adding some more notes

m2d/converters/utils.js

@@ -2,6 +2,12 @@
 
 const reg = require("../../matrix/read-registration")
 const userRegex = reg.namespaces.users.map(u => new RegExp(u.regex))
+const assert = require("assert").strict
+/** @type {import("xxhash-wasm").XXHashAPI} */ // @ts-ignore
+let hasher = null
+// @ts-ignore
+require("xxhash-wasm")().then(h => hasher = h)
+
 /**
  * Determine whether an event is the bridged representation of a discord message.
  * Such messages shouldn't be bridged again.
@@ -29,5 +35,25 @@ function getPublicUrlForMxc(mxc) {
 	else return null
 }
 
+/**
+ * Event IDs are really big and have more entropy than we need.
+ * If we want to store the event ID in the database, we can store a more compact version by hashing it with this.
+ * Choosing a 64-bit non-cryptographic hash as only a 32-bit hash will see birthday collisions unreasonably frequently: https://en.wikipedia.org/wiki/Birthday_attack#Mathematics
+ * xxhash outputs an unsigned 64-bit integer.
+ * Converting to a signed 64-bit integer with no bit loss so that it can be stored in an SQLite integer field as-is: https://www.sqlite.org/fileformat2.html#record_format
+ * This should give very efficient storage with sufficient entropy.
+ * @param {string} eventID
+ */
+function getEventIDHash(eventID) {
+	assert(hasher, "xxhash is not ready yet")
+	if (eventID[0] === "$" && eventID.length >= 13) {
+		eventID = eventID.slice(1) // increase entropy per character to potentially help xxhash
+	}
+	const unsignedHash = hasher.h64(eventID)
+	const signedHash = unsignedHash - 0x8000000000000000n // shifting down to signed 64-bit range
+	return signedHash
+}
+
 module.exports.eventSenderIsFromDiscord = eventSenderIsFromDiscord
 module.exports.getPublicUrlForMxc = getPublicUrlForMxc
+module.exports.getEventIDHash = getEventIDHash