/**
 * POSIX-ish $(( )) arithmetic used by the guest shell.
 */
import { shellCheckUnboundParam } from './shell-nounset.js'

/**
 * Expand `$VAR`, `${VAR}`, and `$N` inside arithmetic before identifier substitution.
 * @param {string} inner
 * @param {Record<string, string>} env
 */
export function bareOsArithmeticExpandDollarForms(inner, env) {
  const s = String(inner || '')
  if (s.length > 4096) throw new Error('shell: arithmetic: expression too long')
  /** @type {string[]} */
  const parts = []
  let i = 0
  let replacements = 0
  const maxRep = 256
  while (i < s.length) {
    if (s[i] !== '$') {
      parts.push(s[i++])
      continue
    }
    if (++replacements > maxRep)
      throw new Error('shell: arithmetic: expansion limit exceeded')
    if (i + 1 >= s.length) {
      parts.push('$')
      i++
      continue
    }
    if (s[i + 1] === '{') {
      const end = s.indexOf('}', i + 2)
      if (end === -1) {
        parts.push('$')
        i++
        continue
      }
      const name = s.slice(i + 2, end).trim()
      if (!/^[A-Za-z_][A-Za-z0-9_]*$/.test(name))
        throw new Error('shell: arithmetic: invalid ${} in expression')
      shellCheckUnboundParam(name, env)
      const raw = env[name]
      const n = Number.parseInt(String(raw ?? '0').trim(), 10)
      parts.push(String(Number.isFinite(n) ? n : 0))
      i = end + 1
      continue
    }
    const c1 = s[i + 1]
    if (/[0-9]/.test(c1)) {
      shellCheckUnboundParam(c1, env)
      const raw = env[c1]
      const n = Number.parseInt(String(raw ?? '0').trim(), 10)
      parts.push(String(Number.isFinite(n) ? n : 0))
      i += 2
      continue
    }
    if (!/[A-Za-z_]/.test(c1)) {
      parts.push('$')
      i++
      continue
    }
    let k = i + 2
    while (k < s.length && /[A-Za-z0-9_]/.test(s[k])) k++
    const name = s.slice(i + 1, k)
    shellCheckUnboundParam(name, env)
    const raw = env[name]
    const n = Number.parseInt(String(raw ?? '0').trim(), 10)
    parts.push(String(Number.isFinite(n) ? n : 0))
    i = k
  }
  return parts.join('')
}

/**
 * @param {string} expr
 * @param {Record<string, string>} env
 */
export function bareOsEvalArithmeticExpr(expr, env) {
  const pre = bareOsArithmeticExpandDollarForms(String(expr || ''), env)
  const src = pre.trim()
  if (!src) return '0'
  if (!/^[A-Za-z0-9_+\-*/%()\s]+$/.test(src)) {
    throw new Error('shell: arithmetic: invalid token')
  }
  const replaced = src.replace(/\b[A-Za-z_][A-Za-z0-9_]*\b/g, (name) => {
    shellCheckUnboundParam(name, env)
    const raw = env[name]
    const n = Number.parseInt(String(raw ?? '0').trim(), 10)
    return String(Number.isFinite(n) ? n : 0)
  })
  let out = 0
  try {
    out = Function('"use strict"; return (' + replaced + ');')()
  } catch {
    throw new Error('shell: arithmetic: invalid expression')
  }
  const num = Number(out)
  if (!Number.isFinite(num)) throw new Error('shell: arithmetic: non-finite result')
  return String(Math.trunc(num))
}