chore(doc): add strings feature to doctests

Author: nsarlin-zama

tfhe/docs/tutorials/ascii_fhe_string.md

@@ -1,8 +1,13 @@
 # Homomorphic case changing on Ascii string
 
-This tutorial demonstrates how to build a data type that represents an ASCII string in Fully Homomorphic Encryption (FHE) by implementing to\_lower and to\_upper functions.
+This tutorial demonstrates how to build your own data type that represents an ASCII string in Fully Homomorphic Encryption (FHE) by implementing to\_lower and to\_upper functions.
 
-An ASCII character is stored in 7 bits. To store an encrypted ASCII, we use the `FheUint8`:
+{% hint style="info" %}
+Since version 0.11, **TFHE-rs** has introduced the `strings` feature, which provides an easy to use FHE strings API. See the [fhe strings guide](../guides/strings.md) for more information.
+{% endhint %}
+
+
+An ASCII character is stored in 7 bits. In this tutorial, we use the `FheUint8` to store an encrypted ASCII:
 
 * The uppercase letters are in the range \[65, 90]
 * The lowercase letters are in the range \[97, 122]
@@ -24,13 +29,10 @@ To use the `FheUint8` type, enable the `integer` feature:
 # Cargo.toml
 
 [dependencies]
-# Default configuration for x86 Unix machines:
 tfhe = { version = "0.11.1", features = ["integer"] }
 ```
 
-Refer to the [installation guide](../getting\_started/installation.md) for other configurations.
-
-The `FheAsciiString::encrypt` function performs data validation to ensure the input string contains only ASCII characters.
+The `MyFheString::encrypt` function performs data validation to ensure the input string contains only ASCII characters.
 
 In FHE operations, direct branching on encrypted values is not possible. However, you can evaluate a boolean condition to obtain the desired outcome. Here is an example to check and convert the 'char' to a lowercase without using a branch:
 
@@ -83,7 +85,7 @@ use tfhe::{generate_keys, set_server_key, ClientKey, ConfigBuilder, FheUint8};
 
 const UP_LOW_DISTANCE: u8 = 32;
 
-struct FheAsciiString {
+struct MyFheString {
     bytes: Vec<FheUint8>,
 }
 
@@ -95,7 +97,7 @@ fn to_lower(c: &FheUint8) -> FheUint8 {
     c + FheUint8::cast_from(c.gt(64) & c.lt(91)) * UP_LOW_DISTANCE
 }
 
-impl FheAsciiString {
+impl MyFheString {
     fn encrypt(string: &str, client_key: &ClientKey) -> Self {
         assert!(
             string.is_ascii(),
@@ -140,7 +142,7 @@ fn main() {
 
     set_server_key(server_key);
 
-    let my_string = FheAsciiString::encrypt("Hello Zama, how is it going?", &client_key);
+    let my_string = MyFheString::encrypt("Hello Zama, how is it going?", &client_key);
     let verif_string = my_string.decrypt(&client_key);
     println!("Start string: {verif_string}");
 
@@ -155,3 +157,45 @@ fn main() {
     assert_eq!(verif_string, "hello zama, how is it going?");
 }
 ```
+
+## Using **TFHE-rs** strings feature
+This code can be greatly simplified by using the `strings` feature from **TFHE-rs**.
+
+First, add the feature in your `Cargo.toml`
+```toml
+# Cargo.toml
+
+[dependencies]
+tfhe = { version = "0.11.1", features = ["strings"] }
+```
+
+The `FheAsciiString` type allows to simply do homomorphic case changing of encrypted strings (and much more!):
+```rust
+use tfhe::prelude::*;
+use tfhe::{generate_keys, set_server_key, ConfigBuilder, FheAsciiString};
+
+fn main() {
+    let config = ConfigBuilder::default().build();
+
+    let (client_key, server_key) = generate_keys(config);
+
+    set_server_key(server_key);
+
+    let my_string =
+        FheAsciiString::try_encrypt("Hello Zama, how is it going?", &client_key).unwrap();
+    let verif_string = my_string.decrypt(&client_key);
+    println!("Start string: {verif_string}");
+
+    let my_string_upper = my_string.to_uppercase();
+    let verif_string = my_string_upper.decrypt(&client_key);
+    println!("Upper string: {verif_string}");
+    assert_eq!(verif_string, "HELLO ZAMA, HOW IS IT GOING?");
+
+    let my_string_lower = my_string_upper.to_lowercase();
+    let verif_string = my_string_lower.decrypt(&client_key);
+    println!("Lower string: {verif_string}");
+    assert_eq!(verif_string, "hello zama, how is it going?");
+}
+```
+
+You can read more about this in the [FHE strings documentation](../guides/strings.md)

tfhe/src/lib.rs

@@ -120,7 +120,8 @@ mod js_on_wasm_api;
     feature = "shortint",
     feature = "boolean",
     feature = "integer",
-    feature = "zk-pok"
+    feature = "zk-pok",
+    feature = "strings"
 ))]
 mod test_user_docs;