ok(0), u8::from_str(b_str!("0")));
+    ///
+    /// assert_eq!(Ok(0xa2u8), u8::from_str(b_str!("0xa2")));
+    /// assert_eq!(Ok(-0xa2i32), i32::from_str(b_str!("-0xa2")));
+    ///
+    /// assert_eq!(Ok(-0o57i8), i8::from_str(b_str!("-0o57")));
+    /// assert_eq!(Ok(0o57i8), i8::from_str(b_str!("057")));
+    ///
+    /// assert_eq!(Ok(0b1001i16), i16::from_str(b_str!("0b1001")));
+    /// assert_eq!(Ok(-0b1001i16), i16::from_str(b_str!("-0b1001")));
+    ///
+    /// assert_eq!(Ok(127), i8::from_str(b_str!("127")));
+    /// assert!(i8::from_str(b_str!("128")).is_err());
+    /// assert_eq!(Ok(-128), i8::from_str(b_str!("-128")));
+    /// assert!(i8::from_str(b_str!("-129")).is_err());
+    /// assert_eq!(Ok(255), u8::from_str(b_str!("255")));
+    /// assert!(u8::from_str(b_str!("256")).is_err());
+    /// ```
+    pub trait ParseInt: private::FromStrRadix + TryFrom<u64> {
+        /// Parse a string according to the description in [`Self`].
+        fn from_str(src: &BStr) -> Result<Self> {
+            match src.deref() {
+                [b'-', rest @ ..] => {
+                    let (radix, digits) = strip_radix(rest.as_ref());
+                    // 2's complement values range from -2^(b-1) to 2^(b-1)-1.
+                    // So if we want to parse negative numbers as positive and
+                    // later multiply by -1, we have to parse into a larger
+                    // integer. We choose u64 as sufficiently large. NOTE: 128
+                    // bit integers are not available on all platforms, hence
+                    // the choice of 64 bit.
+                    let val = u64::from_str_radix(
+                        core::str::from_utf8(digits).map_err(|_| EINVAL)?,
+                        radix,
+                    )
+                    .map_err(|_| EINVAL)?;
+
+                    if val > Self::abs_min() {
+                        return Err(EINVAL);
+                    }
+
+                    // SAFETY: We checked that `val` will fit in `Self` above.
+                    let val: Self = unsafe { val.try_into().unwrap_unchecked() };
+
+                    Ok(val.complement())
+                }
+                _ => {
+                    let (radix, digits) = strip_radix(src);
+                    Self::from_str_radix(digits, radix).map_err(|_| EINVAL)
+                }
+            }
+        }
+    }
+
+    macro_rules! impl_parse_int {
+        ($ty:ty) => {
+            impl private::FromStrRadix for $ty {
+                fn from_str_radix(src: &BStr, radix: u32) -> Result<Self, crate::error::Error> {
+                    <$ty>::from_str_radix(core::str::from_utf8(src).map_err(|_| EINVAL)?, radix)
+                        .map_err(|_| EINVAL)
+                }
+
+                fn abs_min() -> u64 {
+                    #[allow(unused_comparisons)]
+                    if Self::MIN < 0 {
+                        1u64 << (Self::BITS - 1)
+                    } else {
+                        0
+                    }
+                }
+
+                fn complement(self) -> Self {
+                    (!self).wrapping_add((1 as $ty))
+                }
+            }
+
+            impl ParseInt for $ty {}
+        };
+    }
+
+    impl_parse_int!(i8);
+    impl_parse_int!(u8);
+    impl_parse_int!(i16);
+    impl_parse_int!(u16);
+    impl_parse_int!(i32);
+    impl_parse_int!(u32);
+    impl_parse_int!(i64);
+    impl_parse_int!(u64);
+    impl_parse_int!(isize);
+    impl_parse_int!(usize);
+}