hrynco-notification-service/HrynCo.NotificationService.Worker.Services/EmailProcessing/EmailTemplateRenderingService.cs

namespace HrynCo.NotificationService.Worker.Services.EmailProcessing;

using System.Net;
using System.Text;
using HrynCo.NotificationService.Contracts.Messages;
using HrynCo.NotificationService.DAL.Abstract.Templates;

internal sealed class EmailTemplateRenderingService : IEmailTemplateRenderingService
{
    public RenderedEmail Render(EmailTemplate template, SendEmailMessageData data)
    {
        string[] missingVariables = template.Variables
            .Where(variable => variable.Required)
            .Select(variable => variable.Name)
            .Where(name => !data.Variables.TryGetValue(name, out string? value) || string.IsNullOrWhiteSpace(value))
            .ToArray();

        if (missingVariables.Length > 0)
        {
            throw new InvalidDataException(
                $"Required template variables are missing: {string.Join(", ", missingVariables)}.");
        }

        return new RenderedEmail(
            Interpolate(template.Subject, data.Variables),
            InterpolateHtml(template.HtmlBody, data.Variables),
            Interpolate(template.TextBody, data.Variables));
    }

    private static string InterpolateHtml(string text, IReadOnlyDictionary<string, string> variables)
    {
        return Interpolate(text, variables, WebUtility.HtmlEncode);
    }

    private static string Interpolate(string text, IReadOnlyDictionary<string, string> variables)
    {
        return Interpolate(text, variables, static value => value);
    }

    private static string Interpolate(
        string text,
        IReadOnlyDictionary<string, string> variables,
        Func<string, string> encodeValue)
    {
        var sb = new StringBuilder(text);
        foreach (var (key, value) in variables)
            sb.Replace($"{{{{{key}}}}}", encodeValue(value));
        return sb.ToString();
    }
}