@the-little-books/little/src/env/env-next.ts

import * as Env from "../env"
import * as Node from "../node"
import * as Pattern from "../pattern"

function put_back_entry(env: Env.Env, entry: Env.ReturnEntry): Env.Env {
  const { nodes, index } = entry
  // NOTE Handle proper tail call.
  // - put back entry unless at the tail of its nodes.
  if (index + 1 < nodes.length) {
    env.return_stack.push({ ...entry, index: index + 1 })
  }
  return env
}

export async function next(env: Env.Env): Promise<Node.Node> {
  const entry = env.return_stack.pop()
  if (entry === undefined) {
    throw new Error("The return_stack is empty.")
  }
  const { nodes, index } = entry
  const node = nodes[index]
  if (node.kind === "Node.Element") {
    if (node.tag === "jump") {
      return await next_jump(env, entry, node)
    } else if (node.tag === "call") {
      return await next_call(env, entry, node)
    } else if (node.tag === "match") {
      return await next_match(env, entry, node)
    } else {
      put_back_entry(env, entry)
      return node
    }
  } else {
    put_back_entry(env, entry)
    return node
  }
}

function find_label_index(nodes: Array<Node.Node>, label: string): number {
  return nodes.findIndex(
    (node) => node.kind === "Node.Element" && node.attributes.label === label
  )
}

async function next_jump(
  env: Env.Env,
  entry: Env.ReturnEntry,
  node: Node.Element
): Promise<Node.Node> {
  const module = node.attributes.module || entry.module
  const nodes = await env.loader(env.book, module)
  const index = node.attributes.label
    ? find_label_index(nodes, node.attributes.label)
    : 0
  env.return_stack.push({ module, nodes, index })
  return await next(env)
}

async function next_call(
  env: Env.Env,
  entry: Env.ReturnEntry,
  node: Node.Element
): Promise<Node.Node> {
  put_back_entry(env, entry)
  return await next_jump(env, entry, node)
}

async function next_match(
  env: Env.Env,
  entry: Env.ReturnEntry,
  node: Node.Element
): Promise<Node.Node> {
  const top_node = env.node_stack.pop()
  // NOTE better log.
  // console.log("top_node:", top_node)
  if (top_node === undefined) {
    throw new Error("Empty env.node_stack.")
  }
  for (const case_node of node.contents) {
    if (case_node.kind === "Node.Element") {
      if (case_node.tag === "case") {
        const pattern = Pattern.from_node(case_node.contents[0])
        const result = Pattern.match(pattern, top_node)
        if (result !== null) {
          // TODO use `result` to subst pattern variables in body.
          const body = case_node.contents.slice(1)
          put_back_entry(env, entry)
          env.return_stack.push({ ...entry, nodes: body, index: 0 })
          return await next(env)
        }
      } else if (case_node.tag === "default") {
        const body = case_node.contents
        put_back_entry(env, entry)
        env.return_stack.push({ ...entry, nodes: body, index: 0 })
        return await next(env)
      }
    }
  }
  throw new Error("Mismatch.")
}