orca/src/shared/utf8-byte-limits.test.ts

import { describe, expect, it } from 'vitest'
import {
  clampUtf8TextPrefix,
  getUtf8ChunkEndIndex,
  isUtf8ByteLengthWithinLimit,
  measureUtf8ByteLength,
  readUtf8CodePointAt
} from './utf8-byte-limits'

// Once V8 optimizes the calling function, `String.prototype.codePointAt` on a sliced string
// pairs a trailing high surrogate with the code unit that follows the SLICE inside its parent
// (reproduced on Node 24 and 26). A prefix slice cut mid-pair then reads a code point the
// string does not contain, so a scan reports one byte too many — but only after tier-up, which
// is why it surfaced as an intermittent CI failure rather than a deterministic one.
// Build a rope, cut it mid-pair, and hammer the scan so the optimizing tier is under test.
const SLICE_BOUNDARY_PARENT_UNITS = [
  0x72, 0x2e6, 0x7b, 0x54, 0xda9b, 0x568, 0x52, 0x26, 0x46, 0xc15b, 0x7, 0x768, 0xd9f6, 0xdcde
]
// V8 only creates a sliced string (rather than copying) at 13 code units or more.
const SLICE_BOUNDARY_UNITS = 13
const TIER_UP_ITERATIONS = 200_000

function buildSliceEndingInLoneHighSurrogate(): string {
  let text = ''
  for (const unit of SLICE_BOUNDARY_PARENT_UNITS) {
    text += String.fromCharCode(unit)
  }
  return text.slice(0, SLICE_BOUNDARY_UNITS)
}

describe('scanning a prefix slice whose parent continues past the slice', () => {
  const sliced = buildSliceEndingInLoneHighSurrogate()

  it('is a sliced string that ends in a lone high surrogate', () => {
    expect(sliced.length).toBe(SLICE_BOUNDARY_UNITS)
    expect(sliced.charCodeAt(SLICE_BOUNDARY_UNITS - 1)).toBe(0xd9f6)
    expect(Buffer.byteLength(sliced, 'utf8')).toBe(22)
  })

  it('reads the trailing lone surrogate without pairing past the end in every JIT tier', () => {
    const observed = new Set<number>()
    for (let iteration = 0; iteration < TIER_UP_ITERATIONS; iteration += 1) {
      observed.add(readUtf8CodePointAt(sliced, SLICE_BOUNDARY_UNITS - 1))
    }
    expect([...observed]).toEqual([0xd9f6])
  })

  it('measures the same byte length as the encoder in every JIT tier', () => {
    const expected = Buffer.byteLength(sliced, 'utf8')
    const observed = new Set<number>()
    for (let iteration = 0; iteration < TIER_UP_ITERATIONS; iteration += 1) {
      observed.add(measureUtf8ByteLength(sliced).byteLength)
    }
    expect([...observed]).toEqual([expected])
  })

  it('does not report an exceeded limit at the true byte length in every JIT tier', () => {
    const limit = Buffer.byteLength(sliced, 'utf8')
    const observed = new Set<boolean>()
    for (let iteration = 0; iteration < TIER_UP_ITERATIONS; iteration += 1) {
      observed.add(measureUtf8ByteLength(sliced, { stopAfterBytes: limit }).exceededLimit)
    }
    expect([...observed]).toEqual([false])
  })

  it('keeps the whole slice when clamping to its true byte length in every JIT tier', () => {
    const limit = Buffer.byteLength(sliced, 'utf8')
    const observed = new Set<number>()
    for (let iteration = 0; iteration < TIER_UP_ITERATIONS; iteration += 1) {
      observed.add(clampUtf8TextPrefix(sliced, limit).length)
    }
    expect([...observed]).toEqual([SLICE_BOUNDARY_UNITS])
  })
})

describe('getUtf8ChunkEndIndex', () => {
  it.each([
    { name: 'ASCII', text: 'abcdef', maxBytes: 3, endIndex: 3 },
    { name: 'multibyte characters', text: 'aé中z', maxBytes: 6, endIndex: 3 }
  ])('respects the byte budget for $name', ({ text, maxBytes, endIndex }) => {
    expect(getUtf8ChunkEndIndex(text, 0, maxBytes)).toBe(endIndex)
  })

  it('never splits valid surrogate pairs', () => {
    expect(getUtf8ChunkEndIndex('a😀b', 0, 4)).toBe(1)
    expect(getUtf8ChunkEndIndex('a😀b', 1, 4)).toBe(3)
  })

  it.each(['\ud800a', '\udc00a'])('counts a lone surrogate as three bytes', (text) => {
    expect(getUtf8ChunkEndIndex(text, 0, 3)).toBe(1)
  })

  it('starts measuring at the requested offset', () => {
    expect(getUtf8ChunkEndIndex('skipé中tail', 4, 5)).toBe(6)
  })

  it('returns the starting index when no text remains', () => {
    expect(getUtf8ChunkEndIndex('', 0, 1)).toBe(0)
    expect(getUtf8ChunkEndIndex('abc', 3, 1)).toBe(3)
  })

  it.each([0, -1])('consumes one code point when the %s-byte budget is exceeded', (maxBytes) => {
    expect(getUtf8ChunkEndIndex('😀a', 0, maxBytes)).toBe(2)
  })

  it('preserves raw non-finite budget comparisons', () => {
    expect(getUtf8ChunkEndIndex('abc', 0, Number.NaN)).toBe(3)
    expect(getUtf8ChunkEndIndex('abc', 0, Number.POSITIVE_INFINITY)).toBe(3)
    expect(getUtf8ChunkEndIndex('abc', 0, Number.NEGATIVE_INFINITY)).toBe(1)
  })
})

describe('isUtf8ByteLengthWithinLimit', () => {
  it.each([
    { name: 'ASCII at a finite limit', text: 'abc', maxBytes: 3, expected: true },
    { name: 'ASCII over a finite limit', text: 'abcd', maxBytes: 3, expected: false },
    { name: 'multibyte text at its limit', text: 'é中', maxBytes: 5, expected: true },
    { name: 'multibyte text over its limit', text: 'é中', maxBytes: 4, expected: false },
    { name: 'empty text with a negative limit', text: '', maxBytes: -1, expected: true },
    {
      name: 'nonempty text with negative infinity',
      text: 'a',
      maxBytes: -Infinity,
      expected: false
    },
    { name: 'text with a NaN limit', text: '😀', maxBytes: Number.NaN, expected: true },
    { name: 'text with positive infinity', text: '😀', maxBytes: Infinity, expected: true }
  ])('$name', ({ text, maxBytes, expected }) => {
    expect(isUtf8ByteLengthWithinLimit(text, maxBytes)).toBe(expected)
  })
})