using System.Collections.Generic;
using System.Linq;

namespace nqueens
{
	public class NBoard
	{
		public readonly int[] Columns;

		public int Size { get { return Columns.Length; } }
		
		public readonly int[] Conflicts;

		public NBoard(int[] columns)
		{
			Columns = columns;
			Conflicts = 
				(from n in Enumerable.Range(0, Size)
				select GetConflictCount(n)).ToArray();
		}

		public int GetQueenRow(int column)
		{
			return Columns[column];
		}

		public bool IsFeasible()
		{
			return Conflicts.All(c => c == 0);
		}

		public int GetConflictCount(int column)
		{
			return GetConflicts(column).Count;
		}

		public int TestConflictCount(int column, int row)
		{
			var originalRow = Columns[column];
			Columns[column] = row;

			var conflicts = GetConflictCount(column);

			Columns[column] = originalRow;

			return conflicts;
		}

		private List<int> GetConflicts(int column)
		{
			var queenRow = GetQueenRow(column);
			var conflicts = new List<int>();
			for (var iColumn = 0; iColumn < Size; iColumn++)
			{
				if (iColumn == column)
					continue;
				var iRow = Columns[iColumn];
				if (iRow == queenRow)
				{
					conflicts.Add(iColumn);
					continue;
				}

				var columnDiff = column - iColumn;
				if (queenRow - columnDiff == iRow || queenRow + columnDiff == iRow)
					conflicts.Add(iColumn);
			}
			return conflicts;
		}

		public void UpdateRow(int column, int row)
		{
			var oldConflictColumns = GetConflicts(column);
			foreach (var i in oldConflictColumns)
				Conflicts[i]--;

			var sum = Conflicts.Any(c => c < 0);

			Columns[column] = row;
			
			var newConflictColumns = GetConflicts(column);
			Conflicts[column] = newConflictColumns.Count;
			foreach (var i in newConflictColumns)
				Conflicts[i]++;
		}
	}
}