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\(context\.(.+?),\s*"(.*)"\)$/
const PATH_RE = /^context\.(.+)$/

export type ReverseMapToken = {
  kind: "path" | "content"
  result: string
  token: string
  contextPath?: string
  outputPath: string
  templatePath: string
  range?: {
    start: number
    end: number
  }
}

export type ReverseMapFile = {
  outputPath: string
  templatePath: string
  tokens: ReverseMapToken[]
}

export type ReverseMapManifest = {
  version: 1
  files: ReverseMapFile[]
  tokens: Record<string, string[]>
}

export type RenderOptions = {
  reverseMap?: boolean | string
}

type RenderState = {
  sourceRoot: string
  destRoot: string
  manifest?: ReverseMapManifest
}

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 createReverseMapManifest(): ReverseMapManifest {
  return { version: 1, files: [], tokens: {} }
}

function addReverseMapToken(
  state: RenderState | undefined,
  file: ReverseMapFile | undefined,
  token: ReverseMapToken,
) {
  if (!state?.manifest || !file) return
  file.tokens.push(token)
  const tokens = state.manifest.tokens[token.result] ?? []
  if (!tokens.includes(token.token)) tokens.push(token.token)
  state.manifest.tokens[token.result] = tokens
}

function getReverseMapPath(destRoot: string, reverseMap: true | string): string {
  if (reverseMap === true) return resolveOutputPath(destRoot, ".tdir-map.json")
  return resolveOutputPath(destRoot, reverseMap)
}

function getOutputName(
  entry: string,
  context: Record<string, unknown>,
  state?: RenderState,
  file?: ReverseMapFile,
): string | null {
  const ifMatch = entry.match(IF_PATH_RE)
  let outputName = entry
  if (ifMatch) {
    if (!evalCondition(ifMatch[1], context)) return null
    outputName = ifMatch[2]!
    if (ifMatch[0] !== outputName) {
      addReverseMapToken(state, file, {
        kind: "path",
        result: outputName,
        token: ifMatch[0],
        outputPath: file?.outputPath ?? "",
        templatePath: file?.templatePath ?? "",
      })
    }
  }

  return outputName.replace(VAR_RE, (_match, path: string) => {
    const result = String(resolveContext(context, path) ?? "")
    addReverseMapToken(state, file, {
      kind: "path",
      result,
      token: _match,
      contextPath: path,
      outputPath: file?.outputPath ?? "",
      templatePath: file?.templatePath ?? "",
    })
    return result
  })
}

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 {
  return renderContentWithMap(content, context)
}

function renderContentWithMap(
  content: string,
  context: Record<string, unknown>,
  state?: RenderState,
  file?: ReverseMapFile,
): string {
  const processed = processIfBlocks(content, context)
  let result = ""
  let pos = 0

  for (const match of processed.matchAll(VAR_RE)) {
    const token = match[0]
    const path = match[1]!
    const rendered = String(resolveContext(context, path) ?? "")
    result += processed.slice(pos, match.index)
    const start = result.length
    result += rendered
    addReverseMapToken(state, file, {
      kind: "content",
      result: rendered,
      token,
      contextPath: path,
      outputPath: file?.outputPath ?? "",
      templatePath: file?.templatePath ?? "",
      range: { start, end: start + rendered.length },
    })
    pos = match.index! + token.length
  }

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

function renderDir(
  srcDir: string,
  destDir: string,
  context: Record<string, unknown>,
  options: RenderOptions = {},
) {
  const destRoot = resolvePath(destDir)
  const sourceRoot = resolvePath(srcDir)
  const state: RenderState = {
    sourceRoot,
    destRoot,
    manifest: options.reverseMap ? createReverseMapManifest() : undefined,
  }
  const mapPath = options.reverseMap ? getReverseMapPath(destRoot, options.reverseMap) : undefined
  assertSafeRenderTarget(srcDir, destDir)
  validateOutputPaths(srcDir, destRoot, context)
  rmSync(destDir, { recursive: true, force: true })
  renderDirInner(srcDir, destRoot, context, state)
  if (state.manifest && mapPath) {
    mkdirSync(dirname(mapPath), { recursive: true })
    writeFileSync(mapPath, `${JSON.stringify(state.manifest, null, 2)}\n`)
  }
}

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>,
  state: RenderState,
) {
  mkdirSync(destDir, { recursive: true })
  const entries = readdirSync(srcDir).sort()

  for (const entry of entries) {
    const srcPath = resolvePath(srcDir, entry)
    const stat = statSync(srcPath)
    const templatePath = relative(state.sourceRoot, srcPath)
    const tempFile: ReverseMapFile = {
      outputPath: "",
      templatePath,
      tokens: [],
    }
    const outputName = getOutputName(entry, context, state, tempFile)
    if (outputName === null) continue

    const destPath = resolveOutputPath(destDir, outputName)
    const outputPath = relative(state.destRoot, destPath)
    tempFile.outputPath = outputPath
    for (const token of tempFile.tokens) {
      token.outputPath = outputPath
      token.templatePath = templatePath
    }

    if (stat.isDirectory()) {
      if (tempFile.tokens.length > 0) state.manifest?.files.push(tempFile)
      renderDirInner(srcPath, destPath, context, state)
    } else {
      mkdirSync(dirname(destPath), { recursive: true })
      const content = readFileSync(srcPath)
      if (isUtf8Text(content)) {
        const rendered = renderContentWithMap(content.toString("utf-8"), context, state, tempFile)
        writeFileSync(destPath, rendered)
      } else {
        copyFileSync(srcPath, destPath)
      }
      if (tempFile.tokens.length > 0) state.manifest?.files.push(tempFile)
    }
  }
}

export { renderDir, renderContent }