compare_variables_macro 0.2.2

#![cfg_attr(debug_assertions, allow(unused_imports))]

use proc_macro::{self, TokenStream};
use proc_macro_error::abort;
use proc_macro2::{Ident, Span, TokenStream as TokenStream2};
use quote::quote;
use syn::ext::IdentExt;
use syn::parse::Parse;
use syn::{Token, parse_macro_input};

/**
A macro to compare types which implement `PartialOrd`.

# Overview

This macro performs comparison between two or three values of any type `T` which
implements  `PartialOrd`. If the comparison evaluates to `true`, the macro
returns `Result::Ok(())`, otherwise it returns a
`Result::Err(compare_variables::Operator)` which can be formatted into a
string showcasing the failed comparison.

The macro syntax is
```math
compare_variables(x _ y)
```
for comparing two values and
```math
compare_variables(x _ y _ z)
```
for comparing three values with `_` being any of the comparison operators
`<, <=, ==, !=, >, >=`.

`x`, `y` and `z` can be either a literal (e.g. `3.141` or `1e10`) or a variable:

```rust
use compare_variables::compare_variables;

assert!(compare_variables!(2.0 > 1.5).is_ok());

let x = 1;
let y = 2;
assert!(compare_variables!(x < 2 == y).is_ok());
assert!(compare_variables!(x >= 2).is_err());
assert!(compare_variables!(x != y).is_ok());
```

It is possible to combine the macro with the question mark operator:
```rust
use compare_variables::{compare_variables, Operator};

fn checked_sub(left: u16, right: u16) -> Result<u16, Operator<u16>> {
    compare_variables!(left >= right)?;
    return Ok(left - right);
}

assert_eq!(checked_sub(2, 1).unwrap(), 1);
assert_eq!(checked_sub(2, 2).unwrap(), 0);
assert!(checked_sub(2, 3).is_err());
```

It is also possible to use named and anonymous struct fields as inputs:

```
use compare_variables::compare_variables;

struct NamedField {
   x: f64
}
let n = NamedField {x: 1.0};
assert!(compare_variables!(n.x > -1.0).is_ok());
assert!(compare_variables!(n.x > 1.0).is_err());

struct AnonymousField(i32);
let a = AnonymousField(-5);
assert!(compare_variables!(a.0 > -6).is_ok());
assert!(compare_variables!(a.0 > 1).is_err());
```

# Error message

The error message is created via the struct [`Operator`](https://docs.rs/compare_variables/0.1.0/compare_variables/struct.Operator.html).
Please refer to its documentation for more details. The keywords `val` and `as` allow to customize the treatment of variable names in the error message:

```
use compare_variables::compare_variables;

// Error message with literals only
let err = compare_variables!(5i32 <= -1i32).unwrap_err();
assert_eq!(err.to_string(), "`5 <= -1` is false");

let x = 1;
let y = 2;

// Default error message
let err = compare_variables!(x > y).unwrap_err();
assert_eq!(err.to_string(), "`x (value: 1) > y (value: 2)` is false");

// Rename x in the error message
let err = compare_variables!(x as variable > y).unwrap_err();
assert_eq!(err.to_string(), "`variable (value: 1) > y (value: 2)` is false");

// Only display the underlying value, not the variable name:
let err = compare_variables!(val x > y).unwrap_err();
assert_eq!(err.to_string(), "`1 > y (value: 2)` is false");

// `as` is ignored if used together with `val`:
let err = compare_variables!(val x as variable > y).unwrap_err();
assert_eq!(err.to_string(), "`1 > y (value: 2)` is false");
```

# Examples

```rust
use compare_variables::compare_variables;

// Different float types:
assert!(compare_variables!(1.5 < 2.0 == 3.0).is_err());
assert!(compare_variables!(1.7f32 == 1.7f32).is_ok());
let f = 2.0;
assert!(compare_variables!(f < 5.2).is_ok());
assert!(compare_variables!(f as f_var == val f).is_ok());

// Signed and unsigned integers
assert!(compare_variables!(1i32 >= 2i32).is_err());
let u = 3usize;
assert!(compare_variables!(2usize < u < 4usize).is_ok());
let i = 15i64;
assert!(compare_variables!(-10i64 <= i).is_ok());
assert!(compare_variables!(-10i64 <= i <= 10i64).is_err());

// Custom types implementing `PartialOrd`.
// Clone and Copy are not required and are only used here for the example.
#[derive(PartialEq, Clone, Copy)]
struct MyFloat64(f64);

impl PartialOrd for MyFloat64 {
   fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
       return self.0.partial_cmp(&other.0);
   }
}

let myfloat1 = MyFloat64(1.0);
let myfloat2 = MyFloat64(2.0);
assert!(compare_variables!(myfloat1 == myfloat1).is_ok());
assert!(compare_variables!(myfloat1 <= myfloat2).is_ok());
assert!(compare_variables!(myfloat1 >= myfloat2).is_err());
```
 */
#[proc_macro]
pub fn compare_variables(input: TokenStream) -> TokenStream {
    let comparison_error_info: ComparisonErrorInfo = parse_macro_input!(input);

    let first_arg = comparison_error_info.first_arg.as_token_stream();
    let relation_first_to_second = comparison_error_info
        .relation_first_to_second
        .as_token_stream();
    let second_arg = comparison_error_info.second_arg.as_token_stream();
    let relation_second_to_third = comparison_error_info
        .relation_second_to_third
        .as_token_stream();
    let third_arg = match comparison_error_info.third_arg {
        Some(arg) => {
            let ts = arg.as_token_stream();
            quote! {Some(#ts)}
        }
        None => quote! {None},
    };

    // Build the input for the compare_variables function
    let stream = quote! {
        compare_variables::Operator::new(
            #first_arg,
            #relation_first_to_second,
            #second_arg,
            #relation_second_to_third,
            #third_arg,
        )
    };

    return TokenStream::from(stream);
}

#[repr(u8)]
enum Operator {
    Lesser,
    LesserOrEqual,
    Equal,
    Inequal,
    GreaterOrEqual,
    Greater,
}

impl Operator {
    fn as_token_stream(&self) -> proc_macro2::TokenStream {
        match self {
            Operator::Lesser => {
                quote! {
                    compare_variables::ComparisonOperator::Lesser
                }
            }
            Operator::LesserOrEqual => {
                quote! {
                    compare_variables::ComparisonOperator::LesserOrEqual
                }
            }
            Operator::Equal => {
                quote! {
                    compare_variables::ComparisonOperator::Equal
                }
            }
            Operator::Inequal => {
                quote! {
                    compare_variables::ComparisonOperator::Inequal
                }
            }
            Operator::GreaterOrEqual => {
                quote! {
                    compare_variables::ComparisonOperator::GreaterOrEqual
                }
            }
            Operator::Greater => {
                quote! {
                    compare_variables::ComparisonOperator::Greater
                }
            }
        }
    }
}

impl Parse for Operator {
    fn parse(input: syn::parse::ParseStream) -> syn::Result<Self> {
        // If Token![<] is tested before Token![<=], then "<" is parsed, leaving only
        // "=". This will then lead to a compile error.
        if input.peek(Token![<=]) {
            input.parse::<Token![<=]>()?;
            Ok(Operator::LesserOrEqual)
        } else if input.peek(Token![>=]) {
            input.parse::<Token![>=]>()?;
            Ok(Operator::GreaterOrEqual)
        } else if input.peek(Token![==]) {
            input.parse::<Token![==]>()?;
            Ok(Operator::Equal)
        } else if input.peek(Token![!=]) {
            input.parse::<Token![!=]>()?;
            Ok(Operator::Inequal)
        } else if input.peek(Token![<]) {
            input.parse::<Token![<]>()?;
            Ok(Operator::Lesser)
        } else if input.peek(Token![>]) {
            input.parse::<Token![>]>()?;
            Ok(Operator::Greater)
        } else {
            Err(syn::Error::new(
                input.span(),
                "no comparison operator could be identified. Valid
                    operators are \"<\", \"<=\", \"==\", \"!=\", \">=\" or \">\".",
            ))
        }
    }
}

enum VariableOrLiteral {
    Other {
        arg_names: Vec<String>,
        arg_names_display: Vec<String>,
    },
    LitFloat(syn::LitFloat),
    LitInt(syn::LitInt),
}

impl VariableOrLiteral {
    fn as_token_stream(&self) -> proc_macro2::TokenStream {
        match self {
            VariableOrLiteral::Other {
                arg_names,
                arg_names_display,
            } => {
                // Build a token stream out of arg_name and arg_name_display, using . as a
                // delimiter
                let arg_value = arg_names.join(".");
                let arg_value_ts: TokenStream2 = match str::parse::<TokenStream2>(&arg_value) {
                    Ok(ts) => ts,
                    Err(_) => abort!(
                        Span::call_site(),
                        format!("could not interpret {arg_value} as rust code")
                    ),
                };
                if arg_names_display.is_empty() {
                    quote! {
                        compare_variables::ComparisonValue::new(#arg_value_ts, None)
                    }
                } else {
                    let arg_name_display = arg_names_display.join(".");
                    quote! {
                        compare_variables::ComparisonValue::new(#arg_value_ts, Some(#arg_name_display))
                    }
                }
            }
            VariableOrLiteral::LitFloat(lit) => {
                quote! {
                    compare_variables::ComparisonValue::new(#lit, None)
                }
            }
            VariableOrLiteral::LitInt(lit) => {
                quote! {
                    compare_variables::ComparisonValue::new(#lit, None)
                }
            }
        }
    }
}

impl Parse for VariableOrLiteral {
    fn parse(input: syn::parse::ParseStream) -> syn::Result<Self> {
        fn parse_composite_varname(
            input: &syn::parse::ParseStream,
            vec: &mut Vec<String>,
        ) -> syn::Result<()> {
            loop {
                if input.peek(syn::LitInt) {
                    let lit = input.parse::<syn::LitInt>()?;
                    vec.push(lit.to_string());
                } else {
                    let ident: syn::Ident = input.call(Ident::parse_any)?;
                    vec.push(ident.to_string()); // parse_any also handles stuff like self
                }

                if input.peek(Token![.]) {
                    // Throw the token away
                    let _ = input.parse::<Token![.]>()?;
                } else {
                    // Field access is done ==> Finish the loop
                    break;
                }
            }
            return Ok(());
        } // parse_composite_varname

        if input.peek(syn::LitFloat) {
            // Parse the float literal
            let val = input.parse::<syn::LitFloat>()?;
            return Ok(VariableOrLiteral::LitFloat(val));
        } else if input.peek(syn::LitInt) {
            // Parse the float literal
            let val = input.parse::<syn::LitInt>()?;
            return Ok(VariableOrLiteral::LitInt(val));
        } else {
            let mut display_arg_names = true;

            // Input is possibly a variable name.
            let mut arg_names: Vec<String> = Vec::new();

            // First check if the first identifier is "val":
            let first_ident: Ident = input.call(Ident::parse_any)?; // parse_any also handles stuff like self

            // Next identifier is not a "." -> Check if first_ident is "val". If not, the
            // macro is used wrong
            if input.peek(Token![.]) {
                // Throw the token away
                let _ = input.parse::<Token![.]>()?;

                // Try continuing to parse and keep the first identifier
                arg_names.push(first_ident.to_string());
                parse_composite_varname(&input, &mut arg_names)?;
            } else {
                if input.peek(syn::Ident) {
                    if first_ident == "val" {
                        display_arg_names = false;

                        // Try continuing to parse
                        parse_composite_varname(&input, &mut arg_names)?;
                    } else {
                        abort!(
                            Span::call_site(),
                            format!("found unexpected tokens behind {first_ident}")
                        )
                    }
                } else {
                    arg_names.push(first_ident.to_string());
                }
            }

            // Resolve the alias, if the variable name should be displayed
            let arg_names_display: Vec<String> = if input.peek(Token![as]) {
                input.parse::<Token![as]>()?;
                let mut arg_names_display: Vec<String> = Vec::new();
                parse_composite_varname(&input, &mut arg_names_display)?;
                if display_arg_names {
                    arg_names_display
                } else {
                    Vec::new()
                }
            } else {
                if display_arg_names {
                    arg_names.clone()
                } else {
                    Vec::new()
                }
            };

            return Ok(VariableOrLiteral::Other {
                arg_names,
                arg_names_display,
            });
        }
    }
}

// Parser for the compare_variables macro
struct ComparisonErrorInfo {
    first_arg: VariableOrLiteral,
    relation_first_to_second: Operator,
    second_arg: VariableOrLiteral,
    relation_second_to_third: Operator,
    third_arg: Option<VariableOrLiteral>,
}

impl Parse for ComparisonErrorInfo {
    fn parse(input: syn::parse::ParseStream) -> syn::Result<Self> {
        // Read the arguments
        let first_arg = VariableOrLiteral::parse(&input)?;
        let relation_first_to_second = Operator::parse(&input)?;
        let second_arg = VariableOrLiteral::parse(&input)?;

        // If the input continues, parse the third argument
        let (relation_second_to_third, third_arg) = if let Ok(operator) = Operator::parse(&input) {
            (operator, Some(VariableOrLiteral::parse(&input)?))
        } else {
            (Operator::Equal, None)
        };

        return Ok(ComparisonErrorInfo {
            first_arg,
            relation_first_to_second,
            second_arg,
            relation_second_to_third,
            third_arg,
        });
    }
}

#[cfg(test)]
mod tests {

    use super::*;

    #[test]
    fn test_parse_check_bounds_info() {
        // Assert that the parse was successfull

        // Float
        let _: ComparisonErrorInfo = syn::parse_quote!(0.0 < arg);
        let _: ComparisonErrorInfo = syn::parse_quote!(0.0 <= arg);
        let _: ComparisonErrorInfo = syn::parse_quote!(0.0 <= arg as alternative_arg);
        let _: ComparisonErrorInfo = syn::parse_quote!(0.0 < arg <= 1.0);
        let _: ComparisonErrorInfo = syn::parse_quote!(0.0 < arg as alternative_arg <= 1.0);
        let _: ComparisonErrorInfo = syn::parse_quote!(arg < 1.0);
        let _: ComparisonErrorInfo = syn::parse_quote!(arg <= 1.0);
        let _: ComparisonErrorInfo = syn::parse_quote!(arg as alternative_arg <= 1.0);

        // Int
        let _: ComparisonErrorInfo = syn::parse_quote!(-1 < arg);
        let _: ComparisonErrorInfo = syn::parse_quote!(-1 < -2);
        let _: ComparisonErrorInfo = syn::parse_quote!(-1 < arg as alternative_arg <= 2);
    }
}