[POC] zstd decompression

crates/core/Cargo.toml

@@ -23,6 +23,7 @@ const_format = "0.2.34"
 futures-lite = { version = "2.6.0", default-features = false, features = ["alloc"] }
 rustc-hash = { version = "2.1", default-features = false }
 streaming-iterator = { version = "0.1.9", default-features = false, features = ["alloc"] }
+zstd-safe = { version = "7.2.4", default-features = false }
 
 [dependencies.uuid]
 version = "1.4.1"

crates/core/src/lib.rs

@@ -34,6 +34,7 @@ mod version;
 mod view_admin;
 mod views;
 mod vtab_util;
+mod zstd;
 
 #[no_mangle]
 pub extern "C" fn sqlite3_powersync_init(
@@ -62,6 +63,7 @@ fn init_extension(db: *mut sqlite::sqlite3) -> Result<(), ResultCode> {
     crate::view_admin::register(db)?;
     crate::checkpoint::register(db)?;
     crate::kv::register(db)?;
+    crate::zstd::register(db)?;
     sync::register(db)?;
 
     crate::schema::register(db)?;

crates/core/src/zstd.rs

@@ -0,0 +1,46 @@
+extern crate alloc;
+
+use alloc::string::String;
+use core::ffi::c_int;
+
+use sqlite::ResultCode;
+use sqlite_nostd::{self as sqlite, Value};
+use sqlite_nostd::{Connection, Context};
+
+use crate::create_sqlite_text_fn;
+use crate::error::SQLiteError;
+use zstd_safe::DCtx;
+
+fn powersync_zstd_impl(
+    _ctx: *mut sqlite::context,
+    args: &[*mut sqlite::value],
+) -> Result<String, ResultCode> {
+    let arg = args.get(0).ok_or(ResultCode::MISMATCH)?.blob();
+    let dict = args.get(1).ok_or(ResultCode::MISMATCH)?.blob();
+    // TODO: Use a form of streaming decompression to avoid pre-allocating a large buffer.
+    let mut dest = alloc::vec![0u8; 1024 * 20];
+    let mut ctx = DCtx::create();
+    let size = ctx
+        .decompress_using_dict(&mut dest[..], arg, dict)
+        .map_err(|_| ResultCode::CORRUPT)?;
+    dest.truncate(size);
+    let text = String::from_utf8(dest).map_err(|_| ResultCode::MISUSE)?;
+    Ok(text)
+}
+
+create_sqlite_text_fn!(powersync_zstd, powersync_zstd_impl, "zstd_decompress_text");
+
+pub fn register(db: *mut sqlite::sqlite3) -> Result<(), ResultCode> {
+    db.create_function_v2(
+        "zstd_decompress_text",
+        2,
+        sqlite::UTF8,
+        None,
+        Some(powersync_zstd),
+        None,
+        None,
+        None,
+    )?;
+
+    Ok(())
+}