FileSystemPromptProvider.cs

// Copyright (c) August 2026, devMobile Software
// 
using FoehnAIBuilder.Configuration;

namespace FoehnAIBuilder.Chat;

/// <summary>
/// Reads the system prompt from the path in <see cref="FoehnAIBuilderOptions.SystemMessageFile"/>.
/// Missing / empty / unreadable files log a warning and produce no prompt.
/// </summary>
public sealed class FileSystemPromptProvider : ISystemPromptProvider
{
    private readonly FoehnAIBuilderOptions _options;
    private readonly ILogger<FileSystemPromptProvider> _logger;
    private readonly Lazy<string?> _cached;

    public FileSystemPromptProvider(
        IOptions<FoehnAIBuilderOptions> options,
        ILogger<FileSystemPromptProvider> logger)
    {
        _options = options.Value;
        _logger = logger;
        _cached = new Lazy<string?>(Load);
    }

    public string? GetSystemPrompt() => _cached.Value;

    private string? Load()
    {
        var path = _options.SystemMessageFile;

        if (string.IsNullOrWhiteSpace(path))
            return null;

        if (!File.Exists(path))
        {
            _logger.LogWarning("System message file '{Path}' was not found; continuing without a system prompt.", path);
            return null;
        }

        try
        {
            var content = File.ReadAllText(path);
            if (string.IsNullOrWhiteSpace(content))
            {
                _logger.LogWarning("System message file '{Path}' is empty; continuing without a system prompt.", path);
                return null;
            }
            return content;
        }
        catch (IOException ex)
        {
            _logger.LogError(ex, "Failed to read system message file '{Path}'.", path);
            return null;
        }
    }
}