forked from Fediversity/Fediversity
98 lines
3.9 KiB
JavaScript
98 lines
3.9 KiB
JavaScript
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const BREAK = Symbol('break visit');
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const SKIP = Symbol('skip children');
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const REMOVE = Symbol('remove item');
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/**
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* Apply a visitor to a CST document or item.
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*
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* Walks through the tree (depth-first) starting from the root, calling a
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* `visitor` function with two arguments when entering each item:
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* - `item`: The current item, which included the following members:
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* - `start: SourceToken[]` – Source tokens before the key or value,
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* possibly including its anchor or tag.
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* - `key?: Token | null` – Set for pair values. May then be `null`, if
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* the key before the `:` separator is empty.
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* - `sep?: SourceToken[]` – Source tokens between the key and the value,
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* which should include the `:` map value indicator if `value` is set.
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* - `value?: Token` – The value of a sequence item, or of a map pair.
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* - `path`: The steps from the root to the current node, as an array of
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* `['key' | 'value', number]` tuples.
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*
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* The return value of the visitor may be used to control the traversal:
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* - `undefined` (default): Do nothing and continue
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* - `visit.SKIP`: Do not visit the children of this token, continue with
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* next sibling
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* - `visit.BREAK`: Terminate traversal completely
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* - `visit.REMOVE`: Remove the current item, then continue with the next one
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* - `number`: Set the index of the next step. This is useful especially if
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* the index of the current token has changed.
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* - `function`: Define the next visitor for this item. After the original
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* visitor is called on item entry, next visitors are called after handling
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* a non-empty `key` and when exiting the item.
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*/
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function visit(cst, visitor) {
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if ('type' in cst && cst.type === 'document')
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cst = { start: cst.start, value: cst.value };
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_visit(Object.freeze([]), cst, visitor);
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}
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// Without the `as symbol` casts, TS declares these in the `visit`
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// namespace using `var`, but then complains about that because
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// `unique symbol` must be `const`.
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/** Terminate visit traversal completely */
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visit.BREAK = BREAK;
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/** Do not visit the children of the current item */
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visit.SKIP = SKIP;
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/** Remove the current item */
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visit.REMOVE = REMOVE;
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/** Find the item at `path` from `cst` as the root */
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visit.itemAtPath = (cst, path) => {
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let item = cst;
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for (const [field, index] of path) {
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const tok = item?.[field];
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if (tok && 'items' in tok) {
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item = tok.items[index];
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}
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else
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return undefined;
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}
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return item;
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};
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/**
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* Get the immediate parent collection of the item at `path` from `cst` as the root.
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*
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* Throws an error if the collection is not found, which should never happen if the item itself exists.
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*/
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visit.parentCollection = (cst, path) => {
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const parent = visit.itemAtPath(cst, path.slice(0, -1));
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const field = path[path.length - 1][0];
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const coll = parent?.[field];
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if (coll && 'items' in coll)
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return coll;
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throw new Error('Parent collection not found');
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};
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function _visit(path, item, visitor) {
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let ctrl = visitor(item, path);
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if (typeof ctrl === 'symbol')
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return ctrl;
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for (const field of ['key', 'value']) {
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const token = item[field];
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if (token && 'items' in token) {
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for (let i = 0; i < token.items.length; ++i) {
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const ci = _visit(Object.freeze(path.concat([[field, i]])), token.items[i], visitor);
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if (typeof ci === 'number')
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i = ci - 1;
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else if (ci === BREAK)
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return BREAK;
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else if (ci === REMOVE) {
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token.items.splice(i, 1);
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i -= 1;
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}
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}
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if (typeof ctrl === 'function' && field === 'key')
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ctrl = ctrl(item, path);
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}
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}
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return typeof ctrl === 'function' ? ctrl(item, path) : ctrl;
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}
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export { visit };
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