/**
 * Request-header reconstruction utilities over full `request/header` session
 * events. Anyone holding a session log reconstructs the {@link EpochHeader}
 * any request was built under by taking the latest canonical snapshot; the
 * loop uses the same equality helper to avoid logging unchanged headers.
 *
 * @module dsh-session/request-header
 */

import { callConfigEquals } from '@deepseek-ai/dsh-llm'
import type { Message, ToolSchema } from '@deepseek-ai/dsh-llm'
import type { EpochHeader, SessionEvent } from './types.ts'

/**
 * Normalize a header to canonical form: an empty system prompt, an empty tool
 * list, and an empty session prefix become absent fields, matching how requests
 * are built. Logging, folding, and comparison use this one representation.
 * @param header - the header to normalize (not mutated).
 * @returns the canonical header.
 */
export function canonicalHeader(header: EpochHeader): EpochHeader {
  return {
    config: header.config,
    ...header.system !== undefined && header.system.length > 0 ? { system: header.system } : {},
    ...header.tools !== undefined && header.tools.length > 0 ? { tools: header.tools } : {},
    ...header.messagePrefix !== undefined && header.messagePrefix.length > 0 ? { messagePrefix: header.messagePrefix } : {},
  }
}

/** Canonical JSON equality for tool schemas assembled through the same path. */
function sameSchema(a: ToolSchema, b: ToolSchema): boolean {
  return JSON.stringify(a) === JSON.stringify(b)
}

/** Canonical JSON equality over session-prefix arrays; absence equals empty. */
function sameMessages(a: readonly Message[] | undefined, b: readonly Message[] | undefined): boolean {
  return JSON.stringify(a ?? []) === JSON.stringify(b ?? [])
}

/**
 * Field-wise equality over canonical headers. Tool schemas compare in order;
 * the session prefix compares as canonical JSON.
 * @param a - one canonical header.
 * @param b - the other.
 * @returns whether config, system, tools, and session prefix all match.
 */
export function headerEquals(a: EpochHeader, b: EpochHeader): boolean {
  if (!callConfigEquals(a.config, b.config) || a.system !== b.system) return false
  if (!sameMessages(a.messagePrefix, b.messagePrefix)) return false
  const at = a.tools ?? []
  const bt = b.tools ?? []
  return at.length === bt.length && at.every((tool, i) => sameSchema(tool, bt[i] as ToolSchema))
}

/**
 * Fold the header events of a log (or any prefix) into the
 * {@link EpochHeader} in force after the last snapshot. Non-header events are
 * skipped. This is the pure offline reconstruction path; the live session
 * tracks the same fold incrementally.
 * @param events - session events in log order.
 * @param from - a previously folded state to continue from.
 * @returns the latest canonical header, or undefined when none exists yet.
 */
export function foldRequestHeader(events: readonly SessionEvent[], from?: EpochHeader): EpochHeader | undefined {
  let state = from
  for (const event of events) {
    if (event.type === 'request/header') state = canonicalHeader(event.data.header)
  }
  return state
}