using System;

namespace IslaApocalypse.Tools
{
	/// <summary>
	/// ⭐ COASTAL FRAGMENTATION (chat2/09, EXPLORATION) — a perimeter-wide, band-limited, zero-mean noise
	/// on the PRE-power falloff, applied only inside the coastal window.
	///
	/// ═══ WHY A DEDICATED TERM AND NOT THE EDGE NOISE SCALED ═══
	///
	/// Pass 1's edge noise is <c>(noise+1)/2 · 0.15 · squircle</c>: positive-only (it only ever PUSHES
	/// the falloff up), modulated by the squircle (strongest at the rim, zero at the centre), sampled at
	/// 2.5× the base frequency. Scaling it would roughen the whole rim and bias the coast inward; it is
	/// the coastline's jitter, not a margin-targeted cutter. This term is separate: its own deterministic
	/// field (seed offset + a coordinate offset in map widths), its own frequency (the neck/lobe scale),
	/// zero-mean (it bites AND builds, so the coast is redrawn rather than eroded), and weighted by a
	/// smooth WINDOW on the falloff value itself:
	///
	///     w(f) = 1 − smoothstep(|f − centre| / halfWidth)      (exactly 0 beyond ± halfWidth)
	///     falloff += amp · w(f) · noise(x + off, y + off)        noise ∈ [−1, 1]
	///
	/// The coast sits where rawBase − falloff^2.5 crosses sea, i.e. near f ≈ 0.66 for a median rawBase
	/// (chat2/08 diagnosis), so a window centred there covers the barely-land / barely-sea margin
	/// around the whole perimeter, north and south. Cells whose falloff is clear of the window — the
	/// interior, the massif, the deep sea — get w = 0 and are bit-identical by construction (asserted).
	///
	/// SELF-TARGETING: every lobe hangs off a neck of barely-land, cells whose height sits a hair above
	/// sea. A bite of the same Δfalloff flips those first; solid land inside the window (a coastal hill)
	/// moves in height but does not flip. No neck is detected; the margin selects itself. Amplitude is
	/// the ladder; frequency is fixed and exposed as the secondary dial.
	/// </summary>
	public static class CoastalFragment
	{
		/// <summary>Periods per map width. The base noise is ≈ 4/map, the edge noise ≈ 10/map; lobes in the 08 plates are 2–7 % of the map. Chosen by the chat2/09 probe.</summary>
		public const float DefaultFreqPerMapWidth = 12f;

		/// <summary>The coastal window's centre in pre-power falloff units (the coast's falloff for a median base noise).</summary>
		public const float DefaultBandCentre = 0.66f;

		/// <summary>Half-width of the window. 0.18 spans f ∈ [0.48, 0.84] — the whole fringe, nothing of the interior.</summary>
		public const float DefaultBandHalfWidth = 0.18f;

		/// <summary>Bites only by default? Set by the chat2/09 probe (see the report): zero-mean redraws the margin and can bridge islands back; bites-only only cuts.</summary>
		public const bool DefaultBitesOnly = false;

		/// <summary>The field's seed offset (a SEED offset, like the islet layer's 7607).</summary>
		public const int SeedOffset = 9109;

		/// <summary>The field's coordinate offset, IN MAP WIDTHS (D-059) — decorrelates it from the base field's realization at every size.</summary>
		public const float OffsetMapWidths = 0.37f;

		/// <summary>The window weight for a pre-power falloff value.</summary>
		public static float Window(float falloff, float centre, float halfWidth)
		{
			float t = MathF.Abs(falloff - centre) / halfWidth;
			if (t >= 1f) return 0f;
			return 1f - t * t * (3f - 2f * t);
		}
	}
}