package tools

import (
	"context"
	"encoding/json"
	"fmt"
	"sort"
	"sync"
	"time"
)

// MaxToolsPerAgent is §8's cap.
//
// Not a technical limit. Past roughly twenty tools a model's choice degrades
// faster than the extra capability helps, and an agent that needs more is an
// agent that should be a parent with subagents. Enforced at resolution so a
// spec cannot quietly exceed it.
const MaxToolsPerAgent = 20

// Registry holds every tool this service can run.
//
// One registry, not one per agent: a spec selects from it by name, and a name
// that resolves to nothing fails at publish rather than at run time. That is
// the same rule the frontend registry already applies to skills.
type Registry struct {
	mu    sync.RWMutex
	tools map[string]Tool

	// confirmations is where a pending write waits for a person. Held by the
	// registry rather than passed to Dispatch so that no call site can supply
	// its own — a store is the thing that says a write was approved, and a
	// caller able to swap it is a caller able to approve on the user's behalf.
	confirmations Store
}

// NewRegistry builds an empty registry with an in-process confirmation store.
//
// Fine for one instance and for tests. A deployment with more than one replica
// must pass a shared store — see NewRegistryWithStore and the note on
// MemoryStore.
func NewRegistry() *Registry {
	return NewRegistryWithStore(NewMemoryStore())
}

// NewRegistryWithStore builds a registry over a specific confirmation store.
func NewRegistryWithStore(s Store) *Registry {
	if s == nil {
		s = NewMemoryStore()
	}
	return &Registry{tools: make(map[string]Tool), confirmations: s}
}

// Register adds a tool, or returns why it cannot be added.
//
// Called at startup, so a malformed tool is a boot failure rather than a
// surprise on the first run that reaches it.
func (r *Registry) Register(t Tool) error {
	if t.Name == "" {
		return fmt.Errorf("tools: a tool needs a name")
	}
	if t.Handler == nil {
		return fmt.Errorf("tools: %s has no handler", t.Name)
	}
	if t.Description == "" {
		// The description is what the model reads instead of documentation. A
		// tool without one is a tool that will be called wrongly.
		return fmt.Errorf("tools: %s has no description", t.Name)
	}
	switch t.Effect {
	case EffectRead:
	case EffectWrite:
		// I4, and the reason RequiresConfirmation is not left to the author:
		// a write that forgot to set it would run unconfirmed forever, and
		// nothing downstream could tell it apart from a deliberate choice.
		t.RequiresConfirmation = true
	default:
		return fmt.Errorf("tools: %s declares effect %q, want read or write", t.Name, t.Effect)
	}
	// §8 step 3. A write with no renderer could only ever be confirmed by
	// showing someone its raw arguments, and nobody can meaningfully approve
	// a pair of uuids. Refused at registration, so it is a boot failure rather
	// than a bad dialog discovered in production.
	if t.RequiresConfirmation && t.Confirm == nil {
		return fmt.Errorf(
			"tools: %s needs confirmation but has no Confirm renderer; "+
				"a write must be able to say in plain language what it will do", t.Name)
	}
	if t.Effect == EffectRead && t.Confirm != nil && !t.RequiresConfirmation {
		// A renderer that will never run is a renderer nobody maintains, and
		// the day the tool becomes a write it will be wrong.
		return fmt.Errorf("tools: %s has a Confirm renderer but never asks for confirmation", t.Name)
	}
	if t.MaxResultBytes <= 0 {
		t.MaxResultBytes = DefaultMaxResultBytes
	}

	r.mu.Lock()
	defer r.mu.Unlock()
	if _, exists := r.tools[t.Name]; exists {
		return fmt.Errorf("tools: %s is already registered", t.Name)
	}
	r.tools[t.Name] = t
	return nil
}

// MustRegister adds a tool or panics. For startup wiring, where the alternative
// to a panic is a service that boots without a capability it claims to have.
func (r *Registry) MustRegister(t Tool) {
	if err := r.Register(t); err != nil {
		panic(err)
	}
}

// Get returns a tool by name.
func (r *Registry) Get(name string) (Tool, bool) {
	r.mu.RLock()
	defer r.mu.RUnlock()
	t, ok := r.tools[name]
	return t, ok
}

// Names lists every registered tool, sorted.
//
// Sorted because this list is rendered into the prompt, and the prompt is a
// cache prefix: an unstable order would invalidate the cache on every request
// for no reason at all.
func (r *Registry) Names() []string {
	r.mu.RLock()
	defer r.mu.RUnlock()

	names := make([]string, 0, len(r.tools))
	for name := range r.tools {
		names = append(names, name)
	}
	sort.Strings(names)
	return names
}

// Resolve turns a spec's tool names into tools.
//
// Unknown names are returned rather than dropped: §3 says a spec naming a tool
// that does not exist fails validation at publish, and this is the function
// that lets publish say which one. At run time the caller decides — dropping a
// missing tool is better than refusing the run, but only if someone is told.
func (r *Registry) Resolve(names []string) (resolved []Tool, unknown []string, err error) {
	if len(names) > MaxToolsPerAgent {
		return nil, nil, fmt.Errorf(
			"tools: %d tools requested, the cap is %d — split this agent into a parent with subagents",
			len(names), MaxToolsPerAgent)
	}

	r.mu.RLock()
	defer r.mu.RUnlock()

	seen := make(map[string]bool, len(names))
	for _, name := range names {
		if seen[name] {
			continue
		}
		seen[name] = true

		t, ok := r.tools[name]
		if !ok {
			unknown = append(unknown, name)
			continue
		}
		resolved = append(resolved, t)
	}
	return resolved, unknown, nil
}

// Dispatch runs one tool call and returns its result.
//
// This is the one place a tool is invoked, and it holds the two gates a handler
// must not be trusted to hold itself:
//
//  1. **The confirmation gate.** A write with no resolved confirmation never
//     reaches its handler. The model cannot argue its way past this because the
//     model is not consulted — the check is on the tool's declared effect and
//     the token in the context, both of which are set outside the conversation.
//  2. **The truncation cap.** Applied to what the handler returned, with the
//     flag set. A handler that forgets to bound its own output cannot flood the
//     next turn.
//
// A panicking handler is contained here too. A tool is the least trusted code
// in the runtime — it is where new integrations land — and one bad handler
// must cost its own call, not the run.
func (r *Registry) Dispatch(ctx context.Context, tc Context, name string, inputs json.RawMessage) (res Result) {
	t, ok := r.Get(name)
	if !ok {
		return Failf(CodeUnavailable, "there is no tool called %q", name)
	}

	// The gate. Everything past this line has either no effect or an approval.
	if t.RequiresConfirmation {
		if pending, blocked := r.gate(ctx, tc, t, inputs); blocked {
			return pending
		}
	}

	defer func() {
		if p := recover(); p != nil {
			res = Failf(CodeFailed, "%s failed unexpectedly", name)
		}
	}()

	res = t.Handler(ctx, tc, inputs)
	return truncate(res, t.MaxResultBytes)
}

// gate decides whether a confirmed tool may run now.
//
// Two outcomes, and the interesting thing is how few:
//
//   - **A token bound to exactly this call.** Consumed, and the handler runs.
//   - **Anything else.** Render what the call would do, issue a token bound to
//     it, and return that as a pending Result. Nothing is written. The run ends
//     at ConfirmationPending and a person is asked.
//
// "Anything else" deliberately includes a token that does not match: expired,
// already spent, issued to somebody else, or issued for a different worker on a
// different day. None of those is an error to report — they all mean the same
// thing, which is that nobody has approved THIS call, and the honest response
// to that is to describe it and ask. Returning a failure instead would leave
// the model holding an error it cannot act on, in a run whose token is fixed
// for its whole duration.
//
// A non-matching token is NOT consumed. An earlier version spent it on any
// attempt, reasoning that a mismatch was a replay or a guess. It is neither —
// the token is 24 random bytes, so guessing is not on the table — and the cost
// was real: a model that makes two write calls in one turn would destroy a
// perfectly good approval with whichever call happened to be dispatched first.
//
// Returns (result, true) when the call must not proceed.
func (r *Registry) gate(ctx context.Context, tc Context, t Tool, inputs json.RawMessage) (Result, bool) {
	b := bind(tc, t.Name, inputs)

	if tc.Confirmation != "" && r.confirmations != nil &&
		r.confirmations.Resolve(ctx, tc.Confirmation, b) {
		return Result{}, false
	}

	// Rendered under the same authorization as the write. A renderer that
	// resolved a name the caller may not see would have leaked it in the act
	// of asking permission not to.
	confirmation, denied := r.render(ctx, tc, t, inputs)
	if denied != nil {
		return *denied, true
	}

	token, err := newToken()
	if err != nil {
		return Failf(CodeUnavailable, "%s could not be prepared for confirmation", t.Name), true
	}
	confirmation.Token = token
	confirmation.Tool = t.Name
	if confirmation.ExpiresAt.IsZero() {
		confirmation.ExpiresAt = time.Now().Add(ConfirmationTTL)
	}

	if r.confirmations == nil {
		return Failf(CodeUnavailable,
			"%s cannot run: there is nowhere to record a confirmation", t.Name), true
	}
	if err := r.confirmations.Issue(ctx, b, confirmation); err != nil {
		// Failing closed. A confirmation that was shown but not recorded is one
		// that can never be honoured, and asking a person a question whose
		// answer will be discarded is worse than saying the tool is unavailable.
		return Failf(CodeUnavailable, "%s could not be prepared for confirmation", t.Name), true
	}

	return Result{Confirmation: confirmation}, true
}

// render runs a tool's Confirm renderer, containing its failures.
//
// A renderer is author-written code that runs before any approval exists, so it
// gets the same panic containment a handler does — and a renderer that returns
// nothing at all is treated as a refusal rather than as an empty dialog.
func (r *Registry) render(ctx context.Context, tc Context, t Tool, inputs json.RawMessage) (c *Confirmation, denied *Result) {
	defer func() {
		if p := recover(); p != nil {
			failed := Failf(CodeFailed, "%s could not describe what it would do", t.Name)
			c, denied = nil, &failed
		}
	}()

	if t.Confirm == nil {
		// Register refuses this, so reaching it means a Tool was built by hand
		// and bypassed registration. Fail closed rather than trust it.
		failed := Failf(CodeUnavailable, "%s cannot describe what it would do", t.Name)
		return nil, &failed
	}

	c, denied = t.Confirm(ctx, tc, inputs)
	if denied != nil {
		return nil, denied
	}
	if c == nil {
		failed := Failf(CodeUnavailable, "%s cannot describe what it would do", t.Name)
		return nil, &failed
	}
	return c, nil
}

// truncate bounds a result, marking it when it had to.
//
// The data is replaced wholesale rather than cut mid-encoding: a JSON document
// sliced at a byte offset is not a JSON document, and a model handed one will
// either fail to parse it or — worse — parse the fragment and reason about it
// as if it were the whole.
func truncate(res Result, maxBytes int) Result {
	if res.Data == nil || maxBytes <= 0 {
		return res
	}
	encoded, err := json.Marshal(res.Data)
	if err != nil {
		return Failf(CodeFailed, "the result could not be encoded")
	}
	if len(encoded) <= maxBytes {
		return res
	}
	return Result{
		Data: map[string]any{
			"note": fmt.Sprintf(
				"This result was %d bytes, over the %d-byte limit, and has been withheld rather than cut. "+
					"Ask again with a narrower filter, a period, or a smaller limit.",
				len(encoded), maxBytes),
		},
		Truncated: true,
	}
}

/* ── Redeeming an approval ──────────────────────────────────────────────── */

// Redeemed is what an approved write did.
type Redeemed struct {
	// Tool is the tool that ran, so the caller can record and report it.
	Tool string

	// Inputs are the arguments a person approved, replayed verbatim.
	Inputs json.RawMessage

	Result Result
}

// DispatchApproved performs the call a token authorised.
//
// The other half of the confirmation flow, and the one that makes it reliable.
//
// The original design had only one path: the model, on a resumed turn, makes
// the same tool call again, and the token is matched against it. That works
// when it works and fails silently when it does not — a model is not
// deterministic, and asked a second time it may reasonably seek clarification
// instead of repeating itself. Observed: a person clicked Approve, the model
// asked a follow-up question, the token was never presented, and nothing
// happened. No error. No write. Nothing to tell the user why.
//
// So this path does not ask the model anything. It takes the token, gets back
// the exact call that was described to the person, and performs THAT. What
// somebody approved and what happens are the same thing by construction rather
// than by the model's cooperation.
//
// The gate is not bypassed — this IS the gate. Redeem checks the caller, the
// tenant and the expiry and consumes the token atomically, so an approval still
// buys exactly one write and only for the person who was asked.
//
// Returns ok=false when the token authorises nothing: unknown, expired, spent,
// or somebody else's. Indistinguishably, as everywhere else.
func (r *Registry) DispatchApproved(ctx context.Context, tc Context, token string) (out Redeemed, ok bool) {
	if r.confirmations == nil || token == "" {
		return Redeemed{}, false
	}

	approved, ok := r.confirmations.Redeem(ctx, token, Principal{
		UserID: tc.Principal.UserID,
		OrgID:  tc.Principal.OrgID,
	})
	if !ok {
		return Redeemed{}, false
	}

	t, exists := r.Get(approved.Tool)
	if !exists {
		// The tool was withdrawn between the asking and the answering. The
		// token is spent either way — it has been consumed by Redeem — which is
		// correct: re-offering it would let a person approve something this
		// service can no longer describe.
		return Redeemed{
			Tool:   approved.Tool,
			Inputs: approved.Inputs,
			Result: Failf(CodeUnavailable, "%s is no longer available", approved.Tool),
		}, true
	}

	res := func() (res Result) {
		defer func() {
			if p := recover(); p != nil {
				res = Failf(CodeFailed, "%s failed unexpectedly", t.Name)
			}
		}()
		return t.Handler(ctx, tc, approved.Inputs)
	}()

	return Redeemed{
		Tool:   approved.Tool,
		Inputs: approved.Inputs,
		Result: truncate(res, t.MaxResultBytes),
	}, true
}

// ToolInfo is what a tool looks like to somebody choosing one, rather than to
// the model calling it.
//
// The InputSchema is deliberately absent: an author picks a capability, and the
// schema is the model's business. Effect is present because it is the one thing
// an author must understand — a write tool means their agent can propose
// changes, which a person will then be asked to approve.
type ToolInfo struct {
	Name                 string `json:"name"`
	Description          string `json:"description"`
	Effect               string `json:"effect"`
	RequiresConfirmation bool   `json:"requiresConfirmation"`
}

// Catalogue lists every registered tool, sorted, as choosable metadata.
//
// This exists so an agent author can be shown the real tool set rather than a
// hand-maintained copy of it in the frontend. A second list would drift, and
// the failure would be silent: an author picks a tool that no longer exists and
// gets an agent that quietly cannot do the thing they picked.
func (r *Registry) Catalogue() []ToolInfo {
	r.mu.RLock()
	defer r.mu.RUnlock()

	out := make([]ToolInfo, 0, len(r.tools))
	for _, t := range r.tools {
		out = append(out, ToolInfo{
			Name:                 t.Name,
			Description:          t.Description,
			Effect:               string(t.Effect),
			RequiresConfirmation: t.RequiresConfirmation,
		})
	}
	sort.Slice(out, func(i, j int) bool { return out[i].Name < out[j].Name })
	return out
}

// Known reports whether every name is a registered tool, returning the ones
// that are not.
//
// §3 requires an unknown tool name to fail validation at PUBLISH. Without this
// the runtime records and drops the name, so a typo becomes an agent that is
// silently missing a capability its author believes it has.
func (r *Registry) Known(names []string) (unknown []string) {
	r.mu.RLock()
	defer r.mu.RUnlock()

	for _, n := range names {
		if _, ok := r.tools[n]; !ok {
			unknown = append(unknown, n)
		}
	}
	return unknown
}