dev: ideas on caching.

packages/cspell-lib/docs/cache.md

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+# Caching Spell Checking Results
+
+Spell checking a large project can take a lot of time.
+
+In most cases, the files do not change from run to run, so the results are not expected to change.
+
+The idea is to cache the results of the previous runs and use those same results if nothing has changed.
+
+This is also related to ignoring known issues, see [Feature: Support Known Spelling Issues · Issue #1297 · streetsidesoftware/cspell](https://github.com/streetsidesoftware/cspell/issues/1297).

packages/cspell-lib/src/Cache/cspell.cache.ts

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+/**
+ * Proposed CSpell Cache file.
+ * To be stored at `./.cspell/cache/cache.json`
+ */
+
+export interface CSpellCache {
+    version: Version;
+    signature: Hash;
+    files: CachedFile[];
+}
+
+type Version = '0.1';
+
+/**
+ * Hash used. Starts with hash id. i.e. `sha1-` or `sha512-`.
+ */
+type Hash = string;
+
+type UriRelPath = string;
+
+enum IssueCode {
+    UnknownWord = 1 << 0,
+    ForbiddenWord = 1 << 1,
+    KnownIssue = 1 << 2,
+}
+
+interface CachedFile {
+    hash: Hash;
+    path: UriRelPath;
+    issues: Issue[];
+}
+
+type Issue = IssueEntry | IssueLine;
+
+interface IssueEntry {
+    line: number;
+    character: number;
+    code: IssueCode;
+    text: string;
+    len: number;
+}
+
+type IssueLine = [
+    line: IssueEntry['line'],
+    character: IssueEntry['character'],
+    code: IssueEntry['code'],
+    text: IssueEntry['text'],
+    len: IssueEntry['len']
+];