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use crate::{BuiltinType, IntRepr, NamedType, RecordDatatype, Type, TypeRef, Variant};
use std::collections::HashMap;

// A lattice. Eq + Eq = Eq, SuperSet + any = NotEq, NotEq + any = NotEq.
#[derive(Debug, Copy, Clone, PartialEq, Eq)]
pub enum RepEquality {
    Eq,
    Superset,
    NotEq,
}

impl RepEquality {
    pub fn join(&self, rhs: &Self) -> Self {
        match (self, rhs) {
            (RepEquality::Eq, RepEquality::Eq) => RepEquality::Eq,
            _ => RepEquality::NotEq,
        }
    }
}

pub trait Representable {
    fn representable(&self, by: &Self) -> RepEquality;
}

impl Representable for BuiltinType {
    fn representable(&self, by: &Self) -> RepEquality {
        // An unsigned integer can be used to represent an unsigned integer of smaller width.
        // Otherwise, types must be equal.
        if self == by {
            return RepEquality::Eq;
        }
        match self {
            BuiltinType::U8 { .. } => match by {
                BuiltinType::U64 | BuiltinType::U32 { .. } | BuiltinType::U16 => {
                    RepEquality::Superset
                }
                _ => RepEquality::NotEq,
            },
            BuiltinType::U16 => match by {
                BuiltinType::U64 | BuiltinType::U32 { .. } => RepEquality::Superset,
                _ => RepEquality::NotEq,
            },
            BuiltinType::U32 { .. } => match by {
                BuiltinType::U64 => RepEquality::Superset,
                _ => RepEquality::NotEq,
            },
            _ => RepEquality::NotEq,
        }
    }
}

impl Representable for IntRepr {
    fn representable(&self, by: &Self) -> RepEquality {
        if self == by {
            return RepEquality::Eq;
        }
        // An unsigned integer can be used to represent an unsigned integer of smaller width.
        match self {
            IntRepr::U16 => match by {
                IntRepr::U32 | IntRepr::U64 => RepEquality::Superset,
                _ => RepEquality::NotEq,
            },
            IntRepr::U32 => match by {
                IntRepr::U64 => RepEquality::Superset,
                _ => RepEquality::NotEq,
            },
            _ => RepEquality::NotEq,
        }
    }
}

impl Representable for Variant {
    fn representable(&self, by: &Self) -> RepEquality {
        let mut superset = false;
        // Integer representation must be compatible
        match self.tag_repr.representable(&by.tag_repr) {
            RepEquality::NotEq => return RepEquality::NotEq,
            RepEquality::Eq => {}
            RepEquality::Superset => superset = true,
        }
        let other_by_name = by
            .cases
            .iter()
            .enumerate()
            .map(|(i, c)| (&c.name, (c, i)))
            .collect::<HashMap<_, _>>();
        // For each variant in self, must have variant of same name in by:
        for (i, v) in self.cases.iter().enumerate() {
            let other_ty = match other_by_name.get(&v.name) {
                Some((_, j)) if i != *j => return RepEquality::NotEq,
                Some((other, _)) => &other.tref,
                None => return RepEquality::NotEq,
            };
            match (&v.tref, other_ty) {
                (Some(me), Some(other)) => match me.representable(other) {
                    RepEquality::NotEq => return RepEquality::NotEq,
                    RepEquality::Eq => {}
                    RepEquality::Superset => superset = true,
                },
                // We added fields, that's not ok
                (Some(_), None) => return RepEquality::NotEq,
                // Fields were deleted, that's ok
                (None, Some(_)) => superset = true,
                (None, None) => {}
            }
        }
        if superset || self.cases.len() < by.cases.len() {
            RepEquality::Superset
        } else {
            RepEquality::Eq
        }
    }
}

impl Representable for RecordDatatype {
    fn representable(&self, by: &Self) -> RepEquality {
        // Records must have exact structural equality - same members, must
        // be Eq, in the same order.
        // We would require require a more expressive RepEquality enum to describe which members
        // might be supersets.
        if self.members.len() != by.members.len() {
            return RepEquality::NotEq;
        }
        for (m, bym) in self.members.iter().zip(by.members.iter()) {
            if m.name != bym.name {
                return RepEquality::NotEq;
            }
            if m.tref.type_().representable(&*bym.tref.type_()) != RepEquality::Eq {
                return RepEquality::NotEq;
            }
        }
        RepEquality::Eq
    }
}

impl Representable for TypeRef {
    fn representable(&self, by: &Self) -> RepEquality {
        self.type_().representable(&*by.type_())
    }
}

impl Representable for NamedType {
    fn representable(&self, by: &Self) -> RepEquality {
        self.tref.representable(&by.tref)
    }
}

impl Representable for Type {
    fn representable(&self, by: &Self) -> RepEquality {
        match (&self, &by) {
            (Type::Variant(s), Type::Variant(b)) => s.representable(b),
            (Type::Record(s), Type::Record(b)) => s.representable(b),
            (Type::Handle(_), Type::Handle(_)) => RepEquality::Eq, // Handles are nominal, not structural
            (Type::List(s), Type::List(b)) => s.representable(b),
            (Type::Pointer(s), Type::Pointer(b)) => s.representable(b),
            (Type::ConstPointer(s), Type::ConstPointer(b)) => s.representable(b),
            (Type::Builtin(s), Type::Builtin(b)) => s.representable(b),
            _ => RepEquality::NotEq,
        }
    }
}