llm-pii-poc/src/PiiRedaction.Core/Redaction/PlaceholderPiiRedactor.cs

using PiiRedaction.Core.Abstractions;
using PiiRedaction.Core.Models;

namespace PiiRedaction.Core.Redaction;

public sealed class PlaceholderPiiRedactor : IPiiRedactor
{
    public RedactionResult Redact(string text, IReadOnlyList<PiiEntity> entities)
    {
        ArgumentException.ThrowIfNullOrWhiteSpace(text);
        ArgumentNullException.ThrowIfNull(entities);

        var placeholderByValue = new Dictionary<string, string>(StringComparer.Ordinal);
        var counters = new Dictionary<PiiEntityType, int>();
        var orderedEntities = entities.OrderByDescending(entity => entity.StartIndex).ToList();
        var sanitized = text;

        foreach (var entity in orderedEntities)
        {
            var mapKey = CreateValueKey(entity);
            if (!placeholderByValue.TryGetValue(mapKey, out var placeholder))
            {
                counters.TryGetValue(entity.Type, out var count);
                count++;
                counters[entity.Type] = count;
                placeholder = $"<{ToPlaceholderPrefix(entity.Type)}_{count}>";
                placeholderByValue[mapKey] = placeholder;
            }

            sanitized = string.Concat(
                sanitized.AsSpan(0, entity.StartIndex),
                placeholder,
                sanitized.AsSpan(entity.EndIndex));
        }

        var placeholderMap = placeholderByValue
            .ToDictionary(pair => pair.Value, pair => pair.Key.Split('|', 2)[1], StringComparer.Ordinal);

        return new RedactionResult(sanitized, placeholderMap);
    }

    private static string CreateValueKey(PiiEntity entity) =>
        $"{entity.Type}|{entity.Value}";

    private static string ToPlaceholderPrefix(PiiEntityType type) => type switch
    {
        PiiEntityType.Person => "PERSON",
        PiiEntityType.Email => "EMAIL",
        PiiEntityType.Phone => "PHONE",
        PiiEntityType.Pan => "PAN",
        PiiEntityType.Aadhaar => "AADHAAR",
        PiiEntityType.CreditCard => "CREDIT_CARD",
        PiiEntityType.LoanNumber => "LOAN_NUMBER",
        PiiEntityType.CustomerId => "CUSTOMER_ID",
        PiiEntityType.AccountNumber => "ACCOUNT_NUMBER",
        _ => type.ToString().ToUpperInvariant()
    };
}