// Package anthropic is a minimal Anthropic Messages API HTTP+SSE
// client. Works against the real Anthropic API and any Anthropic-
// compatible endpoint (MiniMax exposes one at
// https://api.minimax.io/anthropic).
//
// The MiniMax provider plugin's only consumer; if other providers
// land later they can reuse this package.
package anthropic

import (
	"bufio"
	"bytes"
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"net/http"
	"strings"
	"time"
)

// DefaultAPIVersion matches what Anthropic + Anthropic-compat servers expect.
const DefaultAPIVersion = "2023-06-01"

// Client is a thin HTTP wrapper. Stateless across calls.
type Client struct {
	BaseURL    string // e.g. "https://api.kimi.com/coding" or "https://api.minimax.io/anthropic"
	APIKey     string
	APIVersion string // default "2023-06-01"
	// UserAgent, if set, replaces Go's default UA header. Kimi's
	// coding endpoint expects "claude-code/0.1.0"; other backends
	// (MiniMax etc.) are flexible.
	UserAgent string
	// ExtraHeaders are sent verbatim on every request.
	ExtraHeaders map[string]string
	HTTP         *http.Client
}

// New constructs a Client with sensible defaults.
func New(baseURL, apiKey string) *Client {
	return &Client{
		BaseURL:    strings.TrimRight(baseURL, "/"),
		APIKey:     apiKey,
		APIVersion: DefaultAPIVersion,
		// 5 min ceiling per turn — long enough for a slow reasoning
		// response, short enough that wedged calls don't hang forever.
		HTTP: &http.Client{Timeout: 5 * time.Minute},
	}
}

// MessagesRequest is the wire shape POSTed to /v1/messages.
type MessagesRequest struct {
	Model         string           `json:"model"`
	System        any              `json:"system,omitempty"` // string OR []ContentBlock
	Messages      []Message        `json:"messages"`
	MaxTokens     int              `json:"max_tokens"`
	Temperature   *float64         `json:"temperature,omitempty"`
	StopSequences []string         `json:"stop_sequences,omitempty"`
	Tools         []ToolDef        `json:"tools,omitempty"`
	ToolChoice    *ToolChoice      `json:"tool_choice,omitempty"`
	Thinking      *ThinkingConfig  `json:"thinking,omitempty"`
	Stream        bool             `json:"stream"`
	Metadata      *RequestMetadata `json:"metadata,omitempty"`
}

// Message is one entry in messages[].
type Message struct {
	Role    string         `json:"role"` // "user" | "assistant"
	Content []ContentBlock `json:"content"`
}

// ContentBlock covers text / tool_use / tool_result / thinking. We
// keep the discriminated union loose (map[string]any) on the request
// side so callers can pass whatever shape the API accepts; on the
// response side the SSE parser produces typed deltas.
type ContentBlock = map[string]any

// ToolDef is one tool entry the model can call.
type ToolDef struct {
	Name        string          `json:"name"`
	Description string          `json:"description,omitempty"`
	InputSchema json.RawMessage `json:"input_schema"`
}

// ToolChoice constrains how the model picks tools.
type ToolChoice struct {
	Type string `json:"type"` // "auto" | "any" | "tool" | "none"
	Name string `json:"name,omitempty"`
}

// ThinkingConfig enables extended-thinking on supported models.
type ThinkingConfig struct {
	Type         string `json:"type"` // "enabled" | "disabled"
	BudgetTokens int    `json:"budget_tokens,omitempty"`
}

// RequestMetadata is optional user-supplied tracing.
type RequestMetadata struct {
	UserID string `json:"user_id,omitempty"`
}

// ---- Streaming SSE response ----

// Event is one parsed SSE event from the streaming endpoint.
// Direct mirror of Anthropic's event shapes — caller's responsibility
// to translate into Plexum's canonical.TurnEvent.
type Event struct {
	Type string `json:"type"`
	// message_start: complete initial message envelope
	Message *MessageEnvelope `json:"message,omitempty"`
	// content_block_start: block at .Index
	Index        int           `json:"index,omitempty"`
	ContentBlock *BlockStart   `json:"content_block,omitempty"`
	// content_block_delta: text_delta / thinking_delta / input_json_delta / signature_delta
	Delta *BlockDelta `json:"delta,omitempty"`
	// message_delta: usage update + stop_reason
	Usage *Usage `json:"usage,omitempty"`
	// error event payload
	Error *ErrorBody `json:"error,omitempty"`
}

// MessageEnvelope is the top-level message info in message_start.
type MessageEnvelope struct {
	ID         string `json:"id"`
	Type       string `json:"type"`
	Role       string `json:"role"`
	Model      string `json:"model"`
	StopReason string `json:"stop_reason"`
	Usage      *Usage `json:"usage,omitempty"`
}

// BlockStart describes a block that just started. Type is "text",
// "thinking", or "tool_use". For tool_use the ID + Name are set.
type BlockStart struct {
	Type     string          `json:"type"`
	Text     string          `json:"text,omitempty"`
	Thinking string          `json:"thinking,omitempty"`
	ID       string          `json:"id,omitempty"`
	Name     string          `json:"name,omitempty"`
	Input    json.RawMessage `json:"input,omitempty"`
}

// BlockDelta carries incremental changes. Exactly one of the *_delta
// fields is meaningful per delta.
type BlockDelta struct {
	Type         string `json:"type"` // "text_delta" | "thinking_delta" | "input_json_delta" | "signature_delta"
	Text         string `json:"text,omitempty"`
	Thinking     string `json:"thinking,omitempty"`
	PartialJSON  string `json:"partial_json,omitempty"`
	Signature    string `json:"signature,omitempty"`
	// On message_delta event, the same Delta carries stop_reason.
	StopReason   string `json:"stop_reason,omitempty"`
	StopSequence string `json:"stop_sequence,omitempty"`
}

// Usage is the running input/output token counts.
type Usage struct {
	InputTokens              int `json:"input_tokens,omitempty"`
	OutputTokens             int `json:"output_tokens,omitempty"`
	CacheReadInputTokens     int `json:"cache_read_input_tokens,omitempty"`
	CacheCreationInputTokens int `json:"cache_creation_input_tokens,omitempty"`
}

// ErrorBody is the Anthropic error envelope.
type ErrorBody struct {
	Type    string `json:"type"`
	Message string `json:"message"`
}

// StreamMessages opens a streaming POST /v1/messages call and sends
// parsed events down the returned channel. Channel closes when SSE
// stream terminates (message_stop, server EOF, ctx cancel, or error).
// On HTTP-level error (non-2xx, network failure), an EventType="error"
// event lands on the channel with the wrapped error before close.
func (c *Client) StreamMessages(ctx context.Context, req MessagesRequest) (<-chan Event, error) {
	req.Stream = true
	body, err := json.Marshal(req)
	if err != nil {
		return nil, fmt.Errorf("anthropic: marshal: %w", err)
	}
	httpReq, err := http.NewRequestWithContext(ctx, http.MethodPost,
		c.BaseURL+"/v1/messages", bytes.NewReader(body))
	if err != nil {
		return nil, err
	}
	httpReq.Header.Set("content-type", "application/json")
	httpReq.Header.Set("authorization", "Bearer "+c.APIKey)
	httpReq.Header.Set("anthropic-version", c.APIVersion)
	httpReq.Header.Set("accept", "text/event-stream")
	if c.UserAgent != "" {
		httpReq.Header.Set("User-Agent", c.UserAgent)
	}
	for k, v := range c.ExtraHeaders {
		httpReq.Header.Set(k, v)
	}

	resp, err := c.HTTP.Do(httpReq)
	if err != nil {
		return nil, fmt.Errorf("anthropic: %w", err)
	}
	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
		raw, _ := io.ReadAll(resp.Body)
		resp.Body.Close()
		return nil, fmt.Errorf("anthropic: %s -> %d: %s", c.BaseURL, resp.StatusCode, string(raw))
	}

	ch := make(chan Event, 32)
	go func() {
		defer close(ch)
		defer resp.Body.Close()
		if err := parseSSE(ctx, resp.Body, ch); err != nil && !errors.Is(err, io.EOF) {
			select {
			case ch <- Event{Type: "error", Error: &ErrorBody{Type: "stream_error", Message: err.Error()}}:
			default:
			}
		}
	}()
	return ch, nil
}

// parseSSE reads SSE frames (`event:` + `data:` lines, blank-line
// separator) and dispatches Event values onto ch. Returns nil on
// normal EOF.
func parseSSE(ctx context.Context, r io.Reader, ch chan<- Event) error {
	scanner := bufio.NewScanner(r)
	scanner.Buffer(make([]byte, 64*1024), 1<<20) // 1 MiB line cap

	var dataBuf bytes.Buffer
	for scanner.Scan() {
		select {
		case <-ctx.Done():
			return ctx.Err()
		default:
		}
		line := scanner.Bytes()
		if len(line) == 0 {
			// End-of-frame: dispatch buffered data, reset.
			if dataBuf.Len() == 0 {
				continue
			}
			var ev Event
			if err := json.Unmarshal(dataBuf.Bytes(), &ev); err == nil {
				select {
				case ch <- ev:
				case <-ctx.Done():
					return ctx.Err()
				}
			}
			dataBuf.Reset()
			continue
		}
		if bytes.HasPrefix(line, []byte("data:")) {
			payload := bytes.TrimSpace(line[5:])
			if dataBuf.Len() > 0 {
				dataBuf.WriteByte('\n')
			}
			dataBuf.Write(payload)
		}
		// `event:` lines describe the event name. JSON `data:` payload
		// also carries `type` field — we use that. Ignore event: lines.
	}
	if err := scanner.Err(); err != nil {
		return fmt.Errorf("sse scan: %w", err)
	}
	// Flush any trailing data without final blank line.
	if dataBuf.Len() > 0 {
		var ev Event
		if err := json.Unmarshal(dataBuf.Bytes(), &ev); err == nil {
			select {
			case ch <- ev:
			case <-ctx.Done():
				return ctx.Err()
			}
		}
	}
	return nil
}