const greeting = `Hello`
console.log(greeting)

Edit the buffer, then press Cmd/Ctrl+S. Unsaved changes: no. Saved 47 bytes.

svelte<script lang="ts">
  import 'svelte-widgets/code-editor/editor.css'
  import { CodeEditor } from 'svelte-widgets'
  import { create_editor_model, TOKEN_CLASS_NAMES } from 'svelte-widgets/code-editor'
  import type { EditorBackend, EditorUpdate, TextEdit } from 'svelte-widgets/code-editor'

  const initial_text = `const greeting = \`Hello\`\nconsole.log(greeting)\n`
  const model = create_editor_model({ uri: `greeting.ts`, text: initial_text })
  let saved_text = $state(initial_text)
  let status = $state(`Edit the buffer, then press Cmd/Ctrl+S`)
  let dirty = $state(false)
  let backend_text = initial_text
  let backend_revision = 0

  const keyword_class = TOKEN_CLASS_NAMES.indexOf(`keyword`)
  const spans_for = (line: string) => {
    const match = /\b(?:const|let|function|return)\b/u.exec(line)
    return match
      ? [0, 0, match.index, keyword_class, match.index + match[0].length, 0]
      : []
  }
  const apply_text_edits = (text: string, edits: readonly TextEdit[]) => {
    for (const { from, to, insert } of edits)
      text = text.slice(0, from) + insert + text.slice(to)
    return text
  }

  const backend: EditorBackend = {
    open_doc: ({ text, revision }) => {
      backend_text = text
      backend_revision = revision
      return Promise.resolve({
        language: `TypeScript`,
        highlightable: true,
        editable: true,
      })
    },
    highlight_lines: ({ start_line, end_line }) =>
      Promise.resolve(
        backend_text.split(`\n`).slice(start_line, end_line).map(spans_for),
      ),
    apply_edits: ({
      base_revision,
      revision,
      edits,
      expected_line_count,
      expected_length,
    }) => {
      if (base_revision !== backend_revision)
        return Promise.reject(
          new Error(`Expected revision ${backend_revision}, received ${base_revision}`),
        )
      const next_text = apply_text_edits(backend_text, edits)
      if (
        next_text.split(`\n`).length !== expected_line_count ||
        next_text.length !== expected_length
      )
        return Promise.reject(new Error(`Editor buffers diverged`))
      backend_text = next_text
      backend_revision = revision
      return Promise.resolve(revision)
    },
    set_text: ({ text, revision }) => {
      backend_text = text
      backend_revision = revision
      return Promise.resolve(revision)
    },
    cancel_highlight: () => undefined,
    close_doc: () => Promise.resolve(),
  }

  const save = (buffer: string) => {
    saved_text = buffer
    status = `Saved ${buffer.length} characters in host memory`
  }
  const on_update = (update: EditorUpdate) => {
    dirty = update.dirty
  }
  const load_large = () => {
    const text = Array.from(
      { length: 100_000 },
      (_unused, line_idx) => `line ${line_idx + 1}`,
    ).join(`\n`)
    model.transact([{ from: 0, to: model.length, insert: text }], {
      selection: { anchor: text.length, head: text.length },
      source: `external`,
    })
    status = `Loaded a generated 100,000-line document`
  }
</script>

<div style="height: 17rem; border: 1px solid lightgray">
  <CodeEditor {backend} {model} {on_update} on_save={save} />
</div>

<button data-load-large onclick={load_large}>Load 100k generated lines</button>
<p>{status}. Unsaved changes: {dirty ? `yes` : `no`}. Saved {saved_text.length} bytes.</p>

svelte<script lang="ts">
  import 'svelte-widgets/code-editor/editor.css'
  import { DiffView } from 'svelte-widgets'
  import { EMPHASIS_BIT, TOKEN_CLASS_NAMES } from 'svelte-widgets/code-editor'
  import type {
    DiffBackend,
    DiffResult,
    DiffRow,
    RowKind,
  } from 'svelte-widgets/code-editor'

  const unchanged_prefix = [
    `type GreetingOptions = {`,
    `  excited?: boolean`,
    `}`,
    ``,
    `const default_name = \`world\``,
    ``,
  ]
  const old_lines = [
    ...unchanged_prefix,
    `import { format } from './format'`,
    ``,
    `export function greet(name: string) {`,
    `  const message = \`Hello, \${name}!\``,
    `  console.log(message)`,
    `  return message`,
    `}`,
    ``,
    `greet(\`world\`)`,
  ]
  const new_lines = [
    ...unchanged_prefix,
    `import { format } from './format'`,
    ``,
    `export function greet(name: string, excited = false) {`,
    `  const punctuation = excited ? \`!\` : \`.\``,
    `  const message = \`Hello, \${name}\${punctuation}\``,
    `  console.info(message)`,
    `  return message`,
    `}`,
    ``,
    `greet(\`world\`, true)`,
  ]
  const old_text = `${old_lines.join(`\n`)}\n`
  const new_text = `${new_lines.join(`\n`)}\n`

  const token = (name: (typeof TOKEN_CLASS_NAMES)[number], emphasized = false) =>
    TOKEN_CLASS_NAMES.indexOf(name) | (emphasized ? EMPHASIS_BIT : 0)
  // fixture tokenizer: shows token color and changed-word emphasis without bundling a
  // language grammar or diff engine into the docs site
  const spans_for = (text: string) => {
    const match = /\b(?:log|info|function|const)\b/u.exec(text)
    if (!match) return []
    const emphasized = match[0] === `log` || match[0] === `info`
    return [
      0,
      token(`plain`),
      match.index,
      token(emphasized ? `function` : `keyword`, emphasized),
      match.index + match[0].length,
      token(`plain`),
    ]
  }
  const line = (lines: string[], line_no: number) => {
    const text = lines[line_no - 1] ?? ``
    return { line_no, text, spans: spans_for(text) }
  }
  const row = (kind: RowKind, old_no: number | null, new_no: number | null): DiffRow => ({
    kind,
    old: old_no === null ? null : line(old_lines, old_no),
    new: new_no === null ? null : line(new_lines, new_no),
  })

  const result = {
    hunks: [
      {
        old_start: 7,
        new_start: 7,
        skipped_before: 6,
        rows: [
          row(`equal`, 7, 7),
          row(`equal`, 8, 8),
          row(`replace`, 9, 9),
          row(`insert`, null, 10),
          row(`replace`, 10, 11),
          row(`replace`, 11, 12),
          row(`equal`, 12, 13),
          row(`equal`, 13, 14),
          row(`equal`, 14, 15),
          row(`replace`, 15, 16),
        ],
      },
    ],
    added: 5,
    removed: 4,
    language: `typescript`,
    old_line_count: old_lines.length,
    new_line_count: new_lines.length,
    skipped_after: 0,
    old_ends_with_newline: true,
    new_ends_with_newline: true,
    truncated: false,
  } satisfies DiffResult

  const backend: DiffBackend = {
    // fixed fixture; a production backend inspects all DiffTextArgs fields
    diff_text: () => Promise.resolve(result),
  }
</script>

<DiffView
  {backend}
  {old_text}
  {new_text}
  options={{ font_size: 13, context_lines: 2, layout: `side-by-side` }}
  filename="greeting.ts"
  old_label="Before"
  new_label="After"
  style="height: 12rem; border: 1px solid lightgray"
/>