// Lossless grouping: only meshes with byte-identical primitives, attributes and material
// dependencies become instances. Geometry is neither rescaled nor decimated here.
import assert from 'node:assert/strict'
import { createHash } from 'node:crypto'

export function parseGlb(bytes) {
    assert.equal(bytes.toString('ascii', 0, 4), 'glTF')
    assert.equal(bytes.readUInt32LE(4), 2)
    assert.equal(bytes.readUInt32LE(8), bytes.length)
    const size = bytes.readUInt32LE(12), offset = 20 + size
    assert.equal(bytes.toString('ascii', 16, 20), 'JSON')
    assert.equal(bytes.toString('ascii', offset + 4, offset + 8), 'BIN\0')
    return { json: JSON.parse(bytes.toString('utf8', 20, offset)), bin: bytes.subarray(offset + 8, offset + 8 + bytes.readUInt32LE(offset)) }
}
export function packGlb(json, bin) {
    let text = Buffer.from(JSON.stringify(json))
    text = Buffer.concat([text, Buffer.alloc((-text.length) & 3, 0x20)])
    bin = Buffer.concat([bin, Buffer.alloc((-bin.length) & 3)])
    const header = Buffer.alloc(20), tail = Buffer.alloc(8)
    header.write('glTF'); header.writeUInt32LE(2, 4); header.writeUInt32LE(28 + text.length + bin.length, 8)
    header.writeUInt32LE(text.length, 12); header.write('JSON', 16)
    tail.writeUInt32LE(bin.length); tail.write('BIN\0', 4)
    return Buffer.concat([header, text, tail, bin])
}
export function accessorValues(json, bin, index) {
    const a = json.accessors[index], view = json.bufferViews[a.bufferView]
    assert.ok(!a.sparse && !a.normalized)
    const components = { SCALAR: 1, VEC2: 2, VEC3: 3, VEC4: 4 }[a.type]
    assert.equal(a.componentType, 5126)
    const start = (view.byteOffset || 0) + (a.byteOffset || 0), stride = view.byteStride || components * 4
    return Array.from({ length: a.count }, (_, i) => Array.from({ length: components }, (_, c) => bin.readFloatLE(start + i * stride + c * 4)))
}
export function meshSignature(json, bin, mesh) {
    function viewData(index) {
        const view = json.bufferViews[index], bytes = bin.subarray(view.byteOffset || 0, (view.byteOffset || 0) + view.byteLength)
        return createHash('sha256').update(bytes).digest('hex')
    }
    function attribute(index) {
        const a = structuredClone(json.accessors[index]); delete a.name
        if (a.bufferView !== undefined) { a.bufferView = viewData(a.bufferView) }
        return a
    }
    return JSON.stringify(mesh.primitives.map(p => ({
        mode: p.mode ?? 4, material: json.materials[p.material],
        attributes: Object.fromEntries(Object.entries(p.attributes).map(([k, i]) => [k, attribute(i)])),
        indices: attribute(p.indices),
        extensions: p.extensions?.KHR_draco_mesh_compression ? {
            attributes: p.extensions.KHR_draco_mesh_compression.attributes,
            payload: viewData(p.extensions.KHR_draco_mesh_compression.bufferView),
        } : undefined,
    })))
}
export function instanceRepeatedMeshes(bytes) {
    const { json, bin } = parseGlb(bytes)
    assert.ok(!json.skins?.length && !json.animations?.length)
    assert.equal(json.scenes.length,1,'only one static scene supported')
    assert.ok(json.accessors.every(a=>!a.sparse),'sparse attributes unsupported')
    assert.ok((json.extensionsUsed||[]).every(e=>e==='KHR_draco_mesh_compression'),'unsupported extension')
    for(const mesh of json.meshes){
        assert.ok(!mesh.weights,'static morph weights unsupported')
        for(const p of mesh.primitives){
            assert.ok(!p.targets,'morph targets unsupported')
            assert.ok(p.indices!==undefined,'indexed meshes required')
            assert.ok(Object.keys(p.extensions||{}).every(e=>e==='KHR_draco_mesh_compression'))
        }
    }
    const roots = json.scenes[json.scene || 0].nodes
    assert.deepEqual([...roots].sort((a,b) => a-b), json.nodes.map((_, i) => i), 'all source nodes must be scene roots')
    const groups = new Map(), originalNodes = json.nodes.length
    for (const node of json.nodes) {
        assert.ok(node.mesh !== undefined && !node.children?.length && !node.matrix && !node.extensions && !node.weights)
        const key = meshSignature(json, bin, json.meshes[node.mesh])
        if (!groups.has(key)) groups.set(key, [])
        groups.get(key).push(node)
    }
    const chunks = [bin], nodes = [], meshes = [], records = []
    let length = bin.length
    function append(values, components) {
        const padding = (-length) & 3
        if (padding) { chunks.push(Buffer.alloc(padding)); length += padding }
        const data = Buffer.alloc(values.length * 4)
        values.forEach((v, i) => { assert.ok(Number.isFinite(v)); data.writeFloatLE(v, i * 4) })
        const view = json.bufferViews.length
        json.bufferViews.push({ buffer: 0, byteOffset: length, byteLength: data.length })
        chunks.push(data); length += data.length
        const accessor = json.accessors.length
        json.accessors.push({ bufferView: view, componentType: 5126, type: components === 4 ? 'VEC4' : 'VEC3', count: values.length / components,
            ...(components === 3 ? { min: [0,1,2].map(a => Math.min(...values.filter((_,i)=>i%3===a))), max: [0,1,2].map(a=>Math.max(...values.filter((_,i)=>i%3===a))) } : {}) })
        return accessor
    }
    for (const group of groups.values()) {
        const meshIndex = meshes.length
        meshes.push(json.meshes[group[0].mesh])
        if (group.length === 1) { nodes.push({ ...group[0], mesh: meshIndex }); continue }
        const trs = { TRANSLATION: [], ROTATION: [], SCALE: [] }
        for (const node of group) {
            const t = node.translation || [0,0,0], r = node.rotation || [0,0,0,1], s = node.scale || [1,1,1]
            assert.ok(s.every(v => Math.abs(v-1)<1e-7), 'bake nonuniform scale before instancing')
            assert.ok(r.slice(0,3).every(v=>Math.abs(v)<1e-7)&&Math.abs(r[3]-1)<1e-7, 'bake rotation before instancing')
            trs.TRANSLATION.push(...t); trs.ROTATION.push(...r); trs.SCALE.push(...s)
        }
        const attributes = { TRANSLATION: append(trs.TRANSLATION, 3) }
        nodes.push({ mesh: meshIndex, name: 'Instances_' + meshIndex, extensions: { EXT_mesh_gpu_instancing: { attributes } },
            extras: { instanceNames: group.map(n => n.name), instanceExtras: group.map(n => n.extras || {}) } })
        records.push({ node: nodes.length - 1, count: group.length, names: group.map(n => n.name) })
    }
    json.nodes = nodes; json.meshes = meshes; json.scenes[json.scene || 0].nodes = nodes.map((_,i)=>i)
    if (records.length) {
        json.extensionsUsed = [...new Set([...(json.extensionsUsed || []), 'EXT_mesh_gpu_instancing'])]
        json.extensionsRequired = [...new Set([...(json.extensionsRequired || []), 'EXT_mesh_gpu_instancing'])]
    }
    // Prune mesh/accessor/view closure, including instance attributes; discard duplicate compressed payloads.
    const accessorIds = new Set(), viewIds = new Set(), materialIds = new Set()
    for (const mesh of meshes) for (const p of mesh.primitives) {
        Object.values(p.attributes).forEach(i => accessorIds.add(i)); accessorIds.add(p.indices); materialIds.add(p.material)
        if (p.extensions?.KHR_draco_mesh_compression) viewIds.add(p.extensions.KHR_draco_mesh_compression.bufferView)
    }
    for (const node of nodes) Object.values(node.extensions?.EXT_mesh_gpu_instancing?.attributes || {}).forEach(i => accessorIds.add(i))
    for (const i of accessorIds) if (json.accessors[i].bufferView !== undefined) viewIds.add(json.accessors[i].bufferView)
    assert.ok(!json.images?.length && !json.textures?.length, 'city asset conversion only supports untextured palette')
    const accessors = [...accessorIds].map(i => json.accessors[i]), accessorMap = new Map([...accessorIds].map((i,n)=>[i,n]))
    const all = Buffer.concat(chunks), views = [], payloads = []; let offset = 0
    const viewMap = new Map()
    for (const i of viewIds) {
        const v = json.bufferViews[i], padding = (-offset)&3
        if (padding) { payloads.push(Buffer.alloc(padding)); offset += padding }
        viewMap.set(i, views.length)
        views.push({ ...v, byteOffset: offset }); payloads.push(all.subarray(v.byteOffset || 0, (v.byteOffset || 0)+v.byteLength)); offset += v.byteLength
    }
    for (const a of accessors) if (a.bufferView !== undefined) a.bufferView = viewMap.get(a.bufferView)
    const materials = [...materialIds].map(i => json.materials[i]), materialMap = new Map([...materialIds].map((i,n)=>[i,n]))
    for (const mesh of meshes) for (const p of mesh.primitives) {
        p.attributes = Object.fromEntries(Object.entries(p.attributes).map(([k,i])=>[k,accessorMap.get(i)]))
        p.indices = accessorMap.get(p.indices); p.material = materialMap.get(p.material)
        if (p.extensions?.KHR_draco_mesh_compression) p.extensions.KHR_draco_mesh_compression.bufferView = viewMap.get(p.extensions.KHR_draco_mesh_compression.bufferView)
    }
    for (const node of nodes) if (node.extensions?.EXT_mesh_gpu_instancing) node.extensions.EXT_mesh_gpu_instancing.attributes = Object.fromEntries(Object.entries(node.extensions.EXT_mesh_gpu_instancing.attributes).map(([k,i])=>[k,accessorMap.get(i)]))
    json.accessors = accessors; json.bufferViews = views; json.materials = materials; json.buffers = [{byteLength:offset}]
    return { bytes: packGlb(json, Buffer.concat(payloads)), report: { originalNodes, outputNodes: nodes.length, instanceGroups: records.length,
        instancedPlacements: records.reduce((s,g)=>s+g.count,0), records } }
}