NextExpense/internal/ai/openai.go

package ai

import (
	"bytes"
	"encoding/base64"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"log"
	"net/http"
	"os"
	"strings"
	"time"
)

const openaiTimeout = 30 * time.Second

type openaiRequest struct {
	Model       string          `json:"model"`
	Messages    []openaiMessage `json:"messages"`
	Temperature float64         `json:"temperature"`
}

type openaiMessage struct {
	Role    string          `json:"role"`
	Content []openaiContent `json:"content"`
}

type openaiContent struct {
	Type     string       `json:"type"`
	Text     string       `json:"text,omitempty"`
	ImageURL *openaiImage `json:"image_url,omitempty"`
}

type openaiImage struct {
	URL string `json:"url"`
}

type openaiResponse struct {
	Choices []struct {
		Message struct {
			Content string `json:"content"`
		} `json:"message"`
	} `json:"choices"`
	Error *struct {
		Message string `json:"message"`
	} `json:"error,omitempty"`
}

type openaiProvider struct {
	apiKey  string
	model   string
	baseURL string
}

func newOpenAIProvider() openaiProvider {
	return openaiProvider{
		apiKey:  os.Getenv("OPENAI_API_KEY"),
		model:   envOrDefault("AI_MODEL", "gpt-4o-mini"),
		baseURL: strings.TrimRight(envOrDefault("AI_BASE_URL", "https://api.openai.com/v1"), "/"),
	}
}

func (p openaiProvider) ExtractReceipt(imagePath string) (*ReceiptData, error) {
	if p.apiKey == "" {
		return &ReceiptData{}, errors.New("OPENAI_API_KEY environment variable not set")
	}

	imageData, err := readFile(imagePath)
	if err != nil {
		return &ReceiptData{}, fmt.Errorf("read file: %w", err)
	}

	mimeType := detectMimeType(imageData)
	if mimeType == "" {
		mimeType = "image/jpeg"
	}

	b64Data := base64.StdEncoding.EncodeToString(imageData)
	dataURL := fmt.Sprintf("data:%s;base64,%s", mimeType, b64Data)

	payload := openaiRequest{
		Model:       p.model,
		Temperature: 0.1,
		Messages: []openaiMessage{{
			Role: "user",
			Content: []openaiContent{
				{Type: "text", Text: fmt.Sprintf("Analyze this receipt image. Extract the following fields as a strict JSON object with these exact keys: \"merchant\" (string, store or business name), \"amount\" (number, total paid), \"currency\" (string, 3-letter code like KES, USD, EUR), \"category\" (string, MUST be exactly one of: %s), \"date\" (string, YYYY-MM-DD format). Return ONLY valid JSON. No markdown, no explanation, no code fences.", categoryPrompt())},
				{Type: "image_url", ImageURL: &openaiImage{URL: dataURL}},
			},
		}},
	}

	body, err := json.Marshal(payload)
	if err != nil {
		return &ReceiptData{}, fmt.Errorf("marshal request: %w", err)
	}

	apiURL := p.baseURL + "/chat/completions"
	req, err := http.NewRequest(http.MethodPost, apiURL, bytes.NewReader(body))
	if err != nil {
		return &ReceiptData{}, fmt.Errorf("create request: %w", err)
	}
	req.Header.Set("Content-Type", "application/json")
	req.Header.Set("Authorization", "Bearer "+p.apiKey)

	client := &http.Client{Timeout: openaiTimeout}
	resp, err := client.Do(req)
	if err != nil {
		return &ReceiptData{}, fmt.Errorf("API request: %w", err)
	}
	defer resp.Body.Close()

	respBody, err := io.ReadAll(resp.Body)
	if err != nil {
		return &ReceiptData{}, fmt.Errorf("read response: %w", err)
	}

	if resp.StatusCode != http.StatusOK {
		return &ReceiptData{}, fmt.Errorf("API status %d: %s", resp.StatusCode, strings.TrimSpace(string(respBody)))
	}

	var apiResp openaiResponse
	if err := json.Unmarshal(respBody, &apiResp); err != nil {
		return &ReceiptData{}, fmt.Errorf("parse response: %w", err)
	}

	if apiResp.Error != nil {
		return &ReceiptData{}, fmt.Errorf("API error: %s", apiResp.Error.Message)
	}
	if len(apiResp.Choices) == 0 {
		return &ReceiptData{}, errors.New("no response choices from API")
	}

	contentStr := stripMarkdownFences(apiResp.Choices[0].Message.Content)
	var receipt ReceiptData
	if err := json.Unmarshal([]byte(contentStr), &receipt); err != nil {
		return &ReceiptData{}, fmt.Errorf("parse receipt JSON: %w (content: %s)", err, contentStr)
	}

	log.Printf("ExtractReceipt [openai-%s]: merchant=%q amount=%.2f %s",
		p.model, receipt.Merchant, receipt.Amount, receipt.Currency)
	return &receipt, nil
}