// The chunker: one text into citation-sized pieces. Pure TypeScript, unit-tested without a store
// or a model. Chunks split on blank lines where they can, so a snippet reads as a sentence and not
// as a byte range; a chunk that fits stays whole.

import { validationFailed } from "@conusai-kit/core"
import type { KnowledgeChunk } from "./ports.js"

export const CHUNK_SIZE = 2_000
const CHUNK_OVERLAP = 200

/**
 * Splits one document's text into chunks of at most {@link CHUNK_SIZE} characters. Paragraphs
 * longer than the size are hard-split; the overlap keeps a sentence split across two chunks
 * retrievable from either side.
 */
export function chunkText(text: string): KnowledgeChunk[] {
  if (text.length === 0)
    throw validationFailed([{ path: "file", message: "the file has no text to index" }], "nothing to index")
  const paragraphs = text.split(/\r?\n\r?\n/).flatMap((paragraph) => {
    if (paragraph.length <= CHUNK_SIZE) return [paragraph]
    const pieces: string[] = []
    for (let at = 0; at < paragraph.length; at += CHUNK_SIZE - CHUNK_OVERLAP) {
      pieces.push(paragraph.slice(at, at + CHUNK_SIZE))
    }
    return pieces
  })
  const chunks: KnowledgeChunk[] = []
  let current = ""
  for (const paragraph of paragraphs) {
    if (current !== "" && current.length + paragraph.length + 2 > CHUNK_SIZE) {
      chunks.push({ position: chunks.length + 1, text: current })
      current = ""
    }
    current = current === "" ? paragraph : `${current}\n\n${paragraph}`
  }
  if (current !== "") chunks.push({ position: chunks.length + 1, text: current })
  return chunks
}