import {
  copyFileSync,
  mkdirSync,
  readFileSync,
  readdirSync,
  rmSync,
  statSync,
  writeFileSync,
} from "node:fs"
import { dirname, isAbsolute, relative, resolve as resolvePath } from "node:path"
import { homedir } from "node:os"
import { TextDecoder } from "node:util"

const IF_PATH_RE = /^<@if\((.+?)\)>(.*)$/
const VAR_RE = /<@var\(context\.(.+?)(?::(\w+))?\)>/g
const DIRECTIVE_RE = /<@(if|elseif|else|endif)(?:\((.+?)\))?>/g
const EQ_RE = /^eq\(context\.(.+?),\s*"(.*)"\)$/
const PATH_RE = /^context\.(.+)$/

function evalCondition(expr: string | undefined, context: Record<string, unknown>): boolean {
  if (!expr) throw new Error("Missing condition expression")
  const eqMatch = expr.match(EQ_RE)
  if (eqMatch) {
    return resolveContext(context, eqMatch[1]!) === eqMatch[2]
  }
  const pathMatch = expr.match(PATH_RE)
  if (pathMatch) {
    return !!resolveContext(context, pathMatch[1]!)
  }
  throw new Error(`Invalid condition expression: ${expr}`)
}

function resolveContext(context: Record<string, unknown>, path: string): unknown {
  const segments = path.split(".")
  let current: unknown = context
  for (const seg of segments) {
    if (current === null || current === undefined || typeof current !== "object") return undefined
    current = (current as Record<string, unknown>)[seg]
  }
  return current
}

function isInsidePath(parent: string, child: string): boolean {
  const rel = relative(parent, child)
  return rel === "" || (!rel.startsWith("..") && !isAbsolute(rel))
}

function assertSafeRenderTarget(srcDir: string, destDir: string) {
  const sourceRoot = resolvePath(srcDir)
  const destRoot = resolvePath(destDir)

  if (destRoot === resolvePath(destRoot, "..")) {
    throw new Error("Refusing to render into filesystem root")
  }

  if (destRoot === resolvePath(process.cwd())) {
    throw new Error("Refusing to render into the current working directory")
  }

  if (destRoot === resolvePath(homedir())) {
    throw new Error("Refusing to render into the home directory")
  }

  if (isInsidePath(sourceRoot, destRoot) || isInsidePath(destRoot, sourceRoot)) {
    throw new Error("Refusing to render when source and target directories overlap")
  }
}

function resolveOutputPath(destRoot: string, outputName: string): string {
  const outputPath = resolvePath(destRoot, outputName)
  if (!isInsidePath(destRoot, outputPath)) {
    throw new Error(`Refusing to write outside target directory: ${outputName}`)
  }
  return outputPath
}

function getOutputName(entry: string, context: Record<string, unknown>): string | null {
  const ifMatch = entry.match(IF_PATH_RE)
  let outputName = entry
  if (ifMatch) {
    if (!evalCondition(ifMatch[1], context)) return null
    outputName = ifMatch[2]!
  }

  return outputName.replace(VAR_RE, (_match, path: string) => {
    return String(resolveContext(context, path) ?? "")
  })
}

function isUtf8Text(buffer: Buffer): boolean {
  if (buffer.indexOf(0) !== -1) return false
  try {
    new TextDecoder("utf-8", { fatal: true }).decode(buffer)
    return true
  } catch {
    return false
  }
}

// Each stack frame tracks: did any branch match yet, is the current branch active
type IfFrame = { matched: boolean; active: boolean; sawElse: boolean }

function processIfBlocks(content: string, context: Record<string, unknown>): string {
  let result = ""
  let pos = 0
  const stack: IfFrame[] = []

  const re = new RegExp(DIRECTIVE_RE.source, "g")
  let match: RegExpExecArray | null

  function isEmitting(): boolean {
    return stack.every(f => f.active)
  }

  while ((match = re.exec(content)) !== null) {
    const directive = match[1]!
    const condPath = match[2]

    if (directive === "if") {
      if (isEmitting()) result += content.slice(pos, match.index)
      const truthy = evalCondition(condPath, context)
      stack.push({ matched: truthy, active: truthy, sawElse: false })
      pos = re.lastIndex
    } else if (directive === "elseif") {
      if (stack.length === 0) throw new Error("Unexpected <@elseif> without <@if>")
      const top = stack[stack.length - 1]!
      if (top.sawElse) throw new Error("Unexpected <@elseif> after <@else>")
      // Emit text before this directive if current branch was active
      if (isEmitting()) result += content.slice(pos, match.index)
      if (top.matched) {
        // A previous branch already matched — skip this one
        top.active = false
      } else {
        const truthy = evalCondition(condPath, context)
        top.matched = truthy
        top.active = truthy
      }
      pos = re.lastIndex
    } else if (directive === "else") {
      if (stack.length === 0) throw new Error("Unexpected <@else> without <@if>")
      const top = stack[stack.length - 1]!
      if (top.sawElse) throw new Error("Unexpected duplicate <@else>")
      if (isEmitting()) result += content.slice(pos, match.index)
      top.active = !top.matched
      top.matched = true
      top.sawElse = true
      pos = re.lastIndex
    } else if (directive === "endif") {
      if (stack.length === 0) throw new Error("Unexpected <@endif> without <@if>")
      if (isEmitting()) result += content.slice(pos, match.index)
      stack.pop()
      pos = re.lastIndex
    }
  }

  if (stack.length > 0) {
    throw new Error("Unmatched <@if> without <@endif>")
  }

  result += content.slice(pos)
  return result
}

function renderContent(content: string, context: Record<string, unknown>): string {
  const processed = processIfBlocks(content, context)
  return processed.replace(VAR_RE, (_match, path: string) => {
    return String(resolveContext(context, path) ?? "")
  })
}

function renderDir(
  srcDir: string,
  destDir: string,
  context: Record<string, unknown>,
) {
  const destRoot = resolvePath(destDir)
  assertSafeRenderTarget(srcDir, destDir)
  validateOutputPaths(srcDir, destRoot, context)
  rmSync(destDir, { recursive: true, force: true })
  renderDirInner(srcDir, destRoot, context)
}

function validateOutputPaths(
  srcDir: string,
  destDir: string,
  context: Record<string, unknown>,
) {
  const entries = readdirSync(srcDir).sort()

  for (const entry of entries) {
    const srcPath = resolvePath(srcDir, entry)
    const stat = statSync(srcPath)
    const outputName = getOutputName(entry, context)
    if (outputName === null) continue

    const destPath = resolveOutputPath(destDir, outputName)
    if (stat.isDirectory()) {
      validateOutputPaths(srcPath, destPath, context)
    }
  }
}

function renderDirInner(
  srcDir: string,
  destDir: string,
  context: Record<string, unknown>,
) {
  mkdirSync(destDir, { recursive: true })
  const entries = readdirSync(srcDir).sort()

  for (const entry of entries) {
    const srcPath = resolvePath(srcDir, entry)
    const stat = statSync(srcPath)

    const outputName = getOutputName(entry, context)
    if (outputName === null) continue

    const destPath = resolveOutputPath(destDir, outputName)
    if (stat.isDirectory()) {
      renderDirInner(srcPath, destPath, context)
    } else {
      mkdirSync(dirname(destPath), { recursive: true })
      const content = readFileSync(srcPath)
      if (isUtf8Text(content)) {
        writeFileSync(destPath, renderContent(content.toString("utf-8"), context))
      } else {
        copyFileSync(srcPath, destPath)
      }
    }
  }
}

export { renderDir, renderContent }