extern crate serde_json;

use serde_json::Value;
use std::convert::TryFrom;
use std::ffi::OsString;
use std::io::{stderr, stdout, Error, ErrorKind, Write};
use std::os::unix::ffi::{OsStrExt, OsStringExt};
use std::process::Command;

fn render_nix_string(s: &OsString) -> OsString {
    let mut rendered = Vec::new();

    rendered.extend(b"\"");

    for b in s.as_os_str().as_bytes() {
        match char::from(*b) {
            '\"' => rendered.extend(b"\\\""),
            '\\' => rendered.extend(b"\\\\"),
            '$' => rendered.extend(b"\\$"),
            _ => rendered.push(*b),
        }
    }

    rendered.extend(b"\"");

    OsString::from_vec(rendered)
}

fn render_nix_list(arr: &[OsString]) -> OsString {
    let mut rendered = Vec::new();

    rendered.extend(b"[ ");

    for el in arr {
        rendered.extend(render_nix_string(el).as_os_str().as_bytes());
        rendered.extend(b" ");
    }

    rendered.extend(b"]");

    OsString::from_vec(rendered)
}

/// Slightly overkill helper macro which takes a `Map<String, Value>` obtained
/// from `Value::Object` and an output name (`stderr` or `stdout`) as an
/// identifier. If a value exists for the given output in the object it gets
/// written to the appropriate output.
macro_rules! handle_set_output {
    ($map_name:ident, $output_name:ident) => {
        match $map_name.get(stringify!($output_name)) {
            Some(Value::String(s)) => $output_name().write_all(s.as_bytes()),
            Some(_) => Err(Error::new(
                ErrorKind::Other,
                format!("Attribute {} must be a string!", stringify!($output_name)),
            )),
            None => Ok(()),
        }
    };
}

fn main() -> std::io::Result<()> {
    let mut nix_args = Vec::new();

    let mut args = std::env::args_os().into_iter();
    let mut in_args = true;

    let mut argv: Vec<OsString> = Vec::new();

    // skip argv[0]
    args.next();

    loop {
        let arg = match args.next() {
            Some(a) => a,
            None => break,
        };

        if !arg.to_str().map(|s| s.starts_with("-")).unwrap_or(false) {
            in_args = false;
        }

        if in_args {
            match (arg.to_str()) {
                Some("--arg") | Some("--argstr") => {
                    nix_args.push(arg);
                    nix_args.push(args.next().unwrap());
                    nix_args.push(args.next().unwrap());
                    Ok(())
                }
                _ => Err(Error::new(ErrorKind::Other, "unknown argument")),
            }?
        } else {
            argv.push(arg);
        }
    }

    if argv.len() < 1 {
        Err(Error::new(ErrorKind::Other, "missing argv"))
    } else {
        let cd = std::env::current_dir()?.into_os_string();

        nix_args.push(OsString::from("--arg"));
        nix_args.push(OsString::from("currentDir"));
        nix_args.push(cd);

        nix_args.push(OsString::from("--arg"));
        nix_args.push(OsString::from("argv"));
        nix_args.push(render_nix_list(&argv[..]));

        nix_args.push(OsString::from("--eval"));
        nix_args.push(OsString::from("--strict"));
        nix_args.push(OsString::from("--json"));

        nix_args.push(argv[0].clone());

        let run = Command::new("nix-instantiate").args(nix_args).output()?;

        match serde_json::from_slice(&run.stdout[..]) {
            Ok(Value::String(s)) => stdout().write_all(s.as_bytes()),
            Ok(Value::Object(m)) => {
                handle_set_output!(m, stdout)?;
                handle_set_output!(m, stderr)?;

                match m.get("exit") {
                    Some(Value::Number(n)) => {
                        let code = n.as_i64().and_then(|v| i32::try_from(v).ok());

                        match code {
                            Some(i) => std::process::exit(i),
                            None => {
                                Err(Error::new(ErrorKind::Other, "Attribute exit is not an i32"))
                            }
                        }
                    }
                    Some(_) => Err(Error::new(ErrorKind::Other, "exit must be a number")),
                    None => Ok(()),
                }
            }
            Ok(_) => Err(Error::new(
                ErrorKind::Other,
                "output must be a string or an object",
            )),
            _ => {
                stderr().write_all(&run.stderr[..]);
                Err(Error::new(ErrorKind::Other, "internal nix error"))
            }
        }
    }
}
