parse.rs

use crate::internal_macros::write_err;
use crate::impl_std_error;
use core::fmt;
use core::str::FromStr;
use core::convert::TryFrom;
use crate::prelude::*;

/// Error with rich context returned when a string can't be parsed as an integer.
///
/// This is an extension of [`core::num::ParseIntError`], which carries the input that failed to
/// parse as well as type information. As a result it provides very informative error messages that
/// make it easier to understand the problem and correct mistakes.
///
/// Note that this is larger than the type from `core` so if it's passed through a deep call stack
/// in a performance-critical application you may want to box it or throw away the context by
/// converting to `core` type.
#[derive(Debug, Clone, Eq, PartialEq)]
pub struct ParseIntError {
    input: String,
    // for displaying - see Display impl with nice error message below
    bits: u8,
    // We could represent this as a single bit but it wouldn't actually derease the cost of moving
    // the struct because String contains pointers so there will be padding of bits at least
    // pointer_size - 1 bytes: min 1B in practice.
    is_signed: bool,
    source: core::num::ParseIntError,
}

impl ParseIntError {
    /// Returns the input that was attempted to be parsed.
    pub fn input(&self) -> &str {
        &self.input
    }
}

impl From<ParseIntError> for core::num::ParseIntError {
    fn from(value: ParseIntError) -> Self {
        value.source
    }
}

impl AsRef<core::num::ParseIntError> for ParseIntError {
    fn as_ref(&self) -> &core::num::ParseIntError {
        &self.source
    }
}

impl fmt::Display for ParseIntError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        let signed = if self.is_signed { "signed" } else { "unsigned" };
        let n = if self.bits == 8 { "n" } else { "" };
        write_err!(f, "failed to parse '{}' as a{} {}-bit {} integer", self.input, n, self.bits, signed; self.source)
    }
}

/// Not strictly neccessary but serves as a lint - avoids weird behavior if someone accidentally
/// passes non-integer to the `parse()` function.
pub(crate) trait Integer: FromStr<Err=core::num::ParseIntError> + TryFrom<i8> + Sized {}

macro_rules! impl_integer {
    ($($type:ty),* $(,)?) => {
        $(
        impl Integer for $type {}
        )*
    }
}

impl_integer!(u8, i8, u16, i16, u32, i32, u64, i64, u128, i128);

/// Parses the input string as an integer returning an error carrying rich context.
///
/// If the caller owns `String` or `Box<str>` which is not used later it's better to pass it as
/// owned since it avoids allocation in error case.
pub(crate) fn int<T: Integer, S: AsRef<str> + Into<String>>(s: S) -> Result<T, ParseIntError> {
    s.as_ref().parse().map_err(|error| {
        ParseIntError {
            input: s.into(),
            bits: u8::try_from(core::mem::size_of::<T>() * 8).expect("max is 128 bits for u128"),
            // We detect if the type is signed by checking if -1 can be represented by it
            // this way we don't have to implement special traits and optimizer will get rid of the
            // computation.
            is_signed: T::try_from(-1i8).is_ok(),
            source: error,
        }
    })
}

impl_std_error!(ParseIntError, source);