use crate::error::Error;
use crate::expr::*;
use dhall_core::*;
use std::fs::File;
use std::io::Read;
use std::path::Path;
use std::path::PathBuf;

#[derive(Debug)]
pub enum ImportError {
    Recursive(Import, Box<Error>),
    UnexpectedImport(Import),
}

/// A root from which to resolve relative imports.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum ImportRoot {
    LocalDir(PathBuf),
}

fn resolve_import(
    import: &Import,
    root: &ImportRoot,
) -> Result<Normalized<'static>, ImportError> {
    use self::ImportRoot::*;
    use dhall_core::FilePrefix::*;
    use dhall_core::ImportLocation::*;
    let cwd = match root {
        LocalDir(cwd) => cwd,
    };
    match &import.location_hashed.location {
        Local(prefix, path) => {
            let path = match prefix {
                // TODO: fail gracefully
                Parent => cwd.parent().unwrap().join(path),
                Here => cwd.join(path),
                _ => unimplemented!("{:?}", import),
            };
            Ok(load_import(&path).map_err(|e| {
                ImportError::Recursive(import.clone(), Box::new(e))
            })?)
        }
        _ => unimplemented!("{:?}", import),
    }
}

fn load_import(f: &Path) -> Result<Normalized<'static>, Error> {
    Ok(Parsed::parse_file(f)?.resolve()?.typecheck()?.normalize())
}

fn resolve_expr<'a>(
    Parsed(expr, root): Parsed<'a>,
    allow_imports: bool,
) -> Result<Resolved<'a>, ImportError> {
    let resolve =
        |import: &Import| -> Result<Normalized<'static>, ImportError> {
            if allow_imports {
                let expr = resolve_import(import, &root)?;
                Ok(expr)
            } else {
                Err(ImportError::UnexpectedImport(import.clone()))
            }
        };
    let expr = expr.as_ref().traverse_embed(&resolve)?;
    Ok(Resolved(rc(expr)))
}

impl<'a> Parsed<'a> {
    pub fn parse_file(f: &Path) -> Result<Parsed<'a>, Error> {
        let mut buffer = String::new();
        File::open(f)?.read_to_string(&mut buffer)?;
        let expr = parse_expr(&*buffer)?;
        let root = ImportRoot::LocalDir(f.parent().unwrap().to_owned());
        Ok(Parsed(expr.unnote().note_absurd(), root))
    }

    pub fn parse_str(s: &'a str) -> Result<Parsed<'a>, Error> {
        let expr = parse_expr(s)?;
        let root = ImportRoot::LocalDir(std::env::current_dir()?);
        Ok(Parsed(expr, root))
    }

    pub fn parse_binary_file(f: &Path) -> Result<Parsed<'a>, Error> {
        let mut buffer = Vec::new();
        File::open(f)?.read_to_end(&mut buffer)?;
        let expr = crate::binary::decode(&buffer)?;
        let root = ImportRoot::LocalDir(f.parent().unwrap().to_owned());
        Ok(Parsed(expr.note_absurd(), root))
    }

    pub fn resolve(self) -> Result<Resolved<'a>, ImportError> {
        crate::imports::resolve_expr(self, true)
    }
    pub fn skip_resolve(self) -> Result<Resolved<'a>, ImportError> {
        crate::imports::resolve_expr(self, false)
    }
}