import type {ImportSource,ImportLine} from './imports.ts';
import {fault,validJson} from '../canvas-core/values.ts';
type Range={start:number;end:number};
/** Index the envelope without decoding element payloads. Original bytes remain the source.
 * Preparation scans O(bytes) and retains O(records) ranges; this is not constant memory. */
export async function openDocumentImportSource(options:{bytes:Blob;fingerprint:string;signal:AbortSignal;maxRecordBytes?:number;yieldTask?:()=>Promise<void>}):Promise<ImportSource>{
 const {bytes,signal}=options,max=options.maxRecordBytes??256*1024;
 if(!(bytes instanceof Blob)||bytes.size>64*1024*1024||!options.fingerprint||!Number.isSafeInteger(max)||max<1)throw fault('invalid_import_source');
 let cursor=0,cache={start:-1,bytes:new Uint8Array()},work=0;
 const yieldTask=options.yieldTask??(()=>new Promise<void>(resolve=>setTimeout(resolve,0))),space=(b:number)=>[9,10,13,32].includes(b);
 async function peek(){if(signal.aborted)throw fault('import_aborted');if(cursor>=bytes.size)return -1;if(cursor<cache.start||cursor>=cache.start+cache.bytes.length){cache={start:cursor,bytes:new Uint8Array(await bytes.slice(cursor,cursor+16384).arrayBuffer())};if(signal.aborted)throw fault('import_aborted');}return cache.bytes[cursor-cache.start];}
 async function step(){cursor++;if(++work>=16384){work=0;await yieldTask();if(signal.aborted)throw fault('import_aborted');}}
 async function spaces(){while(space(await peek()))await step();}
 async function take(byte:number){await spaces();if(await peek()!==byte)throw fault('invalid_document_import');await step();}
 async function value():Promise<Range>{await spaces();const start=cursor,stack:number[]=[];let quoted=false,escaped=false;
  while(true){const b=await peek();if(b<0){if(quoted||stack.length)throw fault('invalid_document_import');break;}if(!quoted&&!stack.length&&(b===44||b===93||b===125||space(b)))break;
   if(quoted){if(escaped)escaped=false;else if(b===92)escaped=true;else if(b===34)quoted=false;}else if(b===34)quoted=true;else if(b===91||b===123){stack.push(b);if(stack.length>256)throw fault('json_nesting_limit');}else if(b===93||b===125){if(stack.pop()!==(b===93?91:123))throw fault('invalid_document_import');}
   const ended=!quoted&&stack.length===0&&(b===34||b===93||b===125);await step();if(ended)break;
  }if(cursor===start)throw fault('invalid_document_import');return{start,end:cursor};
 }
 async function decode(range:Range,limit=max){if(range.end-range.start>limit)throw fault('record_too_large');let text:string;try{text=new TextDecoder('utf-8',{fatal:true}).decode(await bytes.slice(range.start,range.end).arrayBuffer());}catch{throw fault('invalid_utf8');}try{const result=JSON.parse(text);if(!validJson(result))throw Error();return result;}catch{throw fault('invalid_json');}}
 const fields=new Map<string,unknown>(),elements:Range[]=[];
 await take(123);await spaces();if(await peek()===125)throw fault('invalid_document_import');
 while(true){const key=await decode(await value(),1024);if(typeof key!=='string'||fields.has(key))throw fault('invalid_document_import');await take(58);
  if(key==='elements'){fields.set(key,true);await take(91);await spaces();if(await peek()!==93)while(true){elements.push(await value());await spaces();if(await peek()===93)break;await take(44);}await take(93);}
  else fields.set(key,await decode(await value(),4*1024*1024));
  await spaces();if(await peek()===125){await step();break;}await take(44);
 }
 await spaces();if(cursor!==bytes.size||fields.get('documentFormatVersion')!==1||typeof fields.get('projectId')!=='string'||!fields.get('projectId')||!Array.isArray(fields.get('canvases'))||!fields.has('elements')||[...fields.keys()].some(key=>!['documentFormatVersion','projectId','canvases','elements','tombstones'].includes(key)))throw fault('invalid_document_import');
 const canvases=fields.get('canvases') as {id:string;parent:null|{canvasId:string;entryElementId:string}}[],ids=new Set<string>();
 for(const canvas of canvases){if(!canvas||typeof canvas.id!=='string'||!canvas.id||ids.has(canvas.id)||canvas.parent!==null&&(!canvas.parent||typeof canvas.parent.canvasId!=='string'||typeof canvas.parent.entryElementId!=='string'))throw fault('invalid_document_import');ids.add(canvas.id);}
 const roots=canvases.filter(canvas=>canvas.parent===null);if(roots.length!==1)throw fault('invalid_document_import');const rootId=roots[0].id;
 const byId=new Map(canvases.map(canvas=>[canvas.id,canvas]));let topologyWork=0;
 for(const canvas of canvases){const seen=new Set<string>();let current=canvas;while(current.parent){if(seen.has(current.id))throw fault('invalid_document_import');seen.add(current.id);if(++topologyWork%128===0){await yieldTask();if(signal.aborted)throw fault('import_aborted');}const parent=byId.get(current.parent.canvasId);if(!parent)throw fault('invalid_document_import');current=parent;}}
 const tombstones=fields.get('tombstones');if(tombstones!==undefined&&(!tombstones||typeof tombstones!=='object'||Array.isArray(tombstones)||Object.values(tombstones).some(n=>!Number.isSafeInteger(n)||(n as number)<1)))throw fault('invalid_document_import');
 const children=canvases.filter(canvas=>canvas.id!==rootId),count=children.length+elements.length;
 return Object.freeze({fingerprint:options.fingerprint,parserVersion:4,async read(position:number,readSignal:AbortSignal):Promise<ImportLine|null>{
  if(readSignal.aborted)throw fault('import_aborted');if(!Number.isSafeInteger(position)||position<0||position>count)throw fault('invalid_import_cursor');if(position===count)return null;
  const base={cursor:position,nextCursor:position+1,key:`record:${position}`};let record:any,kind:string;
  try{if(position<children.length){kind='canvas';record=children[position];}else{kind='element';record=await decode(elements[position-children.length]);}if(readSignal.aborted)throw fault('import_aborted');if(!record||typeof record!=='object'||Array.isArray(record)||typeof record.id!=='string'||!record.id)throw fault('invalid_native_import_record');return{...base,value:{kind,key:record.id,record,sourceRootId:rootId}};}catch(error){if(readSignal.aborted)throw fault('import_aborted');return{...base,error:(error as {code?:string}).code??'invalid_document_import'};}
 }});
}