#include "globals.hh"
#include "installables.hh"
#include "outputs-spec.hh"
#include "util.hh"
#include "command.hh"
#include "attr-path.hh"
#include "common-eval-args.hh"
#include "derivations.hh"
#include "eval-inline.hh"
#include "eval.hh"
#include "get-drvs.hh"
#include "store-api.hh"
#include "shared.hh"
#include "flake/flake.hh"
#include "eval-cache.hh"
#include "url.hh"
#include "registry.hh"
#include "build-result.hh"

#include <regex>
#include <queue>

#include <nlohmann/json.hpp>

namespace nix {

MixFlakeOptions::MixFlakeOptions()
{
    auto category = "Common flake-related options";

    addFlag({
        .longName = "recreate-lock-file",
        .description = "Recreate the flake's lock file from scratch.",
        .category = category,
        .handler = {&lockFlags.recreateLockFile, true}
    });

    addFlag({
        .longName = "no-update-lock-file",
        .description = "Do not allow any updates to the flake's lock file.",
        .category = category,
        .handler = {&lockFlags.updateLockFile, false}
    });

    addFlag({
        .longName = "no-write-lock-file",
        .description = "Do not write the flake's newly generated lock file.",
        .category = category,
        .handler = {&lockFlags.writeLockFile, false}
    });

    addFlag({
        .longName = "no-registries",
        .description =
          "Don't allow lookups in the flake registries. This option is deprecated; use `--no-use-registries`.",
        .category = category,
        .handler = {[&]() {
            lockFlags.useRegistries = false;
            warn("'--no-registries' is deprecated; use '--no-use-registries'");
        }}
    });

    addFlag({
        .longName = "commit-lock-file",
        .description = "Commit changes to the flake's lock file.",
        .category = category,
        .handler = {&lockFlags.commitLockFile, true}
    });

    addFlag({
        .longName = "update-input",
        .description = "Update a specific flake input (ignoring its previous entry in the lock file).",
        .category = category,
        .labels = {"input-path"},
        .handler = {[&](std::string s) {
            lockFlags.inputUpdates.insert(flake::parseInputPath(s));
        }},
        .completer = {[&](size_t, std::string_view prefix) {
            needsFlakeInputCompletion = {std::string(prefix)};
        }}
    });

    addFlag({
        .longName = "override-input",
        .description = "Override a specific flake input (e.g. `dwarffs/nixpkgs`). This implies `--no-write-lock-file`.",
        .category = category,
        .labels = {"input-path", "flake-url"},
        .handler = {[&](std::string inputPath, std::string flakeRef) {
            lockFlags.writeLockFile = false;
            lockFlags.inputOverrides.insert_or_assign(
                flake::parseInputPath(inputPath),
                parseFlakeRef(flakeRef, absPath("."), true));
        }},
        .completer = {[&](size_t n, std::string_view prefix) {
            if (n == 0)
                needsFlakeInputCompletion = {std::string(prefix)};
            else if (n == 1)
                completeFlakeRef(getEvalState()->store, prefix);
        }}
    });

    addFlag({
        .longName = "inputs-from",
        .description = "Use the inputs of the specified flake as registry entries.",
        .category = category,
        .labels = {"flake-url"},
        .handler = {[&](std::string flakeRef) {
            auto evalState = getEvalState();
            auto flake = flake::lockFlake(
                *evalState,
                parseFlakeRef(flakeRef, absPath(".")),
                { .writeLockFile = false });
            for (auto & [inputName, input] : flake.lockFile.root->inputs) {
                auto input2 = flake.lockFile.findInput({inputName}); // resolve 'follows' nodes
                if (auto input3 = std::dynamic_pointer_cast<const flake::LockedNode>(input2)) {
                    overrideRegistry(
                        fetchers::Input::fromAttrs({{"type","indirect"}, {"id", inputName}}),
                        input3->lockedRef.input,
                        {});
                }
            }
        }},
        .completer = {[&](size_t, std::string_view prefix) {
            completeFlakeRef(getEvalState()->store, prefix);
        }}
    });
}

void MixFlakeOptions::completeFlakeInput(std::string_view prefix)
{
    auto evalState = getEvalState();
    for (auto & flakeRefS : getFlakesForCompletion()) {
        auto flakeRef = parseFlakeRefWithFragment(expandTilde(flakeRefS), absPath(".")).first;
        auto flake = flake::getFlake(*evalState, flakeRef, true);
        for (auto & input : flake.inputs)
            if (hasPrefix(input.first, prefix))
                completions->add(input.first);
    }
}

void MixFlakeOptions::completionHook()
{
    if (auto & prefix = needsFlakeInputCompletion)
        completeFlakeInput(*prefix);
}

SourceExprCommand::SourceExprCommand(bool supportReadOnlyMode)
{
    addFlag({
        .longName = "file",
        .shortName = 'f',
        .description =
            "Interpret installables as attribute paths relative to the Nix expression stored in *file*. "
            "If *file* is the character -, then a Nix expression will be read from standard input. "
            "Implies `--impure`.",
        .category = installablesCategory,
        .labels = {"file"},
        .handler = {&file},
        .completer = completePath
    });

    addFlag({
        .longName = "expr",
        .description = "Interpret installables as attribute paths relative to the Nix expression *expr*.",
        .category = installablesCategory,
        .labels = {"expr"},
        .handler = {&expr}
    });

    addFlag({
        .longName = "derivation",
        .description = "Operate on the [store derivation](../../glossary.md#gloss-store-derivation) rather than its outputs.",
        .category = installablesCategory,
        .handler = {&operateOn, OperateOn::Derivation},
    });

    if (supportReadOnlyMode) {
        addFlag({
            .longName = "read-only",
            .description =
                "Do not instantiate each evaluated derivation. "
                "This improves performance, but can cause errors when accessing "
                "store paths of derivations during evaluation.",
            .handler = {&readOnlyMode, true},
        });
    }
}

Strings SourceExprCommand::getDefaultFlakeAttrPaths()
{
    return {
        "packages." + settings.thisSystem.get() + ".default",
        "defaultPackage." + settings.thisSystem.get()
    };
}

Strings SourceExprCommand::getDefaultFlakeAttrPathPrefixes()
{
    return {
        // As a convenience, look for the attribute in
        // 'outputs.packages'.
        "packages." + settings.thisSystem.get() + ".",
        // As a temporary hack until Nixpkgs is properly converted
        // to provide a clean 'packages' set, look in 'legacyPackages'.
        "legacyPackages." + settings.thisSystem.get() + "."
    };
}

void SourceExprCommand::completeInstallable(std::string_view prefix)
{
    try {
        if (file) {
            completionType = ctAttrs;

            evalSettings.pureEval = false;
            auto state = getEvalState();
            Expr *e = state->parseExprFromFile(
                resolveExprPath(state->checkSourcePath(lookupFileArg(*state, *file)))
                );

            Value root;
            state->eval(e, root);

            auto autoArgs = getAutoArgs(*state);

            std::string prefix_ = std::string(prefix);
            auto sep = prefix_.rfind('.');
            std::string searchWord;
            if (sep != std::string::npos) {
                searchWord = prefix_.substr(sep + 1, std::string::npos);
                prefix_ = prefix_.substr(0, sep);
            } else {
                searchWord = prefix_;
                prefix_ = "";
            }

            auto [v, pos] = findAlongAttrPath(*state, prefix_, *autoArgs, root);
            Value &v1(*v);
            state->forceValue(v1, pos);
            Value v2;
            state->autoCallFunction(*autoArgs, v1, v2);

            if (v2.type() == nAttrs) {
                for (auto & i : *v2.attrs) {
                    std::string name = state->symbols[i.name];
                    if (name.find(searchWord) == 0) {
                        if (prefix_ == "")
                            completions->add(name);
                        else
                            completions->add(prefix_ + "." + name);
                    }
                }
            }
        } else {
            completeFlakeRefWithFragment(
                getEvalState(),
                lockFlags,
                getDefaultFlakeAttrPathPrefixes(),
                getDefaultFlakeAttrPaths(),
                prefix);
        }
    } catch (EvalError&) {
        // Don't want eval errors to mess-up with the completion engine, so let's just swallow them
    }
}

void completeFlakeRefWithFragment(
    ref<EvalState> evalState,
    flake::LockFlags lockFlags,
    Strings attrPathPrefixes,
    const Strings & defaultFlakeAttrPaths,
    std::string_view prefix)
{
    /* Look for flake output attributes that match the
       prefix. */
    try {
        auto hash = prefix.find('#');
        if (hash == std::string::npos) {
            completeFlakeRef(evalState->store, prefix);
        } else {
            completionType = ctAttrs;

            auto fragment = prefix.substr(hash + 1);
            auto flakeRefS = std::string(prefix.substr(0, hash));
            auto flakeRef = parseFlakeRef(expandTilde(flakeRefS), absPath("."));

            auto evalCache = openEvalCache(*evalState,
                std::make_shared<flake::LockedFlake>(lockFlake(*evalState, flakeRef, lockFlags)));

            auto root = evalCache->getRoot();

            /* Complete 'fragment' relative to all the
               attrpath prefixes as well as the root of the
               flake. */
            attrPathPrefixes.push_back("");

            for (auto & attrPathPrefixS : attrPathPrefixes) {
                auto attrPathPrefix = parseAttrPath(*evalState, attrPathPrefixS);
                auto attrPathS = attrPathPrefixS + std::string(fragment);
                auto attrPath = parseAttrPath(*evalState, attrPathS);

                std::string lastAttr;
                if (!attrPath.empty() && !hasSuffix(attrPathS, ".")) {
                    lastAttr = evalState->symbols[attrPath.back()];
                    attrPath.pop_back();
                }

                auto attr = root->findAlongAttrPath(attrPath);
                if (!attr) continue;

                for (auto & attr2 : (*attr)->getAttrs()) {
                    if (hasPrefix(evalState->symbols[attr2], lastAttr)) {
                        auto attrPath2 = (*attr)->getAttrPath(attr2);
                        /* Strip the attrpath prefix. */
                        attrPath2.erase(attrPath2.begin(), attrPath2.begin() + attrPathPrefix.size());
                        completions->add(flakeRefS + "#" + concatStringsSep(".", evalState->symbols.resolve(attrPath2)));
                    }
                }
            }

            /* And add an empty completion for the default
               attrpaths. */
            if (fragment.empty()) {
                for (auto & attrPath : defaultFlakeAttrPaths) {
                    auto attr = root->findAlongAttrPath(parseAttrPath(*evalState, attrPath));
                    if (!attr) continue;
                    completions->add(flakeRefS + "#");
                }
            }
        }
    } catch (Error & e) {
        warn(e.msg());
    }
}

void completeFlakeRef(ref<Store> store, std::string_view prefix)
{
    if (!settings.isExperimentalFeatureEnabled(Xp::Flakes))
        return;

    if (prefix == "")
        completions->add(".");

    completeDir(0, prefix);

    /* Look for registry entries that match the prefix. */
    for (auto & registry : fetchers::getRegistries(store)) {
        for (auto & entry : registry->entries) {
            auto from = entry.from.to_string();
            if (!hasPrefix(prefix, "flake:") && hasPrefix(from, "flake:")) {
                std::string from2(from, 6);
                if (hasPrefix(from2, prefix))
                    completions->add(from2);
            } else {
                if (hasPrefix(from, prefix))
                    completions->add(from);
            }
        }
    }
}

DerivedPathWithInfo Installable::toDerivedPath()
{
    auto buildables = toDerivedPaths();
    if (buildables.size() != 1)
        throw Error("installable '%s' evaluates to %d derivations, where only one is expected", what(), buildables.size());
    return std::move(buildables[0]);
}

std::vector<ref<eval_cache::AttrCursor>>
Installable::getCursors(EvalState & state)
{
    auto evalCache =
        std::make_shared<nix::eval_cache::EvalCache>(std::nullopt, state,
            [&]() { return toValue(state).first; });
    return {evalCache->getRoot()};
}

ref<eval_cache::AttrCursor>
Installable::getCursor(EvalState & state)
{
    /* Although getCursors should return at least one element, in case it doesn't,
       bound check to avoid an undefined behavior for vector[0] */
    return getCursors(state).at(0);
}

static StorePath getDeriver(
    ref<Store> store,
    const Installable & i,
    const StorePath & drvPath)
{
    auto derivers = store->queryValidDerivers(drvPath);
    if (derivers.empty())
        throw Error("'%s' does not have a known deriver", i.what());
    // FIXME: use all derivers?
    return *derivers.begin();
}

struct InstallableStorePath : Installable
{
    ref<Store> store;
    DerivedPath req;

    InstallableStorePath(ref<Store> store, DerivedPath && req)
        : store(store), req(std::move(req))
    { }

    std::string what() const override
    {
        return req.to_string(*store);
    }

    DerivedPathsWithInfo toDerivedPaths() override
    {
        return {{.path = req, .info = {} }};
    }

    std::optional<StorePath> getStorePath() override
    {
        return std::visit(overloaded {
            [&](const DerivedPath::Built & bfd) {
                return bfd.drvPath;
            },
            [&](const DerivedPath::Opaque & bo) {
                return bo.path;
            },
        }, req.raw());
    }
};

struct InstallableAttrPath : InstallableValue
{
    SourceExprCommand & cmd;
    RootValue v;
    std::string attrPath;
    ExtendedOutputsSpec extendedOutputsSpec;

    InstallableAttrPath(
        ref<EvalState> state,
        SourceExprCommand & cmd,
        Value * v,
        const std::string & attrPath,
        ExtendedOutputsSpec extendedOutputsSpec)
        : InstallableValue(state)
        , cmd(cmd)
        , v(allocRootValue(v))
        , attrPath(attrPath)
        , extendedOutputsSpec(std::move(extendedOutputsSpec))
    { }

    std::string what() const override { return attrPath; }

    std::pair<Value *, PosIdx> toValue(EvalState & state) override
    {
        auto [vRes, pos] = findAlongAttrPath(state, attrPath, *cmd.getAutoArgs(state), **v);
        state.forceValue(*vRes, pos);
        return {vRes, pos};
    }

    DerivedPathsWithInfo toDerivedPaths() override
    {
        auto v = toValue(*state).first;

        Bindings & autoArgs = *cmd.getAutoArgs(*state);

        DrvInfos drvInfos;
        getDerivations(*state, *v, "", autoArgs, drvInfos, false);

        // Backward compatibility hack: group results by drvPath. This
        // helps keep .all output together.
        std::map<StorePath, OutputsSpec> byDrvPath;

        for (auto & drvInfo : drvInfos) {
            auto drvPath = drvInfo.queryDrvPath();
            if (!drvPath)
                throw Error("'%s' is not a derivation", what());

            auto newOutputs = std::visit(overloaded {
                [&](const ExtendedOutputsSpec::Default & d) -> OutputsSpec {
                    std::set<std::string> outputsToInstall;
                    for (auto & output : drvInfo.queryOutputs(false, true))
                        outputsToInstall.insert(output.first);
                    return OutputsSpec::Names { std::move(outputsToInstall) };
                },
                [&](const ExtendedOutputsSpec::Explicit & e) -> OutputsSpec {
                    return e;
                },
            }, extendedOutputsSpec.raw());

            auto [iter, didInsert] = byDrvPath.emplace(*drvPath, newOutputs);

            if (!didInsert)
                iter->second = iter->second.union_(newOutputs);
        }

        DerivedPathsWithInfo res;
        for (auto & [drvPath, outputs] : byDrvPath)
            res.push_back({
                .path = DerivedPath::Built {
                    .drvPath = drvPath,
                    .outputs = outputs,
                },
            });

        return res;
    }
};

std::vector<std::string> InstallableFlake::getActualAttrPaths()
{
    std::vector<std::string> res;

    for (auto & prefix : prefixes)
        res.push_back(prefix + *attrPaths.begin());

    for (auto & s : attrPaths)
        res.push_back(s);

    return res;
}

Value * InstallableFlake::getFlakeOutputs(EvalState & state, const flake::LockedFlake & lockedFlake)
{
    auto vFlake = state.allocValue();

    callFlake(state, lockedFlake, *vFlake);

    auto aOutputs = vFlake->attrs->get(state.symbols.create("outputs"));
    assert(aOutputs);

    state.forceValue(*aOutputs->value, [&]() { return aOutputs->value->determinePos(noPos); });

    return aOutputs->value;
}

ref<eval_cache::EvalCache> openEvalCache(
    EvalState & state,
    std::shared_ptr<flake::LockedFlake> lockedFlake)
{
    auto fingerprint = lockedFlake->getFingerprint();
    return make_ref<nix::eval_cache::EvalCache>(
        evalSettings.useEvalCache && evalSettings.pureEval
            ? std::optional { std::cref(fingerprint) }
            : std::nullopt,
        state,
        [&state, lockedFlake]()
        {
            /* For testing whether the evaluation cache is
               complete. */
            if (getEnv("NIX_ALLOW_EVAL").value_or("1") == "0")
                throw Error("not everything is cached, but evaluation is not allowed");

            auto vFlake = state.allocValue();
            flake::callFlake(state, *lockedFlake, *vFlake);

            state.forceAttrs(*vFlake, noPos, "while parsing cached flake data");

            auto aOutputs = vFlake->attrs->get(state.symbols.create("outputs"));
            assert(aOutputs);

            return aOutputs->value;
        });
}

static std::string showAttrPaths(const std::vector<std::string> & paths)
{
    std::string s;
    for (const auto & [n, i] : enumerate(paths)) {
        if (n > 0) s += n + 1 == paths.size() ? " or " : ", ";
        s += '\''; s += i; s += '\'';
    }
    return s;
}

InstallableFlake::InstallableFlake(
    SourceExprCommand * cmd,
    ref<EvalState> state,
    FlakeRef && flakeRef,
    std::string_view fragment,
    ExtendedOutputsSpec extendedOutputsSpec,
    Strings attrPaths,
    Strings prefixes,
    const flake::LockFlags & lockFlags)
    : InstallableValue(state),
      flakeRef(flakeRef),
      attrPaths(fragment == "" ? attrPaths : Strings{(std::string) fragment}),
      prefixes(fragment == "" ? Strings{} : prefixes),
      extendedOutputsSpec(std::move(extendedOutputsSpec)),
      lockFlags(lockFlags)
{
    if (cmd && cmd->getAutoArgs(*state)->size())
        throw UsageError("'--arg' and '--argstr' are incompatible with flakes");
}

DerivedPathsWithInfo InstallableFlake::toDerivedPaths()
{
    Activity act(*logger, lvlTalkative, actUnknown, fmt("evaluating derivation '%s'", what()));

    auto attr = getCursor(*state);

    auto attrPath = attr->getAttrPathStr();

    if (!attr->isDerivation()) {

        // FIXME: use eval cache?
        auto v = attr->forceValue();

        if (v.type() == nPath) {
            PathSet context;
            auto storePath = state->copyPathToStore(context, Path(v.path));
            return {{
                .path = DerivedPath::Opaque {
                    .path = std::move(storePath),
                }
            }};
        }

        else if (v.type() == nString) {
            PathSet context;
            auto s = state->forceString(v, context, noPos, fmt("while evaluating the flake output attribute '%s'", attrPath));
            auto storePath = state->store->maybeParseStorePath(s);
            if (storePath && context.count(std::string(s))) {
                return {{
                    .path = DerivedPath::Opaque {
                        .path = std::move(*storePath),
                    }
                }};
            } else
                throw Error("flake output attribute '%s' evaluates to the string '%s' which is not a store path", attrPath, s);
        }

        else
            throw Error("flake output attribute '%s' is not a derivation or path", attrPath);
    }

    auto drvPath = attr->forceDerivation();

    std::optional<NixInt> priority;

    if (attr->maybeGetAttr(state->sOutputSpecified)) {
    } else if (auto aMeta = attr->maybeGetAttr(state->sMeta)) {
        if (auto aPriority = aMeta->maybeGetAttr("priority"))
            priority = aPriority->getInt();
    }

    return {{
        .path = DerivedPath::Built {
            .drvPath = std::move(drvPath),
            .outputs = std::visit(overloaded {
                [&](const ExtendedOutputsSpec::Default & d) -> OutputsSpec {
                    std::set<std::string> outputsToInstall;
                    if (auto aOutputSpecified = attr->maybeGetAttr(state->sOutputSpecified)) {
                        if (aOutputSpecified->getBool()) {
                            if (auto aOutputName = attr->maybeGetAttr("outputName"))
                                outputsToInstall = { aOutputName->getString() };
                        }
                    } else if (auto aMeta = attr->maybeGetAttr(state->sMeta)) {
                        if (auto aOutputsToInstall = aMeta->maybeGetAttr("outputsToInstall"))
                            for (auto & s : aOutputsToInstall->getListOfStrings())
                                outputsToInstall.insert(s);
                    }

                    if (outputsToInstall.empty())
                        outputsToInstall.insert("out");

                    return OutputsSpec::Names { std::move(outputsToInstall) };
                },
                [&](const ExtendedOutputsSpec::Explicit & e) -> OutputsSpec {
                    return e;
                },
            }, extendedOutputsSpec.raw()),
        },
        .info = {
            .priority = priority,
            .originalRef = flakeRef,
            .resolvedRef = getLockedFlake()->flake.lockedRef,
            .attrPath = attrPath,
            .extendedOutputsSpec = extendedOutputsSpec,
        }
    }};
}

std::pair<Value *, PosIdx> InstallableFlake::toValue(EvalState & state)
{
    return {&getCursor(state)->forceValue(), noPos};
}

std::vector<ref<eval_cache::AttrCursor>>
InstallableFlake::getCursors(EvalState & state)
{
    auto evalCache = openEvalCache(state,
        std::make_shared<flake::LockedFlake>(lockFlake(state, flakeRef, lockFlags)));

    auto root = evalCache->getRoot();

    std::vector<ref<eval_cache::AttrCursor>> res;

    Suggestions suggestions;
    auto attrPaths = getActualAttrPaths();

    for (auto & attrPath : attrPaths) {
        debug("trying flake output attribute '%s'", attrPath);

        auto attr = root->findAlongAttrPath(parseAttrPath(state, attrPath));
        if (attr) {
            res.push_back(ref(*attr));
        } else {
            suggestions += attr.getSuggestions();
        }
    }

    if (res.size() == 0)
        throw Error(
            suggestions,
            "flake '%s' does not provide attribute %s",
            flakeRef,
            showAttrPaths(attrPaths));

    return res;
}

std::shared_ptr<flake::LockedFlake> InstallableFlake::getLockedFlake() const
{
    if (!_lockedFlake) {
        flake::LockFlags lockFlagsApplyConfig = lockFlags;
        lockFlagsApplyConfig.applyNixConfig = true;
        _lockedFlake = std::make_shared<flake::LockedFlake>(lockFlake(*state, flakeRef, lockFlagsApplyConfig));
    }
    return _lockedFlake;
}

FlakeRef InstallableFlake::nixpkgsFlakeRef() const
{
    auto lockedFlake = getLockedFlake();

    if (auto nixpkgsInput = lockedFlake->lockFile.findInput({"nixpkgs"})) {
        if (auto lockedNode = std::dynamic_pointer_cast<const flake::LockedNode>(nixpkgsInput)) {
            debug("using nixpkgs flake '%s'", lockedNode->lockedRef);
            return std::move(lockedNode->lockedRef);
        }
    }

    return Installable::nixpkgsFlakeRef();
}

std::vector<std::shared_ptr<Installable>> SourceExprCommand::parseInstallables(
    ref<Store> store, std::vector<std::string> ss)
{
    std::vector<std::shared_ptr<Installable>> result;

    if (readOnlyMode) {
        settings.readOnlyMode = true;
    }

    if (file || expr) {
        if (file && expr)
            throw UsageError("'--file' and '--expr' are exclusive");

        // FIXME: backward compatibility hack
        if (file) evalSettings.pureEval = false;

        auto state = getEvalState();
        auto vFile = state->allocValue();

        if (file == "-") {
            auto e = state->parseStdin();
            state->eval(e, *vFile);
        }
        else if (file)
            state->evalFile(lookupFileArg(*state, *file), *vFile);
        else {
            auto e = state->parseExprFromString(*expr, absPath("."));
            state->eval(e, *vFile);
        }

        for (auto & s : ss) {
            auto [prefix, extendedOutputsSpec] = ExtendedOutputsSpec::parse(s);
            result.push_back(
                std::make_shared<InstallableAttrPath>(
                    state, *this, vFile,
                    prefix == "." ? "" : std::string { prefix },
                    extendedOutputsSpec));
        }

    } else {

        for (auto & s : ss) {
            std::exception_ptr ex;

            auto [prefix_, extendedOutputsSpec_] = ExtendedOutputsSpec::parse(s);
            // To avoid clang's pedantry
            auto prefix = std::move(prefix_);
            auto extendedOutputsSpec = std::move(extendedOutputsSpec_);

            auto found = prefix.find('/');
            if (found != std::string::npos) {
                try {
                    auto derivedPath = std::visit(overloaded {
                        // If the user did not use ^, we treat the output more liberally.
                        [&](const ExtendedOutputsSpec::Default &) -> DerivedPath {
                            // First, we accept a symlink chain or an actual store path.
                            auto storePath = store->followLinksToStorePath(prefix);
                            // Second, we see if the store path ends in `.drv` to decide what sort
                            // of derived path they want.
                            //
                            // This handling predates the `^` syntax. The `^*` in
                            // `/nix/store/hash-foo.drv^*` unambiguously means "do the
                            // `DerivedPath::Built` case", so plain `/nix/store/hash-foo.drv` could
                            // also unambiguously mean "do the DerivedPath::Opaque` case".
                            //
                            // Issue #7261 tracks reconsidering this `.drv` dispatching.
                            return storePath.isDerivation()
                                ? (DerivedPath) DerivedPath::Built {
                                    .drvPath = std::move(storePath),
                                    .outputs = OutputsSpec::All {},
                                }
                                : (DerivedPath) DerivedPath::Opaque {
                                    .path = std::move(storePath),
                                };
                        },
                        // If the user did use ^, we just do exactly what is written.
                        [&](const ExtendedOutputsSpec::Explicit & outputSpec) -> DerivedPath {
                            return DerivedPath::Built {
                                .drvPath = store->parseStorePath(prefix),
                                .outputs = outputSpec,
                            };
                        },
                    }, extendedOutputsSpec.raw());
                    result.push_back(std::make_shared<InstallableStorePath>(store, std::move(derivedPath)));
                    continue;
                } catch (BadStorePath &) {
                } catch (...) {
                    if (!ex)
                        ex = std::current_exception();
                }
            }

            try {
                auto [flakeRef, fragment] = parseFlakeRefWithFragment(std::string { prefix }, absPath("."));
                result.push_back(std::make_shared<InstallableFlake>(
                        this,
                        getEvalState(),
                        std::move(flakeRef),
                        fragment,
                        extendedOutputsSpec,
                        getDefaultFlakeAttrPaths(),
                        getDefaultFlakeAttrPathPrefixes(),
                        lockFlags));
                continue;
            } catch (...) {
                ex = std::current_exception();
            }

            std::rethrow_exception(ex);
        }
    }

    return result;
}

std::shared_ptr<Installable> SourceExprCommand::parseInstallable(
    ref<Store> store, const std::string & installable)
{
    auto installables = parseInstallables(store, {installable});
    assert(installables.size() == 1);
    return installables.front();
}

std::vector<BuiltPathWithResult> Installable::build(
    ref<Store> evalStore,
    ref<Store> store,
    Realise mode,
    const std::vector<std::shared_ptr<Installable>> & installables,
    BuildMode bMode)
{
    std::vector<BuiltPathWithResult> res;
    for (auto & [_, builtPathWithResult] : build2(evalStore, store, mode, installables, bMode))
        res.push_back(builtPathWithResult);
    return res;
}

std::vector<std::pair<std::shared_ptr<Installable>, BuiltPathWithResult>> Installable::build2(
    ref<Store> evalStore,
    ref<Store> store,
    Realise mode,
    const std::vector<std::shared_ptr<Installable>> & installables,
    BuildMode bMode)
{
    if (mode == Realise::Nothing)
        settings.readOnlyMode = true;

    struct Aux
    {
        ExtraPathInfo info;
        std::shared_ptr<Installable> installable;
    };

    std::vector<DerivedPath> pathsToBuild;
    std::map<DerivedPath, std::vector<Aux>> backmap;

    for (auto & i : installables) {
        for (auto b : i->toDerivedPaths()) {
            pathsToBuild.push_back(b.path);
            backmap[b.path].push_back({.info = b.info, .installable = i});
        }
    }

    std::vector<std::pair<std::shared_ptr<Installable>, BuiltPathWithResult>> res;

    switch (mode) {

    case Realise::Nothing:
    case Realise::Derivation:
        printMissing(store, pathsToBuild, lvlError);

        for (auto & path : pathsToBuild) {
            for (auto & aux : backmap[path]) {
                std::visit(overloaded {
                    [&](const DerivedPath::Built & bfd) {
                        auto outputs = resolveDerivedPath(*store, bfd, &*evalStore);
                        res.push_back({aux.installable, {
                            .path = BuiltPath::Built { bfd.drvPath, outputs },
                            .info = aux.info}});
                    },
                    [&](const DerivedPath::Opaque & bo) {
                        res.push_back({aux.installable, {
                            .path = BuiltPath::Opaque { bo.path },
                            .info = aux.info}});
                    },
                }, path.raw());
            }
        }

        break;

    case Realise::Outputs: {
        if (settings.printMissing)
            printMissing(store, pathsToBuild, lvlInfo);

        for (auto & buildResult : store->buildPathsWithResults(pathsToBuild, bMode, evalStore)) {
            if (!buildResult.success())
                buildResult.rethrow();

            for (auto & aux : backmap[buildResult.path]) {
                std::visit(overloaded {
                    [&](const DerivedPath::Built & bfd) {
                        std::map<std::string, StorePath> outputs;
                        for (auto & path : buildResult.builtOutputs)
                            outputs.emplace(path.first.outputName, path.second.outPath);
                        res.push_back({aux.installable, {
                            .path = BuiltPath::Built { bfd.drvPath, outputs },
                            .info = aux.info,
                            .result = buildResult}});
                    },
                    [&](const DerivedPath::Opaque & bo) {
                        res.push_back({aux.installable, {
                            .path = BuiltPath::Opaque { bo.path },
                            .info = aux.info,
                            .result = buildResult}});
                    },
                }, buildResult.path.raw());
            }
        }

        break;
    }

    default:
        assert(false);
    }

    return res;
}

BuiltPaths Installable::toBuiltPaths(
    ref<Store> evalStore,
    ref<Store> store,
    Realise mode,
    OperateOn operateOn,
    const std::vector<std::shared_ptr<Installable>> & installables)
{
    if (operateOn == OperateOn::Output) {
        BuiltPaths res;
        for (auto & p : Installable::build(evalStore, store, mode, installables))
            res.push_back(p.path);
        return res;
    } else {
        if (mode == Realise::Nothing)
            settings.readOnlyMode = true;

        BuiltPaths res;
        for (auto & drvPath : Installable::toDerivations(store, installables, true))
            res.push_back(BuiltPath::Opaque{drvPath});
        return res;
    }
}

StorePathSet Installable::toStorePaths(
    ref<Store> evalStore,
    ref<Store> store,
    Realise mode, OperateOn operateOn,
    const std::vector<std::shared_ptr<Installable>> & installables)
{
    StorePathSet outPaths;
    for (auto & path : toBuiltPaths(evalStore, store, mode, operateOn, installables)) {
        auto thisOutPaths = path.outPaths();
        outPaths.insert(thisOutPaths.begin(), thisOutPaths.end());
    }
    return outPaths;
}

StorePath Installable::toStorePath(
    ref<Store> evalStore,
    ref<Store> store,
    Realise mode, OperateOn operateOn,
    std::shared_ptr<Installable> installable)
{
    auto paths = toStorePaths(evalStore, store, mode, operateOn, {installable});

    if (paths.size() != 1)
        throw Error("argument '%s' should evaluate to one store path", installable->what());

    return *paths.begin();
}

StorePathSet Installable::toDerivations(
    ref<Store> store,
    const std::vector<std::shared_ptr<Installable>> & installables,
    bool useDeriver)
{
    StorePathSet drvPaths;

    for (const auto & i : installables)
        for (const auto & b : i->toDerivedPaths())
            std::visit(overloaded {
                [&](const DerivedPath::Opaque & bo) {
                    if (!useDeriver)
                        throw Error("argument '%s' did not evaluate to a derivation", i->what());
                    drvPaths.insert(getDeriver(store, *i, bo.path));
                },
                [&](const DerivedPath::Built & bfd) {
                    drvPaths.insert(bfd.drvPath);
                },
            }, b.path.raw());

    return drvPaths;
}

InstallablesCommand::InstallablesCommand()
{
    expectArgs({
        .label = "installables",
        .handler = {&_installables},
        .completer = {[&](size_t, std::string_view prefix) {
            completeInstallable(prefix);
        }}
    });
}

void InstallablesCommand::prepare()
{
    installables = load();
}

Installables InstallablesCommand::load() {
    Installables installables;
    if (_installables.empty() && useDefaultInstallables())
        // FIXME: commands like "nix profile install" should not have a
        // default, probably.
        _installables.push_back(".");
    return parseInstallables(getStore(), _installables);
}

std::vector<std::string> InstallablesCommand::getFlakesForCompletion()
{
    if (_installables.empty()) {
        if (useDefaultInstallables())
            return {"."};
        return {};
    }
    return _installables;
}

InstallableCommand::InstallableCommand(bool supportReadOnlyMode)
    : SourceExprCommand(supportReadOnlyMode)
{
    expectArgs({
        .label = "installable",
        .optional = true,
        .handler = {&_installable},
        .completer = {[&](size_t, std::string_view prefix) {
            completeInstallable(prefix);
        }}
    });
}

void InstallableCommand::prepare()
{
    installable = parseInstallable(getStore(), _installable);
}

}