write-fonts/src/validate.rs

//! The pre-compilation validation pass

use std::{
    collections::BTreeSet,
    fmt::{Debug, Display},
    ops::Deref,
};

use crate::offsets::{NullableOffsetMarker, OffsetMarker};

/// Pre-compilation validation of tables.
///
/// The OpenType specification describes various requirements for different
/// tables that are awkward to encode in the type system, such as requiring
/// certain arrays to have equal lengths. These requirements are enforced
/// via a validation pass.
pub trait Validate {
    /// Ensure that this table is well-formed, reporting any errors.
    ///
    /// This is an auto-generated method that calls to [validate_impl][Self::validate_impl] and
    /// collects any errors.
    fn validate(&self) -> Result<(), ValidationReport> {
        let mut ctx = Default::default();
        self.validate_impl(&mut ctx);
        if ctx.errors.is_empty() {
            Ok(())
        } else {
            Err(ValidationReport { errors: ctx.errors })
        }
    }

    /// Validate this table.
    ///
    /// If you need to implement this directly, it should look something like:
    ///
    /// ```rust
    /// # use write_fonts::validate::{Validate, ValidationCtx};
    /// struct MyRecord {
    ///     my_values: Vec<u16>,
    /// }
    ///
    /// impl Validate for MyRecord {
    ///     fn validate_impl(&self, ctx: &mut ValidationCtx) {
    ///         ctx.in_table("MyRecord", |ctx| {
    ///             ctx.in_field("my_values", |ctx| {
    ///                 if self.my_values.len() > (u16::MAX as usize) {
    ///                     ctx.report("array is too long");
    ///                 }
    ///             })
    ///         })
    ///     }
    /// }
    /// ```
    #[allow(unused_variables)]
    fn validate_impl(&self, ctx: &mut ValidationCtx);
}

/// A context for collecting validation error.
///
/// This is responsible for tracking the position in the tree at which
/// a given error is reported.
///
/// ## paths/locations
///
/// As validation travels down through the object graph, the path is recorded
/// via appropriate calls to methods like [in_table][Self::in_table] and [in_field][Self::in_field].
#[derive(Clone, Debug, Default)]
pub struct ValidationCtx {
    cur_location: Vec<LocationElem>,
    errors: Vec<ValidationError>,
}

#[derive(Debug, Clone)]
struct ValidationError {
    error: String,
    location: Vec<LocationElem>,
}

/// One or more validation errors.
#[derive(Clone)]
pub struct ValidationReport {
    errors: Vec<ValidationError>,
}

#[derive(Debug, Clone)]
enum LocationElem {
    Table(&'static str),
    Field(&'static str),
    Index(usize),
}

impl ValidationCtx {
    /// Run the provided closer in the context of a new table.
    ///
    /// Errors reported in the closure will include the provided identifier
    /// in their path.
    pub fn in_table(&mut self, name: &'static str, f: impl FnOnce(&mut ValidationCtx)) {
        self.with_elem(LocationElem::Table(name), f);
    }

    /// Run the provided closer in the context of a new field.
    ///
    /// Errors reported in the closure will be associated with the field.
    pub fn in_field(&mut self, name: &'static str, f: impl FnOnce(&mut ValidationCtx)) {
        self.with_elem(LocationElem::Field(name), f);
    }

    /// Run the provided closer for each item in an array.
    ///
    /// This handles tracking the active item, so that validation errors can
    /// be associated with the correct index.
    pub fn with_array_items<'a, T: 'a>(
        &mut self,
        iter: impl Iterator<Item = &'a T>,
        mut f: impl FnMut(&mut ValidationCtx, &T),
    ) {
        for (i, item) in iter.enumerate() {
            self.with_elem(LocationElem::Index(i), |ctx| f(ctx, item))
        }
    }

    /// Report a new error, associating it with the current path.
    pub fn report(&mut self, msg: impl Display) {
        self.errors.push(ValidationError {
            location: self.cur_location.clone(),
            error: msg.to_string(),
        });
    }

    fn with_elem(&mut self, elem: LocationElem, f: impl FnOnce(&mut ValidationCtx)) {
        self.cur_location.push(elem);
        f(self);
        self.cur_location.pop();
    }
}

impl Display for ValidationReport {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        if self.errors.len() == 1 {
            return writeln!(f, "Validation error:\n{}", self.errors.first().unwrap());
        }

        writeln!(f, "{} validation errors:", self.errors.len())?;
        for (i, error) in self.errors.iter().enumerate() {
            writeln!(f, "#{}\n{error}", i + 1)?;
        }
        Ok(())
    }
}

impl Debug for ValidationReport {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        <Self as Display>::fmt(self, f)
    }
}

static MANY_SPACES: &str = "                                                                                                        ";

impl Display for ValidationError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        writeln!(f, "\"{}\"", self.error)?;
        let mut indent = 0;
        if self.location.len() < 2 {
            return f.write_str("no parent location available");
        }

        for (i, window) in self.location.windows(2).enumerate() {
            let prev = &window[0];
            let current = &window[1];
            if i == 0 {
                if let LocationElem::Table(name) = prev {
                    write!(f, "in: {name}")?;
                } else {
                    panic!("first item always table");
                }
            }

            match current {
                LocationElem::Table(name) => {
                    indent += 1;
                    let indent_str = &MANY_SPACES[..indent * 2];
                    write!(f, "\n{indent_str}{name}")
                }
                LocationElem::Field(name) => write!(f, ".{name}"),
                LocationElem::Index(idx) => write!(f, "[{idx}]"),
            }?;
        }
        writeln!(f)
    }
}

impl<T: Validate> Validate for Vec<T> {
    fn validate_impl(&self, ctx: &mut ValidationCtx) {
        ctx.with_array_items(self.iter(), |ctx, item| item.validate_impl(ctx))
    }
}

impl<const N: usize, T: Validate> Validate for OffsetMarker<T, N> {
    fn validate_impl(&self, ctx: &mut ValidationCtx) {
        self.deref().validate_impl(ctx)
    }
}

impl<const N: usize, T: Validate> Validate for NullableOffsetMarker<T, N> {
    fn validate_impl(&self, ctx: &mut ValidationCtx) {
        if let Some(b) = self.as_ref() {
            b.validate_impl(ctx);
        }
    }
}

impl<T: Validate> Validate for Option<T> {
    fn validate_impl(&self, ctx: &mut ValidationCtx) {
        match self {
            Some(t) => t.validate_impl(ctx),
            None => (),
        }
    }
}

impl<T: Validate> Validate for BTreeSet<T> {
    fn validate_impl(&self, ctx: &mut ValidationCtx) {
        ctx.with_array_items(self.iter(), |ctx, item| item.validate_impl(ctx))
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn sanity_check_array_validation() {
        #[derive(Clone, Debug, Copy)]
        struct Derp(i16);

        struct DerpStore {
            derps: Vec<Derp>,
        }

        impl Validate for Derp {
            fn validate_impl(&self, ctx: &mut ValidationCtx) {
                if self.0 > 7 {
                    ctx.report("this derp is too big!!");
                }
            }
        }

        impl Validate for DerpStore {
            fn validate_impl(&self, ctx: &mut ValidationCtx) {
                ctx.in_table("DerpStore", |ctx| {
                    ctx.in_field("derps", |ctx| self.derps.validate_impl(ctx))
                })
            }
        }

        let my_derps = DerpStore {
            derps: [1i16, 0, 3, 4, 12, 7, 6].into_iter().map(Derp).collect(),
        };

        let report = my_derps.validate().err().unwrap();
        assert_eq!(report.errors.len(), 1);
        // ensure that the index is being calculated correctly
        assert!(report.to_string().contains(".derps[4]"));
    }
}