harn_rules_hostlib/

lib.rs

//! Host capability exposing the `harn-rules` declarative rule engine to
//! Harn as `rules.search` / `rules.report` / `rules.apply`.
//!
//! This crate lives outside `harn-hostlib` on purpose: `harn-rules` already
//! depends on `harn-hostlib` (for the tree-sitter grammars), so the rules
//! builtins would form a dependency cycle if they lived there. An embedder
//! (harn-cli, harn-serve) calls [`install`] alongside `harn_hostlib::install_default`.
//!
//! ## Builtins
//!
//! - `rules.search` (read-only) — run a rule and return its matches.
//! - `rules.report` (read-only) — run a rule in report-only mode and return
//!   a [`harn_rules::DataTable`] (counts + per-match rows).
//! - `rules.diagnostics` (read-only) — run a **declarative** rule and return
//!   its [`harn_rules::Diagnostic`]s (message + severity + span + fix).
//! - `rules.visit` (read-only, **async**) — the **imperative** escape hatch:
//!   run a rule's matcher, then invoke a `.harn` visitor
//!   `on_match($node, $ctx)` once per match. The visitor returns its
//!   report(s) — `nil`/`false` to skip, a `{message, fix, safety}` dict, or
//!   a list of them — which the engine turns into diagnostics of the same
//!   shape `rules.diagnostics` emits. The visitor has full programmatic
//!   control (compute a message/fix from the captured metavars), which the
//!   declarative form cannot.
//! - `rules.apply` — apply a codemod rule's `fix`; writes only
//!   when `dry_run: false` *and* the rule is safe to auto-apply (or
//!   `allow_unsafe: true`). Possessing `HarnessRules` is the authority to call
//!   it; no process- or thread-local grant state exists.
//!
//! A rule is passed as its TOML source (`rule`), so an agent can author and
//! run a rule — declarative *or* imperative — entirely from `.harn` without
//! recompiling the binary.
//!
//! ### Why `rules.visit` is async, and why it returns rather than mutates
//!
//! A *synchronous* hostlib builtin cannot call a `.harn` closure: the VM's
//! [`Vm::call_closure_pub`] is async-only. So the visitor is registered as an
//! **async** builtin (directly on the VM via [`Vm::register_async_builtin`],
//! bypassing the sync [`HostlibRegistry`]), which can obtain a child VM from
//! its [`AsyncBuiltinCtx`] and call back per match.
//!
//! The visitor **returns** its reports instead of calling a mutating
//! `ctx.report(...)`: `VmValue` has no callable variant that carries captured
//! Rust state, so a stateful `report` method cannot be embedded in `ctx`.
//! Returning is both the sound option and the simpler one.

use std::collections::BTreeMap;
use std::path::{Path, PathBuf};
use std::sync::Arc;

use harn_hostlib::ast::Language;
use harn_hostlib::{
    BuiltinRegistry, HostlibCapability, HostlibError, HostlibRegistry, RegisteredBuiltin,
};
use harn_vm::{AsyncBuiltinCtx, Vm, VmError, VmValue};

use harn_rules::{
    data_table, Applicability, BindingMetadata, CompiledRule, Diagnostic, ResolvedBinding, Rule,
    RuleMatch, Safety, Severity, SourceFile, Span,
};

const SEARCH: &str = "hostlib_rules_search";
const REPORT: &str = "hostlib_rules_report";
const DIAGNOSTICS: &str = "hostlib_rules_diagnostics";
const VISIT: &str = "hostlib_rules_visit";
const APPLY: &str = "hostlib_rules_apply";
const FOLD: &str = "hostlib_rules_fold";
const LINT_RUN: &str = "hostlib_lint_run";

/// The `rules` host capability.
#[derive(Default)]
pub struct RulesCapability;

impl HostlibCapability for RulesCapability {
    fn module_name(&self) -> &'static str {
        "rules"
    }

    fn register_builtins(&self, registry: &mut BuiltinRegistry) {
        registry.register(RegisteredBuiltin {
            name: SEARCH,
            module: "rules",
            method: "search",
            handler: Arc::new(search_run),
        });
        registry.register(RegisteredBuiltin {
            name: REPORT,
            module: "rules",
            method: "report",
            handler: Arc::new(report_run),
        });
        registry.register(RegisteredBuiltin {
            name: DIAGNOSTICS,
            module: "rules",
            method: "diagnostics",
            handler: Arc::new(diagnostics_run),
        });
        registry.register(RegisteredBuiltin {
            name: APPLY,
            module: "rules",
            method: "apply",
            handler: Arc::new(apply_run),
        });
        registry.register(RegisteredBuiltin {
            name: FOLD,
            module: "rules",
            method: "fold",
            handler: Arc::new(fold_run),
        });
    }
}

/// The `lint` host capability (#2851): runs the Harn linter for an
/// agent/IDE/cloud caller, returning the same diagnostics the CLI emits.
#[derive(Default)]
pub struct LintCapability;

impl HostlibCapability for LintCapability {
    fn module_name(&self) -> &'static str {
        "lint"
    }

    fn register_builtins(&self, registry: &mut BuiltinRegistry) {
        // Read-only: lint.run parses + lints in memory, never writes.
        registry.register(RegisteredBuiltin {
            name: LINT_RUN,
            module: "lint",
            method: "run",
            handler: Arc::new(lint_run),
        });
    }
}

/// Install the `rules` + `lint` capabilities into a VM. Call this alongside
/// `harn_hostlib::install_default`.
pub fn install(vm: &mut Vm) {
    HostlibRegistry::new()
        .with(RulesCapability)
        .with(LintCapability)
        .register_into_vm(vm);
    // `rules.visit` invokes a `.harn` closure per match, which only an async
    // builtin can do (`call_closure_pub` is async). It is therefore registered
    // directly on the VM rather than through the sync `HostlibRegistry`.
    vm.register_async_capability_method(harn_builtin_meta::CapabilityId::Rules, "visit", visit_run);
}

// ---------------------------------------------------------------------------
// Builtins
// ---------------------------------------------------------------------------

fn search_run(args: &[VmValue]) -> Result<VmValue, HostlibError> {
    let dict = first_dict(SEARCH, args)?;
    let rule = compile_rule(SEARCH, &dict)?;
    let files = load_files(SEARCH, &dict)?;

    let mut matches = Vec::new();
    for file in &files {
        for m in rule.run(&file.source).map_err(|e| backend(SEARCH, &e))? {
            matches.push(match_to_vm(&file.path, &m));
        }
    }
    Ok(dict_vm([
        ("result", str_vm("ok")),
        ("match_count", VmValue::Int(matches.len() as i64)),
        ("matches", VmValue::List(Arc::new(matches))),
    ]))
}

fn report_run(args: &[VmValue]) -> Result<VmValue, HostlibError> {
    let dict = first_dict(REPORT, args)?;
    let rule = compile_rule(REPORT, &dict)?;
    let files = load_files(REPORT, &dict)?;
    let table = data_table(&rule, &files).map_err(|e| backend(REPORT, &e))?;
    Ok(json_to_vm(&table.to_json_value()))
}

fn diagnostics_run(args: &[VmValue]) -> Result<VmValue, HostlibError> {
    let dict = first_dict(DIAGNOSTICS, args)?;
    let rule = compile_rule(DIAGNOSTICS, &dict)?;
    let files = load_files(DIAGNOSTICS, &dict)?;

    let mut diagnostics = Vec::new();
    for file in &files {
        for d in rule
            .diagnostics(&file.source)
            .map_err(|e| backend(DIAGNOSTICS, &e))?
        {
            diagnostics.push(diagnostic_vm(&file.path, &d));
        }
    }
    Ok(dict_vm([
        ("result", str_vm("ok")),
        ("diagnostic_count", VmValue::Int(diagnostics.len() as i64)),
        ("diagnostics", VmValue::List(Arc::new(diagnostics))),
    ]))
}

/// The imperative escape hatch (#2878): run the rule's matcher, then call the
/// `.harn` visitor `on_match($node, $ctx)` once per match. The visitor's
/// return value becomes diagnostics of the same shape `rules.diagnostics`
/// emits. Read-only — it never writes; the agent applies fixes itself.
async fn visit_run(ctx: AsyncBuiltinCtx, args: Vec<VmValue>) -> Result<VmValue, VmError> {
    let dict = first_dict(VISIT, &args).map_err(host_err)?;
    let rule = compile_rule(VISIT, &dict).map_err(host_err)?;
    let files = load_files(VISIT, &dict).map_err(host_err)?;
    let visitor = match dict.get("on_match") {
        Some(VmValue::Closure(c)) => c.clone(),
        _ => {
            return Err(VmError::Runtime(format!(
                "{VISIT}: `on_match` must be a function `fn(node, ctx)`"
            )))
        }
    };

    let default_severity = rule.severity();
    let default_safety = rule.safety();
    let rule_id = rule.id().to_string();

    let mut vm = ctx.child_vm();
    let mut diagnostics = Vec::new();
    for file in &files {
        let matches = rule
            .run(&file.source)
            .map_err(|e| host_err(backend(VISIT, &e)))?;
        let file_ctx = ctx_vm(&file.path, file.language, &file.source, &rule_id);
        for m in &matches {
            let node = node_vm(m);
            let ret = vm
                .call_closure_pub(&visitor, &[node, file_ctx.clone()])
                .await?;
            ctx.forward_output(&vm.take_output());
            for report in reports_from_return(ret) {
                diagnostics.push(report_to_diagnostic_vm(
                    &file.path,
                    &rule_id,
                    m.span,
                    report,
                    default_severity,
                    default_safety,
                ));
            }
        }
    }
    Ok(dict_vm([
        ("result", str_vm("ok")),
        ("diagnostic_count", VmValue::Int(diagnostics.len() as i64)),
        ("diagnostics", VmValue::List(Arc::new(diagnostics))),
    ]))
}

fn apply_run(args: &[VmValue]) -> Result<VmValue, HostlibError> {
    let dict = first_dict(APPLY, args)?;
    let rule = compile_rule(APPLY, &dict)?;
    let dry_run = optional_bool(&dict, "dry_run", true);
    let allow_unsafe = optional_bool(&dict, "allow_unsafe", false);
    // fmt post-pass (#2847): normalize rewritten `.harn` so a batch lands
    // fmt-stable. On by default; `format: false` opts out.
    let format = optional_bool(&dict, "format", true);
    let files = load_files(APPLY, &dict)?;

    let auto_applicable = rule.safety().is_auto_applicable();
    let mut entries = Vec::new();
    for file in &files {
        let outcome = rule.apply(&file.source).map_err(|e| backend(APPLY, &e))?;
        // Only `.harn` has a formatter; harn_fmt is idempotent, so a later
        // `harn fmt` is a no-op. A formatter error falls back to the raw
        // rewrite rather than failing the codemod.
        let formatted = format && outcome.changed && file.language == Language::Harn;
        let rewritten = if formatted {
            match harn_fmt::format_source(&outcome.rewritten) {
                Ok(canonical) => canonical,
                Err(_) => outcome.rewritten,
            }
        } else {
            outcome.rewritten
        };
        // Write only on a real apply, when the edit is safe to auto-apply
        // (or explicitly allowed), and the rule actually changed the file.
        let applied = !dry_run && outcome.changed && (auto_applicable || allow_unsafe);
        if applied {
            std::fs::write(&file.path, &rewritten).map_err(|e| HostlibError::Backend {
                builtin: APPLY,
                message: format!("write `{}`: {e}", file.path.display()),
            })?;
        }
        entries.push(dict_vm([
            ("path", str_vm(file.path.display().to_string())),
            ("changed", VmValue::Bool(outcome.changed)),
            ("applied", VmValue::Bool(applied)),
            ("idempotent", VmValue::Bool(outcome.idempotent)),
            ("formatted", VmValue::Bool(formatted)),
            ("safety", str_vm(format!("{:?}", outcome.safety))),
            // The original source, so callers can render a diff without a
            // (sandboxed) re-read of the file.
            ("before", str_vm(&file.source)),
            ("preview", str_vm(rewritten)),
        ]));
    }
    Ok(dict_vm([
        ("result", str_vm("ok")),
        ("dry_run", VmValue::Bool(dry_run)),
        ("auto_applicable", VmValue::Bool(auto_applicable)),
        ("files", VmValue::List(Arc::new(entries))),
    ]))
}

/// `rules.fold` (#2824): fold consecutive `let x = src?.x ?? d` runs into a
/// single destructure-with-defaults. A specialized, behavior-preserving
/// codemod (the engine can't fold statement sequences declaratively). Writes
/// only on a real apply (`dry_run: false`); shares the deterministic gate.
fn fold_run(args: &[VmValue]) -> Result<VmValue, HostlibError> {
    let dict = first_dict(FOLD, args)?;
    let dry_run = optional_bool(&dict, "dry_run", true);
    let files = load_files(FOLD, &dict)?;

    let mut entries = Vec::new();
    for file in &files {
        let raw_folded =
            harn_rules::fold::fold_destructure_defaults(&file.source, file.language.name())
                .map_err(|e| backend(FOLD, &e))?;
        let raw_changed = raw_folded != file.source;
        let formatted = raw_changed && file.language == Language::Harn;
        let folded = if formatted {
            match harn_fmt::format_source(&raw_folded) {
                Ok(canonical) => canonical,
                Err(_) => raw_folded,
            }
        } else {
            raw_folded
        };
        let changed = folded != file.source;
        let idempotent = harn_rules::fold::fold_destructure_defaults(&folded, file.language.name())
            .map(|again| again == folded)
            .unwrap_or(false);
        let applied = !dry_run && changed;
        if applied {
            std::fs::write(&file.path, &folded).map_err(|e| HostlibError::Backend {
                builtin: FOLD,
                message: format!("write `{}`: {e}", file.path.display()),
            })?;
        }
        entries.push(dict_vm([
            ("path", str_vm(file.path.display().to_string())),
            ("changed", VmValue::Bool(changed)),
            ("applied", VmValue::Bool(applied)),
            ("idempotent", VmValue::Bool(idempotent)),
            ("formatted", VmValue::Bool(formatted)),
            ("safety", str_vm("BehaviorPreserving")),
            ("before", str_vm(&file.source)),
            ("preview", str_vm(folded)),
        ]));
    }
    Ok(dict_vm([
        ("result", str_vm("ok")),
        ("dry_run", VmValue::Bool(dry_run)),
        ("files", VmValue::List(Arc::new(entries))),
    ]))
}

/// `lint.run` (#2851): lint a Harn source string and return its diagnostics, so
/// an agent / IDE / cloud caller gets the same findings as `harn lint` without
/// shelling out. Read-only. Params: `{source, disabled?, severity?}` where
/// `severity` maps a rule id to `"error"` / `"warning"` / `"info"`.
fn lint_run(args: &[VmValue]) -> Result<VmValue, HostlibError> {
    let dict = first_dict(LINT_RUN, args)?;
    let source = require_string(LINT_RUN, &dict, "source")?;
    let disabled = optional_string_list(&dict, "disabled");
    let severity_overrides = parse_severity_overrides(&dict);

    let program = harn_parser::parse_source(&source).map_err(|e| HostlibError::Backend {
        builtin: LINT_RUN,
        message: format!("parse error: {e}"),
    })?;
    let options = harn_lint::LintOptions {
        severity_overrides,
        ..Default::default()
    };
    let diagnostics = harn_lint::lint_with_options(
        &program,
        &disabled,
        Some(&source),
        &std::collections::HashSet::new(),
        &options,
    );
    let items: Vec<VmValue> = diagnostics.iter().map(lint_diagnostic_vm).collect();
    Ok(dict_vm([
        ("result", str_vm("ok")),
        ("diagnostic_count", VmValue::Int(items.len() as i64)),
        ("diagnostics", VmValue::List(Arc::new(items))),
    ]))
}

/// Parse a `severity` dict param (`{rule: "error"|"warning"|"info"}`) into the
/// linter's override map. Unknown severities are skipped.
fn parse_severity_overrides(
    dict: &harn_vm::value::DictMap,
) -> std::collections::HashMap<String, harn_lint::LintSeverity> {
    let mut out = std::collections::HashMap::new();
    if let Some(VmValue::Dict(map)) = dict.get("severity") {
        for (rule, value) in map.iter() {
            if let VmValue::String(s) = value {
                let severity = match s.to_ascii_lowercase().as_str() {
                    "error" => Some(harn_lint::LintSeverity::Error),
                    "warning" | "warn" => Some(harn_lint::LintSeverity::Warning),
                    "info" => Some(harn_lint::LintSeverity::Info),
                    _ => None,
                };
                if let Some(severity) = severity {
                    out.insert(rule.to_string(), severity);
                }
            }
        }
    }
    out
}

/// Marshal a [`harn_lint::LintDiagnostic`] into a VM dict, mirroring the
/// fields the CLI renders (code, rule, message, severity, span).
fn lint_diagnostic_vm(diag: &harn_lint::LintDiagnostic) -> VmValue {
    let severity = match diag.severity {
        harn_lint::LintSeverity::Error => "error",
        harn_lint::LintSeverity::Warning => "warning",
        harn_lint::LintSeverity::Info => "info",
    };
    dict_vm([
        ("code", str_vm(diag.code.as_str())),
        ("rule", str_vm(diag.rule.as_ref())),
        ("message", str_vm(&diag.message)),
        ("severity", str_vm(severity)),
        ("start_byte", VmValue::Int(diag.span.start as i64)),
        ("end_byte", VmValue::Int(diag.span.end as i64)),
        ("line", VmValue::Int(diag.span.line as i64)),
        ("column", VmValue::Int(diag.span.column as i64)),
    ])
}

// ---------------------------------------------------------------------------
// Shared parsing / conversion
// ---------------------------------------------------------------------------

fn compile_rule(
    builtin: &'static str,
    dict: &harn_vm::value::DictMap,
) -> Result<CompiledRule, HostlibError> {
    let toml = require_string(builtin, dict, "rule")?;
    let rule = Rule::from_toml_str(&toml).map_err(|e| HostlibError::InvalidParameter {
        builtin,
        param: "rule",
        message: format!("invalid rule TOML: {e}"),
    })?;
    CompiledRule::compile(&rule).map_err(|e| HostlibError::InvalidParameter {
        builtin,
        param: "rule",
        message: e.to_string(),
    })
}

/// Load the fileset: inline `source` (+ `language`) for a single buffer, or
/// `paths` read from disk (language inferred per file; non-UTF8 and
/// undetectable files are skipped).
fn load_files(
    builtin: &'static str,
    dict: &harn_vm::value::DictMap,
) -> Result<Vec<SourceFile>, HostlibError> {
    if let Some(source) = optional_string(dict, "source") {
        let language_name = require_string(builtin, dict, "language")?;
        let language =
            Language::from_name(&language_name).ok_or_else(|| HostlibError::InvalidParameter {
                builtin,
                param: "language",
                message: format!("unknown language `{language_name}`"),
            })?;
        let path = optional_string(dict, "path").unwrap_or_else(|| "<inline>".to_string());
        return Ok(vec![SourceFile {
            path: PathBuf::from(path),
            language,
            source,
        }]);
    }

    let paths = optional_string_list(dict, "paths");
    if paths.is_empty() {
        return Err(HostlibError::MissingParameter {
            builtin,
            param: "paths",
        });
    }
    let mut files = Vec::new();
    for path in paths {
        let bytes = std::fs::read(&path).map_err(|e| HostlibError::Backend {
            builtin,
            message: format!("read `{path}`: {e}"),
        })?;
        let Ok(contents) = String::from_utf8(bytes) else {
            continue;
        };
        if let Some(file) = SourceFile::detect(&path, contents) {
            files.push(file);
        }
    }
    Ok(files)
}

fn match_to_vm(path: &std::path::Path, m: &RuleMatch) -> VmValue {
    let captures: harn_vm::value::DictMap = m
        .bindings
        .iter()
        .map(|(name, b)| (name.clone(), str_vm(&b.text)))
        .collect();
    let capture_metadata = capture_metadata_vm(m);
    dict_vm([
        ("path", str_vm(path.display().to_string())),
        ("text", str_vm(&m.text)),
        ("start_row", VmValue::Int(m.span.start_row as i64)),
        ("start_col", VmValue::Int(m.span.start_col as i64)),
        ("end_row", VmValue::Int(m.span.end_row as i64)),
        ("end_col", VmValue::Int(m.span.end_col as i64)),
        ("captures", VmValue::dict(captures)),
        ("capture_metadata", capture_metadata),
    ])
}

fn backend(builtin: &'static str, err: &harn_rules::RulesError) -> HostlibError {
    HostlibError::Backend {
        builtin,
        message: err.to_string(),
    }
}

/// Lower a `HostlibError` into a `VmError` for the async `rules.visit` path
/// (which must return `VmError`, not `HostlibError`).
fn host_err(err: HostlibError) -> VmError {
    VmError::Runtime(err.to_string())
}

/// One report a `.harn` visitor returned for a single match. Every field is
/// optional: an empty report (e.g. the visitor returned `true`) flags the
/// match using the rule's own defaults.
#[derive(Default)]
struct ReportSpec {
    message: Option<String>,
    fix: Option<String>,
    safety: Option<Safety>,
    severity: Option<Severity>,
}

/// The `node` value handed to a visitor: the matched text, its metavar
/// captures, and its span.
fn node_vm(m: &RuleMatch) -> VmValue {
    let captures: harn_vm::value::DictMap = m
        .bindings
        .iter()
        .map(|(name, b)| (name.clone(), str_vm(&b.text)))
        .collect();
    let capture_metadata = capture_metadata_vm(m);
    dict_vm([
        ("text", str_vm(&m.text)),
        ("captures", VmValue::dict(captures)),
        ("capture_metadata", capture_metadata),
        ("start_row", VmValue::Int(m.span.start_row as i64)),
        ("start_col", VmValue::Int(m.span.start_col as i64)),
        ("end_row", VmValue::Int(m.span.end_row as i64)),
        ("end_col", VmValue::Int(m.span.end_col as i64)),
    ])
}

fn capture_metadata_vm(m: &RuleMatch) -> VmValue {
    let metadata: harn_vm::value::DictMap = m
        .bindings
        .iter()
        .filter(|(_, binding)| !binding.metadata.is_empty())
        .map(|(name, binding)| (name.clone(), binding_metadata_vm(&binding.metadata)))
        .collect();
    VmValue::dict(metadata)
}

fn binding_metadata_vm(metadata: &BindingMetadata) -> VmValue {
    let mut entries: BTreeMap<String, harn_vm::VmValue> = BTreeMap::new();
    if let Some(ty) = &metadata.ty {
        entries.insert("type".into(), str_vm(ty));
    }
    if let Some(resolved) = &metadata.resolved {
        entries.insert("resolved".into(), resolved_binding_vm(resolved));
    }
    VmValue::dict(entries)
}

fn resolved_binding_vm(resolved: &ResolvedBinding) -> VmValue {
    dict_vm([
        ("id", str_vm(&resolved.id)),
        ("name", str_vm(&resolved.name)),
        ("kind", str_vm(&resolved.kind)),
        ("start_row", VmValue::Int(resolved.span.start_row as i64)),
        ("start_col", VmValue::Int(resolved.span.start_col as i64)),
        ("end_row", VmValue::Int(resolved.span.end_row as i64)),
        ("end_col", VmValue::Int(resolved.span.end_col as i64)),
    ])
}

/// The read-only `ctx` value handed to a visitor: where the match lives and
/// what produced it.
fn ctx_vm(path: &Path, language: Language, source: &str, rule_id: &str) -> VmValue {
    dict_vm([
        ("path", str_vm(path.display().to_string())),
        ("language", str_vm(language.name())),
        ("source", str_vm(source)),
        ("rule_id", str_vm(rule_id)),
    ])
}

/// Build a diagnostic dict — the one shape both `rules.diagnostics` and
/// `rules.visit` emit, so an equivalent declarative and imperative rule
/// produce identical output.
fn diagnostic_dict(
    path: &Path,
    rule_id: &str,
    message: &str,
    severity: Severity,
    span: Span,
    fix: Option<String>,
    applicability: Applicability,
) -> VmValue {
    dict_vm([
        ("path", str_vm(path.display().to_string())),
        ("rule_id", str_vm(rule_id)),
        ("message", str_vm(message)),
        ("severity", str_vm(severity.as_str())),
        ("start_row", VmValue::Int(span.start_row as i64)),
        ("start_col", VmValue::Int(span.start_col as i64)),
        ("end_row", VmValue::Int(span.end_row as i64)),
        ("end_col", VmValue::Int(span.end_col as i64)),
        ("applicability", str_vm(applicability.as_str())),
        ("fix", fix.map(str_vm).unwrap_or(VmValue::Nil)),
    ])
}

fn diagnostic_vm(path: &Path, d: &Diagnostic) -> VmValue {
    diagnostic_dict(
        path,
        &d.rule_id,
        &d.message,
        d.severity,
        d.span,
        d.fix.clone(),
        d.applicability,
    )
}

/// Turn a visitor's [`ReportSpec`] into the same diagnostic dict, located at
/// the match's span and falling back to the rule's defaults.
fn report_to_diagnostic_vm(
    path: &Path,
    rule_id: &str,
    span: Span,
    report: ReportSpec,
    default_severity: Severity,
    default_safety: Safety,
) -> VmValue {
    let severity = report.severity.unwrap_or(default_severity);
    let safety = report.safety.unwrap_or(default_safety);
    diagnostic_dict(
        path,
        rule_id,
        report.message.as_deref().unwrap_or(""),
        severity,
        span,
        report.fix,
        safety.applicability(),
    )
}

/// Interpret a visitor's return value: `nil`/`false` skips, `true` flags with
/// rule defaults, a dict is one report, a list is many (skipping `nil`/`false`
/// entries).
fn reports_from_return(ret: VmValue) -> Vec<ReportSpec> {
    match ret {
        VmValue::Nil | VmValue::Bool(false) => Vec::new(),
        VmValue::Bool(true) => vec![ReportSpec::default()],
        VmValue::Dict(d) => vec![report_from_dict(&d)],
        VmValue::List(items) => items.iter().filter_map(report_from_item).collect(),
        _ => Vec::new(),
    }
}

fn report_from_item(v: &VmValue) -> Option<ReportSpec> {
    match v {
        VmValue::Nil | VmValue::Bool(false) => None,
        VmValue::Bool(true) => Some(ReportSpec::default()),
        VmValue::Dict(d) => Some(report_from_dict(d)),
        _ => None,
    }
}

fn report_from_dict(d: &harn_vm::value::DictMap) -> ReportSpec {
    ReportSpec {
        message: optional_string(d, "message"),
        fix: optional_string(d, "fix"),
        safety: optional_string(d, "safety").and_then(|s| parse_safety(&s)),
        severity: optional_string(d, "severity").and_then(|s| parse_severity(&s)),
    }
}

fn parse_severity(s: &str) -> Option<Severity> {
    match s {
        "info" => Some(Severity::Info),
        "warning" => Some(Severity::Warning),
        "error" => Some(Severity::Error),
        _ => None,
    }
}

fn parse_safety(s: &str) -> Option<Safety> {
    match s {
        "format-only" => Some(Safety::FormatOnly),
        "behavior-preserving" => Some(Safety::BehaviorPreserving),
        "scope-local" => Some(Safety::ScopeLocal),
        "surface-changing" => Some(Safety::SurfaceChanging),
        "capability-changing" => Some(Safety::CapabilityChanging),
        "needs-human" => Some(Safety::NeedsHuman),
        _ => None,
    }
}

fn json_to_vm(value: &serde_json::Value) -> VmValue {
    match value {
        serde_json::Value::Null => VmValue::Nil,
        serde_json::Value::Bool(b) => VmValue::Bool(*b),
        serde_json::Value::Number(n) => n
            .as_i64()
            .map(VmValue::Int)
            .unwrap_or_else(|| VmValue::Float(n.as_f64().unwrap_or(0.0))),
        serde_json::Value::String(s) => str_vm(s),
        serde_json::Value::Array(items) => {
            VmValue::List(Arc::new(items.iter().map(json_to_vm).collect()))
        }
        serde_json::Value::Object(map) => VmValue::dict(
            map.iter()
                .map(|(k, v)| (k.clone(), json_to_vm(v)))
                .collect::<harn_vm::value::DictMap>(),
        ),
    }
}

// ---------------------------------------------------------------------------
// Minimal arg/value helpers (harn-hostlib's `tools::args` is crate-private)
// ---------------------------------------------------------------------------

fn first_dict(
    builtin: &'static str,
    args: &[VmValue],
) -> Result<Arc<harn_vm::value::DictMap>, HostlibError> {
    match args.first() {
        Some(VmValue::Dict(dict)) => Ok(dict.clone()),
        Some(VmValue::Nil) | None => Ok(Arc::new(harn_vm::value::DictMap::new())),
        Some(_) => Err(HostlibError::InvalidParameter {
            builtin,
            param: "params",
            message: "expected a dict argument".into(),
        }),
    }
}

fn require_string(
    builtin: &'static str,
    dict: &harn_vm::value::DictMap,
    key: &'static str,
) -> Result<String, HostlibError> {
    match dict.get(key) {
        Some(VmValue::String(s)) => Ok(s.to_string()),
        _ => Err(HostlibError::MissingParameter {
            builtin,
            param: key,
        }),
    }
}

fn optional_string(dict: &harn_vm::value::DictMap, key: &str) -> Option<String> {
    match dict.get(key) {
        Some(VmValue::String(s)) => Some(s.to_string()),
        _ => None,
    }
}

fn optional_string_list(dict: &harn_vm::value::DictMap, key: &str) -> Vec<String> {
    match dict.get(key) {
        Some(VmValue::List(items)) => items
            .iter()
            .filter_map(|v| match v {
                VmValue::String(s) => Some(s.to_string()),
                _ => None,
            })
            .collect(),
        _ => Vec::new(),
    }
}

fn optional_bool(dict: &harn_vm::value::DictMap, key: &str, default: bool) -> bool {
    match dict.get(key) {
        Some(VmValue::Bool(b)) => *b,
        _ => default,
    }
}

fn str_vm(s: impl AsRef<str>) -> VmValue {
    VmValue::string(s)
}

fn dict_vm<const N: usize>(entries: [(&str, VmValue); N]) -> VmValue {
    let map: harn_vm::value::DictMap = entries
        .into_iter()
        .map(|(k, v)| (k.to_string(), v))
        .collect();
    VmValue::dict(map)
}

#[cfg(test)]
mod tests {
    use super::*;

    fn dict(pairs: &[(&str, VmValue)]) -> VmValue {
        let map: harn_vm::value::DictMap = pairs
            .iter()
            .map(|(k, v)| (k.to_string(), v.clone()))
            .collect();
        VmValue::dict(map)
    }

    fn get<'a>(v: &'a VmValue, key: &str) -> &'a VmValue {
        match v {
            VmValue::Dict(d) => d.get(key).unwrap_or_else(|| panic!("missing {key}")),
            _ => panic!("not a dict"),
        }
    }

    fn int(v: &VmValue) -> i64 {
        match v {
            VmValue::Int(i) => *i,
            other => panic!("not int: {other:?}"),
        }
    }

    fn s(v: &VmValue) -> String {
        match v {
            VmValue::String(s) => s.to_string(),
            other => panic!("not string: {other:?}"),
        }
    }

    fn b(v: &VmValue) -> bool {
        match v {
            VmValue::Bool(b) => *b,
            other => panic!("not bool: {other:?}"),
        }
    }

    const SEARCH_RULE: &str = r#"
        id = "find-calls"
        language = "typescript"
        [rule]
        pattern = "$FN()"
    "#;

    #[test]
    fn search_returns_matches_with_captures() {
        let result = search_run(&[dict(&[
            ("rule", str_vm(SEARCH_RULE)),
            ("source", str_vm("foo();\nbar();\n")),
            ("language", str_vm("typescript")),
        ])])
        .unwrap();
        assert_eq!(int(get(&result, "match_count")), 2);
        let matches = match get(&result, "matches") {
            VmValue::List(l) => l.clone(),
            _ => panic!(),
        };
        assert_eq!(s(get(get(&matches[0], "captures"), "FN")), "foo");
    }

    #[test]
    fn search_skips_non_utf8_paths() {
        let dir = tempfile::tempdir().unwrap();
        let source_path = dir.path().join("calls.ts");
        let binary_path = dir.path().join(".DS_Store");
        std::fs::write(&source_path, b"foo();\n").unwrap();
        std::fs::write(&binary_path, [0xff, 0xfe, 0xfd]).unwrap();

        let result = search_run(&[dict(&[
            ("rule", str_vm(SEARCH_RULE)),
            (
                "paths",
                VmValue::List(Arc::new(vec![
                    str_vm(source_path.display().to_string()),
                    str_vm(binary_path.display().to_string()),
                ])),
            ),
        ])])
        .unwrap();

        assert_eq!(int(get(&result, "match_count")), 1);
        let matches = match get(&result, "matches") {
            VmValue::List(l) => l.clone(),
            _ => panic!(),
        };
        assert_eq!(
            s(get(&matches[0], "path")),
            source_path.display().to_string()
        );
    }

    #[test]
    fn search_returns_harn_capture_metadata() {
        let rule = r#"
            id = "int-logs"
            language = "harn"
            [rule]
            pattern = "log($VALUE)"
        "#;
        let result = search_run(&[dict(&[
            ("rule", str_vm(rule)),
            (
                "source",
                str_vm("fn main() {\n  let count: int = 1\n  log(count)\n}\n"),
            ),
            ("language", str_vm("harn")),
        ])])
        .unwrap();
        let matches = match get(&result, "matches") {
            VmValue::List(l) => l.clone(),
            _ => panic!(),
        };
        let metadata = get(get(&matches[0], "capture_metadata"), "VALUE");
        assert_eq!(s(get(metadata, "type")), "int");
        assert_eq!(s(get(get(metadata, "resolved"), "name")), "count");
        assert_eq!(s(get(get(metadata, "resolved"), "kind")), "let");
    }

    #[test]
    fn report_returns_a_data_table() {
        let result = report_run(&[dict(&[
            ("rule", str_vm(SEARCH_RULE)),
            ("source", str_vm("foo();\nbar();\n")),
            ("language", str_vm("typescript")),
            ("path", str_vm("a.ts")),
        ])])
        .unwrap();
        assert_eq!(int(get(get(&result, "summary"), "total_rows")), 2);
        assert_eq!(s(get(&result, "rule_id")), "find-calls");
    }

    #[test]
    fn apply_dry_run_previews_without_writing() {
        let rule = r#"
            id = "rename"
            language = "typescript"
            safety = "behavior-preserving"
            fix = "bar()"
            [rule]
            pattern = "foo()"
        "#;
        let result = apply_run(&[dict(&[
            ("rule", str_vm(rule)),
            ("source", str_vm("foo();\n")),
            ("language", str_vm("typescript")),
            ("dry_run", VmValue::Bool(true)),
        ])])
        .unwrap();
        let files = match get(&result, "files") {
            VmValue::List(l) => l.clone(),
            _ => panic!(),
        };
        assert!(b(get(&files[0], "changed")));
        assert!(!b(get(&files[0], "applied")));
        assert_eq!(s(get(&files[0], "preview")), "bar();\n");
    }

    const UGLY_HARN_CODEMOD: &str = r#"
        id = "dd"
        language = "harn"
        safety = "scope-local"
        fix = "let {$K=$D}=$X"
        [rule]
        pattern = "let $K = $X?.$K ?? $D"
    "#;

    #[test]
    fn apply_formats_harn_output_by_default() {
        // The fix template is deliberately ugly; the #2847 fmt post-pass
        // normalizes the rewritten `.harn` (so a batch lands fmt-stable).
        let result = apply_run(&[dict(&[
            ("rule", str_vm(UGLY_HARN_CODEMOD)),
            (
                "source",
                str_vm("fn main() {\n  let timeout = cfg?.timeout ?? 30\n}\n"),
            ),
            ("language", str_vm("harn")),
            ("dry_run", VmValue::Bool(true)),
        ])])
        .unwrap();
        let files = match get(&result, "files") {
            VmValue::List(l) => l.clone(),
            _ => panic!(),
        };
        assert!(b(get(&files[0], "changed")));
        assert!(b(get(&files[0], "formatted")));
        let preview = s(get(&files[0], "preview"));
        assert!(preview.contains("= 30"), "preview not formatted: {preview}");
    }

    #[test]
    fn apply_format_false_leaves_raw_output() {
        let result = apply_run(&[dict(&[
            ("rule", str_vm(UGLY_HARN_CODEMOD)),
            (
                "source",
                str_vm("fn main() {\n  let timeout = cfg?.timeout ?? 30\n}\n"),
            ),
            ("language", str_vm("harn")),
            ("dry_run", VmValue::Bool(true)),
            ("format", VmValue::Bool(false)),
        ])])
        .unwrap();
        let files = match get(&result, "files") {
            VmValue::List(l) => l.clone(),
            _ => panic!(),
        };
        assert!(!b(get(&files[0], "formatted")));
        let preview = s(get(&files[0], "preview"));
        assert!(preview.contains("{timeout=30}"), "expected raw: {preview}");
    }

    #[test]
    fn diagnostics_returns_lint_findings() {
        let lint = r#"
            id = "calls"
            language = "typescript"
            message = "function call"
            [rule]
            pattern = "$FN()"
        "#;
        let result = diagnostics_run(&[dict(&[
            ("rule", str_vm(lint)),
            ("source", str_vm("foo();\nbar();\n")),
            ("language", str_vm("typescript")),
            ("path", str_vm("a.ts")),
        ])])
        .unwrap();
        assert_eq!(int(get(&result, "diagnostic_count")), 2);
        let diags = match get(&result, "diagnostics") {
            VmValue::List(l) => l.clone(),
            _ => panic!(),
        };
        assert_eq!(s(get(&diags[0], "message")), "function call");
        assert_eq!(s(get(&diags[0], "severity")), "warning");
        // No `fix` and default safety → a suggestion, not machine-applicable.
        assert_eq!(s(get(&diags[0], "applicability")), "suggestion");
        assert_eq!(int(get(&diags[1], "start_row")), 1);
        assert!(matches!(get(&diags[0], "fix"), VmValue::Nil));
    }

    #[test]
    fn report_helpers_round_trip_severity_and_safety() {
        // The string<->enum mapping used by `rules.visit` reports.
        assert_eq!(parse_severity("error"), Some(Severity::Error));
        assert_eq!(parse_severity("bogus"), None);
        assert_eq!(parse_safety("format-only"), Some(Safety::FormatOnly));
        assert_eq!(parse_safety("needs-human"), Some(Safety::NeedsHuman));
        assert_eq!(parse_safety("nope"), None);
        // `true` flags with defaults; nil/false skip; a dict carries fields.
        assert_eq!(reports_from_return(VmValue::Bool(true)).len(), 1);
        assert_eq!(reports_from_return(VmValue::Nil).len(), 0);
        assert_eq!(reports_from_return(VmValue::Bool(false)).len(), 0);
        let list = VmValue::List(Arc::new(vec![
            dict(&[("message", str_vm("a"))]),
            VmValue::Nil,
            dict(&[("message", str_vm("b"))]),
        ]));
        assert_eq!(reports_from_return(list).len(), 2);
    }

    #[test]
    fn capability_does_not_register_the_async_visitor() {
        // `rules.visit` is async, so it is installed directly on the VM in
        // `install`, not through the sync capability registry.
        let mut registry = BuiltinRegistry::new();
        RulesCapability.register_builtins(&mut registry);
        let names: Vec<_> = registry.iter().map(|b| b.name).collect();
        assert!(!names.contains(&VISIT));
        assert!(names.contains(&DIAGNOSTICS));
    }

    #[test]
    fn missing_rule_is_an_error() {
        let err = search_run(&[dict(&[
            ("source", str_vm("x")),
            ("language", str_vm("rust")),
        ])]);
        assert!(matches!(
            err,
            Err(HostlibError::MissingParameter { param: "rule", .. })
        ));
    }

    #[test]
    fn capability_registers_the_sync_builtins() {
        let mut registry = BuiltinRegistry::new();
        RulesCapability.register_builtins(&mut registry);
        let names: Vec<_> = registry.iter().map(|b| b.name).collect();
        assert_eq!(names, vec![SEARCH, REPORT, DIAGNOSTICS, APPLY, FOLD]);
    }

    #[test]
    fn lint_capability_registers_run() {
        let mut registry = BuiltinRegistry::new();
        LintCapability.register_builtins(&mut registry);
        let names: Vec<_> = registry.iter().map(|b| b.name).collect();
        assert_eq!(names, vec![LINT_RUN]);
    }

    #[test]
    fn lint_run_returns_the_linter_findings() {
        let result =
            lint_run(&[dict(&[("source", str_vm("fn f() {\n  let x = (1)\n}\n"))])]).unwrap();
        assert_eq!(s(get(&result, "result")), "ok");
        let diags = match get(&result, "diagnostics") {
            VmValue::List(l) => l.clone(),
            _ => panic!(),
        };
        assert!(
            diags
                .iter()
                .any(|d| s(get(d, "rule")) == "unnecessary-parentheses"),
            "expected unnecessary-parentheses, got {diags:?}"
        );
    }

    #[test]
    fn lint_run_applies_a_severity_override() {
        let result = lint_run(&[dict(&[
            ("source", str_vm("fn f() {\n  let x = (1)\n}\n")),
            (
                "severity",
                dict(&[("unnecessary-parentheses", str_vm("error"))]),
            ),
        ])])
        .unwrap();
        let diags = match get(&result, "diagnostics") {
            VmValue::List(l) => l.clone(),
            _ => panic!(),
        };
        let d = diags
            .iter()
            .find(|d| s(get(d, "rule")) == "unnecessary-parentheses")
            .expect("rule present");
        assert_eq!(s(get(d, "severity")), "error");
    }
}