ReefVM/tests/tail-call.test.ts

import { test, expect } from "bun:test"
import { toBytecode } from "#bytecode"
import { VM } from "#vm"

test("TAIL_CALL - basic tail recursive countdown", async () => {
  // Tail recursive function that counts down to 0
  // function countdown(n) {
  //   if (n === 0) return "done"
  //   return countdown(n - 1)  // tail call
  // }
  const bytecode = toBytecode(`
    MAKE_FUNCTION (n) #8
    STORE countdown
    LOAD countdown
    PUSH 5
    PUSH 1
    PUSH 0
    CALL
    HALT
    LOAD n
    PUSH 0
    EQ
    JUMP_IF_FALSE #2
    PUSH "done"
    RETURN
    LOAD countdown
    LOAD n
    PUSH 1
    SUB
    PUSH 1
    PUSH 0
    TAIL_CALL
  `)

  const result = await new VM(bytecode).run()
  expect(result).toEqual({ type: 'string', value: 'done' })
})

test("TAIL_CALL - tail recursive sum with accumulator", async () => {
  // function sum(n, acc = 0) {
  //   if (n === 0) return acc
  //   return sum(n - 1, acc + n)  // tail call
  // }
  const bytecode = toBytecode(`
    MAKE_FUNCTION (n acc) #9
    STORE sum
    LOAD sum
    PUSH 10
    PUSH 0
    PUSH 2
    PUSH 0
    CALL
    HALT
    LOAD n
    PUSH 0
    EQ
    JUMP_IF_FALSE #2
    LOAD acc
    RETURN
    LOAD sum
    LOAD n
    PUSH 1
    SUB
    LOAD acc
    LOAD n
    ADD
    PUSH 2
    PUSH 0
    TAIL_CALL
  `)

  const result = await new VM(bytecode).run()
  expect(result).toEqual({ type: 'number', value: 55 })  // sum of 1..10
})

test("TAIL_CALL - doesn't overflow stack with deep recursion", async () => {
  // This would overflow the stack with regular CALL
  // but should work fine with TAIL_CALL
  const bytecode = toBytecode(`
    MAKE_FUNCTION (n) #8
    STORE deep
    LOAD deep
    PUSH 10000
    PUSH 1
    PUSH 0
    CALL
    HALT
    LOAD n
    PUSH 0
    LTE
    JUMP_IF_FALSE #2
    PUSH "success"
    RETURN
    LOAD deep
    LOAD n
    PUSH 1
    SUB
    PUSH 1
    PUSH 0
    TAIL_CALL
  `)

  const result = await new VM(bytecode).run()
  expect(result).toEqual({ type: 'string', value: 'success' })
})

test("TAIL_CALL - tail call to different function", async () => {
  // TAIL_CALL can call a different function (mutual recursion)
  // function even(n) { return n === 0 ? true : odd(n - 1) }
  // function odd(n) { return n === 0 ? false : even(n - 1) }
  const bytecode = toBytecode(`
    MAKE_FUNCTION (n) #10
    STORE even
    MAKE_FUNCTION (n) #23
    STORE odd
    LOAD even
    PUSH 7
    PUSH 1
    PUSH 0
    CALL
    HALT
    LOAD n
    PUSH 0
    EQ
    JUMP_IF_FALSE #2
    PUSH true
    RETURN
    LOAD odd
    LOAD n
    PUSH 1
    SUB
    PUSH 1
    PUSH 0
    TAIL_CALL
    LOAD n
    PUSH 0
    EQ
    JUMP_IF_FALSE #2
    PUSH false
    RETURN
    LOAD even
    LOAD n
    PUSH 1
    SUB
    PUSH 1
    PUSH 0
    TAIL_CALL
  `)

  const result = await new VM(bytecode).run()
  expect(result).toEqual({ type: 'boolean', value: false })  // 7 is odd
})