char/fjson

git clone https://git.t4t.associates/char/fjson

Charlotte Somrestructurebb038202026-10-12

import { FjsonError, type Span } from "./errors.ts";
import { type Options, resolveMaxDepth } from "./options.ts";
import {
  type ArrayNode,
  type JsonObject,
  type JsonValue,
  nodeToValue,
  type ObjectNode,
  parseObjectTree,
  parseTree,
  type SyntaxNode,
  toJsonValue,
} from "./parse.ts";
import { keyText, printJson } from "./stringify.ts";

/** path segments are object keys or array indices */
export type PathSegment = string | number;

type Edit = [span: Span, text: string];

function lineStart(source: string, at: number): number {
  const slice = source.slice(0, at);
  return Math.max(slice.lastIndexOf("\r"), slice.lastIndexOf("\n")) + 1;
}

function lineIndent(source: string, at: number): string {
  const start = lineStart(source, at);
  let end = start;
  while (
    end < at && (source[end] === " " || source[end] === "\t")
  ) end += 1;
  return source.slice(start, end);
}

function newlineOf(source: string): string {
  if (source.includes("\r\n")) return "\r\n";
  if (source.includes("\r")) return "\r";
  return "\n";
}

function isBlank(source: string, from: number, to: number): boolean {
  for (let i = from; i < to; i += 1) {
    if (source[i] !== " " && source[i] !== "\t") return false;
  }
  return true;
}

function findPunctuation(
  source: string,
  from: number,
  to: number,
  target: string,
): number | undefined {
  let at = from;
  while (at < to) {
    if (source.startsWith("//", at)) {
      const end = source.slice(at, to).search(/[\r\n]/);
      at += end === -1 ? to - at : end;
    } else if (source.startsWith("/*", at)) {
      const close = source.indexOf("*/", at + 2);
      if (close === -1 || close + 2 > to) return undefined;
      at = close + 2;
    } else if (source[at] === target) {
      return at;
    } else {
      at += 1;
    }
  }
  return undefined;
}

function jsonEqual(a: JsonValue, b: JsonValue): boolean {
  if (a === b) return true;
  if (Array.isArray(a) || Array.isArray(b)) {
    return Array.isArray(a) && Array.isArray(b) &&
      a.length === b.length &&
      a.every((item, index) => jsonEqual(item, (b as JsonValue[])[index]));
  }
  if (
    typeof a === "object" && a !== null && typeof b === "object" && b !== null
  ) {
    const left = a as JsonObject;
    const right = b as JsonObject;
    const keys = Object.keys(left);
    return keys.length === Object.keys(right).length &&
      keys.every((key) =>
        Object.hasOwn(right, key) && jsonEqual(left[key], right[key])
      );
  }
  return false;
}

function mismatch(span: Span): FjsonError {
  return new FjsonError(
    "syntax",
    "Path segment does not match its container",
    span,
  );
}

function child(node: SyntaxNode, segment: PathSegment): SyntaxNode | undefined {
  if (node.kind === "object" && typeof segment === "string") {
    return node.members.find((member) => member.key === segment)?.value;
  }
  if (
    node.kind === "array" && typeof segment === "number" &&
    Number.isInteger(segment) && segment >= 0
  ) {
    return node.items[segment];
  }
  throw mismatch(node.span);
}

/** an fJSON document with source-preserving mutations */
export class Document {
  #text: string;
  #tree: SyntaxNode;
  readonly #resolvedOptions: Options;

  private constructor(source: string, root: SyntaxNode, options: Options) {
    this.#text = source;
    this.#tree = root;
    this.#resolvedOptions = options;
  }

  /** parse an fJSON document */
  static parse(source: string, options?: Options): Document {
    return Document.#fromTree(source, parseTree(source, options), options);
  }

  /** parse a document which **must** be an object */
  static parseObject(source: string, options?: Options): Document {
    return Document.#fromTree(
      source,
      parseObjectTree(source, options),
      options,
    );
  }

  static #fromTree(
    source: string,
    root: SyntaxNode,
    options?: Options,
  ): Document {
    return new Document(source, root, {
      maxDepth: resolveMaxDepth(options),
    });
  }

  /** options this document was parsed with (for reuse by edits) */
  get options(): Options {
    return { ...this.#resolvedOptions };
  }

  /** source text. */
  get source(): string {
    return this.#text;
  }

  /** spanned AST syntax tree */
  root(): SyntaxNode {
    return this.#tree;
  }

  /** convert to simple data */
  toValue(): JsonValue {
    return nodeToValue(this.#tree);
  }

  /** replace a value (source-preserving) */
  set(path: readonly PathSegment[], value: unknown): void {
    const json = toJsonValue(value, this.#resolvedOptions);
    let edits: Edit[] | undefined;
    if (path.length === 0) {
      edits = this.#replacement(this.#tree, json);
    } else {
      const parent = this.#locate(path.slice(0, -1));
      const last = path[path.length - 1];
      const existing = child(parent, last);
      if (existing !== undefined) {
        edits = this.#replacement(existing, json);
      } else if (parent.kind === "object" && typeof last === "string") {
        edits = [this.#insertion(parent, `${keyText(last)}: `, json)];
      } else if (
        parent.kind === "array" && typeof last === "number" &&
        last === parent.items.length
      ) {
        edits = [this.#insertion(parent, "", json)];
      } else {
        throw new FjsonError(
          "syntax",
          "Array index out of bounds",
          parent.span,
        );
      }
    }
    if (edits !== undefined) this.#applyEdits(edits);
  }

  /** remove a value (source-preserving) */
  remove(path: readonly PathSegment[]): boolean {
    if (path.length === 0) {
      throw new FjsonError("syntax", "Cannot remove the root", this.#tree.span);
    }
    const parent = this.#locate(path.slice(0, -1));
    const last = path[path.length - 1];
    const target = child(parent, last);
    if (target === undefined) return false;
    if (parent.kind !== "object" && parent.kind !== "array") {
      throw mismatch(parent.span); // Unreachable: child() rejects other shapes.
    }
    const edits: Edit[] = [[target.span, ""]];
    let next: number | undefined;
    if (parent.kind === "object" && typeof last === "string") {
      const index = parent.members.findIndex((member) => member.key === last);
      const member = parent.members[index];
      edits.push([member.keySpan, ""]);
      const colon = findPunctuation(
        this.#text,
        member.keySpan.end,
        target.span.start,
        ":",
      );
      if (colon === undefined) throw new Error("unreachable: missing colon");
      edits.push([{ start: colon, end: colon + 1 }, ""]);
      next = parent.members[index + 1]?.keySpan.start;
    } else if (parent.kind === "array" && typeof last === "number") {
      next = parent.items[last + 1]?.span.start;
    }
    const comma = findPunctuation(
      this.#text,
      target.span.end,
      next ?? this.#containerEnd(parent),
      ",",
    );
    if (comma !== undefined) edits.push([{ start: comma, end: comma + 1 }, ""]);
    this.#applyEdits(edits);
    return true;
  }

  /** merge overrides into the document. see {@linkcode apply} */
  apply(...overrides: unknown[]): void {
    for (const override of overrides) this.#merge(override, [], []);
  }

  #locate(path: readonly PathSegment[]): SyntaxNode {
    let node = this.#tree;
    for (const segment of path) {
      const next = child(node, segment);
      if (next === undefined) {
        throw new FjsonError("syntax", "Path does not exist", node.span);
      }
      node = next;
    }
    return node;
  }

  #replacement(
    existing: SyntaxNode,
    value: JsonValue,
  ): Edit[] | undefined {
    if (jsonEqual(nodeToValue(existing), value)) return undefined;
    const implicit = existing.kind === "object" &&
      !this.#text.startsWith("{", existing.span.start);
    const indent = lineIndent(this.#text, existing.span.start);
    let text: string;
    if (
      typeof value === "string" &&
      this.#text.startsWith("'", existing.span.start)
    ) {
      const json = JSON.stringify(value);
      text = `'${json.slice(1, -1).replaceAll("'", "\\'")}'`;
    } else {
      text = reindent(
        printJson(value, implicit),
        indent,
        newlineOf(this.#text),
      );
    }
    if (
      existing.span.start === existing.span.end && existing.span.start > 0 &&
      !/[\r\n]$/.test(this.#text.slice(0, existing.span.start))
    ) {
      text = `${newlineOf(this.#text)}${indent}${text}`;
    }
    const edits: Edit[] = [[existing.span, text]];
    if (implicit) {
      const comma = findPunctuation(
        this.#text,
        existing.span.end,
        this.#text.length,
        ",",
      );
      if (comma !== undefined) {
        edits.push([{ start: comma, end: comma + 1 }, ""]);
      }
    }
    return edits;
  }

  #containerEnd(container: ArrayNode | ObjectNode): number {
    if (
      container.kind === "object" &&
      !this.#text.startsWith("{", container.span.start)
    ) {
      return this.#text.length;
    }
    return container.span.end;
  }

  #insertion(
    container: ArrayNode | ObjectNode,
    prefix: string,
    value: JsonValue,
  ): Edit {
    const braced = this.#text.startsWith("{", container.span.start) ||
      this.#text.startsWith("[", container.span.start);
    const end = this.#containerEnd(container) - (braced ? 1 : 0);
    const base = lineIndent(this.#text, container.span.start);
    const first = container.kind === "object"
      ? container.members[0]?.keySpan.start
      : container.items[0]?.span.start;
    const indent = first !== undefined &&
        isBlank(this.#text, lineStart(this.#text, first), first)
      ? lineIndent(this.#text, first)
      : braced
      ? `${base}  `
      : base;
    const line = lineStart(this.#text, end);
    const atLineStart = isBlank(this.#text, line, end);
    const at = braced && atLineStart ? line : end;
    const newline = newlineOf(this.#text);
    let text = "";
    if (at > 0 && !/[\r\n]$/.test(this.#text.slice(0, at))) text += newline;
    text += indent + prefix +
      reindent(printJson(value, false), indent, newline);
    if (braced) {
      text += newline;
      if (!atLineStart) text += base;
    } else if (/[\r\n]$/.test(this.#text)) {
      text += newline;
    }
    return [{ start: at, end: at }, text];
  }

  #applyEdits(edits: Edit[]): void {
    const ordered = [...edits].sort((a, b) => b[0].start - a[0].start);
    let source = this.#text;
    for (const [span, text] of ordered) {
      source = source.slice(0, span.start) + text + source.slice(span.end);
    }
    try {
      this.#tree = parseTree(source, this.#resolvedOptions);
    } catch (error) {
      if (error instanceof FjsonError) {
        throw new FjsonError(
          error.kind,
          `Edit would produce invalid fJSON: ${error.message}`,
          this.#tree.span,
        );
      }
      throw error;
    }
    this.#text = source;
  }

  #merge(value: unknown, path: string[], ancestors: object[]): void {
    if (value === null || typeof value !== "object" || Array.isArray(value)) {
      throw new TypeError("fJSON overrides must be objects");
    }
    if (ancestors.includes(value)) {
      throw new TypeError("Cannot apply cyclic overrides");
    }
    const maxDepth = resolveMaxDepth(this.#resolvedOptions);
    if (path.length > maxDepth) {
      throw new RangeError(
        `fJSON nesting exceeds ${maxDepth} levels below the root`,
      );
    }
    ancestors.push(value);
    for (const key of Object.keys(value)) {
      const prop = (value as Record<string, unknown>)[key];
      path.push(key);
      const segments: PathSegment[] = [...path];
      const nested = prop !== null && typeof prop === "object" &&
        !Array.isArray(prop) && typeof prop !== "function";
      try {
        if (prop === undefined) {
          this.remove(segments);
        } else if (nested) {
          if (lookup(this.#tree, path)?.kind !== "object") {
            this.set(segments, {});
          }
        } else {
          // roundtrip to JSON for semantic filtering
          const json = JSON.stringify(prop);
          if (json !== undefined) this.set(segments, JSON.parse(json));
        }
      } catch (error) {
        if (error instanceof FjsonError) {
          if (error.kind === "nesting") throw new RangeError(error.message);
          throw new TypeError(error.message);
        }
        throw error;
      }
      if (nested) this.#merge(prop, path, ancestors);
      path.pop();
    }
    ancestors.pop();
  }
}

function lookup(
  root: SyntaxNode,
  path: readonly string[],
): SyntaxNode | undefined {
  let node: SyntaxNode | undefined = root;
  for (const key of path) {
    if (node?.kind !== "object") return undefined;
    node = node.members.find((member) => member.key === key)?.value;
  }
  return node;
}

function reindent(printed: string, indent: string, newline: string): string {
  return printed.split("\n").join(newline + indent);
}