ROLAC/API/ROLAC.API/Services/Ai/GeminiExpenseAiService.cs

using System.Net.Http.Json;
using System.Text.Json;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Options;
using ROLAC.API.Data;
using ROLAC.API.DTOs.Expense;

namespace ROLAC.API.Services.Ai;

/// <summary>
/// Calls the Google Gemini <c>generateContent</c> API to translate an expense description and
/// classify it into the church's existing expense category catalog (大項 / 系項). The full active
/// catalog is sent in the prompt so the model can only choose from real ids; any id it returns is
/// re-validated against the catalog before being surfaced, so a hallucinated id is dropped, not echoed.
/// </summary>
public sealed class GeminiExpenseAiService : IExpenseAiService
{
    private readonly HttpClient _http;
    private readonly GeminiOptions _options;
    private readonly AppDbContext _db;
    private readonly ILogger<GeminiExpenseAiService> _logger;

    public GeminiExpenseAiService(
        HttpClient http,
        IOptions<GeminiOptions> options,
        AppDbContext db,
        ILogger<GeminiExpenseAiService> logger)
    {
        _http = http;
        _options = options.Value;
        _db = db;
        _logger = logger;
    }

    public async Task<ExpenseAiSuggestion> SuggestAsync(string chineseText, decimal amount, CancellationToken ct = default)
    {
        // Load the active catalog: the allow-list the model must classify into.
        var groups = await _db.ExpenseCategoryGroups
            .AsNoTracking()
            .Where(group => group.IsActive)
            .OrderBy(group => group.SortOrder)
            .Select(group => new
            {
                group.Id,
                group.Name_en,
                group.Name_zh,
                Subs = group.SubCategories
                    .Where(sub => sub.IsActive)
                    .OrderBy(sub => sub.SortOrder)
                    .Select(sub => new { sub.Id, sub.Name_en, sub.Name_zh })
                    .ToList(),
            })
            .ToListAsync(ct);

        if (string.IsNullOrWhiteSpace(_options.ApiKey))
        {
            _logger.LogWarning("Gemini API key is not configured; expense AI assist is disabled.");
            return new ExpenseAiSuggestion();
        }

        try
        {
            var catalogJson = JsonSerializer.Serialize(groups);
            var prompt =
                "You are a bookkeeping assistant for a church. Given an expense description (often in " +
                "Traditional Chinese) and its amount, do two things:\n" +
                "1. Translate the description into concise, natural accounting English (a short noun phrase, " +
                "not a full sentence).\n" +
                "2. Choose the single best matching major category (group) and sub-category from the catalog " +
                "below. You MUST pick a groupId and subCategoryId that appear in the catalog, and the " +
                "subCategoryId must belong to that groupId. If nothing fits well, choose the closest " +
                "\"Other / 其他\" option and lower your confidence.\n\n" +
                $"Expense description: {chineseText}\n" +
                $"Amount: {amount}\n\n" +
                $"Category catalog (JSON; each group has an id, English/Chinese names, and its sub-categories):\n{catalogJson}\n\n" +
                "Respond with JSON: englishDescription (string), groupId (integer), subCategoryId (integer), " +
                "confidence (number 0..1).";

            var payload = new
            {
                contents = new[]
                {
                    new { parts = new[] { new { text = prompt } } },
                },
                generationConfig = new
                {
                    responseMimeType = "application/json",
                    responseSchema = new
                    {
                        type = "object",
                        properties = new
                        {
                            englishDescription = new { type = "string" },
                            groupId = new { type = "integer" },
                            subCategoryId = new { type = "integer" },
                            confidence = new { type = "number" },
                        },
                        required = new[] { "englishDescription", "groupId", "subCategoryId", "confidence" },
                    },
                },
            };

            var url = $"{_options.BaseUrl}/models/{_options.Model}:generateContent";
            using var request = new HttpRequestMessage(HttpMethod.Post, url)
            {
                Content = JsonContent.Create(payload),
            };
            request.Headers.Add("X-goog-api-key", _options.ApiKey);

            using var response = await _http.SendAsync(request, ct);
            if (!response.IsSuccessStatusCode)
            {
                var body = await response.Content.ReadAsStringAsync(ct);
                _logger.LogWarning("Gemini returned {Status}: {Body}", (int)response.StatusCode, body);
                return new ExpenseAiSuggestion();
            }

            // Navigate candidates[0].content.parts[0].text — the model's JSON answer as a string.
            using var doc = JsonDocument.Parse(await response.Content.ReadAsStreamAsync(ct));
            var text = doc.RootElement
                .GetProperty("candidates")[0]
                .GetProperty("content")
                .GetProperty("parts")[0]
                .GetProperty("text")
                .GetString();

            if (string.IsNullOrWhiteSpace(text))
            {
                _logger.LogWarning("Gemini response contained no text part.");
                return new ExpenseAiSuggestion();
            }

            var parsed = JsonSerializer.Deserialize<GeminiAnswer>(
                text, new JsonSerializerOptions { PropertyNameCaseInsensitive = true });
            if (parsed is null) return new ExpenseAiSuggestion();

            var suggestion = new ExpenseAiSuggestion
            {
                EnglishDescription = string.IsNullOrWhiteSpace(parsed.EnglishDescription)
                    ? null
                    : parsed.EnglishDescription.Trim(),
                Confidence = parsed.Confidence,
            };

            // Re-validate the returned ids against the catalog; drop anything that doesn't line up.
            var group = groups.FirstOrDefault(candidate => candidate.Id == parsed.GroupId);
            if (group is not null)
            {
                suggestion.GroupId = group.Id;
                suggestion.GroupLabel = Label(group.Name_en, group.Name_zh);

                var sub = group.Subs.FirstOrDefault(candidate => candidate.Id == parsed.SubCategoryId);
                if (sub is not null)
                {
                    suggestion.SubCategoryId = sub.Id;
                    suggestion.SubLabel = Label(sub.Name_en, sub.Name_zh);
                }
            }

            return suggestion;
        }
        catch (Exception ex)
        {
            _logger.LogError(ex, "Expense AI assist failed.");
            return new ExpenseAiSuggestion();
        }
    }

    /// <summary>Mirror the frontend's bilingual() convention: "English / 中文" (or just English).</summary>
    private static string Label(string nameEn, string? nameZh)
        => string.IsNullOrWhiteSpace(nameZh) ? nameEn : $"{nameEn} / {nameZh}";

    /// <summary>Shape of the model's JSON answer (constrained by responseSchema).</summary>
    private sealed class GeminiAnswer
    {
        public string? EnglishDescription { get; set; }
        public int     GroupId            { get; set; }
        public int     SubCategoryId      { get; set; }
        public double  Confidence         { get; set; }
    }
}