using System;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.Serialization;

public class GrassField : MonoBehaviour {
    public int size = 100;
    public int numChunks = 20;
    public int bladesPerSquareMeter = 100;
    public GrassChunk chunkPrefab;
    public bool ignoreHeightForLOD = true;
    [Header("LOD 0")]
    public Material grassMaterialLOD0;
    public float minDistanceLOD0 = 2f;
    [Range(0f, 1f)] public float densityLOD0 = 1f;
    public bool enableShadowsLOD0 = true;
    [Header("LOD 1")]
    public float distanceLOD1 = 5f;
    public Material grassMaterialLOD1;
    [Range(0f, 1f)] public float densityLOD1 = 0.5f;
    public bool enableShadowsLOD1 = true;
    [Header("LOD 2")]
    public float distanceLOD2 = 15f;
    public Material grassMaterialLOD2;
    [Range(0f, 1f)] public float densityLOD2 = 0.25f;
    public bool enableShadowsLOD2 = false;
    [Header("LOD 3")]
    public float distanceLOD3 = 25f;
    public Material grassMaterialLOD3;
    [Range(0f, 1f)] public float densityLOD3 = 0.125f;
    public bool enableShadowsLOD3 = false;
    
    private const float SQRT_2 = 1.4142135623730f;
    private static readonly int pId_WorldSpaceCameraPosEditor = Shader.PropertyToID("_WorldSpaceCameraPosEditor");
    private static readonly int pId_TransitionRange = Shader.PropertyToID("_TransitionRange");

    // a 2D array would be cleaner, but it can't be serialized
    // serialize it, because the chunks are generated offline and used during runtime
    [SerializeField, HideInInspector] private GrassChunk[] _chunks;

    public void GenerateGrassField() {
        var bladesLOD0 = GrassMeshGeneration.GenerateBladesRandom(
            size, size * size * (int)(bladesPerSquareMeter * densityLOD0)
        );
        var bladesLOD1 = GrassMeshGeneration.GenerateBladesReduced(bladesLOD0, densityLOD1);
        var bladesLOD2 = GrassMeshGeneration.GenerateBladesReduced(bladesLOD1, densityLOD2 / densityLOD1);
        var bladesLOD3 = GrassMeshGeneration.GenerateBladesReduced(bladesLOD2, densityLOD3 / densityLOD2);
        CreateChunks(bladesLOD0, bladesLOD1, bladesLOD2, bladesLOD3);

        var camPosWorld = Camera.main ? Camera.main.transform.position : Vector3.zero;
        UpdateLODs(camPosWorld);
        UpdateMaterials(camPosWorld);
    }

    public void UpdateLODsAndMaterials() {
        var camPosWorld = Camera.main ? Camera.main.transform.position : Vector3.zero;
        UpdateMaterials(camPosWorld);
        UpdateLODs(camPosWorld);
    }

    private void OnEnable() {
        var chunkSize = ((float)size) / numChunks;
        var minDistLOD2 = distanceLOD1 + chunkSize * SQRT_2;
        if (distanceLOD2 < minDistLOD2) {
            Debug.LogWarning("It is recommended that minDistLOD2 is greater than " + minDistLOD2 + ".");
        }
        var minDistLOD3 = distanceLOD2 + chunkSize * SQRT_2;
        if (distanceLOD3 < minDistLOD3) {
            Debug.LogWarning("It is recommended that minDistLOD3 is greater than " + minDistLOD3 + ".");
        }
        
        if (_chunks.Length != numChunks * numChunks) {
            Debug.LogError(this.name + ": Existing chunks does not match numChunks.");
            gameObject.SetActive(false);
        }

        UpdateLODsAndMaterials();
    }

    private void OnDisable() {
        UpdateLODsAndMaterials();
    }

    private void FixedUpdate() {
        var cam = Camera.main;
        if (!cam) return;
        var camPosWorld = cam.transform.position;

#if UNITY_EDITOR
        // update LOD distances and camera position
        UpdateMaterials(camPosWorld);
#endif

        if (_chunks.Length != numChunks * numChunks) return;
        UpdateLODs(camPosWorld, false); // while the game is running, only the chunks near the camera are updated
    }

    private void UpdateLODs(Vector3 camPosWorld, bool forceCompleteCheck = true) {
        var camPos = camPosWorld - transform.position; // cam position relative to grass field
        if (ignoreHeightForLOD) {
            camPos.y = 0;
        }
        
        var chunkSize = ((float)size) / numChunks;
        var bounds = new Bounds(
            new Vector3(0.5f * chunkSize, 0f, 0.5f * chunkSize),
            new Vector3(chunkSize, 0f, chunkSize)
        );

        // small performance difference (0.13ms vs 0.02ms on a 100m 30x30 grid, LOD dist 26.6, camera at corner)
        var range = forceCompleteCheck
            ? new GridRange{ fromX = 0, toX = numChunks-1, fromZ = 0, toZ = numChunks-1 }
            : GetLODGridRange(camPos, chunkSize);

        for (int x = range.fromX; x <= range.toX; x++)
        for (int y = range.fromZ; y <= range.toZ; y++) {
            var chunkPos = new Vector3(x, 0f, y) * chunkSize;
            var localCamPos = camPos - chunkPos; // cam position relative to chunk

            var sqrCamDist = bounds.SqrDistance(localCamPos);
            if (sqrCamDist > distanceLOD3 * distanceLOD3) {
                _chunks[x + y * numChunks].LOD = 3;
            }
            else if (sqrCamDist > distanceLOD2 * distanceLOD2) {
                _chunks[x + y * numChunks].LOD = 2;
            }
            else if (sqrCamDist > distanceLOD1 * distanceLOD1) {
                _chunks[x + y * numChunks].LOD = 1;
            }
            else {
                _chunks[x + y * numChunks].LOD = 0;
            }
        }
    }

    private struct GridRange {
        public int fromX;
        public int toX;
        public int fromZ;
        public int toZ;
    }

    private GridRange GetLODGridRange(Vector3 camPos, float chunkSize) {
        GridRange range;
        
        var maxCamMove = 3f; // assume that the camera did not move further than this distance
        var fromXWorld = camPos.x - distanceLOD3 - maxCamMove;
        var toXWorld = camPos.x + distanceLOD3 + maxCamMove;
        var fromZWorld = camPos.z - distanceLOD3 - maxCamMove;
        var toZWorld = camPos.z + distanceLOD3 + maxCamMove;
        
        range.fromX = Math.Max( 0, (int) (fromXWorld / chunkSize) );
        range.toX = Math.Max( 0, (int) (toXWorld / chunkSize) );
        range.fromZ = Math.Max( 0, (int) (fromZWorld / chunkSize) );
        range.toZ = (int) (toZWorld / chunkSize);
        range.fromX = Math.Clamp(range.fromX, 0, numChunks - 1);
        range.toX = Math.Clamp(range.toX, 0, numChunks - 1);
        range.fromZ = Math.Clamp(range.fromZ, 0, numChunks - 1);
        range.toZ = Math.Clamp(range.toZ, 0, numChunks - 1);

        return range;
    }

    private void UpdateMaterials(Vector3 camPosWorld) {
        var chunkSize = ((float)size) / numChunks;
        var chunkDiagonalLength = chunkSize * SQRT_2;

        var transition0From = minDistanceLOD0;
        var transition0To   = distanceLOD1;
        var transition1From = distanceLOD1 + chunkDiagonalLength;
        var transition1To   = distanceLOD2;
        var transition2From = distanceLOD2 + chunkDiagonalLength;
        var transition2To   = distanceLOD3;
        grassMaterialLOD0.SetVector(pId_TransitionRange, new Vector2(transition0From, transition0To));
        grassMaterialLOD1.SetVector(pId_TransitionRange, new Vector2(transition1From, transition1To));
        grassMaterialLOD2.SetVector(pId_TransitionRange, new Vector2(transition2From, transition2To));
#if UNITY_EDITOR
        if (ignoreHeightForLOD) {
            camPosWorld.y = 0;
        }
        grassMaterialLOD0.SetVector(pId_WorldSpaceCameraPosEditor, camPosWorld);
        grassMaterialLOD1.SetVector(pId_WorldSpaceCameraPosEditor, camPosWorld);
        grassMaterialLOD2.SetVector(pId_WorldSpaceCameraPosEditor, camPosWorld);
        grassMaterialLOD3.SetVector(pId_WorldSpaceCameraPosEditor, camPosWorld);
#endif
    }

    private void CreateChunks(
        BladeData[] bladesLOD0, BladeData[] bladesLOD1, BladeData[] bladesLOD2, BladeData[] bladesLOD3
    ) {
        GameObject chunksContainer;
        // If a child called "Chunks" exists, destroy it -> children are also destroyed
        var chunksTransform = gameObject.transform.Find("Chunks");
        if (chunksTransform != null) {
            // Destroy(obj) does not work in edit mode
            DestroyImmediate(chunksTransform.gameObject);
        }
        // create GameObject "Chunks" as a container for all GrassChunk objects
        chunksContainer = new GameObject("Chunks");
        chunksContainer.transform.SetParent(this.transform, false);

        var chunkSize = ((float)size) / numChunks;
        // bounds of the chunk (relative to chunk) - do not consider y position
        var chunkBounds = new Bounds(
            new Vector3(0.5f * chunkSize, 0.0f, 0.5f * chunkSize),
            new Vector3(chunkSize, 10000f, chunkSize)
        );
        _chunks = new GrassChunk[numChunks * numChunks];
        for (int x = 0; x < numChunks; x++)
        for (int y = 0; y < numChunks; y++) {
            var chunkPos = new Vector3(x, 0.0f, y) * chunkSize;

            var meshLOD0 = GenerateChunkMesh(chunkPos, chunkBounds, bladesLOD0, "grass_chunk_" + x + "_" + y + "_lod0");
            var meshLOD1 = GenerateChunkMesh(chunkPos, chunkBounds, bladesLOD1, "grass_chunk_" + x + "_" + y + "_lod1");
            var meshLOD2 = GenerateChunkMesh(chunkPos, chunkBounds, bladesLOD2, "grass_chunk_" + x + "_" + y + "_lod2");
            var meshLOD3 = GenerateChunkMesh(chunkPos, chunkBounds, bladesLOD3, "grass_chunk_" + x + "_" + y + "_lod3");

            var chunk = Instantiate(chunkPrefab, chunksContainer.transform);
            chunk.name = "Chunk [" + x + ", " + y +"]";
            chunk.transform.localPosition = chunkPos;

            chunk.MaterialLOD0 = grassMaterialLOD0;
            chunk.MaterialLOD1 = grassMaterialLOD1;
            chunk.MaterialLOD2 = grassMaterialLOD2;
            chunk.MaterialLOD3 = grassMaterialLOD3;
            chunk.MeshLOD0 = meshLOD0;
            chunk.MeshLOD1 = meshLOD1;
            chunk.MeshLOD2 = meshLOD2;
            chunk.MeshLOD3 = meshLOD3;
            chunk.EnableShadowsLOD0 = enableShadowsLOD0;
            chunk.EnableShadowsLOD1 = enableShadowsLOD1;
            chunk.EnableShadowsLOD2 = enableShadowsLOD2;
            chunk.EnableShadowsLOD3 = enableShadowsLOD3;

            _chunks[x + y * numChunks] = chunk;
        }
    }

    private Mesh GenerateChunkMesh(Vector3 chunkPos, Bounds chunkBounds, BladeData[] blades, string meshName) {
        List<Vector3> rootPositions = new(); // vertices
        List<Vector4> tipOffsets_missingInNextLOD = new(); // uv0
        List<int> indices = new();
        foreach (var blade in blades) {
            // position relative to chunk
            var localRootPos = blade.rootPosition - chunkPos;
            if (chunkBounds.Contains(localRootPos)) {
                rootPositions.Add(localRootPos);
                tipOffsets_missingInNextLOD.Add(new Vector4(
                    blade.tipOffset.x, blade.tipOffset.y, blade.tipOffset.z,
                    blade.missingInNextLOD ? 1f : 0f)
                );
                indices.Add(rootPositions.Count - 1);
            }
        }

        var mesh = new Mesh();
        mesh.name = meshName;
        if (rootPositions.Count > 65536) {
            // 16 bit (default) supports 65536 vertices, 32 bit supports 4 billion
            mesh.indexFormat = UnityEngine.Rendering.IndexFormat.UInt32;
            Debug.LogWarning(meshName + " has more than 65536 grass blades. Mesh.indexFormat set to UInt32.");
        }
        mesh.SetVertices(rootPositions);
        mesh.SetUVs(0, tipOffsets_missingInNextLOD); // Vector4 UVs
        // AABB gets calculated automatically (considers all root positions)
        mesh.SetIndices(indices, MeshTopology.Points, 0);
        // update the AABB to match the actual grass (not just the root positions)
        // use mesh.bounds rather than meshRenderer.localBounds,
        // because the latter gets overwritten automatically
        var previousBounds = mesh.bounds;
        const float width = 1f; // a blade should never bend sideways farther than this
        const float height = 1f; // a blade should not be higher than this
        mesh.bounds = new Bounds(
            previousBounds.center + new Vector3(0f, height * 0.5f, 0f),
            previousBounds.size + new Vector3(2f*width, height, 2f*width)
        );

        return mesh;
    }
    
    private void OnDrawGizmos() {
        var chunkSize = ((float)size) / numChunks;
        var tPos = transform.position;

        // grid
        for (int i = 0; i < numChunks + 1; i++) {
            Gizmos.color = new Color(0f, 0.3f, 1f, 1f);
            Gizmos.DrawLine(
                tPos + new Vector3(chunkSize * i, 0.01f, 0),
                tPos + new Vector3(chunkSize * i, 0.01f, 0) + new Vector3(0, 0, size)
            );
            Gizmos.DrawLine(
                tPos + new Vector3(0, 0.01f, chunkSize * i),
                tPos + new Vector3(0, 0.01f, chunkSize * i) + new Vector3(size, 0, 0)
            );
        }

        var cam = Camera.main;
        if (!cam) return;
        var center = cam.transform.position;
        var oldMatrix = Gizmos.matrix;
        if (ignoreHeightForLOD) {
            center.y = 0.01f;
            // draw a circle instead of a sphere
            Gizmos.matrix = Matrix4x4.TRS(Vector3.zero, Quaternion.identity, new Vector3(1f, 0.001f, 1f));
        }
        
        Gizmos.color = new Color(0f, 0.3f, 1f, 1f);
        Gizmos.DrawWireSphere(center, distanceLOD1);
        Gizmos.DrawWireSphere(center, distanceLOD2);
        Gizmos.DrawWireSphere(center, distanceLOD3);
        
        Gizmos.color = new Color(1.0f, 1.0f, 0.0f, 1f);
        var minDistLOD0 = minDistanceLOD0;
        var minDistLOD1 = distanceLOD1 + chunkSize * SQRT_2;
        var minDistLOD2 = distanceLOD2 + chunkSize * SQRT_2;
        Gizmos.DrawWireSphere(center, minDistLOD0);
        Gizmos.DrawWireSphere(center, minDistLOD1);
        Gizmos.DrawWireSphere(center, minDistLOD2);
        
        Gizmos.matrix = oldMatrix;
    }
}