use std::collections::HashSet;

use proc_macro2::{Ident, Span, TokenStream};
use quote::{quote, ToTokens};
use syn::{DataEnum, Fields};
use syn::spanned::Spanned;

use crate::{CASE_INSENSITIVE, CRATE_ATTR, IGNORE, NAME, OTHER};
use crate::attribute::{Dict, Value};
use crate::error::{MacroError, MacroResult};

#[derive(Clone)]
pub(crate) struct Enum<'a> {
    pub(crate) variants: Vec<Variant<'a>>,
}

#[derive(Clone)]
pub(crate) struct Variant<'a> {
    pub(crate) data: &'a syn::Variant,
    pub(crate) v_type: VariantType<'a>,
    pub(crate) span: Span,
}

#[derive(Clone)]
pub(crate) enum VariantType<'a> {
    Ignore,
    Named {
        name: String,
        constructor: VariantConstructor,
        case_insensitive: bool,
    },
    Other {
        field_name: Option<&'a Ident> //use {} for constructor if Some, use () if None
    },
}

#[derive(Clone, Copy, Debug)]
pub(crate) enum VariantConstructor {
    None,
    Paren,
    Brace,
}

impl<'a> Variant<'a> {
    pub(crate) fn match_variant<F, G>(
        &self,
        enum_ident: &Ident,
        named_fn: &F,
        other_fn: &G
    ) -> MacroResult<Option<(TokenStream, TokenStream)>>
        where
            F: Fn(&Variant, &Ident, &str) -> MacroResult<TokenStream>,
            G: Fn(&Variant, &Ident, TokenStream) -> MacroResult<TokenStream>
    {
        let variant_ident = &self.data.ident;

        match &self.v_type {
            VariantType::Ignore => Ok(None),

            VariantType::Named { name, constructor, .. } => {
                let pattern = match constructor {
                    VariantConstructor::None => quote! { #enum_ident::#variant_ident },
                    VariantConstructor::Paren => quote! { #enum_ident::#variant_ident() },
                    VariantConstructor::Brace => quote! { #enum_ident::#variant_ident{} }
                };
                Ok(Some((pattern, named_fn(self, enum_ident, name)?)))
            }

            VariantType::Other { field_name } => {
                let field_name_tokens = match field_name {
                    Some(field_name) => field_name.to_token_stream(),
                    None => quote! { __enumscribe_other_inner }
                };
                let pattern = match field_name {
                    Some(_) => quote! { #enum_ident::#variant_ident{#field_name_tokens} },
                    None => quote! { #enum_ident::#variant_ident(#field_name_tokens) }
                };
                Ok(Some((pattern, other_fn(self, enum_ident, field_name_tokens)?)))
            }
        }
    }
}

pub(crate) fn parse_enum(data: &DataEnum) -> MacroResult<Enum> {
    let mut variants = Vec::with_capacity(data.variants.len());
    let mut taken_names = HashSet::new();
    let mut other_variant = false;

    for variant in data.variants.iter() {
        let variant_span = variant.span();

        // Parse the `#[enumscribe(...)]` attributes for this variant into a single Dict
        let mut dict = Dict::from_attrs(CRATE_ATTR, &variant.attrs)?;

        // Convert the values in the Dict to the appropriate types
        let name_opt = dict.remove_typed(NAME, Value::value_string)?;
        let (other, other_span) = dict.remove_typed_or_default(OTHER, (false, variant_span), Value::value_bool)?;
        let (ignore, _) = dict.remove_typed_or_default(IGNORE, (false, variant_span), Value::value_bool)?;
        let (case_insensitive, _) = dict.remove_typed_or_default(CASE_INSENSITIVE, (false, variant_span), Value::value_bool)?;

        // Return an error if there are any unrecognised keys in the Dict
        dict.assert_empty()?;

        let scribe_variant = if ignore {
            Variant {
                data: variant,
                v_type: VariantType::Ignore,
                span: variant_span,
            }
        } else if other {
            // Return an error if there is already an "other" variant for this enum
            if other_variant {
                return Err(MacroError::new(
                    format!(
                        "cannot have multiple variants marked as {}",
                        OTHER
                    ),
                    other_span,
                ));
            }

            other_variant = true;

            // Return an error if a str name is provided for this variant
            if let Some((_, name_span)) = name_opt {
                return Err(MacroError::new(
                    format!(
                        "cannot use {} for variant {} because it is marked as {}",
                        NAME, variant.ident.to_string(), OTHER
                    ),
                    name_span,
                ));
            }

            // Return an error if this variant doesn't have exactly one field
            if variant.fields.len() != 1 {
                return Err(MacroError::new(
                    format!(
                        "the variant {} must have exactly one field because it is marked as {}",
                        variant.ident.to_string(), OTHER
                    ),
                    variant_span,
                ));
            }

            // Get the name of the variant's field (or None if it is unnamed)
            let field_name = variant.fields.iter().next()
                .and_then(|field| field.ident.as_ref());

            Variant {
                data: variant,
                v_type: VariantType::Other { field_name },
                span: variant_span,
            }
        } else {
            // Use the str name if one is provided, otherwise use the variant's name
            let (name, name_span) = match name_opt {
                Some((name, name_span)) => (name, name_span),
                None => (variant.ident.to_string(), variant.ident.span())
            };

            // Do not allow duplicate names
            if taken_names.contains(&name) {
                return Err(MacroError::new(
                    format!("duplicate name \"{}\"", name),
                    name_span,
                ));
            }

            // Return an error if the variant has any fields
            if variant.fields.len() != 0 {
                let variant_ident = variant.ident.to_string();
                return Err(MacroError::new(
                    format!(
                        "the variant {} must not have any fields\n\
                         hint: if you do not want to remove {}\'s fields, try using \
                         #[enumscribe(ignore)] for {}",
                        variant_ident, variant_ident, variant_ident
                    ),
                    variant_span,
                ));
            }

            // The variant is allowed to have an empty constructor, so find out if it has one
            // and, if so, what type of constructor (parentheses or braces)
            let constructor = match variant.fields {
                Fields::Named(_) => VariantConstructor::Brace,
                Fields::Unnamed(_) => VariantConstructor::Paren,
                Fields::Unit => VariantConstructor::None,
            };

            taken_names.insert(name.clone());

            Variant {
                data: variant,
                v_type: VariantType::Named { name, constructor, case_insensitive },
                span: variant_span,
            }
        };

        variants.push(scribe_variant);
    }

    Ok(Enum { variants })
}