@nextgensoftwares/folio-formatter

A smart formatting pass: rules read the document and propose layout hints. It never mutates the document. 2 rules

interface RuleContext { prev: FolioNode | undefined; next: FolioNode | undefined }

interface FormatRule {
  name: string;
  apply(block: FolioNode, ctx: RuleContext): BlockHint | undefined; // BlockHint from @nextgensoftwares/folio-layout
}

A rule may only depend on the block and its immediate neighbours: results are cached by those three nodes' identities (per rules array), so a keystroke re-evaluates three blocks, not the whole book. Hosts ship their own rule packs.

Export	Signature	Description
format	(doc: FolioDocument, rules: readonly FormatRule[]) => Map<number, BlockHint>	hints by top-level block index; later rules' fields override earlier ones for the same block. Pass the same rules array each time to keep the cache
h1StartsPage	FormatRule	every top-level H1 after the first gets pageBreakBefore
introKeepsWithNext	FormatRule	a paragraph ending in : (or ：) keeps with a following list, table, code block, media or math block
defaultRules	FormatRule[]	[introKeepsWithNext]

const hints = format(doc, defaultRules);
const layout = layoutDocument(doc, { measurer, hints });

@nextgensoftwares/folio-editor re-declares the rule shape structurally as FormatRuleLike, so plugins can contribute rules without depending on this package.