using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using DCIT.Core.Models;

namespace DCIT.Core.Services
{
    public sealed class BatchExecutionPlanner
    {
        public BatchExecutionPlan CreatePlan(IList<FileScanItem> selectedItems, int processorCount)
        {
            var executionItems = new List<FileScanItem>();
            var skippedItems = new List<BatchExecutionSkipItem>();
            var sourcePaths = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
            var outputPaths = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
            var diffPaths = new HashSet<string>(StringComparer.OrdinalIgnoreCase);

            if (selectedItems != null)
            {
                foreach (var item in selectedItems)
                {
                    if (item == null)
                    {
                        continue;
                    }

                    var sourcePath = NormalizePath(item.SourcePath);
                    var outputPath = NormalizePath(item.OutputPath);
                    var diffPath = NormalizePath(item.DiffPath);

                    var collision = GetCollision(sourcePath, sourcePaths, BatchExecutionSkipReason.DuplicateSourcePath);
                    if (collision == null)
                    {
                        collision = GetCollision(outputPath, outputPaths, BatchExecutionSkipReason.DuplicateOutputPath);
                    }

                    if (collision == null && !string.IsNullOrWhiteSpace(diffPath))
                    {
                        collision = GetCollision(diffPath, diffPaths, BatchExecutionSkipReason.DuplicateDiffPath);
                    }

                    if (collision != null)
                    {
                        collision.Item = item;
                        skippedItems.Add(collision);
                        continue;
                    }

                    AddIfPresent(sourcePaths, sourcePath);
                    AddIfPresent(outputPaths, outputPath);
                    AddIfPresent(diffPaths, diffPath);
                    executionItems.Add(item);
                }
            }

            var requiresSerialExecution = executionItems.Any(item => Path.GetExtension(item.SourcePath).Equals(".doc", StringComparison.OrdinalIgnoreCase));
            var degreeOfParallelism = DetermineDegreeOfParallelism(executionItems.Count, processorCount, requiresSerialExecution);

            return new BatchExecutionPlan
            {
                ExecutionItems = executionItems,
                SkippedItems = skippedItems,
                DegreeOfParallelism = degreeOfParallelism,
                RequiresSerialExecution = requiresSerialExecution,
            };
        }

        private static BatchExecutionSkipItem GetCollision(string path, HashSet<string> existingPaths, BatchExecutionSkipReason reason)
        {
            if (string.IsNullOrWhiteSpace(path) || !existingPaths.Contains(path))
            {
                return null;
            }

            return new BatchExecutionSkipItem
            {
                Reason = reason,
                ConflictingPath = path,
                ExistingPath = path,
            };
        }

        private static void AddIfPresent(HashSet<string> set, string path)
        {
            if (!string.IsNullOrWhiteSpace(path))
            {
                set.Add(path);
            }
        }

        private static int DetermineDegreeOfParallelism(int itemCount, int processorCount, bool requiresSerialExecution)
        {
            if (itemCount <= 0 || requiresSerialExecution)
            {
                return 1;
            }

            var normalizedProcessorCount = processorCount > 0 ? processorCount : Environment.ProcessorCount;
            normalizedProcessorCount = Math.Max(1, normalizedProcessorCount);
            return Math.Max(1, Math.Min(itemCount, normalizedProcessorCount));
        }

        private static string NormalizePath(string path)
        {
            if (string.IsNullOrWhiteSpace(path))
            {
                return string.Empty;
            }

            try
            {
                return Path.GetFullPath(path)
                    .TrimEnd(Path.DirectorySeparatorChar, Path.AltDirectorySeparatorChar);
            }
            catch
            {
                return path.Trim();
            }
        }
    }
}