8bitworkshop/src/ide/views/tabs.ts

import { indentLess, indentMore, insertTab } from "@codemirror/commands";
import { getIndentUnit } from "@codemirror/language";
import { countColumn, EditorSelection, EditorState } from "@codemirror/state";
import { EditorView, KeyBinding } from "@codemirror/view";

// Spaces to insert when pressing Tab at document position 'head': up to the
// next multiple of the indent unit in leading whitespace, otherwise up to the
// next multiple of the tab size.
export function spacesToTabStop(state: EditorState, head: number): string {
  const line = state.doc.lineAt(head);
  const before = line.text.slice(0, head - line.from);
  const col = countColumn(before, state.tabSize);
  const stop = /^\s*$/.test(before) ? getIndentUnit(state) : state.tabSize;
  return " ".repeat(stop - (col % stop));
}

// Insert spaces from the cursor up to the next tab stop.
// A non-empty selection indents the selected lines instead.
function insertSpacesToTabStop(view: EditorView): boolean {
  const state = view.state;
  if (state.selection.ranges.some(r => !r.empty)) return indentMore(view);
  view.dispatch(state.changeByRange(range => {
    const insert = spacesToTabStop(state, range.head);
    return {
      changes: { from: range.head, insert },
      range: EditorSelection.cursor(range.head + insert.length),
    };
  }), { scrollIntoView: true, userEvent: "input" });
  return true;
}

export const insertSpacesKeymap: KeyBinding[] = [
  { key: "Tab", run: insertSpacesToTabStop },
  { key: "Shift-Tab", run: indentLess },
];

export const insertTabKeymap: KeyBinding[] = [
  { key: "Tab", run: insertTab },
  { key: "Shift-Tab", run: indentLess },
];

// Number of lines scanned in each direction when inferring tab stops.
const ASM_TABSTOP_SCAN_LINES = 50;
// Existing tab characters are assumed to advance to the next multiple of this column.
const TAB_EXPAND_WIDTH = 8;
// Print tab stop inference results to the browser console (for debugging).
const ASM_TAB_DEBUG = true;

function dbgTab(msg: string) {
  if (ASM_TAB_DEBUG) console.log("[asm-tab] " + msg);
}

// Compute the visual column of character position 'index' within a line,
// expanding tab characters to the next multiple of TAB_EXPAND_WIDTH.
function visualCol(text: string, index: number): number {
  let col = 0;
  for (let i = 0; i < index; i++) {
    col = text.charCodeAt(i) == 9 ? (Math.floor(col / TAB_EXPAND_WIDTH) + 1) * TAB_EXPAND_WIDTH : col + 1;
  }
  return col;
}

// Add a candidate stop column to the frequency tally.
function addStop(col: number, freq: Map<number, number>) {
  if (col > 0) freq.set(col, (freq.get(col) || 0) + 1);
}

// Collect candidate tab stops from one asm line: the opcode start, operand
// start, and comment start columns (in visual columns, tabs expanded to 8).
// Lines starting at column 0 are assumed to have a label, so the opcode is the
// second word on those lines.
function collectStopsFromLine(text: string, freq: Map<number, number>) {
  // Visual columns of word starts, stopping at the comment.
  const words: number[] = [];
  const re = /\S+/g;
  let m;
  while ((m = re.exec(text)) !== null) {
    if (text[m.index] == ';') break;
    words.push(visualCol(text, m.index));
  }
  // Comment start column (';' preceded by whitespace or line start).
  const cm = /(^|\s);/.exec(text);
  if (cm) addStop(visualCol(text, cm.index + cm[1].length), freq);
  // Skip the label if the line starts in column 0.
  const fields = words.length && words[0] == 0 ? words.slice(1) : words;
  if (fields.length >= 1) addStop(fields[0], freq); // opcode column
  if (fields.length >= 2) addStop(fields[1], freq); // operand column
}

// Minimum number of non-empty lines scanned after the cursor before we also
// merge in lines from before it (e.g. when the cursor is near the end of file).
const ASM_TABSTOP_MIN_LINES = 3;

// Infer tab stops for asm files: scan the lines after the current line first;
// if there aren't many of them (or they yield no stops), also merge in the lines
// before it. Returns a frequency tally of stop columns (in visual columns,
// with tabs expanded to 8).
export function inferAsmTabStops(state: EditorState, pos: number): Map<number, number> {
  const cur = state.doc.lineAt(pos);
  const total = state.doc.lines;
  const scan = (from: number, to: number, step: number, freq: Map<number, number>): number => {
    let lines = 0;
    for (let n = from; n != to && lines < ASM_TABSTOP_SCAN_LINES; n += step) {
      const line = state.doc.line(n);
      if (line.text.trim()) {
        collectStopsFromLine(line.text, freq);
        lines++;
      }
    }
    return lines;
  };
  const freq: Map<number, number> = new Map();
  // Prefer lines after the current line, but merge in lines before it when
  // there are too few lines below (cursor near end of file).
  const after = scan(cur.number + 1, Math.min(total, cur.number + ASM_TABSTOP_SCAN_LINES) + 1, 1, freq);
  if (after < ASM_TABSTOP_MIN_LINES) {
    scan(cur.number - 1, Math.max(1, cur.number - ASM_TABSTOP_SCAN_LINES) - 1, -1, freq);
  }
  return freq;
}

// Most frequent stop greater than col (ties: smallest column),
// or the next grid stop as a fallback.
function nextStop(freq: Map<number, number>, col: number, grid: number): number {
  let best = -1, bestFreq = 0;
  for (const [s, f] of freq) {
    if (s > col && (f > bestFreq || (f == bestFreq && s < best))) {
      best = s; bestFreq = f;
    }
  }
  if (best >= 0) return best;
  return (Math.floor(col / grid) + 1) * grid;
}

// Most frequent stop less than col (ties: largest column),
// or the previous grid stop as a fallback.
function prevStop(freq: Map<number, number>, col: number, grid: number): number {
  let best = -1, bestFreq = 0;
  for (const [s, f] of freq) {
    if (s < col && (f > bestFreq || (f == bestFreq && s > best))) {
      best = s; bestFreq = f;
    }
  }
  if (best >= 0) return best;
  return Math.max(0, Math.min(Math.floor((col - 1) / grid) * grid, col - 1));
}

// True if the document region near pos contains literal tab characters;
// in that case tab stops fall back to a hard grid of 8 columns.
function docHasTabs(state: EditorState, pos: number): boolean {
  const cur = state.doc.lineAt(pos);
  const fromLine = Math.max(1, cur.number - ASM_TABSTOP_SCAN_LINES);
  const toLine = Math.min(state.doc.lines, cur.number + ASM_TABSTOP_SCAN_LINES);
  for (let n = fromLine; n <= toLine; n++) {
    if (state.doc.line(n).text.includes('\t')) return true;
  }
  return false;
}

// Compute the insertion needed for pressing Tab at document position 'head':
// spaces from the cursor up to the next inferred tab stop.
function fmtStops(freq: Map<number, number>): string {
  return "[" + [...freq.entries()].sort((a, b) => a[0] - b[0]).map(([s, f]) => `${s}:${f}`).join(" ") + "]";
}

export function asmTabChange(state: EditorState, head: number): { from: number, insert: string } | null {
  const line = state.doc.lineAt(head);
  const col = visualCol(line.text, head - line.from);
  const hasTabs = docHasTabs(state, head);
  const grid = hasTabs ? TAB_EXPAND_WIDTH : state.tabSize;
  const stops = inferAsmTabStops(state, head);
  const target = nextStop(stops, col, grid);
  dbgTab(`Tab @ line ${line.number} col ${col}: stops(col:freq)=${fmtStops(stops)} ${hasTabs ? "grid=8 (file has tabs)" : `grid=${state.tabSize}`} -> target=${target}`
    + ` | line=${JSON.stringify(line.text.slice(0, col))}+CURSOR`);
  if (target <= col) return null;
  return { from: head, insert: " ".repeat(target - col) };
}

// Compute the deletion range for Backspace at document position 'head'
// (spaces removed back to the previous inferred tab stop, or a single tab char),
// or null if default backspace behavior should apply.
export function asmBackspaceRange(state: EditorState, head: number): { from: number, to: number } | null {
  const line = state.doc.lineAt(head);
  const index = head - line.from;
  if (index == 0) {
    dbgTab(`Backspace @ line ${line.number} col 0: start of line, using default`);
    return null;
  }
  // If the character before the cursor is a tab, just delete it.
  if (line.text[index - 1] == '\t') {
    dbgTab(`Backspace @ line ${line.number} col ${visualCol(line.text, index)}: deleting one tab char`);
    return { from: head - 1, to: head };
  }
  // Only handle cursors inside a pure-whitespace indent region.
  if (!/^[ \t]+$/.test(line.text.slice(0, index))) {
    dbgTab(`Backspace @ line ${line.number} col ${visualCol(line.text, index)}: cursor not in leading whitespace, using default`);
    return null;
  }
  const hasTabs = docHasTabs(state, head);
  const grid = hasTabs ? TAB_EXPAND_WIDTH : state.tabSize;
  const col = visualCol(line.text, index);
  const stops = inferAsmTabStops(state, head);
  const target = prevStop(stops, col, grid);
  if (target >= col) {
    dbgTab(`Backspace @ line ${line.number} col ${col}: no stop before cursor, using default`);
    return null;
  }
  // Convert target visual column back to a character index (only spaces ahead of us).
  const from = head - (col - target);
  dbgTab(`Backspace @ line ${line.number} col ${col}: stops(col:freq)=${fmtStops(stops)} ${hasTabs ? "grid=8 (file has tabs)" : `grid=${state.tabSize}`} -> deleting cols ${target}..${col}`
    + ` | line=${JSON.stringify(line.text.slice(0, index))}+CURSOR`);
  return { from, to: head };
}

// Insert spaces from the cursor up to the next inferred tab stop.
function asmInsertSpaces(view: EditorView): boolean {
  const state = view.state;
  if (state.selection.ranges.some(r => !r.empty)) return false; // let indentMore handle selections
  const tr = state.changeByRange(range => {
    const change = asmTabChange(state, range.head);
    if (!change) return { range };
    return {
      changes: change,
      range: EditorSelection.cursor(range.head + change.insert.length),
    };
  });
  view.dispatch(tr);
  return true;
}

// Delete spaces from the cursor back to the previous inferred tab stop,
// but only when the cursor sits in leading whitespace.
function asmDeleteSpacesBackward(view: EditorView): boolean {
  const state = view.state;
  const range = state.selection.main;
  if (!range.empty) return false;
  const del = asmBackspaceRange(state, range.head);
  if (!del) return false;
  view.dispatch(state.update({
    changes: del,
    selection: EditorSelection.cursor(del.from),
  }));
  return true;
}

// Keymap for asm editors when "insert spaces when pressing tab" is enabled.
// The callback lets the caller gate the behavior on current settings.
export function asmSpacesKeymap(tabsToSpaces: () => boolean): KeyBinding[] {
  return [
    { key: "Tab", run: v => tabsToSpaces() && asmInsertSpaces(v) },
    { key: "Backspace", run: v => tabsToSpaces() && asmDeleteSpacesBackward(v) },
  ];
}