use std::env;
use std::fs;
use std::io;
use std::path::{Path, PathBuf};

const BUILD_TEMPLATE: &str = include_str!("templates/BUILD.template");
const CARGO_TEMPLATE: &str = include_str!("templates/Cargo.toml.template");
const LIBRARY_TEMPLATE: &str = include_str!("templates/library.rs.template");
const BINARY_TEMPLATE: &str = include_str!("templates/binary.rs.template");
const TEST_TEMPLATE: &str = include_str!("templates/test.rs.template");
const CRATE_NAME_PLACEHOLDER: &str = "{CRATE_NAME}";

/// Creates a Rust package at `destination`.
pub fn run(destination: &Path) -> io::Result<PathBuf> {
    scaffold(&workspace_root()?, destination)
}

fn scaffold(workspace_root: &Path, destination: &Path) -> io::Result<PathBuf> {
    let workspace_root = fs::canonicalize(workspace_root)?;
    let destination = resolve_destination(&workspace_root, destination)?;
    let crate_name = crate_name(&destination)?;
    let workspace_member = workspace_member(&workspace_root, &destination)?;
    let files = render_files(&crate_name);
    let cargo_path = workspace_root.join("Cargo.toml");
    let cargo_manifest = fs::read_to_string(&cargo_path)?;
    let cargo_manifest = update_workspace_members(&cargo_manifest, &workspace_member)?;

    fs::create_dir(&destination)?;
    for file in files {
        fs::write(destination.join(file.name), file.contents)?;
    }
    fs::write(cargo_path, cargo_manifest)?;

    Ok(destination)
}

fn workspace_root() -> io::Result<PathBuf> {
    match env::var_os("BUILD_WORKSPACE_DIRECTORY") {
        Some(path) => Ok(PathBuf::from(path)),
        None => env::current_dir(),
    }
}

fn resolve_destination(workspace_root: &Path, destination: &Path) -> io::Result<PathBuf> {
    let destination = if destination.is_absolute() {
        destination.to_path_buf()
    } else {
        workspace_root.join(destination)
    };
    let parent = destination.parent().ok_or_else(|| {
        io::Error::new(
            io::ErrorKind::InvalidInput,
            "the destination must have a parent directory",
        )
    })?;
    let parent = fs::canonicalize(parent)?;

    if !parent.starts_with(workspace_root) {
        return Err(io::Error::new(
            io::ErrorKind::InvalidInput,
            format!(
                "the destination is outside the workspace: {}",
                destination.display()
            ),
        ));
    }

    let destination = parent.join(destination.file_name().ok_or_else(|| {
        io::Error::new(
            io::ErrorKind::InvalidInput,
            "the destination must name a directory",
        )
    })?);
    if destination.try_exists()? {
        return Err(io::Error::new(
            io::ErrorKind::AlreadyExists,
            format!("the destination already exists: {}", destination.display()),
        ));
    }

    Ok(destination)
}

fn crate_name(destination: &Path) -> io::Result<String> {
    let crate_name = destination
        .file_name()
        .and_then(|name| name.to_str())
        .ok_or_else(|| {
            io::Error::new(
                io::ErrorKind::InvalidInput,
                "the destination name must be valid UTF-8",
            )
        })?;

    if crate_name.is_empty()
        || !crate_name.bytes().all(|character| {
            character.is_ascii_lowercase() || character.is_ascii_digit() || character == b'_'
        })
    {
        return Err(io::Error::new(
            io::ErrorKind::InvalidInput,
            format!("invalid crate name: {crate_name}"),
        ));
    }

    Ok(crate_name.to_owned())
}

fn workspace_member(workspace_root: &Path, destination: &Path) -> io::Result<String> {
    let member = destination.strip_prefix(workspace_root).map_err(|_| {
        io::Error::new(
            io::ErrorKind::InvalidInput,
            "the destination is outside the workspace",
        )
    })?;
    let member = member.to_str().ok_or_else(|| {
        io::Error::new(
            io::ErrorKind::InvalidInput,
            "the destination must be valid UTF-8",
        )
    })?;

    Ok(member.replace(std::path::MAIN_SEPARATOR, "/"))
}

fn render_files(crate_name: &str) -> Vec<RenderedFile> {
    vec![
        render_file("BUILD", BUILD_TEMPLATE, crate_name),
        render_file("Cargo.toml", CARGO_TEMPLATE, crate_name),
        render_file(&format!("{crate_name}.rs"), LIBRARY_TEMPLATE, crate_name),
        render_file(
            &format!("{crate_name}_main.rs"),
            BINARY_TEMPLATE,
            crate_name,
        ),
        render_file(&format!("{crate_name}_test.rs"), TEST_TEMPLATE, crate_name),
    ]
}

fn render_file(name: &str, template: &str, crate_name: &str) -> RenderedFile {
    RenderedFile {
        name: name.to_owned(),
        contents: template.replace(CRATE_NAME_PLACEHOLDER, crate_name),
    }
}

fn update_workspace_members(manifest: &str, member: &str) -> io::Result<String> {
    let workspace_start = manifest.find("[workspace]").ok_or_else(|| {
        io::Error::new(
            io::ErrorKind::InvalidData,
            "the root Cargo.toml has no [workspace] section",
        )
    })?;
    let members_start = manifest[workspace_start..]
        .find("members = [")
        .map(|offset| workspace_start + offset)
        .ok_or_else(|| {
            io::Error::new(
                io::ErrorKind::InvalidData,
                "the root Cargo.toml has no workspace members array",
            )
        })?;
    let members_end = manifest[members_start..]
        .find(']')
        .map(|offset| members_start + offset)
        .ok_or_else(|| {
            io::Error::new(
                io::ErrorKind::InvalidData,
                "the root Cargo.toml has an unclosed workspace members array",
            )
        })?;

    if manifest[members_start..members_end].contains(&format!("\"{member}\"")) {
        return Err(io::Error::new(
            io::ErrorKind::AlreadyExists,
            format!("the workspace already has member: {member}"),
        ));
    }

    let mut manifest = manifest.to_owned();
    manifest.insert_str(members_end, &format!("    \"{member}\",\n"));
    Ok(manifest)
}

struct RenderedFile {
    name: String,
    contents: String,
}

#[cfg(test)]
#[path = "rust_test.rs"]
mod rust_test;