import type { ClassSchema, MergeSuggestion } from '../../types/auto-group'

const THRESHOLD = 0.95
const MIN_CLASS_SIZE = 4

function functionalDependency(rows: string[][], srcIdx: number, dstIdx: number): number {
  const map = new Map<string, Set<string>>()
  for (const row of rows) {
    const a = row[srcIdx] ?? ''
    const b = row[dstIdx] ?? ''
    const set = map.get(a)
    if (set) set.add(b)
    else map.set(a, new Set([b]))
  }
  if (map.size === 0) return 0
  let functional = 0
  for (const set of map.values()) if (set.size === 1) functional++
  return functional / map.size
}

export function findMerges(schema: ClassSchema, rows: string[][]): MergeSuggestion[] {
  if (rows.length < MIN_CLASS_SIZE) return []

  const out: MergeSuggestion[] = []
  for (let i = 0; i < schema.columns.length - 1; i++) {
    const a = schema.columns[i]
    const b = schema.columns[i + 1]
    if (a.role !== 'factor' && b.role !== 'factor') continue

    const forward = functionalDependency(rows, a.index, b.index)
    const backward = functionalDependency(rows, b.index, a.index)
    if (forward < THRESHOLD || backward < THRESHOLD) continue

    const merged = new Set(rows.map(r => `${r[a.index] ?? ''}_${r[b.index] ?? ''}`))
    if (merged.size < 2) continue

    out.push({
      classKind: schema.classKind,
      subKind: schema.subKind,
      columnIndices: [a.index, b.index],
      proposedName: `Column ${a.index + 1}–${b.index + 1}`,
      confidence: Math.min(forward, backward),
    })
  }
  return out
}