/**
 * Apply-changes, v0 — the write path's planning half. Estimates and priority
 * live as exclusive label axes (`est/*`, `p/*`); a change swaps the axis label.
 * This computes the resulting label set + a human-readable diff *purely*, so the
 * UI can show a propose-approve consequence before the write and tests can pin
 * the semantics. The actual PUT (name→id resolution + network) is the bridge's
 * job — additive here, never assumed.
 */

import {
  type EstimateLabel,
  ESTIMATE_LABELS,
  type PriorityLabel,
  PRIORITY_LABELS,
} from '../labels/label-schema.js'

export type IssueChange =
  | { kind: 'reestimate'; issue: number; estimate: EstimateLabel | null }
  | { kind: 'reprioritize'; issue: number; priority: PriorityLabel | null }
  | { kind: 'assign'; issue: number; assignee: string | null }
  | { kind: 'remilestone'; issue: number; milestone: number | null; milestoneTitle?: string | null }

/** The label axes are the only kinds planned as a label swap; the rest are field writes. */
export function isLabelChange(
  change: IssueChange,
): change is Extract<IssueChange, { kind: 'reestimate' | 'reprioritize' }> {
  return change.kind === 'reestimate' || change.kind === 'reprioritize'
}

export interface LabelPlan {
  /** The full resulting label-name set (order: kept labels, then the new axis label). */
  labels: string[]
  /** Axis labels being added (0 or 1). */
  added: string[]
  /** Axis labels being removed (includes clearing a duplicated axis). */
  removed: string[]
  /** true when the change would leave the labels unchanged. */
  noop: boolean
}

/** A change that resolves to a label swap (the only kind `planIssueChange` accepts). */
export type LabelChange = Extract<IssueChange, { kind: 'reestimate' | 'reprioritize' }>

function axisFor(change: LabelChange): { labels: readonly string[]; target: string | null } {
  return change.kind === 'reestimate'
    ? { labels: ESTIMATE_LABELS, target: change.estimate }
    : { labels: PRIORITY_LABELS, target: change.priority }
}

/**
 * Plan the label mutation for a single change. Removes every label on the
 * change's axis except the target, adds the target if absent. Setting the axis
 * to null clears it. Cleans up a duplicated axis (two `est/*`) as a side effect.
 */
export function planIssueChange(current: string[], change: LabelChange): LabelPlan {
  const { labels: axis, target } = axisFor(change)
  const onAxis = current.filter((l) => axis.includes(l))
  const removed = onAxis.filter((l) => l !== target)
  const added = target && !current.includes(target) ? [target] : []
  const kept = current.filter((l) => !axis.includes(l))
  const labels = target ? [...kept, target] : kept
  return { labels, added, removed, noop: added.length === 0 && removed.length === 0 }
}

/** A short "est/2d → est/3d" (or "+p/1" / "−est/5d") summary for the confirm UI. */
export function describeChange(plan: LabelPlan): string {
  if (plan.noop) return 'no change'
  const from = plan.removed.length ? plan.removed.join(', ') : '∅'
  const to = plan.added.length ? plan.added.join(', ') : '∅'
  return `${from} → ${to}`
}

/**
 * One human-readable confirm line for ANY change kind — the unified describe for
 * the propose-approve UI. Label kinds delegate to {@link describeChange} (so they
 * need the issue's current labels); assign/milestone describe the field write
 * directly. Pure; no network.
 */
export function summarizeChange(change: IssueChange, currentLabels: string[] = []): string {
  switch (change.kind) {
    case 'reestimate':
    case 'reprioritize':
      return describeChange(planIssueChange(currentLabels, change))
    case 'assign':
      return change.assignee ? `assign → @${change.assignee}` : 'unassign'
    case 'remilestone':
      return change.milestone == null
        ? 'remove from milestone'
        : `milestone → ${change.milestoneTitle ?? `#${change.milestone}`}`
  }
}

/** A change the agent proposes: the concrete op + its human diff, ready for approve-then-apply. */
export interface ChangeProposal {
  change: IssueChange
  /** One-line diff for the approve UI — always present ({@link summarizeChange}). */
  summary: string
  /** The label plan, for label kinds only (absent for assign/milestone). */
  plan?: LabelPlan
  issueTitle?: string
}

/** What the `propose_change` tool accepts — a target issue and the fields to set. */
export interface ProposeChangeArgs {
  issue: number
  estimate?: EstimateLabel
  priority?: PriorityLabel
  /** Login to assign, or null to unassign. Omit to leave assignee untouched. */
  assignee?: string | null
  /** Milestone id to set, or null to clear. Omit to leave the milestone untouched. */
  milestone?: number | null
}

/** Current state + lookups a proposal needs to skip noops and label the milestone. */
export interface ProposalContext {
  currentAssignee?: string | null
  currentMilestoneId?: number | null
  milestones?: { id: number; title: string }[]
}

function isEstimate(v: unknown): v is EstimateLabel {
  return typeof v === 'string' && (ESTIMATE_LABELS as readonly string[]).includes(v)
}
function isPriority(v: unknown): v is PriorityLabel {
  return typeof v === 'string' && (PRIORITY_LABELS as readonly string[]).includes(v)
}

/**
 * Build the concrete, non-noop proposals for a `propose_change` request. Label
 * axes (est/*, p/*) are validated and never invented; assign/milestone are
 * emitted only when they differ from the issue's current value (via `ctx`). Each
 * proposal carries a human `summary`; label kinds also carry the label plan.
 */
export function proposalsFor(
  args: ProposeChangeArgs,
  currentLabels: string[],
  issueTitle?: string,
  ctx: ProposalContext = {},
): ChangeProposal[] {
  const out: ChangeProposal[] = []
  const pushLabel = (change: LabelChange) => {
    const plan = planIssueChange(currentLabels, change)
    if (!plan.noop) out.push({ change, plan, summary: describeChange(plan), issueTitle })
  }
  if (isEstimate(args.estimate)) pushLabel({ kind: 'reestimate', issue: args.issue, estimate: args.estimate })
  if (isPriority(args.priority)) pushLabel({ kind: 'reprioritize', issue: args.issue, priority: args.priority })

  if (args.assignee !== undefined && args.assignee !== (ctx.currentAssignee ?? null)) {
    const change: IssueChange = { kind: 'assign', issue: args.issue, assignee: args.assignee }
    out.push({ change, summary: summarizeChange(change), issueTitle })
  }
  if (args.milestone !== undefined && args.milestone !== (ctx.currentMilestoneId ?? null)) {
    const milestoneTitle = ctx.milestones?.find((m) => m.id === args.milestone)?.title ?? null
    const change: IssueChange = { kind: 'remilestone', issue: args.issue, milestone: args.milestone, milestoneTitle }
    out.push({ change, summary: summarizeChange(change), issueTitle })
  }
  return out
}