package llm

import (
	"bytes"
	"context"
	"encoding/json"
	"fmt"
	"io"
	"log/slog"
	"math/rand"
	"net/http"
	"strconv"
	"time"

	"textmachine/backend/internal/obs"
)

// httpllm.go is the shared OpenAI-compatible Chat Completions transport: one
// HTTP+retry implementation reused by every OpenAI-compatible adapter
// (DeepSeek, GLM, Kimi, xAI, ru-агрегаторы, llama-server). Порт vojo
// httpllm.go с правками Фазы 0 (03-implementation-notes §2 п.2):
//
//   - все таймауты/ретраи — ПАРАМЕТРЫ клиента из профиля models.yaml, не
//     константы: захардкоженный 60-секундный per-attempt дедлайн донора убивал
//     бы реальные чанки 63–278 с и трижды бесплатно их ретраил;
//   - Retry-After на 429 читается и уважается (донор капил backoff на 8 с —
//     молоток по rate-limit'ам массового прогона глав);
//   - тело ответа читается через LimitReader (у донора кап был только в
//     gemini-native адаптере).

// RetryProfile bounds one client's retry loop. Zero fields fall back to
// defaults; the profile comes from models.yaml per provider (per-role
// overrides — Фаза 1).
type RetryProfile struct {
	AttemptTimeout time.Duration // deadline for ONE HTTP attempt
	MaxAttempts    int
	BackoffBase    time.Duration // first backoff; doubles per attempt
	BackoffCap     time.Duration
}

func (p RetryProfile) withDefaults() RetryProfile {
	if p.AttemptTimeout <= 0 {
		// Полигон намерил 63–278 с на чанк локальной моделью; облачные быстрее,
		// но дефолт обязан переживать длинный чанк. Провайдер-специфика — в
		// models.yaml, это только страховочный потолок.
		p.AttemptTimeout = 300 * time.Second
	}
	if p.MaxAttempts <= 0 {
		p.MaxAttempts = 3
	}
	if p.BackoffBase <= 0 {
		p.BackoffBase = 500 * time.Millisecond
	}
	if p.BackoffCap <= 0 {
		p.BackoffCap = 30 * time.Second
	}
	return p
}

// maxResponseBytes caps one completion body read. A translated chapter chunk
// is ~10–50 KiB; 16 MiB leaves two orders of magnitude of headroom while
// keeping a misbehaving endpoint from exhausting memory.
const maxResponseBytes = 16 << 20

// openAIClient performs OpenAI-compatible /chat/completions calls with retry.
type openAIClient struct {
	name    string // provider label for logs/errors ("deepseek", "glm", "local")
	base    string
	key     string
	http    *http.Client
	profile RetryProfile
	headers map[string]string // extra static headers (provider-specific), may be nil
	log     *slog.Logger
}

// newOpenAIClient builds the shared transport. httpc may be nil (default
// client); the local provider passes an explicit no-proxy client (прокси-грабли
// стенда: env-прокси перехватывает не-loopback локальные адреса вроде 172.x).
func newOpenAIClient(name, base, key string, profile RetryProfile, headers map[string]string, httpc *http.Client, logger *slog.Logger) *openAIClient {
	if httpc == nil {
		httpc = &http.Client{}
	}
	return &openAIClient{
		name:    name,
		base:    base,
		key:     key,
		http:    httpc,
		profile: profile.withDefaults(),
		headers: headers,
		log:     logger,
	}
}

// --- OpenAI-compatible wire types -------------------------------------------------

type openAIMessage struct {
	Role    string `json:"role"`
	Content string `json:"content"`
}

type openAIRequest struct {
	Model       string          `json:"model"`
	Messages    []openAIMessage `json:"messages"`
	MaxTokens   int             `json:"max_tokens"`
	Temperature float64         `json:"temperature,omitempty"`
	Stream      bool            `json:"stream"`
	// Optional; omitempty keeps the plain body minimal.
	ReasoningEffort string `json:"reasoning_effort,omitempty"`
	ResponseFormat  any    `json:"response_format,omitempty"`
	// extra carries per-model provider-specific fields from models.yaml
	// (например GLM {"thinking":{"type":"disabled"}}). Merged into the JSON
	// body at marshal time — the neutral request stays vendor-free.
	extra map[string]any
}

// MarshalJSON merges extra into the standard body. Standard fields win on
// key collision (extra is config-supplied tuning, not an override channel).
func (r openAIRequest) MarshalJSON() ([]byte, error) {
	type plain openAIRequest
	base, err := json.Marshal(plain(r))
	if err != nil {
		return nil, err
	}
	if len(r.extra) == 0 {
		return base, nil
	}
	var m map[string]any
	if err := json.Unmarshal(base, &m); err != nil {
		return nil, err
	}
	for k, v := range r.extra {
		if _, exists := m[k]; !exists {
			m[k] = v
		}
	}
	return json.Marshal(m)
}

type openAIUsage struct {
	PromptTokens        int `json:"prompt_tokens"`
	CompletionTokens    int `json:"completion_tokens"`
	PromptTokensDetails struct {
		CachedTokens int `json:"cached_tokens"`
	} `json:"prompt_tokens_details"`
	// xAI reports reasoning tokens here SEPARATELY from completion_tokens and
	// bills them at the output rate; OpenAI-spec providers count them inside
	// completion_tokens. The adapter decides which semantics apply.
	CompletionTokensDetails struct {
		ReasoningTokens int `json:"reasoning_tokens"`
	} `json:"completion_tokens_details"`
	// DeepSeek historically reports cache usage in its own top-level fields
	// instead of prompt_tokens_details. Parse both so the Phase-0 cache
	// experiment can't show a false zero (03-implementation-notes §3.5).
	PromptCacheHitTokens  int `json:"prompt_cache_hit_tokens"`
	PromptCacheMissTokens int `json:"prompt_cache_miss_tokens"`
}

// cacheRead returns the cache-hit token count whichever field the provider
// used.
func (u openAIUsage) cacheRead() int {
	if u.PromptTokensDetails.CachedTokens > 0 {
		return u.PromptTokensDetails.CachedTokens
	}
	return u.PromptCacheHitTokens
}

type openAIResponse struct {
	ID      string `json:"id"`
	Model   string `json:"model"`
	Choices []struct {
		Message struct {
			Content string `json:"content"`
		} `json:"message"`
		FinishReason string `json:"finish_reason"`
	} `json:"choices"`
	Usage openAIUsage `json:"usage"`
}

func (r *openAIResponse) Text() string {
	if len(r.Choices) == 0 {
		return ""
	}
	return r.Choices[0].Message.Content
}

func (r *openAIResponse) FinishReason() string {
	if len(r.Choices) == 0 {
		return ""
	}
	// OpenAI-compat wire values map 1:1 onto the neutral constants.
	return r.Choices[0].FinishReason
}

// complete calls Chat Completions with retry on transient failures (429 / 5xx /
// network, exponential backoff + jitter, Retry-After honoured). Non-retryable
// 4xx fail immediately. On exhaustion the caller releases the reservation, so
// a transient failure is never silently swallowed.
func (c *openAIClient) complete(ctx context.Context, reqBody openAIRequest) (*openAIResponse, error) {
	payload, err := json.Marshal(reqBody)
	if err != nil {
		return nil, err
	}

	var lastErr error
	for attempt := 0; attempt < c.profile.MaxAttempts; attempt++ {
		if attempt > 0 {
			backoff := c.profile.BackoffBase << uint(attempt-1)
			if backoff > c.profile.BackoffCap {
				backoff = c.profile.BackoffCap
			}
			// A server-provided Retry-After overrides our schedule: the provider
			// knows its rate-limit window better than our exponential guess.
			if ra := retryAfterOf(lastErr); ra > 0 {
				backoff = ra
			}
			backoff += time.Duration(rand.Intn(250)) * time.Millisecond
			select {
			case <-ctx.Done():
				return nil, ctx.Err()
			case <-time.After(backoff):
			}
		}

		resp, retryable, err := c.attempt(ctx, payload)
		if err == nil {
			return resp, nil
		}
		lastErr = err
		if ctx.Err() != nil {
			return nil, ctx.Err()
		}
		if !retryable {
			return nil, err
		}
		if c.log != nil {
			c.log.WarnContext(ctx, c.name+" attempt failed, will retry", "attempt", attempt+1, "max", c.profile.MaxAttempts, "err", err)
		}
	}
	return nil, fmt.Errorf("%s: exhausted %d attempts: %w", c.name, c.profile.MaxAttempts, lastErr)
}

// attempt performs one HTTP call. Returns retryable=true for 429/5xx and
// network errors, false for other non-2xx (terminal 4xx). The per-attempt
// deadline bounds a single hung connection; the overall per-request deadline
// (set by the caller via ctx) bounds the whole retry loop.
func (c *openAIClient) attempt(ctx context.Context, payload []byte) (*openAIResponse, bool, error) {
	attemptCtx, cancel := context.WithTimeout(ctx, c.profile.AttemptTimeout)
	defer cancel()

	req, err := http.NewRequestWithContext(attemptCtx, http.MethodPost, c.base+"/chat/completions", bytes.NewReader(payload))
	if err != nil {
		return nil, false, err
	}
	req.Header.Set("Content-Type", "application/json")
	// A local backend (ollama / llama-server) usually runs without auth; an
	// empty key means "no Authorization header", not "Bearer " with an empty token.
	if c.key != "" {
		req.Header.Set("Authorization", "Bearer "+c.key)
	}
	for k, v := range c.headers {
		req.Header.Set(k, v)
	}

	resp, err := c.http.Do(req)
	if err != nil {
		// Network error / timeout — retryable (unless the parent ctx is done).
		return nil, ctx.Err() == nil, err
	}
	defer resp.Body.Close()
	data, _ := io.ReadAll(io.LimitReader(resp.Body, maxResponseBytes))

	obs.LogLLMExchange(ctx, c.log, c.name, payload, resp.StatusCode, data)

	if resp.StatusCode == http.StatusTooManyRequests || resp.StatusCode >= 500 {
		return nil, true, &HTTPStatusError{Provider: c.name, Status: resp.StatusCode, Body: snippet(data), RetryAfter: parseRetryAfter(resp.Header.Get("Retry-After"))}
	}
	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
		return nil, false, &HTTPStatusError{Provider: c.name, Status: resp.StatusCode, Body: snippet(data)}
	}

	var out openAIResponse
	if err := json.Unmarshal(data, &out); err != nil {
		return nil, false, fmt.Errorf("%s decode: %w", c.name, err)
	}
	// A 2xx is a billed call even when the model returns empty content
	// (content filter, finish_reason=length with no text). Return it as a
	// success so the caller settles the real cost via the ledger instead of
	// releasing the reservation and losing the spend — which would let empty
	// replies bypass the book/day ceilings. Gates deal with the empty text.
	return &out, false, nil
}

// HTTPStatusError is a non-2xx completion failure, carrying the status code as
// a typed field so callers (the failover decorator) can classify it
// structurally — a terminal 4xx is a config/request error that must fail loud,
// not be masked by a fallback — instead of parsing the message.
type HTTPStatusError struct {
	Provider   string
	Status     int
	Body       string
	RetryAfter time.Duration // from the Retry-After header; 0 = absent
}

func (e *HTTPStatusError) Error() string {
	return fmt.Sprintf("%s http %d: %s", e.Provider, e.Status, e.Body)
}

func retryAfterOf(err error) time.Duration {
	if se, ok := err.(*HTTPStatusError); ok {
		return se.RetryAfter
	}
	return 0
}

func parseRetryAfter(v string) time.Duration {
	if v == "" {
		return 0
	}
	if secs, err := strconv.Atoi(v); err == nil && secs > 0 {
		return time.Duration(secs) * time.Second
	}
	if t, err := http.ParseTime(v); err == nil {
		if d := time.Until(t); d > 0 {
			return d
		}
	}
	return 0
}

func snippet(b []byte) string {
	const max = 300
	if len(b) > max {
		return string(b[:max]) + "…"
	}
	return string(b)
}