From 82dac384cd13d97e07a1bca446c0af826696fd27 Mon Sep 17 00:00:00 2001
From: coco <1285015889@qq.com>
Date: Sat, 28 Sep 2024 23:53:48 +0800
Subject: [PATCH] create 2024.09.28 note

---
 River.md | 79 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 79 insertions(+)

diff --git a/River.md b/River.md
index db71459..ad0efd2 100644
--- a/River.md
+++ b/River.md
@@ -916,4 +916,83 @@ balances.get('Maria');
 
 当运行“字典的基础用法”一节中的例子时，并不会执行字典压缩的操作，但是程序还是会编译成功。这是因为压缩操作会通过`Felt252Dict<T>`所实现的`Destruct<T>`特征来自动调用，这个调用仅仅在字典离开其作用域前执行
 
+### 2024.09.28
+
+#### 字典的`Entry`和`Finalize`
+
+`Cairo`提供了`entry`和`finalize`方法来复制字典数据。
+
+* `entry`：通过给定的`key`来创建一个新的entry。一旦被调用，这个方法会获得字典的所有权，并返回要更新的entry。方法签名如下：
+
+  ~~~rust
+  fn entry(self: Felt252Dict<T>, key: felt252) -> (Felt252DictEntry<T>, T) nopanic
+  ~~~
+
+  第一个参数来获取字典的所有权，第二个参数是用来创建合适的entry。该方法返回一个元组：`Felt252DictEntry<T>`表示字典的entry，`T`表示entry的前一个值。`nopanic`表示该方法不会发生panic
+
+* `finalize`：向字典中插入entry，并返回其所有权。方法签名如下：
+
+  ~~~rust
+  fn finalize(self: Felt252DictEntry<T>, new_value: T) -> Felt252Dict<T>
+  ~~~
+
+🌰例子：
+
+自定义`get`方法：
+
+~~~rust
+use core::dict::{Felt252Dict, Felt252DictEntryTrait};
+
+fn custom_get<T, +Felt252DictValue<T>, +Drop<T>, +Copy<T>>(
+    ref dict: Felt252Dict<T>, key: felt252
+) -> T {
+    // Get the new entry and the previous value held at `key`
+    let (entry, prev_value) = dict.entry(key);
+
+    // Store the value to return
+    let return_value = prev_value;
+
+    // Update the entry with `prev_value` and get back ownership of the dictionary
+    dict = entry.finalize(prev_value);
+
+    // Return the read value
+    return_value
+}
+~~~
+
+自定义`insert`方法：
+
+~~~rust
+use core::dict::{Felt252Dict, Felt252DictEntryTrait};
+
+fn custom_insert<T, +Felt252DictValue<T>, +Destruct<T>, +Drop<T>>(
+    ref dict: Felt252Dict<T>, key: felt252, value: T
+) {
+    // Get the last entry associated with `key`
+    // Notice that if `key` does not exist, `_prev_value` will
+    // be the default value of T.
+    let (entry, _prev_value) = dict.entry(key);
+
+    // Insert `entry` back in the dictionary with the updated value,
+    // and receive ownership of the dictionary
+    dict = entry.finalize(value);
+}
+~~~
+
+使用上面自定义的`get`和`insert`方法：
+
+~~~rust
+fn main() {
+    let mut dict: Felt252Dict<u64> = Default::default();
+
+    custom_insert(ref dict, '0', 100);
+
+    let val = custom_get(ref dict, '0');
+
+    assert!(val == 100, "Expecting 100");
+}
+~~~
+
+
+
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