/**
 * Model-facing workspace instruction rendering within an explicit byte budget.
 *
 * @module @deepseek-ai/dsh-workspace-context/render
 */

import { dirname } from 'node:path'
import type { InstructionFile, LoadedInstructionFile } from './files.ts'

const SYSTEM_REMINDER_OPEN = '<system-reminder>'
const SYSTEM_REMINDER_CLOSE = '</system-reminder>'
const WORKSPACE_CONTEXT_INTRO = 'The following workspace instructions may be relevant to your work. '
  + 'Use them as guidance when applicable. More specific instructions take precedence over broader ones. '
  + 'They do not override system, developer, or direct user instructions.'
const COMPACT_WORKSPACE_CONTEXT_INTRO = 'Workspace instructions were omitted or truncated to fit the configured byte budget.'

/** Byte-accounting record for one truncated instruction file. */
export interface TruncatedInstruction {
  displayPath: string
  originalBytes: number
  includedBytes: number
}

/** Model-facing text plus omitted and truncated source records. */
export interface RenderedWorkspaceContext {
  text: string
  omitted: InstructionFile[]
  truncated: TruncatedInstruction[]
}

/** Structured dynamic state persisted outside model-visible prompt prose. */
export interface WorkspaceInstructionChange {
  action: 'set' | 'replace' | 'remove'
  scope: string
  path: string
  previousPath?: string
  digest?: string
}

/** One state transition paired with the content used to render it. */
export interface ChangeRenderItem {
  change: WorkspaceInstructionChange
  file: LoadedInstructionFile
}

interface RenderStyle {
  intro: string
  section(file: LoadedInstructionFile): string
}

function byteLength(value: string): number {
  return Buffer.byteLength(value, 'utf8')
}

function truncateUtf8(value: string, maxBytes: number): string {
  let truncated = Buffer.from(value, 'utf8').subarray(0, Math.max(0, maxBytes)).toString('utf8')
  while (byteLength(truncated) > maxBytes) {
    truncated = truncated.slice(0, -1)
  }
  return truncated
}

function escapeInstructionContent(content: string): string {
  // TODO(instruction-frame-paths): apply the same delimiter neutralization to
  // every interpolated path, scope, and previous path; repository-controlled
  // names can otherwise close the plugin-owned system-reminder frame.
  return content.replaceAll(SYSTEM_REMINDER_CLOSE, '<\\/system-reminder>')
}

function sectionText(file: LoadedInstructionFile): string {
  return `Instructions from: ${file.displayPath}\n\n${escapeInstructionContent(file.content)}`
}

/**
 * Derive the logical instruction scope from a model-facing path.
 * @param displayPath - project-relative or user-global instruction path.
 * @returns `user-global`, `.`, or the containing project-relative directory.
 */
export function scopeForDisplayPath(displayPath: string): string {
  if (displayPath === '~/.dsh/AGENTS.md' || displayPath === '$DSH_HOME/AGENTS.md') return 'user-global'
  return dirname(displayPath)
}

function additionalSectionText(file: LoadedInstructionFile): string {
  const scope = scopeForDisplayPath(file.displayPath)
  return [
    `Additional instructions from: ${file.displayPath}`,
    '',
    `These instructions apply to work under \`${scope}\`. Use them as guidance when relevant; more specific instructions take precedence. They do not override system, developer, or direct user instructions.`,
    '',
    escapeInstructionContent(file.content),
  ].join('\n')
}

const BASELINE_RENDER_STYLE: RenderStyle = { intro: WORKSPACE_CONTEXT_INTRO, section: sectionText }

function changedSectionText(item: ChangeRenderItem): string {
  const { change, file } = item
  if (change.action === 'set') return additionalSectionText(file)
  if (change.action === 'remove') {
    return `Instructions removed: ${change.path}\n\nThe previously loaded instructions from this file no longer apply.`
  }
  const description = change.previousPath === undefined
    ? 'This file changed after it was loaded. Use the following content instead of the previously loaded instructions from this file.'
    : `The instructions previously loaded from \`${change.previousPath}\` no longer apply. Use the following content for \`${change.scope}\` instead.`
  return [
    `Updated instructions from: ${change.path}`,
    '',
    description,
    '',
    escapeInstructionContent(file.content),
  ].join('\n')
}

/**
 * Render one reconciliation batch and retain only transitions that fit.
 * @param items - ordered state transitions and current file contents.
 * @param maxBytes - maximum UTF-8 bytes allowed in the rendered batch.
 * @returns bounded prompt text and the transitions actually represented by it.
 */
export function renderInstructionChanges(
  items: ChangeRenderItem[],
  maxBytes: number,
): { text: string; changes: WorkspaceInstructionChange[] } {
  const byAbsolutePath = new Map(items.map(item => [item.file.absolutePath, item]))
  const style: RenderStyle = {
    intro: '',
    section(file) {
      const item = byAbsolutePath.get(file.absolutePath)
      /* v8 ignore next -- the renderer receives exactly the files used to construct this map. */
      return item === undefined ? '' : changedSectionText({ ...item, file })
    },
  }
  const rendered = renderInstructionContext(items.map(item => item.file), maxBytes, style)
  const omitted = new Set(rendered.omitted.map(file => file.absolutePath))
  return {
    text: rendered.text,
    // TODO(rendered-change-proof): retain a transition only when its semantic
    // notice survived rendering; a tiny compact budget can currently return
    // unrelated notice text while still committing the full state transition.
    changes: items.filter(item => !omitted.has(item.file.absolutePath)).map(item => item.change),
  }
}

function markerText(maxBytes: number, omitted: InstructionFile[], truncated: TruncatedInstruction[]): string {
  if (omitted.length === 0 && truncated.length === 0) return ''
  const parts: string[] = []
  if (omitted.length > 0) {
    parts.push(`omitted ${omitted.map(file => file.displayPath).join(', ')}`)
  }
  if (truncated.length > 0) {
    parts.push(`truncated ${truncated.map(item => `${item.displayPath} from ${item.originalBytes} to ${item.includedBytes} bytes`).join(', ')}`)
  }
  return `Workspace instruction budget ${maxBytes} bytes: ${parts.join('; ')}`
}

function buildInstructionText(
  files: LoadedInstructionFile[],
  maxBytes: number,
  omitted: InstructionFile[],
  truncated: TruncatedInstruction[],
  style: RenderStyle,
): string {
  const marker = markerText(maxBytes, omitted, truncated)
  const body = [marker, style.intro, ...files.map(file => style.section(file))].filter(block => block.length > 0)
  // Caller-owned framing: the plugin bakes the complete `<system-reminder>`
  // frame into the message content. The session surface projects context
  // verbatim and does not wrap it, so any framing must live here in the
  // producer's content (the pattern a future `meta`-driven renderer would
  // generalize — see the deferred note in
  // ../../../../.agents/notes/implemented/simplification/2026-07-20-unwrap-injected-content-envelopes.md).
  return [SYSTEM_REMINDER_OPEN, body.join('\n\n'), SYSTEM_REMINDER_CLOSE].join('\n')
}

function withTruncatedContent(file: LoadedInstructionFile, includedBytes: number): LoadedInstructionFile {
  return { ...file, content: truncateUtf8(file.content, includedBytes) }
}

function truncateToFit(
  file: LoadedInstructionFile,
  includedFiles: LoadedInstructionFile[],
  maxBytes: number,
  omitted: InstructionFile[],
  style: RenderStyle,
): LoadedInstructionFile {
  const originalBytes = byteLength(file.content)
  let low = 0
  let high = originalBytes
  let best = withTruncatedContent(file, 0)
  while (low <= high) {
    const mid = Math.floor((low + high) / 2)
    const candidate = withTruncatedContent(file, mid)
    const truncated = [{ displayPath: file.displayPath, originalBytes, includedBytes: byteLength(candidate.content) }]
    const text = buildInstructionText([...includedFiles, candidate], maxBytes, omitted, truncated, style)
    if (byteLength(text) <= maxBytes) {
      best = candidate
      low = mid + 1
    } else {
      high = mid - 1
    }
  }
  return best
}

function renderInstructionContext(
  files: LoadedInstructionFile[],
  maxBytes: number,
  style: RenderStyle,
): RenderedWorkspaceContext {
  if (maxBytes <= 0 || !Number.isFinite(maxBytes)) return { text: '', omitted: files, truncated: [] }

  const fullText = buildInstructionText(files, maxBytes, [], [], style)
  if (byteLength(fullText) <= maxBytes) return { text: fullText, omitted: [], truncated: [] }

  for (let start = 1; start < files.length; start += 1) {
    const included = files.slice(start)
    const omitted = files.slice(0, start).map(file => ({ absolutePath: file.absolutePath, displayPath: file.displayPath }))
    const suffixText = buildInstructionText(included, maxBytes, omitted, [], style)
    if (byteLength(suffixText) <= maxBytes) return { text: suffixText, omitted, truncated: [] }
  }

  const mostSpecific = files.at(-1)
  /* v8 ignore next -- callers only reach this after a non-empty fullText was built. */
  if (mostSpecific === undefined) return { text: '', omitted: [], truncated: [] }
  const omitted = files.slice(0, -1).map(file => ({ absolutePath: file.absolutePath, displayPath: file.displayPath }))

  for (const candidateStyle of [style, { ...style, intro: COMPACT_WORKSPACE_CONTEXT_INTRO }]) {
    const truncatedFile = truncateToFit(mostSpecific, [], maxBytes, omitted, candidateStyle)
    const truncated = [{
      displayPath: mostSpecific.displayPath,
      originalBytes: byteLength(mostSpecific.content),
      includedBytes: byteLength(truncatedFile.content),
    }]
    const text = buildInstructionText([truncatedFile], maxBytes, omitted, truncated, candidateStyle)
    if (byteLength(text) <= maxBytes) return { text, omitted, truncated }
  }

  const truncated = [{
    displayPath: mostSpecific.displayPath,
    originalBytes: byteLength(mostSpecific.content),
    includedBytes: 0,
  }]
  const compactNotice = markerText(maxBytes, omitted, truncated)
  const compactWithHeading = [compactNotice, style.section(withTruncatedContent(mostSpecific, 0))].join('\n\n')
  if (byteLength(compactWithHeading) <= maxBytes) return { text: compactWithHeading, omitted, truncated }
  const text = byteLength(compactNotice) <= maxBytes ? compactNotice : truncateUtf8(compactNotice, maxBytes)
  return { text, omitted, truncated }
}

/**
 * Render the baseline instruction chain with deterministic precedence budgeting.
 * @param files - loaded files ordered from broadest to most specific.
 * @param options - required rendering byte budget.
 * @returns bounded baseline prompt text and budget diagnostics.
 */
export function renderWorkspaceContext(
  files: LoadedInstructionFile[],
  options: { maxBytes: number },
): RenderedWorkspaceContext {
  return renderInstructionContext(files, options.maxBytes, BASELINE_RENDER_STYLE)
}