//! Implements `builtins.derivation`, the core of what makes Nix build packages.
use crate::builtins::DerivationError;
use crate::known_paths::KnownPaths;
use nix_compat::derivation::{Derivation, Output};
use nix_compat::nixhash;
use std::cell::RefCell;
use std::collections::{btree_map, BTreeSet};
use std::rc::Rc;
use tvix_eval::builtin_macros::builtins;
use tvix_eval::generators::{self, emit_warning_kind, GenCo};
use tvix_eval::{
    AddContext, CatchableErrorKind, CoercionKind, ErrorKind, NixAttrs, NixContext,
    NixContextElement, NixList, Value, WarningKind,
};

// Constants used for strangely named fields in derivation inputs.
const STRUCTURED_ATTRS: &str = "__structuredAttrs";
const IGNORE_NULLS: &str = "__ignoreNulls";

/// Helper function for populating the `drv.outputs` field from a
/// manually specified set of outputs, instead of the default
/// `outputs`.
async fn populate_outputs(
    co: &GenCo,
    drv: &mut Derivation,
    outputs: NixList,
) -> Result<(), ErrorKind> {
    // Remove the original default `out` output.
    drv.outputs.clear();

    for output in outputs {
        let output_name = generators::request_force(co, output)
            .await
            .to_str()
            .context("determining output name")?;

        if drv
            .outputs
            .insert(output_name.as_str().into(), Default::default())
            .is_some()
        {
            return Err(DerivationError::DuplicateOutput(output_name.as_str().into()).into());
        }
    }

    Ok(())
}

/// Populate the inputs of a derivation from the build references
/// found when scanning the derivation's parameters and extracting their contexts.
fn populate_inputs(drv: &mut Derivation, full_context: NixContext) {
    for element in full_context.iter() {
        match element {
            NixContextElement::Plain(source) => {
                drv.input_sources.insert(source.clone());
            }

            NixContextElement::Single { name, derivation } => {
                match drv.input_derivations.entry(derivation.clone()) {
                    btree_map::Entry::Vacant(entry) => {
                        entry.insert(BTreeSet::from([name.clone()]));
                    }

                    btree_map::Entry::Occupied(mut entry) => {
                        entry.get_mut().insert(name.clone());
                    }
                }
            }

            NixContextElement::Derivation(_drv_path) => {
                // This is a hard one, it means that
                // we are depending on a drvPath of ourselves
                // *or* another derivation's drvPath.
                // What to do here?
                panic!("please do not depend on drvPath, I have 2 hours of sleep in blood");
            }
        }
    }
}

/// Populate the output configuration of a derivation based on the
/// parameters passed to the call, configuring a fixed-output derivation output
/// if necessary.
///
/// This function handles all possible combinations of the
/// parameters, including invalid ones.
///
/// Due to the support for SRI hashes, and how these are passed along to
/// builtins.derivation, outputHash and outputHashAlgo can have values which
/// need to be further modified before constructing the Derivation struct.
///
/// If outputHashAlgo is an SRI hash, outputHashAlgo must either be an empty
/// string, or the hash algorithm as specified in the (single) SRI (entry).
/// SRI strings with multiple hash algorithms are not supported.
///
/// In case an SRI string was used, the (single) fixed output is populated
/// with the hash algo name, and the hash digest is populated with the
/// (lowercase) hex encoding of the digest.
///
/// These values are only rewritten for the outputs, not what's passed to env.
///
/// The return value may optionally contain a warning.
fn handle_fixed_output(
    drv: &mut Derivation,
    hash_str: Option<String>,      // in nix: outputHash
    hash_algo_str: Option<String>, // in nix: outputHashAlgo
    hash_mode_str: Option<String>, // in nix: outputHashmode
) -> Result<Option<WarningKind>, ErrorKind> {
    // If outputHash is provided, ensure hash_algo_str is compatible.
    // If outputHash is not provided, do nothing.
    if let Some(hash_str) = hash_str {
        // treat an empty algo as None
        let hash_algo_str = match hash_algo_str {
            Some(s) if s.is_empty() => None,
            Some(s) => Some(s),
            None => None,
        };

        // construct a NixHash.
        let nixhash = nixhash::from_str(&hash_str, hash_algo_str.as_deref())
            .map_err(DerivationError::InvalidOutputHash)?;
        let algo = nixhash.algo();

        // construct the fixed output.
        drv.outputs.insert(
            "out".to_string(),
            Output {
                path: "".to_string(),
                ca_hash: match hash_mode_str.as_deref() {
                    None | Some("flat") => Some(nixhash::CAHash::Flat(nixhash)),
                    Some("recursive") => Some(nixhash::CAHash::Nar(nixhash)),
                    Some(other) => {
                        return Err(DerivationError::InvalidOutputHashMode(other.to_string()))?
                    }
                },
            },
        );

        // Peek at hash_str once more.
        // If it was a SRI hash, but is not using the correct length, this means
        // the padding was wrong. Emit a warning in that case.
        let sri_prefix = format!("{}-", algo);
        if let Some(rest) = hash_str.strip_prefix(&sri_prefix) {
            if data_encoding::BASE64.encode_len(algo.digest_length()) != rest.len() {
                return Ok(Some(WarningKind::SRIHashWrongPadding));
            }
        }
    }
    Ok(None)
}

/// Handles derivation parameters which are not just forwarded to
/// the environment. The return value indicates whether the
/// parameter should be included in the environment.
async fn handle_derivation_parameters(
    drv: &mut Derivation,
    co: &GenCo,
    name: &str,
    value: &Value,
    val_str: &str,
) -> Result<Result<bool, CatchableErrorKind>, ErrorKind> {
    match name {
        IGNORE_NULLS => return Ok(Ok(false)),

        // Command line arguments to the builder.
        "args" => {
            let args = value.to_list()?;
            for arg in args {
                match strong_importing_coerce_to_string(co, arg).await? {
                    Err(cek) => return Ok(Err(cek)),
                    Ok(s) => drv.arguments.push(s),
                }
            }

            // The arguments do not appear in the environment.
            return Ok(Ok(false));
        }

        // Explicitly specified drv outputs (instead of default [ "out" ])
        "outputs" => {
            let outputs = value
                .to_list()
                .context("looking at the `outputs` parameter of the derivation")?;

            populate_outputs(co, drv, outputs).await?;
        }

        "builder" => {
            drv.builder = val_str.to_string();
        }

        "system" => {
            drv.system = val_str.to_string();
        }

        _ => {}
    }

    Ok(Ok(true))
}

async fn strong_importing_coerce_to_string(
    co: &GenCo,
    val: Value,
) -> Result<Result<String, CatchableErrorKind>, ErrorKind> {
    let val = generators::request_force(co, val).await;
    match generators::request_string_coerce(
        co,
        val,
        CoercionKind {
            strong: true,
            import_paths: true,
        },
    )
    .await
    {
        Err(cek) => Ok(Err(cek)),
        Ok(val_str) => Ok(Ok(val_str.as_str().to_string())),
    }
}

#[builtins(state = "Rc<RefCell<KnownPaths>>")]
pub(crate) mod derivation_builtins {
    use super::*;
    use nix_compat::store_path::hash_placeholder;
    use tvix_eval::generators::Gen;
    use tvix_eval::{NixContext, NixContextElement, NixString};

    #[builtin("placeholder")]
    async fn builtin_placeholder(co: GenCo, input: Value) -> Result<Value, ErrorKind> {
        let placeholder = hash_placeholder(
            input
                .to_str()
                .context("looking at output name in builtins.placeholder")?
                .as_str(),
        );

        Ok(placeholder.into())
    }

    /// Strictly construct a Nix derivation from the supplied arguments.
    ///
    /// This is considered an internal function, users usually want to
    /// use the higher-level `builtins.derivation` instead.
    #[builtin("derivationStrict")]
    async fn builtin_derivation_strict(
        state: Rc<RefCell<KnownPaths>>,
        co: GenCo,
        input: Value,
    ) -> Result<Value, ErrorKind> {
        let input = input.to_attrs()?;
        let name = generators::request_force(&co, input.select_required("name")?.clone())
            .await
            .to_str()
            .context("determining derivation name")?;

        if name.is_empty() {
            return Err(ErrorKind::Abort("derivation has empty name".to_string()));
        }

        // Check whether attributes should be passed as a JSON file.
        // TODO: the JSON serialisation has to happen here.
        if let Some(sa) = input.select(STRUCTURED_ATTRS) {
            if generators::request_force(&co, sa.clone()).await.as_bool()? {
                return Ok(Value::Catchable(CatchableErrorKind::UnimplementedFeature(
                    STRUCTURED_ATTRS.to_string(),
                )));
            }
        }

        // Check whether null attributes should be ignored or passed through.
        let ignore_nulls = match input.select(IGNORE_NULLS) {
            Some(b) => generators::request_force(&co, b.clone()).await.as_bool()?,
            None => false,
        };

        let mut drv = Derivation::default();
        drv.outputs.insert("out".to_string(), Default::default());

        async fn select_string(
            co: &GenCo,
            attrs: &NixAttrs,
            key: &str,
        ) -> Result<Result<Option<String>, CatchableErrorKind>, ErrorKind> {
            if let Some(attr) = attrs.select(key) {
                match strong_importing_coerce_to_string(co, attr.clone()).await? {
                    Err(cek) => return Ok(Err(cek)),
                    Ok(str) => return Ok(Ok(Some(str))),
                }
            }

            Ok(Ok(None))
        }

        let mut input_context = NixContext::new();

        for (name, value) in input.clone().into_iter_sorted() {
            let value = generators::request_force(&co, value).await;
            if ignore_nulls && matches!(value, Value::Null) {
                continue;
            }

            match generators::request_string_coerce(
                &co,
                value.clone(),
                CoercionKind {
                    strong: true,
                    import_paths: true,
                },
            )
            .await
            {
                Err(cek) => return Ok(Value::Catchable(cek)),
                Ok(val_str) => {
                    // Learn about this derivation references
                    // by looking at its context.
                    input_context.mimic(&val_str);

                    let val_str = val_str.as_str().to_string();
                    // handle_derivation_parameters tells us whether the
                    // argument should be added to the environment; continue
                    // to the next one otherwise
                    match handle_derivation_parameters(
                        &mut drv,
                        &co,
                        name.as_str(),
                        &value,
                        &val_str,
                    )
                    .await?
                    {
                        Err(cek) => return Ok(Value::Catchable(cek)),
                        Ok(false) => continue,
                        _ => (),
                    }

                    // Most of these are also added to the builder's environment in "raw" form.
                    if drv
                        .environment
                        .insert(name.as_str().to_string(), val_str.into())
                        .is_some()
                    {
                        return Err(
                            DerivationError::DuplicateEnvVar(name.as_str().to_string()).into()
                        );
                    }
                }
            }
        }

        // Configure fixed-output derivations if required.
        {
            let output_hash = match select_string(&co, &input, "outputHash")
                .await
                .context("evaluating the `outputHash` parameter")?
            {
                Err(cek) => return Ok(Value::Catchable(cek)),
                Ok(s) => s,
            };
            let output_hash_algo = match select_string(&co, &input, "outputHashAlgo")
                .await
                .context("evaluating the `outputHashAlgo` parameter")?
            {
                Err(cek) => return Ok(Value::Catchable(cek)),
                Ok(s) => s,
            };
            let output_hash_mode = match select_string(&co, &input, "outputHashMode")
                .await
                .context("evaluating the `outputHashMode` parameter")?
            {
                Err(cek) => return Ok(Value::Catchable(cek)),
                Ok(s) => s,
            };

            if let Some(warning) =
                handle_fixed_output(&mut drv, output_hash, output_hash_algo, output_hash_mode)?
            {
                emit_warning_kind(&co, warning).await;
            }
        }

        // Each output name needs to exist in the environment, at this
        // point initialised as an empty string because that is the
        // way of Golang ;)
        for output in drv.outputs.keys() {
            if drv
                .environment
                .insert(output.to_string(), String::new().into())
                .is_some()
            {
                emit_warning_kind(&co, WarningKind::ShadowedOutput(output.to_string())).await;
            }
        }

        populate_inputs(&mut drv, input_context);
        let mut known_paths = state.borrow_mut();

        // At this point, derivation fields are fully populated from
        // eval data structures.
        drv.validate(false)
            .map_err(DerivationError::InvalidDerivation)?;

        // Calculate the derivation_or_fod_hash for the current derivation.
        // This one is still intermediate (so not added to known_paths)
        let derivation_or_fod_hash_tmp =
            drv.derivation_or_fod_hash(|drv| known_paths.get_hash_derivation_modulo(drv));

        // Mutate the Derivation struct and set output paths
        drv.calculate_output_paths(&name, &derivation_or_fod_hash_tmp)
            .map_err(DerivationError::InvalidDerivation)?;

        let derivation_path = drv
            .calculate_derivation_path(&name)
            .map_err(DerivationError::InvalidDerivation)?;

        // recompute the hash derivation modulo and add to known_paths
        let derivation_or_fod_hash_final =
            drv.derivation_or_fod_hash(|drv| known_paths.get_hash_derivation_modulo(drv));

        known_paths.add_hash_derivation_modulo(
            derivation_path.to_absolute_path(),
            &derivation_or_fod_hash_final,
        );

        let mut new_attrs: Vec<(String, NixString)> = drv
            .outputs
            .into_iter()
            .map(|(name, output)| {
                (
                    name.clone(),
                    (
                        output.path,
                        Some(
                            NixContextElement::Single {
                                name,
                                derivation: derivation_path.to_absolute_path(),
                            }
                            .into(),
                        ),
                    )
                        .into(),
                )
            })
            .collect();

        new_attrs.push((
            "drvPath".to_string(),
            (
                derivation_path.to_absolute_path(),
                Some(NixContextElement::Derivation(derivation_path.to_absolute_path()).into()),
            )
                .into(),
        ));

        Ok(Value::Attrs(Box::new(NixAttrs::from_iter(
            new_attrs.into_iter(),
        ))))
    }

    #[builtin("toFile")]
    async fn builtin_to_file(co: GenCo, name: Value, content: Value) -> Result<Value, ErrorKind> {
        let name = name
            .to_str()
            .context("evaluating the `name` parameter of builtins.toFile")?;
        let content = content
            .to_contextful_str()
            .context("evaluating the `content` parameter of builtins.toFile")?;

        if content.iter_derivation().count() > 0 || content.iter_single_outputs().count() > 0 {
            return Err(ErrorKind::UnexpectedContext);
        }

        let path = nix_compat::store_path::build_text_path(
            name.as_str(),
            content.as_str(),
            content.iter_plain(),
        )
        .map_err(|_e| {
            nix_compat::derivation::DerivationError::InvalidOutputName(name.as_str().to_string())
        })
        .map_err(DerivationError::InvalidDerivation)?
        .to_absolute_path();

        let context: NixContext = NixContextElement::Plain(path.clone()).into();

        // TODO: actually persist the file in the store at that path ...

        Ok(Value::String(NixString::new_context_from(context, &path)))
    }
}

pub use derivation_builtins::builtins as derivation_builtins;