using System;
using System.Collections.Generic;

namespace IslaApocalypse.Core
{
	/// <summary>One maximal 8-connected component of land, as the region layer exposes it.</summary>
	public sealed class LandRegion
	{
		/// <summary>1-based, assigned in deterministic scan order (x outer, y inner) — stable per seed across runs.</summary>
		public int Id;

		/// <summary>Cells in the component.</summary>
		public long SizeCells;

		/// <summary>Centroid in map cells.</summary>
		public double CentroidX, CentroidY;

		/// <summary>
		/// <see cref="RegionLabeling.HemiNorth"/> / <see cref="RegionLabeling.HemiSouth"/>, decided by the
		/// CENTROID — one label per component; a straddler is decided by where its mass is, never per cell.
		/// </summary>
		public byte Hemisphere;

		/// <summary>True for exactly one component: the one containing the map centre (or the flagged fallback).</summary>
		public bool IsMainland;

		/// <summary>Bounding box, inclusive. Convenience for overlays and guards; not part of the contract.</summary>
		public int MinX, MinY, MaxX, MaxY;
	}

	/// <summary>The result of one labeling: the per-cell id map and the per-component table.</summary>
	public sealed class RegionLabels
	{
		public int MapSize;

		/// <summary>Per cell, <c>x * MapSize + y</c>: the component id, or 0 for water.</summary>
		public int[] Id;

		/// <summary>Every component, indexed by <c>Id - 1</c>, in id order.</summary>
		public List<LandRegion> Regions;

		/// <summary>The mainland's id (0 only if there is no land at all).</summary>
		public int MainlandId;

		/// <summary>
		/// ⚠ Whether the map-centre cell was land. Expected always true (the massif is centred and
		/// stable). When false the mainland fell back to the LARGEST component and the caller must
		/// report it loudly — the contract's mainland definition did not hold on this field.
		/// </summary>
		public bool CentreWasLand;

		public long LandCells;
		public int IslandCount => Regions.Count - (MainlandId > 0 ? 1 : 0);

		public LandRegion Mainland => MainlandId > 0 ? Regions[MainlandId - 1] : null;
		public LandRegion Of(int id) => Regions[id - 1];
		public int IdAt(int x, int y) => Id[x * MapSize + y];
	}

	/// <summary>
	/// ⭐⭐ THE REGION-LABELING LAYER — shared infrastructure (chat2/07). Flood-fills land into distinct
	/// components, identifies mainland vs islands, and exposes per-component data. Islands are its first
	/// consumer; later phases (biomes, placement, rivers, the crater) CONSUME this layer rather than
	/// rebuild it. Engine-free; pure analysis over a <c>float[,]</c>; C++-candidate.
	///
	/// ═══ THE CONTRACT — build to it exactly (recorded at graduation as the shared-infra contract) ═══
	///
	///   FIELD        It runs on the CLASSIFY (raw, uncurved) height — region identity partitions on the
	///                same authoritative field as water bodies and biome regions (D-046), so islands /
	///                water / biomes line up by construction. Raw is authoritative for region identity.
	///                It does NOT run on the render field.
	///
	///   CONNECTIVITY Land is 8-CONNECTED. Deliberately the complement of water's 4-connectivity —
	///                foreground/background using opposite connectivity is the topologically sound
	///                pairing (a diagonal isthmus reads as JOINED; the water on either side of it reads
	///                as SEPARATE), not a conflict with the water model.
	///
	///   COMPONENT    A component = a maximal 8-connected set of land cells (land = classify height ≥ sea).
	///
	///   MAINLAND     The component containing the MAP CENTRE (the mountain/massif is always centred and
	///                stable) — NOT merely the largest component, because a later fragmentation step
	///                could make "largest" flip seed to seed. Every OTHER land component is an island.
	///                ⚠ The crater is NOT central — it is a northern-coastline feature, unrelated to the
	///                centre or the mountain, and plays no part here.
	///                Defensively: if the centre cell is not land, the layer reports it (<see
	///                cref="RegionLabels.CentreWasLand"/> = false) and falls back to the largest
	///                component, FLAGGED — the caller asserts rather than assumes.
	///
	///   PER COMPONENT  id · sizeCells · centroid (x, y) · hemisphere (north / south, BY THE CENTROID —
	///                one label per island; a straddler is decided by its centroid, never per cell) ·
	///                isMainland.
	///
	/// Ids are assigned in deterministic scan order (x outer, y inner, first-seen), so they are stable
	/// per seed across runs. Nothing here knows about "offshore" or "stamped" — it labels land.
	///
	/// ═══ THE HEMISPHERE CONVENTION — read from the code, not invented (chat2/05) ═══
	///
	/// Pass 1's latitude scalar is <c>y / MapSize</c>; the spine's "southern fade" and the "southern
	/// sinker" bite at high y. So y increases SOUTHWARD: NORTH = rows [0, MapSize/2), SOUTH = rows
	/// [MapSize/2, MapSize). The clean row midline, never the wobbled latitude field.
	/// </summary>
	public static class RegionLabeling
	{
		public const byte HemiNone = 0;
		public const byte HemiNorth = 1;
		public const byte HemiSouth = 2;

		/// <summary>The convention, in one place. Every consumer reads hemisphere through this.</summary>
		public static byte HemisphereOfRow(int y, int mapSize) => y < mapSize / 2 ? HemiNorth : HemiSouth;

		public static string HemisphereName(byte h) => h switch
		{
			HemiNorth => "north", HemiSouth => "south", _ => "none",
		};

		// 8-connectivity, fixed order (determinism: the fill order never changes).
		private static readonly int[] DX = { -1, -1, -1, 0, 0, 1, 1, 1 };
		private static readonly int[] DY = { -1, 0, 1, -1, 1, -1, 0, 1 };

		/// <summary>
		/// Label every 8-connected land component of <paramref name="classify"/> (land = height ≥
		/// <paramref name="sea"/>). Pure: the field is read, never written.
		/// </summary>
		public static RegionLabels Label(float[,] classify, int mapSize, float sea)
		{
			int n = mapSize;
			var id = new int[n * n];
			var regions = new List<LandRegion>();
			var stack = new Stack<int>();
			long landCells = 0;

			for (int sx = 0; sx < n; sx++)
			{
				for (int sy = 0; sy < n; sy++)
				{
					if (classify[sx, sy] < sea || id[sx * n + sy] != 0) continue;

					var r = new LandRegion { Id = regions.Count + 1, MinX = sx, MaxX = sx, MinY = sy, MaxY = sy };
					double sumX = 0, sumY = 0;
					id[sx * n + sy] = r.Id;
					stack.Push(sx * n + sy);

					while (stack.Count > 0)
					{
						int cur = stack.Pop();
						int cx = cur / n, cy = cur % n;
						r.SizeCells++; sumX += cx; sumY += cy;
						if (cx < r.MinX) r.MinX = cx; if (cx > r.MaxX) r.MaxX = cx;
						if (cy < r.MinY) r.MinY = cy; if (cy > r.MaxY) r.MaxY = cy;

						for (int k = 0; k < 8; k++)
						{
							int nx = cx + DX[k], ny = cy + DY[k];
							if (nx < 0 || nx >= n || ny < 0 || ny >= n) continue;
							int ni = nx * n + ny;
							if (id[ni] != 0 || classify[nx, ny] < sea) continue;
							id[ni] = r.Id;
							stack.Push(ni);
						}
					}

					r.CentroidX = sumX / r.SizeCells;
					r.CentroidY = sumY / r.SizeCells;
					r.Hemisphere = HemisphereOfRow((int)Math.Round(r.CentroidY), n);
					landCells += r.SizeCells;
					regions.Add(r);
				}
			}

			var labels = new RegionLabels { MapSize = n, Id = id, Regions = regions, LandCells = landCells };

			// ⭐ MAINLAND = the component containing the map centre. Asserted by the caller; the
			// fallback (largest) exists so a run can finish and REPORT the violation rather than crash.
			int centre = (n / 2) * n + (n / 2);
			labels.CentreWasLand = id[centre] != 0;
			if (labels.CentreWasLand) labels.MainlandId = id[centre];
			else
			{
				long best = -1;
				foreach (var r in regions) if (r.SizeCells > best) { best = r.SizeCells; labels.MainlandId = r.Id; }
			}
			if (labels.MainlandId > 0) regions[labels.MainlandId - 1].IsMainland = true;
			return labels;
		}

		/// <summary>
		/// Size statistics over the islands (non-mainland components): count, min / median / mean /
		/// max cells, and a log-spaced histogram — the instrument that turns "nice pieces vs shattered
		/// gravel" into numbers.
		/// </summary>
		public static (int count, long min, long median, double mean, long max, int[] histogram)
			IslandSizes(RegionLabels labels)
		{
			var sizes = new List<long>();
			foreach (var r in labels.Regions) if (!r.IsMainland) sizes.Add(r.SizeCells);
			var hist = new int[HistogramEdges.Length + 1];
			if (sizes.Count == 0) return (0, 0, 0, 0.0, 0, hist);
			sizes.Sort();
			double sum = 0;
			foreach (long s in sizes) { sum += s; hist[HistogramBin(s)]++; }
			return (sizes.Count, sizes[0], sizes[sizes.Count / 2], sum / sizes.Count, sizes[sizes.Count - 1], hist);
		}

		/// <summary>Histogram bin edges (cells): [0,64) [64,256) [256,1024) [1024,4096) [4096,16384) [16384,∞).</summary>
		public static readonly long[] HistogramEdges = { 64, 256, 1024, 4096, 16384 };

		public static int HistogramBin(long cells)
		{
			for (int i = 0; i < HistogramEdges.Length; i++) if (cells < HistogramEdges[i]) return i;
			return HistogramEdges.Length;
		}

		public static string HistogramLabel(int bin) => bin == 0 ? $"<{HistogramEdges[0]}"
			: bin < HistogramEdges.Length ? $"{HistogramEdges[bin - 1]}–{HistogramEdges[bin] - 1}"
			: $"≥{HistogramEdges[^1]}";

		/// <summary>Island counts per hemisphere (non-mainland components, by centroid).</summary>
		public static (int north, int south) IslandsByHemisphere(RegionLabels labels)
		{
			int nN = 0, nS = 0;
			foreach (var r in labels.Regions)
			{
				if (r.IsMainland) continue;
				if (r.Hemisphere == HemiNorth) nN++; else if (r.Hemisphere == HemiSouth) nS++;
			}
			return (nN, nS);
		}
	}
}