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## Implements a least-recently-used cache for prepared statements based on
## https://github.com/jackhftang/lrucache.nim.
import std / [lists, tables]
from .. / sqlite_wrapper as sqlite import nil
type
Node = object
key: string
val: sqlite.Stmt
StmtCache* = object
capacity: int
list: DoublyLinkedList[Node]
table: Table[string, DoublyLinkedNode[Node]]
proc initStmtCache*(capacity: Natural): StmtCache =
## Create a new Least-Recently-Used (LRU) cache that store the last `capacity`-accessed items.
StmtCache(
capacity: capacity,
list: initDoublyLinkedList[Node](),
table: initTable[string, DoublyLinkedNode[Node]](rightSize(capacity))
)
proc resize(cache: var StmtCache) =
while cache.table.len > cache.capacity:
let t = cache.list.tail
cache.table.del(t.value.key)
discard sqlite.finalize(t.value.val)
cache.list.remove t
proc capacity*(cache: StmtCache): int =
## Get the maximum capacity of cache
cache.capacity
proc len*(cache: StmtCache): int =
## Return number of keys in cache
cache.table.len
proc contains*(cache: StmtCache, key: string): bool =
## Check whether key in cache. Does *NOT* update recentness.
cache.table.contains(key)
proc clear*(cache: var StmtCache) =
## remove all items
cache.list = initDoublyLinkedList[Node]()
cache.table.clear()
proc `[]`*(cache: var StmtCache, key: string): sqlite.Stmt =
## Read value from `cache` by `key` and update recentness
## Raise `KeyError` if `key` is not in `cache`.
let node = cache.table[key]
result = node.value.val
cache.list.remove node
cache.list.prepend node
proc `[]=`*(cache: var StmtCache, key: string, val: sqlite.Stmt) =
## Put value `v` in cache with key `k`.
## Remove least recently used value from cache if length exceeds capacity.
var node = cache.table.getOrDefault(key, nil)
if node.isNil:
let node = newDoublyLinkedNode[Node](
Node(key: key, val: val)
)
cache.table[key] = node
cache.list.prepend node
cache.resize()
else:
# set value
node.value.val = val
# move to head
cache.list.remove node
cache.list.prepend node
proc getOrDefault*(cache: StmtCache, key: string, val: sqlite.Stmt = nil): sqlite.Stmt =
## Similar to get, but return `val` if `key` is not in `cache`
let node = cache.table.getOrDefault(key, nil)
if node.isNil:
result = val
else:
result = node.value.val