using UnityEngine;

/// <summary>
/// Swim animation for an inflated paper fish: a sideways wave that grows toward
/// the tail and travels backwards, so tail and rear fins swish while the head
/// stays steady. Deforms a per-instance copy of the shared mesh on the CPU.
/// </summary>
[RequireComponent(typeof(MeshFilter))]
public class FishWiggle : MonoBehaviour
{
    [Tooltip("True if the sprite's nose points +X (tail is at the left of the texture).")]
    public bool spriteFacesRight = true;
    [Tooltip("Sideways swing at the tail tip, as a fraction of fish height.")]
    public float amplitude = 0.10f;
    [Tooltip("Wave cycles per second when cruising.")]
    public float frequency = 1.6f;
    [Tooltip("How far along the body the wave is (0 = only the tail moves, 1 = whole body).")]
    [Range(0f, 1f)] public float bodyShare = 0.25f;
    [Tooltip("Speed multiplier from the controller; 1 = cruising.")]
    public float effort = 1f;

    Mesh mesh;
    Vector3[] baseVerts, verts;
    float[] tail;      // 0 at the nose .. 1 at the tail tip
    float phase;

    void Start()
    {
        var mf = GetComponent<MeshFilter>();
        var src = mf.sharedMesh;
        if (src == null) { enabled = false; return; }
        mesh = Object.Instantiate(src);
        mesh.name = src.name + " (wiggle)";
        mesh.MarkDynamic();
        mf.sharedMesh = mesh;
        baseVerts = mesh.vertices;
        verts = new Vector3[baseVerts.Length];
        var uv = mesh.uv;
        tail = new float[baseVerts.Length];
        for (int i = 0; i < tail.Length; i++)
        {
            float u = uv.Length == baseVerts.Length ? uv[i].x : baseVerts[i].x + 0.5f;
            float t = spriteFacesRight ? 1f - u : u;
            // ease in: head barely moves, last third of the body carries the swing
            tail[i] = Mathf.Pow(Mathf.Clamp01((t - (1f - bodyShare) * 0.6f) / (1f - (1f - bodyShare) * 0.6f)), 2.2f);
        }
        phase = Random.value * Mathf.PI * 2f;
    }

    void Update()
    {
        if (mesh == null) return;
        phase += Time.deltaTime * frequency * Mathf.Max(0.2f, effort) * Mathf.PI * 2f;
        float amp = amplitude * Mathf.Clamp(effort, 0.5f, 1.5f);
        for (int i = 0; i < verts.Length; i++)
        {
            var v = baseVerts[i];
            float k = tail[i];
            // wave travels head -> tail: phase lags with tailness
            v.z += Mathf.Sin(phase - k * 2.5f) * amp * k;
            verts[i] = v;
        }
        mesh.vertices = verts;
        mesh.RecalculateBounds();
    }

    void OnDestroy() { if (mesh != null) Destroy(mesh); }
}