Added Internal ID representation

Author: Hydrocharged

core/id/cache.go

@@ -0,0 +1,130 @@
+// Copyright 2024 Dolthub, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package id
+
+import (
+	"hash/crc32"
+	"strconv"
+)
+
+// builtinOidLimit is the largest OID that Postgres will assign to built-in items, so we use this to mitigate conflicts
+// with existing and future built-in OIDs.
+const builtinOidLimit = 65535
+
+var (
+	// crcTable is the table that is used for our CRC operations.
+	crcTable = crc32.MakeTable(crc32.Castagnoli)
+	// globalCache is the cache structure that is used for the server session.
+	globalCache = &cacheStruct{
+		toOID:      make(map[Internal]uint32),
+		toInternal: make(map[uint32]Internal),
+	}
+)
+
+// cacheStruct is the cache structure that holds mappings between the Internal ID and external OID (used by Postgres).
+// The mappings are temporary, and exist only within a server session. We must discourage users from storing converted
+// OIDs, and to use the actual OID type, since the type uses Internal IDs so long as it's not returned to the user.
+type cacheStruct struct {
+	toOID      map[Internal]uint32
+	toInternal map[uint32]Internal
+}
+
+// Cache returns the global cache that is used for the server session.
+func Cache() *cacheStruct {
+	return globalCache
+}
+
+// ToOID returns the OID associated with the given Internal ID.
+func (cache *cacheStruct) ToOID(id Internal) uint32 {
+	// If the ID is in the cache, then we can just return its associated OID
+	if oid, ok := cache.toOID[id]; ok {
+		return oid
+	}
+	if id.Section() == Section_OID {
+		oid, _ := strconv.ParseUint(id.Segment(0), 10, 32)
+		return uint32(oid)
+	}
+	underlyingBytes := id.UnderlyingBytes()
+	oid := crc32.Checksum(underlyingBytes, crcTable)
+	// If the generated OID is valid, then we'll add it to the cache and return it
+	if _, ok := cache.toInternal[oid]; !ok && oid > builtinOidLimit {
+		cache.toOID[id] = oid
+		cache.toInternal[oid] = id
+		return oid
+	}
+	// In this case, the OID is not valid, so we'll run a small loop to generate an OID based on the actual ID.
+	// This retains some level of determinism for OID to ID relationships.
+	modifiedBytes := make([]byte, len(underlyingBytes)+1)
+	copy(modifiedBytes[1:], underlyingBytes)
+	for i := byte(0); i < 255; i++ {
+		modifiedBytes[0] = i
+		oid = crc32.Checksum(underlyingBytes, crcTable)
+		if _, ok := cache.toInternal[oid]; !ok && oid > builtinOidLimit {
+			cache.toOID[id] = oid
+			cache.toInternal[oid] = id
+			return oid
+		}
+	}
+	// If we're here, then we'll just search for an empty OID as a last resort
+	for i := uint32(4294967295); i > builtinOidLimit; i-- {
+		if _, ok := cache.toInternal[oid]; !ok {
+			cache.toOID[id] = oid
+			cache.toInternal[oid] = id
+			return oid
+		}
+	}
+	// We must have over 4 billion items in the database, so we'll panic since there's nothing we can do
+	panic("all OIDs have been taken")
+}
+
+// ToInternal returns the Internal ID associated with the given OID.
+func (cache *cacheStruct) ToInternal(oid uint32) Internal {
+	if id, ok := cache.toInternal[oid]; ok {
+		return id
+	}
+	// The OID is not in the cache, so it's invalid
+	return ""
+}
+
+// Exists returns whether the given Internal ID exists within the cache. This should primarily be used for the default
+// functions, as it's not guaranteed that user functions will be in the cache, especially after a server restart.
+func (cache *cacheStruct) Exists(id Internal) bool {
+	_, ok := cache.toOID[id]
+	return ok
+}
+
+// setBuiltIn sets the given ID to the OID. This should only be used for the built-in items.
+func (cache *cacheStruct) setBuiltIn(id Internal, oid uint32) {
+	if oid > builtinOidLimit {
+		panic("oid is not a built-in")
+	}
+	cache.toOID[id] = oid
+	cache.toInternal[oid] = id
+}
+
+// update is used to change the OID mapping of an existing Internal ID that has been changed (where the Internal ID
+// points to the same logical item).
+func (cache *cacheStruct) update(old Internal, new Internal) {
+	// If the old ID doesn't exist in the cache, then we don't have anything to update
+	oid, ok := cache.toOID[old]
+	if !ok {
+		return
+	}
+	// We'll delete the old entry and add the new entry, keeping the OID the same for the server session
+	delete(cache.toOID, old)
+	delete(cache.toInternal, oid)
+	cache.toOID[new] = oid
+	cache.toInternal[oid] = new
+}