import { anchorIndexAt } from "@/lib/model/locus"
import type { DocumentModel, HeadingNode } from "@/lib/schemas/model"

/**
 * The AI request envelope (MVP §9, DATA-ARCHITECTURE "Request envelope"): the
 * context an AI call carries alongside the normalized quote — its full containing
 * paragraph and the heading breadcrumbs for its position — plus the nearest
 * preceding locus anchor (the cache row's `locusAnchor`, for the §12 locus index).
 * Pure functions of the Tier-1 model; no DOM.
 */

export interface Envelope {
  /** The full text of the block containing the offset (paragraph or heading). */
  paragraph: string
  /** Heading labels from the outermost down to the active one (Work › … › §). */
  breadcrumbs: string[]
  /** Nearest preceding anchor id, "" if the text has none. */
  locusAnchor: string
}

function pathToHeading(
  nodes: readonly HeadingNode[],
  offset: number,
  trail: HeadingNode[],
): HeadingNode[] {
  // The active child at this level is the last heading whose label starts at or
  // before the offset; descend into it to extend the breadcrumb trail.
  let active: HeadingNode | null = null
  for (const n of nodes) {
    if (n.start <= offset) active = n
    else break
  }
  if (!active) return trail
  trail.push(active)
  return pathToHeading(active.children, offset, trail)
}

export function buildEnvelope(model: DocumentModel, offset: number): Envelope {
  const block = model.blocks.find((b) => b.start <= offset && offset < b.end)
  const paragraph = block ? model.text.slice(block.start, block.end) : ""

  const breadcrumbs = pathToHeading(model.headings, offset, []).map((h) => h.label)

  const idx = anchorIndexAt(model, offset)
  const locusAnchor = idx >= 0 ? (model.anchors[idx]?.id ?? "") : ""

  return { paragraph, breadcrumbs, locusAnchor }
}