gregorlohaus.com/packages/codemirror-helix/src/motions.ts

import type { EditorState } from "@codemirror/state";

export type CharClass = "space" | "word" | "punct";

export function charAt(state: EditorState, i: number): string {
  return i >= 0 && i < state.doc.length ? state.doc.sliceString(i, i + 1) : "";
}

export function classOf(ch: string): CharClass {
  if (ch === "" || /\s/.test(ch)) return "space";
  if (/[\p{L}\p{N}_]/u.test(ch)) return "word";
  return "punct";
}

// For WORD (big) motions only whitespace separates tokens.
function bigClassOf(ch: string): CharClass {
  return ch === "" || /\s/.test(ch) ? "space" : "word";
}

function classAt(state: EditorState, i: number, big: boolean): CharClass {
  const ch = charAt(state, i);
  return big ? bigClassOf(ch) : classOf(ch);
}

/** Helix `w`/`W`: move to the start of the next word. */
export function nextWordStart(state: EditorState, pos: number, big: boolean): number {
  const len = state.doc.length;
  let i = pos;
  if (i >= len) return len;
  if (classAt(state, i, big) === "space") {
    while (i < len && classAt(state, i, big) === "space") i++;
  } else {
    const cls = classAt(state, i, big);
    while (i < len && classAt(state, i, big) === cls) i++;
    while (i < len && classAt(state, i, big) === "space") i++;
  }
  return i;
}

/** Helix `b`/`B`: move to the start of the previous word. */
export function prevWordStart(state: EditorState, pos: number, big: boolean): number {
  let i = pos;
  if (i <= 0) return 0;
  i--;
  while (i > 0 && classAt(state, i, big) === "space") i--;
  const cls = classAt(state, i, big);
  while (i > 0 && classAt(state, i - 1, big) === cls) i--;
  return i;
}

/** Helix `e`/`E`: move to the end of the next word. */
export function nextWordEnd(state: EditorState, pos: number, big: boolean): number {
  const len = state.doc.length;
  let i = pos + 1;
  while (i < len && classAt(state, i, big) === "space") i++;
  const cls = classAt(state, i, big);
  while (i < len && classAt(state, i, big) === cls) i++;
  return Math.min(i, len);
}

export function firstNonBlank(state: EditorState, line: number): number {
  const l = state.doc.line(line);
  return l.from + (l.text.match(/^\s*/)?.[0].length ?? 0);
}

/** f/t/F/T: find `target` from `pos`. Returns null if not found on the line. */
export function findCharOnLine(
  state: EditorState,
  pos: number,
  target: string,
  kind: "f" | "t" | "F" | "T",
): number | null {
  const line = state.doc.lineAt(pos);
  const forward = kind === "f" || kind === "t";
  const till = kind === "t" || kind === "T";

  if (forward) {
    let i = pos + 1;
    // `t` should skip an adjacent target so repeated presses advance.
    if (till && charAt(state, i) === target) i++;
    for (; i <= line.to; i++) {
      if (charAt(state, i) === target) return till ? i - 1 : i;
    }
  } else {
    let i = pos - 1;
    if (till && charAt(state, i) === target) i--;
    for (; i >= line.from; i--) {
      if (charAt(state, i) === target) return till ? i + 1 : i;
    }
  }
  return null;
}

const PAIRS: Record<string, { open: string; close: string }> = {
  "(": { open: "(", close: ")" },
  ")": { open: "(", close: ")" },
  "[": { open: "[", close: "]" },
  "]": { open: "[", close: "]" },
  "{": { open: "{", close: "}" },
  "}": { open: "{", close: "}" },
  "<": { open: "<", close: ">" },
  ">": { open: "<", close: ">" },
};

export function pairFor(ch: string): { open: string; close: string } | null {
  return PAIRS[ch] ?? null;
}

/** Scan forward for the close that matches an open at/after `pos` (nesting-aware). */
export function findClose(state: EditorState, from: number, open: string, close: string): number | null {
  const len = state.doc.length;
  let depth = 0;
  for (let i = from; i < len; i++) {
    const ch = charAt(state, i);
    if (ch === open) depth++;
    else if (ch === close) {
      depth--;
      if (depth === 0) return i;
    }
  }
  return null;
}

/** Scan backward for the open that matches a close at/before `pos` (nesting-aware). */
export function findOpen(state: EditorState, from: number, open: string, close: string): number | null {
  let depth = 0;
  for (let i = from; i >= 0; i--) {
    const ch = charAt(state, i);
    if (ch === close) depth++;
    else if (ch === open) {
      depth--;
      if (depth === 0) return i;
    }
  }
  return null;
}

/** `mm`: position of the bracket matching the one at/under `pos`, or null. */
export function matchingBracket(state: EditorState, pos: number): number | null {
  for (const probe of [pos, pos - 1]) {
    const ch = charAt(state, probe);
    const pair = pairFor(ch);
    if (!pair) continue;
    if (ch === pair.open) return findClose2(state, probe, pair.open, pair.close);
    return findOpen2(state, probe, pair.open, pair.close);
  }
  return null;
}

function findClose2(state: EditorState, openPos: number, open: string, close: string): number | null {
  const len = state.doc.length;
  let depth = 0;
  for (let i = openPos; i < len; i++) {
    const ch = charAt(state, i);
    if (ch === open) depth++;
    else if (ch === close && --depth === 0) return i;
  }
  return null;
}

function findOpen2(state: EditorState, closePos: number, open: string, close: string): number | null {
  let depth = 0;
  for (let i = closePos; i >= 0; i--) {
    const ch = charAt(state, i);
    if (ch === close) depth++;
    else if (ch === open && --depth === 0) return i;
  }
  return null;
}

/** Paragraph bounds (blank-line delimited) containing `pos`. */
export function paragraphAt(state: EditorState, pos: number): { from: number; to: number } {
  const { doc } = state;
  let startLine = doc.lineAt(pos).number;
  let endLine = startLine;
  const blank = (n: number) => doc.line(n).text.trim() === "";
  while (startLine > 1 && !blank(startLine - 1)) startLine--;
  while (endLine < doc.lines && !blank(endLine + 1)) endLine++;
  return { from: doc.line(startLine).from, to: doc.line(endLine).to };
}