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use std::collections::HashSet;

use proc_macro::TokenStream;
use quote::{format_ident, quote};
use syn::{parse_macro_input, Data, DeriveInput, Error};

#[proc_macro_derive(Deinterleave)]
pub fn derive_deinterleave(tokens: TokenStream) -> TokenStream {
  let derive_input = parse_macro_input!(tokens as DeriveInput);

  process_derive_input(derive_input).into()
}

fn process_derive_input(derive_input: DeriveInput) -> proc_macro2::TokenStream {
  // we only support structs for now
  let Data::Struct(data_struct) = derive_input.data else {
    return Error::new_spanned(derive_input, "expected a struct").into_compile_error();
  };

  let struct_ident = derive_input.ident;
  let field_count = data_struct.fields.len();

  let mut seen_field_ty = HashSet::new();
  let mut has_fields = Vec::default();
  let mut push_to = Vec::default();
  let mut remove_from = Vec::default();
  let mut swap_remove_from = Vec::default();
  let mut iter_field_ty = Vec::default();
  let mut iter_field = Vec::default();
  let mut iter_mut_field_ty = Vec::default();
  let mut iter_mut_field = Vec::default();

  for (i, f) in data_struct.fields.into_iter().enumerate() {
    let field_ty = f.ty;
    let field_ident = f.ident;

    if seen_field_ty.contains(&field_ty) {
      return Error::new_spanned(field_ty.clone(), "type seen more than once (you can only use a type once in the fields of a deinterleaved struct)").to_compile_error();
    }

    seen_field_ty.insert(field_ty.clone());

    iter_field_ty.push(quote! {
      #field_ident: std::slice::Iter<'a, #field_ty>
    });

    iter_field.push(quote! {
      #field_ident: unsafe { std::mem::transmute::<&Vec<u8>, &Vec<#field_ty>>(&data[#i]) }.iter()
    });

    iter_mut_field_ty.push(quote! {
      #field_ident: std::slice::IterMut<'a, #field_ty>
    });

    iter_mut_field.push(quote! {
      #field_ident: unsafe { std::mem::transmute::<&mut Vec<u8>, &mut Vec<#field_ty>>(&mut data[#i]) }.iter_mut()
    });

    has_fields.push(quote! {
      impl deinterleave::HasField<#field_ty> for #struct_ident {
        const INDEX: usize = #i;
      }
    });

    push_to.push(quote! {
      unsafe { std::mem::transmute::<&mut Vec<u8>, &mut Vec<#field_ty>>(&mut data[#i]) }.push(value.#field_ident)
    });

    remove_from.push(quote! {
      #field_ident : unsafe { std::mem::transmute::<&mut Vec<u8>, &mut Vec<#field_ty>>(&mut data[#i]) }.remove(i)
    });

    swap_remove_from.push(quote! {
      #field_ident : unsafe { std::mem::transmute::<&mut Vec<u8>, &mut Vec<#field_ty>>(&mut data[#i]) }.swap_remove(i)
    });
  }

  let iter_ident = format_ident!("{struct_ident}Iter");
  let iter_mut_ident = format_ident!("{struct_ident}IterMut");

  quote! {
    #(#has_fields)*

    #[derive(Debug)]
    pub struct #iter_ident<'a> {
      // PhantomData required for empty structs
      _phantom: std::marker::PhantomData<&'a ()>,
      #(pub #iter_field_ty),*
    }

    #[derive(Debug)]
    pub struct #iter_mut_ident<'a> {
      // PhantomData required for empty structs
      _phantom: std::marker::PhantomData<&'a mut ()>,
      #(pub #iter_mut_field_ty),*
    }

    impl deinterleave::Deinterleave for #struct_ident {
      type Iter<'a> = #iter_ident<'a>;
      type IterMut<'a> = #iter_mut_ident<'a>;

      fn new_data() -> Vec<Vec<u8>> {
        vec![Vec::default(); #field_count]
      }

      fn iter(data: &Vec<Vec<u8>>) -> Self::Iter<'_> {
        #iter_ident {
          _phantom: std::marker::PhantomData,
          #(#iter_field),*
        }
      }

      fn iter_mut(data: &mut Vec<Vec<u8>>) -> Self::IterMut<'_> {
        #iter_mut_ident {
          _phantom: std::marker::PhantomData,
          #(#iter_mut_field),*
        }
      }

      fn push_to(data: &mut Vec<Vec<u8>>, value: Self) {
        #(#push_to);*
      }

      fn remove_from(data: &mut Vec<Vec<u8>>, i: usize) -> Self {
        Self {
          #(#remove_from),*
        }
      }

      fn swap_remove_from(data: &mut Vec<Vec<u8>>, i: usize) -> Self {
        Self {
          #(#swap_remove_from),*
        }
      }
    }
  }
}