package gateway

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"strings"
	"time"

	"github.com/anthropics/anthropic-sdk-go"
	"github.com/anthropics/anthropic-sdk-go/option"
)

// Routing is how a tier becomes a model and an effort level.
//
// The model per tier is a deployment knob — a tenant on a different contract,
// or a deployment pinning a version through an incident, changes it without a
// spec edit. The *effort* per tier is not: "fast" and "deep" mean something
// specific about how much work an answer is worth, and letting a deployment
// redefine that would make the same spec behave differently in two places
// while claiming the same tier.
type Routing struct {
	Model  string
	Effort anthropic.OutputConfigEffort
}

// Config is the gateway's whole configuration surface.
//
// Built once at startup from the environment and passed in frozen, per §10.
// Nothing in this package reads the environment itself.
type Config struct {
	APIKey string

	Fast     Routing
	Balanced Routing
	Deep     Routing

	// MaxOutputTokens applies when a request does not set its own.
	MaxOutputTokens int64
}

// AnthropicGateway calls the Claude API.
type AnthropicGateway struct {
	client anthropic.Client
	cfg    Config
}

// Compile-time proof that this satisfies the boundary.
var _ Gateway = (*AnthropicGateway)(nil)

// NewAnthropic builds a gateway over the Claude API.
//
// A missing key is not an error here. The service has to boot without model
// credentials — every endpoint that is not an agent run still works, and a
// developer running migrations should not need a key to do it. The failure
// surfaces at the first Complete, as a structured NotConfigured that the
// runtime can end a run with, rather than as a panic at startup.
func NewAnthropic(cfg Config) *AnthropicGateway {
	opts := []option.RequestOption{}
	if cfg.APIKey != "" {
		opts = append(opts, option.WithAPIKey(cfg.APIKey))
	}
	return &AnthropicGateway{client: anthropic.NewClient(opts...), cfg: cfg}
}

// routing resolves a tier. An unknown tier has already been normalised by
// ParseTier, so the default arm is reached only by a zero value.
func (g *AnthropicGateway) routing(t Tier) Routing {
	switch t {
	case TierFast:
		return g.cfg.Fast
	case TierDeep:
		return g.cfg.Deep
	default:
		return g.cfg.Balanced
	}
}

// Complete sends one request and reports one result.
// MaxAttempts is how many times a transient failure is retried.
//
// Three total, not three retries. Past that the problem is not transient and a
// fourth call is just spending money on the same answer.
const MaxAttempts = 3

// retryBackoff is the pause before each retry.
//
// Short, and deliberately so: this sits inside a run that already has a
// wall-clock deadline, and a backoff long enough to be polite to the API is
// long enough to spend the caller's whole budget waiting. A run that cannot
// afford the wait dies on its deadline instead, which is the correct failure.
var retryBackoff = []time.Duration{400 * time.Millisecond, 1200 * time.Millisecond}

// Complete calls the model, retrying failures that are worth retrying.
//
// THE RETRY IS NOT DEFENSIVE POLISH. Error.Retryable() has existed since this
// package was written and had ZERO callers — the classification was built and
// never used, so a 529 "overloaded" killed a run that would have succeeded four
// hundred milliseconds later. Found by a real overload during live testing,
// where it presented as "the agent could not finish" with nothing to act on.
//
// Only genuinely transient failures qualify: rate limits, timeouts, and 5xx.
// A 400 is a malformed request and will be malformed again; a 401 is a bad
// credential and retrying it three times just gets refused three times.
//
// The run's context governs. A retry that would outlive the caller's deadline
// does not happen — the deadline belongs to the run, not to this function, and
// waiting past it would turn a bounded run into an unbounded one.
func (g *AnthropicGateway) Complete(ctx context.Context, req Request) (*Response, error) {
	var last error
	for attempt := 0; attempt < MaxAttempts; attempt++ {
		if attempt > 0 {
			pause := retryBackoff[min(attempt-1, len(retryBackoff)-1)]
			select {
			case <-time.After(pause):
			case <-ctx.Done():
				// Out of time. The ORIGINAL failure is returned rather than the
				// context error: "the model was overloaded" is what an operator
				// needs to see, and "context deadline exceeded" would hide it.
				return nil, last
			}
		}

		resp, err := g.complete(ctx, req)
		if err == nil {
			return resp, nil
		}
		last = err

		var gwErr *Error
		if !errors.As(err, &gwErr) || !gwErr.Retryable() {
			return resp, err
		}
	}
	return nil, last
}

// complete is one attempt.
func (g *AnthropicGateway) complete(ctx context.Context, req Request) (*Response, error) {
	if g.cfg.APIKey == "" {
		return nil, &Error{
			Code:    CodeNotConfigured,
			Message: "no model credentials are configured for this deployment",
		}
	}
	if err := req.Validate(); err != nil {
		return nil, err
	}

	params, err := g.params(req)
	if err != nil {
		return nil, err
	}

	msg, err := g.client.Messages.New(ctx, params)
	if err != nil {
		return nil, translate(err)
	}
	return g.decode(msg, req)
}

// params builds the request both paths send.
//
// Extracted so the streaming and non-streaming calls cannot drift. They send
// the same model, the same thinking config, the same cache breakpoint and the
// same tools — an answer that differs depending on whether it was streamed
// would be the worst kind of bug to chase, because the transport is the last
// place anybody looks.
func (g *AnthropicGateway) params(req Request) (anthropic.MessageNewParams, error) {
	route := g.routing(req.Tier)

	maxTokens := req.MaxOutputTokens
	if maxTokens <= 0 {
		maxTokens = g.cfg.MaxOutputTokens
	}

	messages, err := encodeMessages(req.Messages)
	if err != nil {
		return anthropic.MessageNewParams{}, err
	}

	params := anthropic.MessageNewParams{
		Model:     anthropic.Model(route.Model),
		MaxTokens: maxTokens,
		Messages:  messages,
		// Adaptive thinking on every tier: the model decides how much to think,
		// and effort sets the ceiling on that. A fixed token budget for
		// reasoning is the deprecated shape and is rejected outright by the
		// current models.
		Thinking: anthropic.ThinkingConfigParamUnion{
			OfAdaptive: &anthropic.ThinkingConfigAdaptiveParam{},
		},
		OutputConfig: anthropic.OutputConfigParam{Effort: route.Effort},
	}

	if len(req.Tools) > 0 {
		params.Tools = encodeTools(req.Tools)
	}

	if s := strings.TrimSpace(req.System); s != "" {
		// One cached block. The system prompt is the stable prefix of every
		// turn in a run, and the render order is tools → system → messages, so
		// a breakpoint here is the one that survives the conversation growing.
		params.System = []anthropic.TextBlockParam{{
			Text:         s,
			CacheControl: anthropic.NewCacheControlEphemeralParam(),
		}}
	}

	return params, nil
}

// decode turns a finished message into a Response.
//
// Shared by both paths for the same reason params() is: a streamed message and
// a non-streamed one are the same object by the time they get here, and reading
// them differently would make streaming a second implementation of the answer.
func (g *AnthropicGateway) decode(msg *anthropic.Message, req Request) (*Response, error) {
	route := g.routing(req.Tier)

	usage := Usage{
		InputTokens:         msg.Usage.InputTokens,
		OutputTokens:        msg.Usage.OutputTokens,
		CacheReadTokens:     msg.Usage.CacheReadInputTokens,
		CacheCreationTokens: msg.Usage.CacheCreationInputTokens,
	}

	// A refusal arrives as a successful HTTP response, so it has to be checked
	// before the content is read. It is still billed, and the usage is carried
	// on the error so the run's budget is charged for a turn that produced no
	// text — a refusal that cost nothing on the ledger is a refusal the loop
	// would happily repeat.
	if msg.StopReason == anthropic.StopReasonRefusal {
		return &Response{
			StopReason: string(msg.StopReason),
			Usage:      usage,
			Model:      route.Model,
			Tier:       req.Tier,
		}, &Error{
			Code:     CodeRefused,
			Message:  "the model declined this request",
			Category: string(msg.StopDetails.Category),
		}
	}

	var (
		text  strings.Builder
		calls []ToolCall
	)
	for _, block := range msg.Content {
		switch b := block.AsAny().(type) {
		case anthropic.TextBlock:
			text.WriteString(b.Text)
		case anthropic.ToolUseBlock:
			// The raw JSON, not a parsed value: current models vary their
			// string escaping inside tool inputs, so this is handed to the
			// handler's own decoder rather than matched on as a string here.
			calls = append(calls, ToolCall{
				ID:    b.ID,
				Name:  b.Name,
				Input: json.RawMessage(b.JSON.Input.Raw()),
			})
		}
	}

	return &Response{
		Text:       text.String(),
		ToolCalls:  calls,
		StopReason: string(msg.StopReason),
		Usage:      usage,
		Model:      route.Model,
		Tier:       req.Tier,
	}, nil
}

// encodeTools renders the tool definitions for the wire.
func encodeTools(defs []ToolDef) []anthropic.ToolUnionParam {
	out := make([]anthropic.ToolUnionParam, 0, len(defs))
	for _, d := range defs {
		schema := anthropic.ToolInputSchemaParam{}
		if props, ok := d.InputSchema["properties"].(map[string]any); ok {
			schema.Properties = props
		}
		if req, ok := d.InputSchema["required"].([]string); ok {
			schema.Required = req
		}
		tool := anthropic.ToolParam{
			Name:        d.Name,
			Description: anthropic.String(d.Description),
			InputSchema: schema,
		}
		out = append(out, anthropic.ToolUnionParam{OfTool: &tool})
	}
	return out
}

// encodeMessages renders a conversation for the wire.
//
// Tool results are variadic within ONE user message. Splitting them across
// several messages is accepted by the API and quietly teaches the model to stop
// making parallel calls — a performance regression with no error to trace it
// to, so the grouping is done here rather than left to callers.
func encodeMessages(msgs []Message) ([]anthropic.MessageParam, error) {
	out := make([]anthropic.MessageParam, 0, len(msgs))

	for i, m := range msgs {
		var blocks []anthropic.ContentBlockParamUnion

		if s := strings.TrimSpace(m.Text); s != "" {
			blocks = append(blocks, anthropic.NewTextBlock(m.Text))
		}
		for _, c := range m.ToolCalls {
			var input any
			if len(c.Input) > 0 {
				if err := json.Unmarshal(c.Input, &input); err != nil {
					return nil, &Error{
						Code:    CodeInvalidRequest,
						Message: fmt.Sprintf("messages[%d]: tool call %s carries invalid JSON", i, c.Name),
					}
				}
			}
			blocks = append(blocks, anthropic.NewToolUseBlock(c.ID, input, c.Name))
		}
		for _, r := range m.ToolResults {
			blocks = append(blocks, anthropic.NewToolResultBlock(r.CallID, r.Content, r.IsError))
		}

		if len(blocks) == 0 {
			continue
		}
		if m.Role == RoleAssistant {
			out = append(out, anthropic.NewAssistantMessage(blocks...))
			continue
		}
		out = append(out, anthropic.NewUserMessage(blocks...))
	}
	return out, nil
}

// translate turns an SDK error into one the runtime can branch on.
//
// A single broad class would lose the distinction the loop actually needs:
// whether sending the same request again could work. So the status is read and
// mapped, and anything unrecognised stays CodeUpstream with its status intact
// rather than being flattened into a generic failure.
func translate(err error) error {
	if errors.Is(err, context.DeadlineExceeded) || errors.Is(err, context.Canceled) {
		return &Error{Code: CodeTimeout, Message: "the model call did not complete in time", Cause: err}
	}

	var apierr *anthropic.Error
	if !errors.As(err, &apierr) {
		return &Error{Code: CodeUpstream, Message: "the model call failed", Cause: err}
	}

	switch apierr.StatusCode {
	case 400:
		return &Error{Code: CodeInvalidRequest, Message: "the model rejected the request", Status: 400, Cause: err}
	case 401, 403:
		return &Error{Code: CodeUnauthorized, Message: "the model credentials were refused", Status: apierr.StatusCode, Cause: err}
	case 408:
		return &Error{Code: CodeTimeout, Message: "the model call timed out", Status: 408, Cause: err}
	case 429:
		return &Error{Code: CodeRateLimited, Message: "the model is rate limiting this deployment", Status: 429, Cause: err}
	case 529:
		// Anthropic's "overloaded" — the service is up and temporarily out of
		// capacity. Named separately from the 500s because it is the one that
		// actually happens, and because a run dying on it is a run that would
		// have succeeded a second later.
		return &Error{Code: CodeUpstream, Message: "the model is temporarily overloaded",
			Status: 529, Cause: err}
	default:
		// The status is IN the message, not only in the field. It cost an hour
		// of debugging to learn that "the model call failed" was a 529 rather
		// than a malformed tool schema, and the trajectory only records the
		// message.
		return &Error{
			Code:    CodeUpstream,
			Message: fmt.Sprintf("the model call failed (http %d)", apierr.StatusCode),
			Status:  apierr.StatusCode, Cause: err,
		}
	}
}

/* ── Streaming ──────────────────────────────────────────────────────────── */

// Stream is Complete, with the assistant's text delivered as it arrives.
//
// §6: "Stream partial assistant text as it arrives; buffer tool calls until
// complete." Both halves of that matter and they pull in opposite directions.
//
// TEXT IS STREAMED because a fifteen-second wait with nothing on screen reads
// as broken. The reader wants the first sentence while the rest is still being
// written, and that is the whole difference between a product and a spinner.
//
// TOOL CALLS ARE NOT. A tool call arrives as JSON assembled character by
// character across many events, and a half-built argument object is not a
// smaller version of the finished one — it is a different object, usually an
// invalid one. Dispatching on a partial call would run a tool with arguments
// the model had not finished choosing. So the accumulated message is decoded
// only once the stream closes, by exactly the same code the non-streaming path
// uses.
//
// onDelta is called from this goroutine, in order, and must not block for long
// — it is on the path between the model and the reader.
func (g *AnthropicGateway) Stream(ctx context.Context, req Request, onDelta func(string)) (*Response, error) {
	if g.cfg.APIKey == "" {
		return nil, &Error{
			Code:    CodeNotConfigured,
			Message: "no model credentials are configured for this deployment",
		}
	}
	if err := req.Validate(); err != nil {
		return nil, err
	}

	params, err := g.params(req)
	if err != nil {
		return nil, err
	}

	stream := g.client.Messages.NewStreaming(ctx, params)
	defer stream.Close()

	var msg anthropic.Message
	for stream.Next() {
		event := stream.Current()
		if err := msg.Accumulate(event); err != nil {
			return nil, &Error{
				Code:    CodeUpstream,
				Message: "the streamed response could not be assembled",
				Cause:   err,
			}
		}

		// Text only. A thinking delta is the model's private reasoning and is
		// not the answer; a tool-input delta is a fragment of JSON. Neither is
		// something to put in front of a reader.
		if event.Type == "content_block_delta" && event.Delta.Type == "text_delta" {
			if d := event.Delta.Text; d != "" && onDelta != nil {
				onDelta(d)
			}
		}
	}
	if err := stream.Err(); err != nil {
		return nil, translate(err)
	}

	return g.decode(&msg, req)
}

// StreamComplete runs a request through whichever path the gateway supports.
//
// A gateway that cannot stream is not a broken gateway — every fake in the test
// suite is one, and so is any future provider without a streaming API. Falling
// back to Complete and delivering the finished text as a single delta keeps the
// caller's code identical either way, which is what stops streaming from
// becoming a second code path through the loop.
func StreamComplete(ctx context.Context, gw Gateway, req Request, onDelta func(string)) (*Response, error) {
	// Normalised once, here, so no implementation has to guard it. A caller
	// that does not want deltas passes nil — every eval and every test does —
	// and an implementation that took that literally would panic on the first
	// fragment. Making each Streamer remember the check is how one of them
	// eventually forgets.
	if onDelta == nil {
		onDelta = func(string) {}
	}
	if s, ok := gw.(Streamer); ok {
		return s.Stream(ctx, req, onDelta)
	}
	resp, err := gw.Complete(ctx, req)
	if err == nil && resp != nil && resp.Text != "" && onDelta != nil {
		onDelta(resp.Text)
	}
	return resp, err
}